Deep Sea Drilling Project Initial Reports Volume 81...CENOZOIC CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY FROM THE NORTH-EASTERN ATLANTIC OCEAN—DEEP SEA DRILLING PROJECT LEG 811 Jan

5. CENOZOIC CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY FROM THE NORTH-EASTERN ATLANTIC OCEAN—DEEP SEA DRILLING PROJECT LEG 811

Jan Backman, University of Stockholm2

ABSTRACT

Using calcareous nannofossils, a biostratigraphic analysis of sedimentary sequences drilled during Leg 81 revealscontinuous upper Neogene deposition at all sites, except at Site 555 where the Pliocene is missing. The series of Pliocenediscoaster disappearances are practically synchronous with their low latitude extinctions. An exception is Discoastersurculus which disappears at the initiation of ice-rafted deposition, 0.1 m.y. prior to its extinction age; D. pentaradiatusdisappears at the beginning of the second lowest cycle of ice rafting, very close to its extinction age. The onset of icerafting is an abrupt event, which reflects the initiation of major northern European glaciation at 2.4 m.y. ago.

The lower and middle Miocene sequences are poorly represented, except at Site 555 which shows an expanded andcontinuous middle Miocene sequence. Site 554 shows the longest Oligocene sequence, but it is condensed (11.5 m) andprobably marred by hiatuses. Manganese formation took place during the early Oligocene (Hole 552A) and during theEocene at Site 554 (overlain by upper Eocene). The upper and middle Eocene intervals are poorly represented; for exam-ple, the Chiasmolithus gigas subzone (Subzone CP13b or Zone NPl5) is only present in a single 2-cm thick horizon(Hole 552A). Site 552 shows the most complete record of the Eocene Zones NPl 1 through NP14, whereas Zone NP10 isconsiderably expanded at Sites 553 and 555. Definite upper Paleocene (Zone NP9) is only recovered at Site 555, wherethis sequence is interbedded between two basalt piles.

INTRODUCTION

Eight holes at four sites were drilled at the southwestmargin of the Rockall Plateau in the northeast AtlanticOcean during Leg 81 of the Deep Sea Drilling Project.Site locations are shown in Figure 1. The sediments re-covered yielded Pleistocene through upper Paleocenecalcareous nannofossils, and their distribution is pre-sented from selected samples in Tables 1-8. Approxi-mately 1200 samples were investigated. The upper Neo-gene and lower Eocene reflect virtually continuous de-position, whereas only fragments of the interveningintervals are preserved. Therefore the Neogene and Pale-ogene sequences are presented and discussed separately.The distribution of taxa is presented through qualita-tively estimated relative abundances. The Neogene ta-bles show three levels of relative abundance: filled cir-cles represent an abundance in excess of 10% of the to-tal assemblage; open circles represent an abundancebetween 1 and 10%; crosses represent an abundance lessthan 1% of the total assemblage.

Smear slides made from the lower Eocene and upperPaleocene shelf sediments contain, as a rule, very lowabundances of coccoliths. In many samples considera-ble difficulties were met in finding enough specimens toestimate relative abundances, and in many other sam-ples this estimate turned out to be completely meaning-less. Because consistent estimates of relative abundancescould not be successfully achieved, Paleogene abun-dances are shown either as "rare" (open squares) or"more commonly occurring" (filled squares). Oligocene

20 10° W

Roberts, D. G., Schnitker, D., et a]., Init. Repts. DSDP, 81: Washington (U.S. Govt.Printing Office).

2 Address: Department of Geology, University of Stockholm, S-106 91 Stockholm, Swe-den.

Figure 1. Locations of sites drilled during Leg 81 (Sites 552-555), Leg48 (Sites 403-404), and Leg 12 (Site 117).

and upper-middle Eocene assemblages are incorporatedin this system for internal consistency in the tables, al-though the way of representing abundances in the Neo-gene samples could have been applied at several samplelevels.

ZONATION

Previous studies of calcareous nannofossil biostratig-raphy from the Rockall area include those of Perch-Nielsen (1972) and Müller (1979). These authors, as wellas the present one, have predominantly relied on bio-stratigraphic zonations suggested by Martini (1971), Bu-kry (1973, 1975), and Okada and Bukry (1980). Theirzonal schemes primarily reflect stratigraphic relation-ships of exits and entries of species in tropical and sub-tropical areas. As a consequence some stratigraphic in-

403

J. BACKMAN

tervals at the Rockall sites, hosting temperate surfacewaters, are characterized by a reduced biostratigraphicresolution or by an absence of marker fossils. The Pa-leocene/Eocene boundary and the lowest Eocene, theupper Oligocene, the middle Miocene, and the lowerPliocene represent such intervals.

Fasciculiths in co-occurrence with Discoaster multi-radiatus are used to denote Zone NP9 of the upper Pa-leocene (Lowrie et al., 1982; Shackleton et al., 1984a),and the boundary between Zones NP10 and NP11 isdrawn at the entry of Tribrachiatus orthostylus (Lowrieet al., 1982). Triquetrorhabdulus carinatus and Cycli-cargolithus abisectus are used to mark the base of ZonesNP25 and NP24, respectively (Müller, 1979). The Neo-gene/Paleogene boundary (Zones NN1/NP25) is placedat the extinction of Dictyococcites bisectus, in accordwith Bukry's (1973) suggestion. Distinction is made be-tween Zones NN6 and NN7 based on the extinctions ofC. abisectus and C. floridanus (Müller, 1981). ZonesNN8 and NN9 are separated on the basis of the extinc-tion of Coccolithus miopelagicus (Bukry, 1973). To datethere are no obvious auxiliary markers that can be usedfor reliable subdivisions of the lower Pliocene in theRockall area.

PRESERVATION

All sites were drilled in moderately deep waters. As arule the Neogene assemblages show varying degrees ofovergrowth of secondary calcite. Dissolution has affect-ed the Neogene assemblages at some levels within thePleistocene and upper Pliocene sequences in conjunc-tion with intense ice-rafted deposition. Samples takenwithin a few centimeters of the upper and middle Mio-cene hiatuses commonly show dissolved assemblages.The condensed Paleogene chalk sequences generally showdissolution, together with some diagenetic overgrowth ofcalcite. For example, whereas the chiasmoliths were dis-solved (central area crosses missing), specimens of Isthmo-lithus recurvus generally appeared as more or less solidbars. The lower Eocene and upper Paleocene assemblagesare primarily affected by dissolution, although diageneticcalcite also played a role in destroying primary morpholog-ical features.

SITE SUMMARIES

Site 552

Two holes (Holes 552 and 552A) were drilled at Site552 (56°02.56'N, 23° 13.88'W, water depth: 2301 m).Hole 552A was hydraulically piston cored, whereas Hole552 was drilled with the conventional rotary drill equip-ment. The first three cores of Hole 552 were spot cored at0, 51, and 108 m respectively, whereafter the hole was con-tinuously cored. Despite being drilled at the same locality,there is a depth discrepancy between the two holes. Corre-latable bio- and lithostratigraphic horizons are locateddeeper in Hole 552A. The difference ranges between ap-proximately 8.4 and 12.4 m. Taking into consideration thatthe individual drilling pipe length is 9.0 m (9.5 m with thecore catcher), it is possible that a single pipe was unac-

counted for during the spot coring of the upper part ofHole 552.

The overlapping sequences of the two holes showpractically identical nannofossil assemblages, so the dis-tribution of the Pleistocene through Miocene taxa areshown from Hole 552A (Table 2) because that sequencehas a better recovery and shows less drilling disturbancethan Hole 552. The Neogene-Paleogene transition isdiscussed below and presented in Figure 2. Table 1 sum-marizes the distribution of Eocene taxa from Hole 552.

Hole 552

160-

Mixed Miocene and Eocene(45-85 cm)

165

-P 170-

175H

180-

Hole 552A

-170

185-

Figure 2. Biostratigraphic correlation across the Neogene/Paleogeneboundary between Holes 552 and 552A. The depth discrepancy isprobably artificial (see text).

404

CENOZOIC CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY

The Neogene of Hole 552

Core 1 of Hole 552 shows an upper Pleistocene as-semblage, lacking Pseudoemiliania lacunosa but withabundant small (<3 µm) placoliths, which are inter-preted as Emiliania huxleyi. The boundary betweenZones NN15 and NN16 is present between 48 and 100cm in Section 552-2-1, with common Reticulofenestrapseudoumbilica at the deeper level. Typical Pliocene dis-coasters occur throughout Core 552-2, including Disco-aster brouweri, D. pentaradiatus, and D. surculus; D.asymmetricus is present in Sample 552-2,CC but not inSample 552-2-1, 100 cm. The reverse relationship is truefor P. lacunosa, which is present in Sample 552-2-1, 100cm but not in the core catcher. Comparison with Hole552A (see below and Figure 5) suggests that the depthdiscrepancy between Holes 552 and 552A is above theextinction level of R. pseudoumbilica.

The range of D. quinqueramus defines Zone NN11,and this species was observed in very low numbers inSamples 552-3-1, 11 cm and 552-3,CC. Zone NN11 con-tinues down to Sample 552-7-1, 143 cm. Ceratolithswere observed at one level in this interval, namely inSample 552-4,CC where Amaurolithus primus and A.tricorniculatus co-occur. Sample 552-7-1, 37 cm con-tains Pliocene contaminants; both Ceratolithus rugosusand D. tamalis are present, although the assemblage hasa predominantly Miocene character. Zone NN9 can sel-dom be identified with accuracy at high latitudes in theNorth Atlantic because D. hamatus is exceedingly rareor absent. Consequently the base of Zone NN10 is diffi-cult to identify with precision. However, Bukry's (1973and subsequent) zonal and subzonal marker species inthis part of the stratigraphic column do appear withsome regularity in the Rockall area. One of these mark-ers is D. loeblichii, which is present from Sample 552-7-1, 143 cm to Sample 552-7-4, 19 cm. The species indi-cates the Discoaster neorectus subzone, which compareswith the upper part of Zone NN10. Discoaster bellus ispresent in Sample 552-7.CC, suggesting a Zone NN9-NN10 assignment for this level. The disappearance ofCoccolithus miopelagicus is a consistent and compara-tively distinct datum event in the Rockall area. Accord-ing to Bukry (1973) it occurs within Zone NN8. In Hole552 this event occurs at Sample 552-8-1, 110 cm, whereCyclicargolithus abisectus is also present in low num-bers, possibly indicating Zone NN6 (Müller, 1981). Re-working of C. abisectus into the upper part of Core 552-8 cannot be excluded, because it is not accompanied byC. ßoridanus.

Zone NN5 with its diagnostic species Sphenolithusheteromorphus can be observed from Sample 552-8-3,20 cm to the Neogene-Paleogene transition at Sample552-8-4, 45 cm, which is marked by a lithologic changeand a hiatus. The occurrence of glauconite in the Mio-cene part of Core 552-8 may indicate that this interval iscondensed, but it has not been possible to determinewhether, above Zone NN5, the NN6-NN8 interval is re-duced or condensed. The nannofossils in the zeoliticnannofossil chalk underlying the indigenous Miocenesediments in Core 552-8 (Sample 552-8-4, 45-85 cm)

represent a mixture of Miocene, Eocene, and probablyOligocene forms. This stratigraphic transition is de-scribed in detail below (see Hole 552A).

The Paleogene of Hole 552The Miocene element between 45 and 85 cm in Sec-

tion 552-8-4 originates from the overlying Zone NN5,whereas the Paleogene element represents a mixture ofprimarily Eocene forms, which are derived from differ-ent stratigraphic levels within the Eocene. The preserva-tion is generally bad in this interval, showing both severedissolution and secondary overgrowth of calcite.

A major part of the lower Eocene and the lowest partof the middle Eocene is represented between the sedi-ment/basalt contact and the Neogene-Paleogene transi-tion in Hole 552 (Table 1). This part of the sequence ismore complete at Site 552 as compared to the corre-sponding sequences at the adjacent Site 403 of Leg 48and Sites 553, 554, and 555 of Leg 81, although it isconceivable that the sequence at Site 552 may also be in-complete.

Zone NP14 is represented from Sample 552-8,CC toSample 552-10,CC (Core 552-11 and its core catcherwere empty). Chiasmoliths and the ubiquitous Coccoli-thus pelagicus dominate the assemblages, together withReticulofenestra dictyoda at some levels. Discoastersublodoensis occurs in all samples in this interval. Thepresence of Nannotetrina cristata throughout the inter-val indicates that only the upper part of Zone NP14 isrepresented between Samples 552-8,CC and 552-10,CC(see Bramlette and Sullivan, 1961; Romein, 1979). Thisis supported by the absence of D. lodoensis in the up-permost sample (Sample 552-8,CC). It is not clearwhether the lower part of Zone NP14 is missing at Site552 or whether it is represented by the underlying inter-val of no recovery.

The assemblage in Zone NP13 differs from that inZone NP14 by showing a comparatively higher abun-dance and a greater diversity of pontosphaerids. Thisfeature also characterizes Zones NP12 and NP11 (see al-so Sites 553 and 555). Similar abundance and diversitypatterns of the pontosphaerids, particularly as concernZone NP12, have been documented by Bramlette andSullivan (1961) from the Lodo Formation in California,by Okada and Thierstein (1979) from Deep Sea DrillingProject (DSDP) Site 386 (western North Atlantic), andby Romein (1979) from the Nahal Advat sequence in Is-rael.

Only one section in Core 552-13 was filled with sedi-ment, and samples from this section were barren of nan-nofossils. Samples from Core 552-14 show poorly pre-served assemblages, affected both by dissolution and di-agenetic overgrowth of calcite. Arguments can be raisedfor placement of the lower boundary of Zone NP13 attwo levels: within Section 552-14-2 or between Sections552-15-1 and 552-16-1.

The guide species, Tribrachiatus orthostylus, is presentas a very rare component of the assemblage at Sample552-14-2, 56-58 cm. This level represents the uppermostoccurrence of T. orthostylus and thus should be consid-ered as the top of Zone NP12, an assignment not dis-

405

Table 1. Distribution of Paleogene calcareous nannofossils at Hole 552.

"Ö 3 o o i5 •S e

•O D t )

Q Ci Q CL CU CL CL Q.

cn en

MIXED MIOCENE AN

CP12 N P K

NP13

CP11

1 2 - 1 .12-3,12, CC1 3 - 1 .K-2,

110131-132

15026-27

R MF MF P

BARRENR M

D D D D O

NP13or

NP12

K-2,U-4,15-1,

56-5825-2610

F PF PF P

CP10 NP12

16-1,18-1,18-1,18-2,

31

8123-2^.

F PC PC MC P

CP9b NP11

18-2,19, CC21-1,21-2,

6011039

C PBARRENF PF PR P

π ππ π α

NP10? 21-3, 60 F P

Note: Open squares = rare, filled squares = more commonly occurring.


agreeing with the magnetostratigraphy. Core 552-14 ischaracterized by normal polarity (Krumsiek, this vol-ume), which probably represents magnetic Anomaly 23.T. orthostylus disappears within Anomaly 23 accordingto Lowrie et al. (1982), which justifies a top Zone NP12assignment at Sample 552-14-2, 56-68 cm, but it shouldbe kept in mind that Cores 552-15 and 552-16 have a re-covery of 14 and 11% respectively, and do not preserveany known magnetic polarity direction. These cores conse-quently also may fit the correlation between Anomaly 23and the T. orthostylus extinction. Unfortunately, the iso-lated occurrence of T. orthostylus in Section 552-14-2 can-not, at least with any degree of certainty, be disregarded asdue to reworking since (1) one may assume that ecologicalgradients have been strong in the shallow-water environ-ment in which these sediments were deposited, thus pro-viding conditions that could favor sporadic occurrences ofmany taxa; (2) isolated, apparently indigenous, occur-rences of taxa are not uncommon (see, for example, thelowest and next to lowest occurrence of D. lodoensis in Ta-ble 1); (3) reworking appears not to be significant in the se-quence. The occurrence of T. orthostylus at Section 552-14-2 thus may represent an indigenous presence.

Another line of reasoning suggests that placing the topof Zone NP12 within Section 552-14-2 would be errone-ous. The first appearance of R. dictyoda was observed inSample 552-14-4, 25-26 cm. Based on material from thevery adjacent Site 404, which shows a sequence and a nan-nofossil assemblage highly comparable to that of Hole552, Müller (1979) observed that R. dictyoda (referred toas R. umbilicá) has its first appearance close to the base,but within, Zone NP13. Her result is in perfect agreementwith that of Romein (1979). Placing the Zone NP12/ZoneNP13 boundary within Section 552-14-2 thus implies thatR. dictyoda appears within Zone NP12, which is inconsis-tent relative to Müller's and Romein's results. The upper-most continuous presence of T. orthostylus is in Section552-16-1, which could justify a placement of the ZoneNP12/Zone NP13 boundary between Sections 552-15-1and 552-16-1 and which would cause a consistent first ap-pearance of R. dictyoda close to the base of Zone NP13.The two possible positions of the Zone NP12/Zone NP13boundary at Site 552 are indicated in Table 1. Interestingly,the base of Bukry's (1973) Zone CPU (first appearance ofC. crassus) coincides with the base of Martini's ZoneNP13 if placed between Sections 552-15-1 and 552-16-1.

Several changes occur in the assemblage between ZoneNP13 and Zone NP12. The latter zone shows the follow-ing characteristics: a downward increase in braarudosphae-rids; a decrease in the diversity of chiasmoliths; C. eo-grandis becoming the most frequently occurring chiasmo-lith taxon; C. cribellum is common at some levels and isone of the dominant taxa in one sample; micrantholiths,fragments mostly, start to occur; sphenoliths begin to occurconsistently, including Sphenolithus anarrhopus whichhas its highest occurrence in this zone (at this site); Zy-godiscus plectopons is restricted to this zone at Site 552;Zygrhablithus bijugatus becomes one of the major ele-ments of the assemblages; Toweius occultatus disap-pears at the top part of the zone; Prinsius bisulcus de-creases significantly in abundance, and last, a few speci-

mens of the distinct species D. robustus were noted forthe first time, going downwards, toward the base ofZone NP12.

An unchanged assemblage character of Zone NP12,as compared to that of Zone NP13, is that D. kuepperiremains the most abundant discoaster.

Zone NP11 is not used in the sense of Martini's origi-nal definition since its base here is defined by the firstappearance of Tribrachiatus orthostylus, following thesuggestion by Lowrie et al. (1982). T. orthostylus ispresent in Sample 552-21-3, 39 cm but was not ob-served, even after long search, in a sample taken 5 cmabove the sediment/basalt contact in Sample 552-21-3,65 cm. The last occurrences of Toweius magnicrassusand Ellipsolithus macellus coincide with the first ap-pearance of Tribrachiatus orthostylus in Sample 552-21-3, 39 cm. The implication of this in relation to the rela-tive age of the sediment/basalt contact is discussed be-low, where it is concluded that it seems likely that thecontact occurs within, but close to the base of, ZoneNP11 as defined here, despite the absence of T. orthos-tylus at the 60-cm level in Section 552-21-3. However,two possible interpretations are indicated in Table 1.

The Neogene of Hole 552A

The greater part of the sequence from Hole 552A isunique in that it preserves the most complete and leastdisturbed upper Neogene sedimentary record ever re-trieved from a high-latitude North Atlantic DSDP site.This sequence consequently provides the best materialpremises from that region for studies of late Neogenepaleoceanography and paleoclimatology. Only two coresare severely disturbed (Cores 552A-6 and 552A-13). Nohiatuses were discovered above Core 552A-35 (lowestlower Miocene), but the resolution decreases below Core552A-29 owing to low sedimentation rates. A remark-able character of the sequence is its long record ofnorthern European glacials-interglacials, including thetransition from preglacial to glacial conditions. The re-cord of ice-rafted material of the Rockall area representsa direct evidence of past ice ages in northern Europe.Studies of this sequence therefore will improve our un-derstanding of how, more or less, indirect indications ofpaleoclimatic and paleoceanographic changes—like sta-ble isotopes, faunal and floral paleobiogeographic pat-terns, and sedimentary processes such as winnowing,erosion, and dissolution—are related to the glacial-in-terglacial climatic system in northern Europe. This isparticularly true for the Pliocene part of the Hole 5 52Asequence since there exists a controversy, or confusion,about the timing of the initial phase of the glacial evolu-tion in the northern hemisphere. Moreover, at presentthere is little consensus about which paleoceanographicchanges represent responses to the northern hemisphereglacial-interglacial system and which changes representresponses to other phenomena; the time interval be-tween 2.5 and 3.2 m.y. ago is of particular interest inthis respect (see, for example, Berggren, 1972; Pooreand Berggren, 1974; 1975; Shackleton and Kennett,1975a; Shackleton and Opdyke, 1977; Ledbetter et al.,1978; Backman, 1979; Keigwin and Thunell, 1979; Shor

407

J. BACKMAN

and Poore, 1979; Hodell et al., 1983). This chapter is fo-cused on the biostratigraphy of Leg 81, and the Hole552A sequence thus is discussed primarily from a bio-stratigraphic and chronologic point of view.

The samples from Hole 552A shown in Table 2 repre-sent only a fraction of the number of samples investi-gated from that sequence. However, those shown illus-trate time-related changes in the assemblages. It appearsimmediately from Table 2 that the qualitatively esti-mated abundances in each sample are characterized by afew dominant taxa, which are accompanied by a num-ber of rare taxa (<1%). Individual discoaster speciesconsistently occupy less than 1% of the total assem-blage, but on the generic level discoasters may reach 1-2% of the assemblage in the occasional sample.

The oxygen isotope stratigraphy of Hole 552A (Shack-leton and Hall, this volume) presents a very refined chro-nostratigraphy for the Pleistocene sequence. Two reli-able nannofossil events in the Pleistocene can be deter-mined using light-microscope techniques: the last occur-rences of Pseudoemiliania lacunosa and Calcidiscus mac-intyrei. Both events have been quantified using themethodology of Backman and Shackleton (1983).

The extinction of P. lacunosa is shown in Figure 3,together with the calcium carbonate percentages. It isevident that the abundance pattern of P. lacunosa variesindependently of the calcium carbonate record. The ex-tinction at Sample 552A-2-3, 60 cm occurs in the middle

8 0 -

4<H

0 J

8 -

4 -

0 J

Section 552A-2-3 2-4i r

S18O Stage 12Section 552A-2-3

13

2-4

40 80 120Depth in section (cm)

10

Figure 3. Abundance of P. lacunosa at the time of extinction. Notethat there is no correlation between calcium carbonate percentage(Zimmerman, this volume) and P. lacunosa's abundance. Oxygenisotope stages are from Shackleton and Hall (this volume).

part of oxygen isotope Stage 12, which is in excellentagreement with the results of Thierstein et al. (1977). Itis notable, however, that although these authors docu-mented a relative abundance of approximately 0.5% ofP. lacunosa in K708-7 immediately prior to its extinc-tion, the figure is much lower in the Hole 552A se-quence. Hole 552A is only about two degrees latitudenorth of K708-7. Percentages of P. lacunosa have notbeen determined from Hole 552A, but it can be esti-mated that 10 specimens of P. lacunosa in Figure 3grossly represent a few per thousand of the total assem-blage; the content of 20 view fields was counted in eachsample and approximately 200 nannofossil specimenswere present in each view field. Thus, for reasons un-known, P. lacunosa is considerably less abundant atHole 552A than at K708-7 near its extinction level.

The last occurrence of Helicosphaera sellii is in Sam-ple 552A-5,CC. Backman and Shackleton (1983) dem-onstrated that this species has a diachronous disappear-ance with latitude. Its last occurrence is not examined indetail in Hole 552A, but nevertheless, it is later in Hole552A than in the equatorial Pacific. There appears to bea difference of several hundred thousand years betweenthe Pacific date (1.37 m.y. ago in V28-239) and the onefrom Hole 552A (0.93-1.17 m.y. ago; Samples 552A-4,CCto 552A-5,CC).

The last occurrence of C. macintyrei is shown in Fig-ure 4, where the calcium carbonate record is also pre-sented. The species disappears within an interval char-acterized by high calcium carbonate values, that is,within an interglacial. This suggests that the disappear-ance of C. macintyrei in Section 552A-7-1 is the true ex-tinction. The rise in abundance at the bottom of Section552A-7-1 coincides with an increased percentage of cal-cium carbonate. From the bottom of Section 552A-7-2

- 80-

40

0 J

<N~30E

Iò•20

10-

0 J

Section 552A-7-1 Section 552A-7-1

120 10 50Depth in section (cm)

130

Figure 4. Abundance of C. macintyrei at the time of extinction. Seetext for discussion of co-variation between calcium carbonate per-centage (Zimmerman, this volume) and abundance of C. macinty-

408


upwards, the abundance of C. macintyrei and the calci-um carbonate values decrease significantly although thehigh carbonate percentage persists up to one sample in-terval (10 cm) above the high abundance interval of C.macintyrei. The lag may be due either to a sample posi-tion bias or to an ecologically governed behavior of C.macintyrei at the transition from interglacial to glacial(cooling of surface waters precedes the short glacialstarting at Sample 552A-7-2, 120 cm?). The high calci-um carbonate values between approximately 60 and 80/90 cm in Section 552A-7-2 are not accompanied by anincrease in abundance of C. macintyrei. However, the60-, 70-, and 80-cm levels in Section 552A-7-2 are char-acterized by a virtually monospecific assemblage of C.pelagicus. This very characteristic monospecific compo-sition was also observed at the outskirts of other glacialintervals. The phenomenon has not been systematicallyinvestigated.

Little attention has been paid to the series of first ap-pearances of gephyrocapsids (Gephyrocapsa aperta, G.caribbeanica, and G. oceanica) during late Pliocene andearly Pleistocene times. Samtleben (1980) demonstratedconvincingly that a thorough morphometrical investiga-tion of gephyrocapsids from many oceanic regions isneeded before the group can provide any reliable bio-stratigraphic information.

The series of disappearances of Pliocene discoasters,a warm(er)-water-preferring group, has been investigatedin detail, partly because it is not fully understood whetherthese disappearances reflect migratory events or genuineextinctions in the high-latitude North Atlantic. The fol-lowing taxa are involved: Discoaster brouweri, and itstriradiate variety, D. pentaradiatus, D. surculus, D.asymmetricus, D. tamalis, and D. variabilis.

The relative abundances of these discoasters are verylow in the investigated interval (from the lower half ofCore 552A-7 through Core 552A-10). Two reasons argueagainst reworking as being the mechanism that explainsthe presence of discoasters in the late Pliocene interval.

First, discoasters are rare throughout the Neogenewhich implies that a considerable amount of pre-latePliocene sediment has to be reworked into the late Plio-cene interval in order to get reworked discoasters. Onlynegligible amounts of pre-late Pliocene contaminantsare present in the late Pliocene interval; for example Re-ticulofenestra pseudoumbilica, which has a late earlyPliocene extinction datum and is very abundant throughmuch of the Neogene, is very rare in the late Pliocene(Fig. 5). On this basis, little reworking of pre-late Plio-cene material into the late Pliocene is considered to haveoccurred. Second, Backman and Shackleton (1983) doc-umented a rise in abundance of the triradiate variety ofD. brouweri relative to D. brouweri', a peak abundancelasting approximately 0.1-0.15 m.y. before their simul-taneous extinction. This increase was found to occur con-sistently whenever the appropriate stratigraphic intervalwas investigated in detail. Moreover, the triradiate varie-ty was observed to be rare during the entire part of itsrange that precedes the last period of peak abundance,despite very high abundances of discoasters. Therefore,the peak abundances of the triradiate variety relative toD. brouweri are considered to be indigenous, becausereworking cannot produce peak abundances of thisform relative to D. brouweri (Backman and Shackleton,1983). This characteristic increase of the triradiate varie-ty also exists in the Hole 552A sequence.

It is estimated that, in the late Pliocene interval ofHole 552A, a discoaster/coccolith ratio of 1/10,000 or

320

280

^240

.§200

"§ 160

<t•£120a.α

| 80-

40

0-

Drilling disturbance

Core552A-12 Core 552 A-13 Core 552A-14

55 60Depth sub-bottom (m)

65

Figure 5. Abundance of R. pseudoumbilica in Cores 552A-11 through 552A-14. Three samples analyzedfrom Core 552A-13 revealed an abundance not exceeding that of Cores 552A-11 and 552A-12. The dis-tinct decrease in abundance at the very top of Core 552A-14 is considered to represent the extinction ofR. pseudoumbilica. Notice low level of reworking in Cores 552A-11 and 552A-12.

409

J. BACKMAN

1/20,000 is the rule rather than the exception. Becauseof their very low abundance, discoasters are of courseeasily overlooked or neglected as being reworked, al-though they are not, according to the above observa-tions.

Samples were investigated every 10 cm, with the fol-lowing results: D. brouweri disappears at Sample 552A-8-1, 40 cm and the triradiate variety 10 cm higher up.The peak abundance of the triradiate variety begins inthe middle part or lower half of Core 552A-8. The lastoccurrence of D. pentaradiatus is at Sample 552A-9-3,20 cm. At Sample 552A-9-4, 10 cm D. surculus disap-pears; D. asymmetricus disappears at Sample 552A-9,CC (1 cm), and D. tamalis disappears at Sample552A-10-1, 20 cm. The highest occurrence of D. variabi-lis is probably situated in the lower part of Core 552A-10, but sporadic occurrences make this difficult to as-certain.

The disappearance of R. pseudoumbilica is the low-ermost event that has been quantified in the Hole 552Asequence (Fig. 5). The profound decrease in abundanceat the top of Core 552A-14 is interpreted as the extinc-tion level of R. pseudoumbilica. Core 552A-13 is se-verely disturbed and the sediment is homogenized; asample from a particular level may represent any levelwithin Core 552A-13. Three samples analyzed fromCore 552A-13 did not show an abundance ofR. pseudo-umbilica exceeding that in Core 552A-12.

Pliocene paleomagnetic reversals of Hole 552A(Shackleton et al., 1984b) and the nannofossil extinctionevents from C. macintyrei to R. pseudoumbilica areplotted versus time in Figure 6. The age assignments ofthe extinctions are derived from low and middle lati-

tudes of the Pacific Ocean (Backman and Shackleton,1983). The disappearance of D. brouweri in Hole 552Acorrelates well with its extinction date from low lati-tudes; the same is true for C. macintyrei. Both D. pen-taradiatus and D. surculus have disappearance levels inHole 5 52A coinciding with transitions from oozes toice-rafted marls; D. surculus at the beginning of thelowest cycle of ice-rafted debris, and D. pentaradiatus atthe beginning of the second cycle. It appears from Fig-ure 6 that the disappearance of D. surculus in Hole552A clearly occurs close to 0.1 m.y. before its Pacificextinction age. Depending on how the sedimentationrate curve is drawn in Figure 6, D. pentaradiatus eitherdisappears 0.05 m.y. prior to its Pacific date or coin-cides, approximately, with that date. Backman andShackleton (1983) did not provide any extinction age forD. asymmetricus, but they suggested that this datumevent occurs between the top of Gauss and the base ofOlduvai. In Hole 5 52A, D. asymmetricus disappears be-tween 43.8 and 44.5 m, which is well within the upperGauss. D. tamalis appears to have a slightly youngerdisappearance age in Hole 552A than in the low-latitudePacific. However, the last relatively substantial abun-dance of D. tamalis is 0.7 m further down in Core 552A-10, and one cannot exclude the possibility that a fewstray specimens were reworked up to the last occurrencelevel shown in Figure 6. If this is the case no differencewould exist between the Hole 552A and the Pacific ages.D. variabilis has an indistinct disappearance in thelower part of Core 552A-10.

Surprisingly, one may conclude that the series of latePliocene discoaster disappearances at this high-latitudelocality agrees well with their low- and midlatitude ex-

30 35Magnetics, cores, and depth (m) in Hole 552A

40 45 50 55

1.6

C. macintyrei

Matuyama/Olduvai

Olduvai/Matuyama

D. pentaradiatus

" JJ nD. surculus

60

Gauss/Kaena

Kaena/Gauss\Gauss/Gilbert

R. pseudoumbilica

3.6

Figure 6. Sedimentation rate plot of the early Pleistocene and late Pliocene of Hole 552A. The timescale ofNess et al. (1980) is used. The magnetostratigraphy of Hole 552A is from Shackleton et al. (1984b). The plotindicates a significant change in sedimentation rate at 3.25 m.y. ago, where the rate decreases by a factor ofapproximately 3.5 going upwards. See text for further discussion of the data shown.

410


tinction ages, although it is far more tedious to deter-mine the extinctions at high latitudes due to low relativeabundances.

The last occurrence of nonbirefringent ceratoliths,which defines the top of Zone NN14, cannot be deter-mined accurately at high latitudes because of their dis-continuous, extremely rare occurrences. The same prob-lem hampers determination of the top of Zone NN12(first appearance of C. rugosus). The top of Zone NN13(first appearance of D. asymmetricus) cannot be deter-mined accurately at any latitude (Backman and Shackle-ton, 1983). The rare findings of these taxa in Hole 552Aare shown in Table 2. The first continuous presence ofD. asymmetricus begins at Sample 552A-14-2, 20 cm(note proximity to R. pseudoumbilica extinction), wherealso the lowest occurrence of D. tamalis was observed.The latter species begins to occur continuously fromSample 552A-14-1, 50 cm, thus shortly overlapping theextinction level of R. pseudoumbilica. This pattern isconsistent with that documented from low latitudes(Backman and Shackleton, 1983).

Specimens of C. rugosus were observed at the bottomof Section 552A-19-2, and Ceratolithus acutus is presentin Sample 552A-22,CC. Zone NN11 ranges from Core552A-27 to Core 552A-33. It has not been possible tosubdivide this zone in Hole 552A since nonbirefringentceratoliths were not recorded, in contrast to observa-tions in Hole 552. The lowermost presence of D. quin-queramus is in Sample 552A-33-3, 100 cm, and D.loeblichii and D. neohamatus are present in Sample552A-33,CC, suggesting that the latter level belongs toZone NN10. The high abundance of large Dictyococci-tes perplexus is conspicuous in the Zone NN9-ZoneNN10 interval. Previously this species has been reportedfrom high latitudes in the southern hemisphere (Burns,1975; Haq, 1976—who referred to the species as D. an-tarcticus) and from the Rockall area (Site 116; Back-man, 1980).

Müller (1981) suggested that C. abisectus and C. flo-ridanus disappear within Zone NN6. The absence ofthese taxa in Sample 552A-35,CC and the presence of C.miopelagicus (see Bukry, 1973) indicate that this levelcan be referred to Zones NN7 or NN8.

At high latitudes the calcareous nannofossil biostrati-graphic resolution is low from the middle Miocenethrough the lower Pliocene, due to the scarcity of mark-er fossils. In order to increase the resolution, it appearsthat one has to make the laborious detour via counts ofrelative abundances and determination of acme hori-zons, as outlined by Haq and Lohmann (1976). This re-quires distinct changes in dominance of assemblage ele-ments over time and well-dated sequences. The mostpromising assemblage elements in this respect are con-sidered to be C. pelagicus, D. productus (Pliocene), D.perplexus (Miocene), and the reticulofenestrids.

The Paleogene of Hole 552AThe Paleogene of Hole 552A is represented from

Sample 552A-36-3, 140 cm down to Sample 552A-38,CC (172.9-183.5 m). The Neogene is separated fromthe Paleogene by a hiatus encompassing the lower Mio-

cene and uppermost Oligocene. Sediment of Oligoceneage characterizes lower Core 552A-36 and upper Core552A-37. Most of Section 552A-37-1 is severely dis-turbed, partly due to intense burrowing; consequently itis difficult to assign this interval to specific biostrati-graphic zones based on primary markers. However, theinterval from Section 552A-37-2 to Sample 552A-38,CCcontains sediments that belong to Zone NP14 of themiddle Eocene. Reworking is common particularly inSection 552A-37-1, in which the stratigraphic order is il-logical at some levels, with a sample providing a certainbiostratigraphic zonal assignment having a stratigraphi-cally higher position than a sample giving a youngerzonal assignment. Thus, detailed stratigraphic analysisis not meaningful in Section 552A-37-1. The stratigra-phy of the short Paleogene sequence in Hole 552A isbased on 35 samples, but the samples discussed beloware those which can be arranged in a logical stratigraph-ic order (see Fig. 2, where the correlation to Hole 552 isshown).

Sample 552A-36-3, 141 cm and several others downto Sample 552A-36,CC (15 cm) show a virtually uniformcomposition, in which Cyclicargolithus floridanus, Cocco-lithus pelagicus, Chiasmolithus altus, Dictyococcites bisec-tus, D. hesslandii, Reticulofenestra daviesi, and Zygrhabli-thus bijugatus are dominating elements. Sphenolithus dis-tentus occurs occasionally and in very low numbers. More-over, Triquetrorhabdulus carinatus is absent and Cyclicar-golithus abisectus is common in this sequence except in thelowest sample, in which it is rare. A Zone NP24 assign-ment therefore appears reasonable for the lowermost partof Core 552A-36 (see Poore et al., 1982; Müller, 1979).

Sample 552A-37-1, 15 cm is taken immediately abovea big manganese nodule. Although this level lacks Isth-molithus recurvus, it is otherwise similar in compositionto the 55- and 70-cm levels in Section 552A-37-1.Among the most abundant taxa are Chiasmolithus al-tus, Coccolithus pelagicus, Cyclicargolithus floridanus,D. bisectus, D. hesslandii, Ericsonia fenestrata, R. dic-tyoda, R. hillae, R. umbilica, and Z. bijugatus. Othertaxa occur sporadically {Chiasmolithus oamaruensis,Coccolithus eopelagicus, D. deflandrei, E. subdisticha,Helicosphaera bramlettei, and S. moriformis). One speci-men of C. formosus (extinction defines Zone NP21/ZoneNP22 boundary) was observed in both the 15- and 70-cm levels, but not at the 55-cm level, in Sample 552A-37-1. This exceedingly rare presence is likely to representreworking and is therefore neglected. The late Eocenediscoasters were not observed.

Bukry (1975) demonstrated that /. recurvus shows apreference for mid to high latitudes, suggesting that itspresence in Samples 552A-37-1, 55 cm and 552A-37-1,70 cm is indigenous. There exists some uncertainty as tothe upper stratigraphic range of /. recurvus, but studiesfrom high latitudes in the North Atlantic region (Marti-ni, 1971; Perch-Nielsen, 1972; Müller, 1979) indicatethat /. recurvus disappears within Zone NP22. Takingthis into account, it appears reasonable to suggest that(1) Sample 552A-37-1, 15 cm belongs to the upper partof Zone NP22 (R. umbilica is present, /. recurvus is ab-sent), (2) the 55- and 70-cm levels in Section 552A-37-1

411

N>

Table 2. Distribution of Neogene calcareous nannofossils at Hole 552A.E

po

ch

PL

EIS

TO

CE

NE

UJoo

Q_Ul

UlzUloo_JQ_

CC<Ul

UlzUlooΣUl

<_l

2] öcr o2 Σ

Zo

ne

(O

kad

a &

Buk

ry,1

98

0)

CN 14b-

CN13-

Ua

CNi2d

CN12

CN11

CN10c

CN 10a

b

CN9

CN7-8

CN5b-6Λ/W<

CN L

Zo

ne

(M

art

ini,

197

1)

NN 20-

NN19

NN18

NN16-

17

NN15

NN13-

K

NN12

NN 11

NN 9-10

NN5

Sa

mp

le

(55

2A

c

ore

-se

cti

on

cm

)

1, CC

2, CC

3, CC

U, CC

5, CC

6, CC

7, CC

8, CC

9, CC

10, CC

11, CC

12, CC

13, CC

U , CC

15, CC

16, CC

17, CC

18, CC

19, CC

20, CC

21, CC

22, CC

23, CC

2k. CC

25, CC

26, CC

27, CC

28, CC

29, CC

30, CC

31, CC

32, CC

33, CC

3k. CC3 5 CC

3 6 - 1 , 100

36-3,130

Ab

un

da

nc

e

Pre

serv

ati

on

A G

A G

C P

A G

A G

A G

A M

A G

A G

A G

A G

A G

A G

A G

A G

A G

A M

A M

A M

A M

A M

A M

A G

A M

A G

A G

A M

A M

A M

A M

A M

A M

A M

A MA M

A M

A M

Am

auro

lith

us

de

lica

tus

A.

pri

mu

sC

atci

dis

cus

lep

top

oru

sC•

m

acin

fyre

iC

era

folit

hu

s a

cu

tus

o

o

+ +

o +

+ +

o +

o + +

o

•f +

+ + •

+

C. r

ug

osu

sC

occ

olit

hu

s m

iop

ela

gic

us

C

pe

lag

icu

sC

ra

dia

tus

Cyc

lica

rgo

lifh

us

abis

ect

us

o

o

o

0

+

0

0

o

1•

|

| +

# +

++ o+ +

C.

flo

rid

an

us

Dic

tyo

cocc

ites

pe

rpte

xus

D• p

ro d

ue tu

sD

isc

oa

sfe

r a

da

ma

nfe

us

D• a

sym

met

ricu

s

oo

oo

o

+

•o

o +

o +

o

. +

•

+ 0

O

#

•

o +

D• b

ellu

sD•

bro

uw

eri

D• c

a I c

ar is

D• c

ha

llen

ge

riD•

d

efl

an

dre

iD•

exi

lis

+

+

+

+ + ++

+ +

D•

inte

rca

lari

sD•

ku

gle

riD

lo

eb

lich

iiD

. n

eo

ha

ma

tus

D.

pe

nta

rad

iatu

s+

+

+

+ +

+ +

D.

pre

pe

nta

rad

iatu

sD•

p

seu

do

vari

ab

ilis

D• q

uin

qu

era

mu

sD

. su

rcu

lus

D• t

am

alis

+

+ +

+

D• v

ari

ab

ilis

Em

ilian

ia

hu

xle

yiG

em

inili

fhe

lla

rotu

laG

ep

hyr

oca

psa

sp

p.

He

lico

sph

ae

ra

ch

ate

ri

• o o

• o

• +

o +

+ + +

+ o

+ + +

+ +

+ ++ + -f

H• g

ran

ula

taH

. s

ellii

Lith

ost

rom

afi

on

pe

rdu

rum

Po

nto

sph

ae

ra

jap

on

ica

P.

jon

esi

+

o +

+ + +

+

+

+

+

+

+

+

P

mu

lfip

ora

Pse

ud

oe

mili

an

ia l

ac

un

osa

Pyr

ocy

clu

s he

rmo

sus

P

in v

ers

us

Re

tic

ulo

fen

est

ra

ha

qii

o

+ o

0

•

-f O

O O

+

+

1 •f

+

1

+o

o

o

o

o

•f -f

+ +

+

R.

min

uta

R• m

inu

tula

R.

pse

udo

umb

itica

Rh

abd

osp

hae

ra

cla

vig

era

Scy

ph

osp

hae

ra

spp

o +

o +

•

• + +• o + +o +

• +

+ o o +

+ o o +

• 0 O

+ +• o +• + +o +• + +o ++ +•+ +

+ +o +

Sp

he

no

lith

us

ab

ies

S• h

eter

omor

phu

sS

m

on

form

isS

ne

oa

bie

sS

yra

co

sph

ae

ra

spp

.

+

+

+

+

+

+

+ +

• + +

Tnq

ue

tro

rhab

du

lus

rug

osu

sZ

ygrh

ab

lith

us

biju

ga

tus

+

+

+

o

Note: filled circles = abundance > 10% of total assemblage, open circles = abundance 1-10% of total assemblage, crosses = abundance of total assemblage.


represent the lower part of this zone (/. recurvus ispresent), and (3) the manganese formation took placeduring the lower part of Zone NP22. A second possibili-ty is provided by the information presented by Poore etal. (1982). Their data—derived from 26°S—suggest that/. recurvus disappears within the upper part of ZoneNP21, at the top of magnetic Anomaly 13. This wouldplace the Sample 552A-37-1, 15 cm in Zone NP22 andthe samples at the 55- and 70-cm levels in Zone NP21.However, the first possibility, based on data derivedfrom 45-55°N, which is supported by data from Site554 (see below), indicates that the observation of Pooreet al. (1982) is not valid for the high-latitude North At-lantic region. Nevertheless, it follows that Zone NP23,which represents a substantial part of the Oligocene,is missing between Samples 552A-36,CC (15 cm) and552A-37-1, 15 cm. It is also noteworthy that the manga-nese nodule was formed at a time characterized by oneof the most pronounced reorganizations of deep watersthat has occurred in the Cenozoic world ocean (see, forexample, Shackleton and Kennett, 1975b).

A downhole change in lithology occurs at approxi-mately Sample 552A-37-1, 75 cm, from foraminifer-nannofossil chalk to zeolitic mudstone. Two samplesfrom the zeolitic mudstone, Samples 552A-37-1, 86 cmand 552A-37-1, 117 cm, are barren of nannofossils, buta 2-cm-thin band of the foraminifer-nannofossil chalk,Sample 552A-37-1, 137-138 cm, contains nannofossilsclearly indicating Zone NP15 of Martini (1971) or sub-zone CB 13b (C. gigas subzone) of Okada and Bukry(1980). This thin band is the only sediment recoveredduring Leg 81 that represents the middle Eocene ZoneNP15. Typical forms include: Chiasmolithus expansus,C. gigas, Chiasmolithus grandis, C. solitus, Coccolithuspelagicus, C. formosus, D. barbadiensis, Markalius spp.,Nannotetrina fulgens, R. dictyoda, R. reticulata, andRhabdosphaera tenuis.

Zones NP16 through NP19 and NP20, or CP13cthrough CP15, are missing (through a hiatus) in Hole552A.

The nannofossils in the sediments from Samples552A-37-2, 20 cm to 552A-38,CC indicate Zone NP14.Consequently the lowest part of Zone NP15 (CP13a) isalso missing in this sequence. Probably only the upperpart of Zone NP14 (CP12b, Rhabdosphaera inflatasubzone of Okada and Bukry, 1980) is represented, sinceR. inflata and N. cristata occur sporadically in the se-quence, the latter including Sample 552A-38,CC. Theassemblage within the Zone NP14 interval of Hole 552A isvery similar to the one that is presented in Table 1 fromHole 552.

Site 553Three holes were drilled at Site 553 (56°05.32'N;

23°20.61' W; water depth: 2329 m): Hole 553 consists ofa single core cut at the mudline; 59 cores were taken atHole 553A, the upper 37 of which are sedimentary andthe remainder of which are basaltic; the four cores ofHole 553B were hydraulically piston cored using the 9-meter core barrel, which unfortunately caused severe dis-turbance.

The Neogene of Holes 553, 553A, and 553B

The Neogene cores of Site 553 have two characteris-tics making them less attractive for study. First, thecores are severely disturbed by drilling processes, includ-ing those cored using the hydraulic piston core (HPC) atHole 553B. Second, a major part of the Neogene se-quence was spot cored. The Miocene/Oligocene bound-ary is determined at 231.5 m, but continuous coringstarted at a depth of 179.5 m (Core 553A-4). Hole 553Bwas continuously cored to the terminal depth of 28.5 m,and Cores 553A-1 to 553A-3 begin at depths of 65.5,103.5, and 151.0 m respectively. The only hiatus detect-ed is in the lower Miocene, where Zones NN2-NN4 aremissing. The nannofossil distribution down to ZoneNN5 is summarized in Table 3.

Samples 553-1,CC, 553B-1,CC, and 553B-2,CC arereferred to the upper Pleistocene (Zones NN20-NN21)owing to the absence of Pseudoemiliania lacunosa. Theremaining part of Hole 553B belongs to Zone NN19.Cores 553A-1 and 553A-2 contain an upper Pliocene as-semblage, Core 553A-3 a lower Pliocene assemblage,and Cores 553A-4 and 553A-5 are referred to the upperMiocene. Coccolithus miopelagicus has its highest oc-currence in Sample 553A-6,CC and co-occurs with C.abisectus in Samples 553A-7,CC and 553A-8-1, 75 cm.In the absence of Sphenolithus heteromorphus, the lat-ter two levels are referred to Zone NN6 (see Bukry, 1973;Müller, 1981). Zone NN5 ranges from Sample 553A-8-2,75 cm to Sample 553A-8-3, 125 cm. The biostratigraphyof the lowest Miocene (Zone NNl) and the Paleogene isshown in Table 4.

The Neogene-Paleogene Transition in Hole 553ASample 553A-8,CC is the highest sample showing

abundant Cyclicargolithus abisectus. Bukry (1973) usedthis characteristic to subdivide the lowest Miocene. How-ever, C. abisectus shows low frequencies in several sam-ples below its highest peak occurrence, and thus thesomewhat vague expression of the C. abisectus acme atHole 553A must be considered with caution; the possi-bility is nevertheless indicated in Table 4. The ZoneNP25/Zone NNl (Oligocene/Miocene) boundary isplaced at the extinction level of Dictyococcites bisectus.This species decreases in abundance by about 90% fromSample 553A-9-4, 10 cm to Sample 553A-9-4, 5 cm, butcontinues above as a rare component throughout Core553A-9. This is compatible with the core's low abun-dance of some obviously reworked Eocene taxa. Un-doubtedly, the extinction of D. bisectus is the most dis-tinct datum event, in terms of abundance prior to ex-tinction, that can be used for consistent recognition ofthe Oligocene/Miocene boundary in the Rockall area.The rare and sporadic occurrences of Helicosphaera re-cta (extinction defines top Zone NP25 according toMartini, 1971) within the lowest Miocene may or maynot be indigenous. In the light of the reworking ob-served, the few specimens recorded, and the distinctlyexpressed D. bisectus' extinction, it is reasonable to ne-glect these H. recta specimens from a biostratigraphicpoint of view.

413

Table 3. Distribution of Neogene calcareous nannofossils at Holes 553, 553A, and 553B.

Epo

chP

LE

IST

OC

EN

ELA

TE

PLI

OC

LAT

EM

IOC

.M

IDD

LE

MIO

CE

NE

Zon

e (O

kada

& B

ukry

, 198

0)

CNttb•15

CN13-tta

CN 12

CN 10-11

CN7-9

CN5b-6

CN5a

Zon

e (M

art

ini,

1971)

NN20-21

NN19

NN16-18

NN 12-15

NN9-11

NN7-8

NN6

NN5

Sam

ple

(553

, 55

3A,

55

3B

)

(co

re-s

ect

ion

, cm

)

1, CC1B.CC2B.CC3B-1, 853B,CC4ß,CC1 A.CC2A.CC3A.CC4A.CC5A,CC6ACC7A,CC8A-1, 758 A-2,758 A-3, 838A-3.125

Abu

ndan

ceP

rese

rvat

ion

A GA GA GA GA GA GA GA MA MA MA MA MA MA MA MA MA P

Cal

cidi

scus

lep

topo

rus

C. m

acin

tyre

iC

occo

lithu

s m

iope

lagi

cus

C. p

elag

icus

C.

radi

atus

o oo o+ o+ oo +o oo + oo +o + ++ ++ o+ + ++ o+ + o+ + +

++

Cor

onoc

yclu

s ni

fesc

ens

Cyc

licar

golit

hus

abis

ectu

sC

flo

ridan

usD

icty

ococ

cite

s he

ssla

ndii

D• p

erpl

exus

o•••

+ + ++ + ++ o

+ + oo o +

D• p

rodu

ctus

Dis

coas

ter

bellu

sD.

bro

uwer

iD.

cha

lteng

eri

D

defla

ndre

i

ooo•oo• +• +

+ ++ ++ +

++

D. e

×ilis

D.

inte

rcal

aris

D. n

eoha

mat

usD.

pen

tara

diat

usD.

sur

culu

s

+ ++ + +

+ + ++ + +

+ ++ ++++

+

D•

varia

bilis

Em

ilian

ia h

u×le

yiG

emin

ilifh

ella

ro

tula

Gep

hyro

caps

a sp

p.

Hel

icos

phae

ra

car t

en

• o +• o +? o

• +• o• o

+ ++ +

++ + ++ ++ + ++ + ++ o ++ + ++ + ++ + +

H. g

ran

ula

taH

. in

term

edia

H. s

elli

iP

onfo

spha

era

japo

nica

P jo

nesi

+

+ +++ ++ +

+ +

+++ ++++++

Pse

udoe

mili

ania

lac

unos

aR

efic

ulof

enes

fra

haqi

iR.

m

i nut

aR.

pse

udou

mbi

lica

Rha

bdos

phae

ra

da

vig

era

++

+o +o ++ o+ + + +

••••o

+ o+

o +o +

Scy

phos

phae

ra s

pp

.S

phen

olith

us

hete

rom

orph

usS•

m

orifo

rmis

S

neoa

bies

Syr

acos

phae

ra

spp•

+++++o+

+

++

+ ++

++ ++ ++

Triq

u e

tror

h a

bdul

us r

ugos

us 1

Zyg

rhab

lithu

s bi

juga

tus

++

+

Note: Filled circles = abundance < 10% of total assemblage, open circles = abundance 1-10% of total assemblage, crosses = abundance > 1% of total assemblage.


Müller (1979) suggested that the first appearance ofTriquetrorhabdulus carinatus can be used to approxi-mately determine the base of Zone NP25 in the NorthAtlantic when lacking Sphenolithus distentus. In Hole553A, however, the first appearance of T. carinatus oc-curs immediately above (5 cm) the extinction level of D.bisectus, which is used to mark the top of Zone NP25.Taking into account the restricted knowledge we haveabout how the above-mentioned markers exactly relateto each other stratigraphically in the Rockall area, it isnot impossible that the interval from Sample 553A-9A-4, 10 cm to Sample 553A-9A-6, 25 cm represents ZoneNP24. This uncertainty is indicated in Table 4.

The Paleogene of Hole 553AA hiatus lasting some 10 to 15 m.y. separates the up-

per Oligocene nannofossil-foraminifer chalk in Sample553A-9-4, 25 cm from the underlying middle Eocenebiosiliceous foraminifer chalk in Sample 553A-9-4, 35cm. Chiasmoliths are abundant in the underlying mid-dle Eocene interval, notably Chiasmolithus nitidus andC. solitus. Discoaster bifax was observed at two levelswithin Core 553A-10, and Reticulofenestra umbilica ispresent from Sample 553A-9-6, 35 cm to Sample 553A-10-3, 100 cm. This interval consequently is referred toZone NP16 or the Discoaster bifax subzone (CP14a).Markalius spp. (M. astroporus in this case) occursthroughout the interval. According to Okada and Thier-stein (1979), M. astroporus disappears in the lowest partof the Discoaster bifax subzone, possibly suggestingthat only the lower part of the subzone is represented inthis interval. This idea is supported by the fact that theinterval is bracketed by hiatuses.

Zone NP15 is missing at Hole 553A, and the entireinterval from Zones NP11-NP14 is represented by lessthan two cores. Nondeposition and/or erosion haveprobably influenced the sedimentation process in theZone NP11 -Zone NP14 interval, thus preserving onlyparts of each zone. Discoaster lodoensis occurs through-out Zone NP14 and Nannotetrina cristata is present atthe base of the zone, which may suggest that both the up-permost and the lowest parts of this zone are missing (seeSite 552). The pontosphaerids begin to increase in abun-dance and diversity in Zone NP13, although the acme ofthese fossils occurs in Zone NP12. Prinsius bisulcus de-creases in abundance at the top of Zone NP12, and thelast occurrence of Toweius occultatus is at the very top ofZone NP12. The first appearance of R. dictyoda occursin the uppermost sample belonging to Zone NP12. Thiscould imply that reworking has displaced the last occur-rence level of Tribrachiatus orthostylus in the Hole 553Asequence. Okada and Thierstein (1979) noticed that thelast occurrence of T. orthostylus and the first appearanceof/?, dictyoda (referred to as R. samodurovii by these au-thors) coincide at Site 386. Obviously these two datumevents virtually coincide over huge geographic distances(Müller, 1979; Okada and Thierstein, 1979; Romein,1979; this chapter). Presence of T. orthostylus far abovethe first appearance level of R. dictyoda thus stronglysuggests that the former species is reworked. On the otherhand, their simultaneous disappearance and appearance

may be regarded as a reliable biostratigraphic indication.Bukry (1973) avoided using the last occurrence of T.orthostylus in his zonation since he had observed the spe-cies as high as Zone NP15 from the Arroy El Bulito se-quence in California. This unusually high stratigraphicposition of T. orthostylus probably can be disregarded asbeing due to reworking. Unfortunately, this led Bukry tochoose the first appearance of Coccolithus crassus, whichis difficult to identify accurately and thus consistently(the opposite is true regarding both T. orthostylus andR. dictyoda), to subdivide the interval between the firstappearances of D. lodoensis (base Zone NP12) and D.sublodoensis (base Zone NP14).

It is on the fringe of an understatement to say that T.contortus and T. nunnii are rare in the Hole 553A (andHole 555) sequence(s). The upper limit of the formerand the lower limit of the latter species define the topand the base of Zone NP10 respectively. The data ofLowrie et al. (1982), Okada and Thierstein (1979), andRomein (1979) indicate that the first appearance of T.orthostylus closely approximates the last occurrence ofT. contortus. The base of Zone NP11 is therefore drawnat the first appearance level of T. orthostylus (see Table4). The short length of Zone NP11 and the many exitsand entrances of taxa in the proximity of the base of thezone is conspicuous {Chiasmolithus bidens, D. kuep-peri, Ellipsolithus macellus, Imperiaster obscurus, Lan-ternithus spp., Rhabdosphaera scabrosa, and T. magni-crassus). The base of the glauconitic unit also occurs atthis level in Core 553A-12, indicating a transgressionover the area (Morton et al., this volume). It seems rea-sonable to suggest that the base of Zone NP11 at Hole553A is slightly younger than the chronostratigraphicage of the Zone NPlO/Zone NP11 boundary.

Zone NP10 is represented in approximately 25 coresoverlying the basalt but roughly 19 of these cores arebarren, and the remaining cores are characterized by lowdiversity and low abundances of nannofossils. Regard-less of the poor control of the exact sedimentation ratein this interval, the rate was clearly very high, compati-ble with the tectonically active situation and the shallowwater depositional environment over this part of the se-quence. It is noteworthy that fasciculiths are absent inCore 553A-37. This genus has its last occurrence in theupper part of Zone NP9 of the upper Paleocene (Shack-leton, in press). The occurrence of Rhomboaster calci-trapa in Sample 553A-37-4, 50 cm may imply that thislevel belongs to the very top of Zone NP9, very close tothe Zone NP9/Zone NP10 boundary (Paleocene/Eo-cene boundary).

Site 554There were two holes drilled at Site 554 (56° 17.41'N;

23°31.69'W; water depth: 2576 m). Drilling commencedat Hole 554A at the level where Hole 554 terminated (76m). The sequence contains calcareous nannofossilsdown to a depth of 123.5 m (Sample 554A-5.CC). Theunderlying cores of Hole 554A (Cores 6-14) consist pri-marily of coarse volcanigenic sediment interbedded withbasalt flows. The uppermost 106 m (Pleistocene-middleMiocene) probably reflect an unbroken record. The

415

Table 4. Distribution of Paleogene calcareous nannofossils at Hole 553A.

a

UJ

z

Σ

<UJ

z

5 8

UJ

U

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1̂

Σ

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t- -I Z<2 ö

<ry

. 19

80)

m

} (O

kad

a

c

N

C N 1 6

C N 1 a

C P 1 8 -

19

C P 1 4 a

^ ^

C P 1 2

CP to-l l

CP9b

CP 9a

C P β b 7

Σ

oM

NN1

CP24-

25

NP16

NP14

NP13

NP12

NP11

NP10

NP9•>

ε

c

ü

53,

<Λ £

8-3, 135

8-4, 12

8. CC

9 - 1 . 59 -2. 5

9-3, 59-4, 5

9-4. 109-4, 125

9-6. 20

9 -6, 25

9-6, 359, CC

10-1, 90

10-3, 9

0-3, 70

0-3, 100

0-3, 130

0-4. 40

0-4, 87

0-6. 59

0-6. 148

1-1, 10

1-1, 63

1-1, 121

1-2, 40

1-2, 80

1-4,135

1-5. 401-5, 112

1-5, 149

1-6 25

1-6, 75"

1, CC

2-1, 26

2-1, 442-1, 742-3.143

12, CC

13-1, 25

3-2, 90

3,CC

4-1, 514-2, 50

4. CC

5-1. 116

5-2, 40

15-3, 90/21,CC22-1, 33

22-3, 7022-5, 82

22, CC

23-1, 106

23-2, 87

23-4, 140

23, CC

24-1. 60

24, CC/36.CC

37-1, 50

37-2. 35

37-3, 4937-4. 13

37-4, 50

37-4, 117

37-5.

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lower half of the Miocene and most of the Eocene aremissing. In contrast to all other sites drilled during Leg81 the Oligocene is well represented at Site 554, albeitcondensed into 11.5 m.

The Neogene of Holes 554 and 554AThe distribution of Neogene nannofossils is shown in

Table 5. Most Neogene cores are severely disturbed bydrilling processes. Core 554-4 was empty. Assemblagesare similar to those observed at Sites 552 and 553: A fewtaxa occur in great abundances, and these are accompa-nied by a number of comparatively rare taxa.

In the Pleistocene, gephyrocapsids tend to be themost prominent assemblage elements, although Cocco-lithus pelagicus and Dictyococcites productus occasion-ally may be abundant. Diagenetic overgrowth of calcitemay blur the morphologic characters used to distinguishbetween gephyrocapsids and D. productus. Reticulo-fenestrids, C. pelagicus, and D. productus dominate thePliocene assemblages. Abundant large D. perplexus char-acterize the assemblages during earliest late and latemiddle Miocene times, together with C. pelagicus andreticulofenestrids.

The last occurrence of Pseudoemiliania lacunosa isin Sample 554-2,CC, that of Calcidiscus macintyrei is inSample 554-3-5, 35 cm, and that of Discoaster brouweriis in Sample 554-3.CC. Zones NN16 and NN17 appar-ently lie within the empty Core 554-4, because the up-permost part of Core 554-5 belongs to Zone NN15. Ce-tatolithus acutus is present in Sample 554-6-5, 46 cmand C. rugosus in Sample 554-5,CC suggesting that theZone NN12/Zone NN13 boundary should be placed be-tween these two levels. Two samples taken at the bottomof Core 554-5 (see Table 5) contain Amaurolithus delica-tus, C. rugosus (one sample), and D. asymmetricus. Asthe lower range of the last taxon is uncertain these twosamples are referred to Zones NN13-NN14, even thoughthe first appearance of D. asymmetricus was used byMartini (1971) to define the base of Zone NN14. ZoneNN11 is represented from 554-6,CC to 554-1,CC. Samples554A-2,CC and 554A-3, 90 cm are referred to ZonesNN9-NN10 since both Coccolithus miopelagicus andD. quinqueramus are absent. Presence of the formerspecies and the absence of Cyclicargolithus abisectusand C. ßoridanus suggest an assignment to Zones NN7and NN8 for Samples 554A-3-3, 145 cm and 554A-4-4,145 cm.

A 25-cm thick unit of glauconitic nannofossil-fora-minifer chalk at the top of Section 554A-4-2 is boundedby hiatuses; the upper one being distinct, encompassingthe interval from Zones NN3 or NN4 to NN6, whereasthe duration of the lower hiatus is shorter (Zone NN1).Bukry (1973) and Martini (1971) give different rangesfor Sphenolithus belemnos and D. druggii. Bukry usesthe first appearances of D. druggii and S. belemnos todefine the base and top of Subzone CNlc respectively,whereas Martini claims that these two events coincide atthe base of Zone NN2. Both these taxa are present atthe 10- and 21-cm levels in Section 554A-4-2, togetherwith Helicosphaera aff. H. ampliaperta which has ashort range in Zone NN2 (see Müller, 1981). The present

material does not allow determination of the exact strati-graphic relationships of the ranges of D. druggii, H. aff.ampliaperta, and S. belemnos. Thus it seems fair to sug-gest a Zone NN2-Zone NN3 (or CNlc-CN2) assign-ment for this 25-cm-thick interval (Sample 554A-4-2, 0-25 cm). Subzones CNla-b are missing.

The Paleogene of Hole 554ADictyococcites bisectus and Triquetrorhabdulus carina-

tus co-occur immediately below the above-discussed 25-cm (lower Miocene) interval. The latter taxon is presentcontinuously to Sample 554A-4-2, 145 cm, suggesting aZone NP25 assignment (Müller, 1979). Since diagnosticsphenoliths are rare at mid and high latitudes Müller al-so suggested that the first appearance of Cyclicargolith-us abisectus could be used to approximately determinethe base of Zone NP24, a suggestion supported by thedata of Poore et al. (1982). The first appearance of C.abisectus is distinct at Hole 554A, occurring between 80and 145 cm in Section 554A-4-3. In practically all sam-ples containing C. abisectus and C. floridanus there arespecimens intermediate between these two taxa. There isno problem in distinguishing between end members ofthese morphologically similar taxa. But, since size is thecharacter that provides the most efficient means to sepa-rate C. abisectus from C. floridanus, it is recommendedto measure the overall size (e.g., placolith diameter) inorder to apply consistent species concepts. This workought to be done on a better sequence than that provid-ed at Site 554.

Reticulofenestra umbilica has its last occurrence be-tween Samples 554A-4,CC (10 cm) and 554A-4-4, 25.Isthmolithus recurvus also disappears at the latter level(the range of /. recurvus above the highest occurrence ofC. formosus is noteworthy). This would imply the pres-ence of a hiatus at the bottom of Core 554A-4, coveringthe upper part of Zone NP22 and a substantial part ofZone NP23. The zonal assignments of the Oligocene atHole 554A (Table 6) should be regarded as tentative be-cause of the condensed nature of the sequence and theabsence of some of the primary zonal markers.

Discoaster saipanensis and R. reticulata disappear inSample 554A-5-3, 104 cm. These taxa co-occur with /.recurvus down to the manganese crust in Sample 554A-5-4, 20 cm. The assemblage immediately above the man-ganese crust can thus be referred to the uppermost Eo-cene (Zone NP19/20 or CP15b). If the biostratigraphyof Hole 552A is correctly interpreted, this implies thatthe manganese formation took place earlier at Hole554A than at Hole 552A.

The sediment below the manganese crust at Hole554A is intensively burrowed, and upper Eocene assem-blage contaminants are mixed into the lower Eocene(Zone NP11). It is estimated that inmixing decreases byapproximately 85-90% within 8 cm from beneath themanganese crust, but the upper Eocene contaminantscontinue to be present in low numbers for roughly 35 cmbelow the manganese thus indicating the depth penetra-tion of the burrowing organisms.

The interval between 39 and 104 cm in Section 554A-5-4 shows a dull assemblage referable to Zone NP11.

417

Table 5. Distribution of Neogene calcareous nannofossils at Holes 554 and 554A.

Epoc

hP

LEIS

TO-

CEN

E

LATEPLIOCENE

EAR

LYP

LIO

CEN

ELA

TEM

IOC

ENE

UJ

a , UJo o z

>- uSSul2 ΣO

Zo

ne

(O

ka

da

& B

ukr

y, 1

98

0)

CNKb-

CN13-

14a

CN12dCN11

CN10c

CN10a-b

CN9b

CN9a

CN7-8

CN5b-6

CN 1c-2

Zo

ne

(M

art

ini,

197

1)

NN 20-

NN19

NN18NN15

NN 13-14

NN12

NN 11

NN 9-10

NN 7-8

NN 2-3

Sa

mp

le

(554

, 5

54

A c

ore

-se

ctio

n,

cm

)

1, CC

2, CC3-4, 503-5, 353, CC5-1, 70

5-6, 1405, CC6-5, 466, CC7, CC8, CC1A, CC

2A.CC3A-3.903A-3,1454A-1.145

4A-2, 104 A-2, 21

Ab

un

da

nc

e

Pre

se

rva

tio

n

A G

A MA GA MA M

A M

A MA MA MA GA MA GA G

A MA GA MA M

A MA M

Am

au

rolif

hu

s d

elic

afu

sC

alc

idis

cus

lep

top

oru

sC.

m

ac

infy

rei

Ce

rato

litu

s a

cu

fus

C.

rug

osu

s

+ooo oo o

o + +

+ o ++ o o +

o + +o +

+ o +o +

Co

cco

lith

us

mio

pe

lag

icu

sC.

p

ela

gic

us

C•

rad

iatu

sC

oron

ocyc

lus

nife

sce

ns

Cyc

lica

rgo

lith

us

ab

ise

ctu

s

o

o

oo

•

•

•o

• +

++

• + +

C•

flo

rid

an

us

Dic

fyoc

occi

fes

he

ssla

nd

iiD•

per

ple×

usD•

pro

du

ct u

sD

isco

aste

r as

ymm

etric

us

oo

•• +

• *

•

oo

•

oo

D.

brou

wer

iD.

ca

lcar

isD

ch

alle

nger

iD.

de

fland

rei

D• d

rugg

ii

++

+

+ +

+ +

* * +

o +

D• e

×ilis

D.

infe

rcal

aris

D• lo

eblic

hii

D•

penf

arad

iatu

sD

pr

epen

tara

diat

us

+ +

+ ++

+ + +

+

D• q

uinq

uera

mus

D•

surc

ulus

D. v

aria

bilis

Em

ilian

ia

hu×l

eyi

Gem

inili

thel

la

rotu

la

•

+ +

+ o+ + +

+ + ++ o+ + o

Gep

hyro

caps

a sp

p.

Hel

icos

phae

ra a

ff.

ampl

iape

rta

H. c

arte

r/H.

gra

nu

lata

H• s

elli

i

• +

• + o• + o

+ oo

o

o +

o ++

o +

+ +

Pon

tosp

haer

a ja

poni

caP.

jonesi

P.

mul

tipor

aP

seud

oem

ilian

ia

lacu

nosa

Pyr

ocyc

lus

herm

osus

+ +o o o

+ o+ o

+ + o+

+

+ +

P. i

nver

sus

Re

ticul

ofen

es tr

a

da vi

esi

R• h

aqii

R. m

inut

aR

. m

inut

ula

•+ + o +

oo + +o +o o

+ + + +

+ o

o+ o

R. p

seud

oum

bilic

aR

habd

osph

aera

cl

avig

era

Scy

phos

phae

ra

spp.

Sph

enol

ithus

ab

ies

S. b

elem

nos

+• + + +

! + 1 +

• +

Φ +

o

:

+

S. m

ori

form

isS.

ne

oa

bie

sS

yrac

osp

hae

ra

sp

p.

Triq

uetr

orh

abd

ulus

c

ari

na

tus

T. m

ilow

ii

+

+

+

+

+ +

T ru

go

sus

Zyg

rha

blif

hu

s b

ijug

atu

s

+

+

o

Note: Filled circles = abundance >10<% of total assemblage, open circles = abundance 1-10% of total assemblage, crosses = abundance < 1 % of total assemblage.

Table 6. Distribution of Paleogene calcareous nannofossils at Hole 554A.

Ep

oc

hE

AR

LY

/

LA

TE

OL

IGO

CE

NE

/O

LIG

OC

EN

EE

AR

LY

S L

AT

EE

OC

EN

E

!> E

OC

EN

E

Zo

ne

(O

kαd

α

&

Bu

kry

, 19

80)

CPi9b

CP19α

CP17-18

CP17

CP16

CP15b

CP9b

CP9α

Zo

ne

(M

art

ini,

19

71)

NP25

NP24

NP23

NP22

NP21

NP19-20

NP11

NP10

Sa

mp

le

(55

4A

c

ore

-se

cti

on

c

m)

4-2, 354-2, 804-2,145

4-3, 80

4-3,145

4-4^254-CC. 105-1, 585-2, 585-2,1455-3, 48

5-3, 845-3,104

5-3,115

5-3. 135

5-4 20

5-4, 395-4, 50

5-4, 925-4,104

5, CC6

Ab

un

da

nc

e

Pre

se

rva

tio

n

A MA MA P

u A M

A MA MA MA MA MA MA P

A M

A M

A P

A P

A P

BARREN

R P

R PR P

R PBARREN

Ch

iasm

olif

hu

s a

ltu

sC•

bid

en

sC.

eo

gra

nd

isC•

oa

ma

rue

nsi

sC•

so

litu

s

ap

p

•• P

• P

• P

• P

•P

P

P P

P

P P

P

P

P

• Pa• P

Coc

cotit

hus

eo

pe

lag

icu

s

C.

form

osu

s

C. p

ela

gic

us

C• s

ub

dis

tic

hu

s

Co

ron

ocy

clu

s n

ife

sce

ns

m

u am

m

m

P P

•••• P

G • P

P P • P

P P •

G P P

P • P

P P P

P

P P

P

p a

Cyc

lica

rgo

lith

us

ab

ise

cfu

s

C.

flo

rid

an

us

Dic

tyo

co

cc

ite

s b

ise

ctu

s

D•

he

ssla

nd

ii

Dis

co

ast

er

ad

am

an

teu

s

m • P P P

• • P P• • a P

• • a• • P• •• • a• • PP • P

• PP • P

• • p

• • P• P

•• P

D• b

arb

ad

ien

sis

D• b

ino

du

sus

D•

de

fla

nd

rei

D• d

iast

ypu

sD•

ke

up

pe

ri

P

pp

p

a

P

P P

P P

0.

mu

ltir

ad

iatu

sD•

no

dif

er

D• s

aip

an

en

sis

D.

tan

iE

llip

solit

hu

s m

ac

ellu

s

a a p

P P

P P

P P D

P

P

Eri

cso

nia

fe

ne

stra

taH

elic

osp

ha

era

b

ram

lett

ei

H.

co

mp

ac

taH

. e

up

hra

tis

H.

refi

cu

lata

aaa

π

p

P

••••• au a

um a

••

Imp

eri

ast

er

ob

scu

rus

Isth

mo

lith

us

rec

urv

us

Ma

rka

lius

spp

.M

icra

nth

olit

hu

s sp

p.

Ne

oc

oc

co

lith

es

du

biu

s

π

p

p

••p

• P•• P

••

a

a

P

Po

nfo

sph

ae

ra

fim

bri

afa

P•

pu

lch

ra

P•

rim

osa

P.

vers

a

Pri

nsi

us

bis

ulc

us

m

m

m

a p P p •

Pyr

oc

yclu

s h

erm

osu

sP.

in

vers

us

P

ora

ng

en

sis

Re

tic

ulo

fen

est

ra

da

vie

siR

. d

icty

od

a

a

p •

p π a

a a

p

P P

p a aa

aP Pa P P

P

p

P P

P P P

P

R.

hill

ae

R.

reti

cu

lata

R.

um

bili

ca

Sp

he

no

lith

us

cip

ero

en

isS

. m

ori

form

is

πa G

p

p

a

p

a P

p • p

P •p • pP • P

G • PP P • PP P •

P P •

a • p

p

Tow

eius

m

agn

icra

ssu

s

T.

oc

cu

lfa

tus

Tn

bra

ch

iatu

s o

rth

ost

ylu

s

Triq

ue

tro

rhab

du

lus

ca

rin

afu

s

Zyg

rha

blit

hu

s b

ijug

afu

s

P P

p •P •

p

••

P

PPP

G

P

•P

•

P G P

P P G

G P P P

• • P


J. BACKMAN

Tribrachiatus orthostylus is not present in the lowestsample containing nannofossils, possibly indicating ZoneNP10 at that level. Several samples were investigated inCore 554A-6, but all calcite proved to be diagenetic.

Site 555The depositional environment of the sequence at Site

555 (56°33.70'N; 20°46.93'W; water depth: 1659 m)differs from those at the other Leg 81 sites in being con-siderably shallower. The Pliocene is missing at Site 555.A hiatus separates the lower Miocene from the lowerEocene, and not even fragmentary occurrences of sedi-ment representing the Oligocene or the middle-upperEocene are preserved. Both these characters make theSite 555 sequence different from those at Sites 552, 553,and 554. Two 19-m intervals, between Cores 555-8 and555-9, and between Cores 555-10 and 555-11, were notcored. The remaining parts of the sequence were contin-uously cored. The Neogene is represented to a depth of281 m (Sample 555-26,CC). The Paleogene, which isrepresented by the lower Eocene and the upper Paleo-cene (the only definite Paleocene recovered during Leg81), is very expanded: It begins at 281 m and continuesto 870 m (terminal depth: 964 m). The six lowest sedi-mentary cores are barren of nannofossils; the sedimentsare mainly comprised of vitric micaceous sandstone andlapilli tuff. Two intervals do not contain sediment, thefirst between Cores 555-68 (Sample 555-68-2, 35 cm)and 555-83 (Sample 555-83-3, 30 cm), and the secondbetween Cores 555-95 (95-1, 35 cm) and 555-98 (termi-nal depth). The former consists of basalt and hyaloclas-tite, the latter wholly basalt.

The Neogene of Site 555Ice-rafted sediment and oozes characterize the Pleis-

tocene to the bottom of Section 555-3-3. The assem-blage undergoes a distinct change between Samples 555-3-4, 80 cm and 555-3-5, 80 cm. The former level can beassigned an age of between 1.45 and 1.88 m.y. (bottomZone NN19) owing to presence of Calcidiscus macinty-rei and absence of Discoaster brouweri (see Backmanand Shackleton, 1983). Samples 555-3-5, 80 cm and555-3,CC are referable to the lower Pliocene. Exactlywhere in the lower Pliocene is more difficult to say. Atentative assignment to Zone NN12 (CNlOa) is suggest-ed, based on presence of Triquetrorhabdulus rugosusSample 555-3,CC. In turn, this implies that Zones NN13through NN18 are not represented.

The interval between Samples 555-3-5, 80 cm and555-24-4, 135 cm represents continuous deposition withan average sedimentation rate of approximately 2.5 cm/1000 years. It is the most complete middle Miocene se-quence recovered during Leg 81. Changes in assem-blages through time are compatible with those docu-mented from Sites 552, 553, and 554. Reworking mayhave affected the nannofossil biostratigraphic resolutionaround the upper/middle Miocene boundary. Sporadicand rare specimens of Coccolithus miopelagicus werenoted above Sample 555-15.CC but this species becomesa more regular member of the assemblages below Sam-ple 555-15,CC possibly implying that Zone NN8 begins

between Samples 555-14,CC and 555-15,CC (see Bukry,1973). The possibility cannot be excluded, however, thatC. miopelagicus is reworked between Samples 555-18,CC and 555-15,CC, since it is still comparatively rarein that interval. This uncertainty is indicated in Table 7.The Zone NN6/Zone NN7 boundary is placed at thelast occurrence level of Cyclicgargolithus abisectus (Sam-ple 555-22,CC). Presence of Sphenolithus heteromor-phus and absence of Helicosphaera ampliaperta in twosamples in Core 555-24 (Table 7) indicate Zone NN5.Zone NN4 is not represented in this sequence, but ZoneNN3 is present between Samples 555-24-5, 100 cm and555-26.CC, as indicated by the presence of both H. am-pliaperta and S. belemnos.

The Paleogene of Site 555The lower Eocene is represented by Zones NP10-

NP12 (Table 8). Two long intervals comprising 18 coresaltogether are barren of nannofossils (Samples 55531,CC to 555-41,CC; Section 555-46-1 to Sample 555-55,CC), and the interval between Sample 555-55.CCand the uppermost sediment/basalt contact in Core555-68 contains impoverished assemblages, character-ized by highly sporadic occurrences of nannofossils, whichare biostratigraphically undiagnostic with respect to theZone NP9/Zone NP10 boundary (Paleocene/Eoceneboundary). The sediments interbedded between the ba-salt piles are richer in nannofossils and provide a dis-tinct Zone NP9 signal.

A drilling breccia consisting of a mixture of lowerEocene zeolitic clays and chalks from the overlyinglower Miocene is present in the uppermost 15 cm ofCore 555-27. A sample was taken at Section 555-27-1, 5cm from the center of a brownish lump clearly belong-ing to the Eocene lithology. No Miocene contaminantswere observed in this sample, in contrast to other sam-ples taken from the breccia. The sample shows a fewlower Eocene taxa, including Discoaster lodoensis andTribrachiatus orthostylus, and no Reticulofenestra dic-tyoda. This composition indicates Zone NP12. Toweiusoccultatus is abundant up to Sample 555-27-2, 132 cm.At Site 552 this species disappears in the middle part ofZone NP12, implying that the Sample 555-27-1, 5 cmprobably represents the upper part of Zone NP12. D.lodoensis was not observed below the drilling breccia inSample 555-27-1, but T. orthostylus continues to Sam-ple 555-3l.CC. The latter level hence is used to mark theboundary between Zones NP10 and NP11 (see above).A single observation of Tribrachiatus contortus in Sam-ple 555-33-2, 88 cm supports the tentative assignment ofthe Zone NPlO/Zone NP11 boundary at Sample 555-31,CC. As at Sites 552 and 553, the disappearance levelof Ellipsolithus macellus coincides with the first appear-ance level of T. orthostylus at Site 555.

The barren intervals are separated by four cores(Cores 555-42 to 555-45) containing nannofossils. Pres-ence of D. diastypus in two samples in Core 555-45places this interval within Zone NP10. A species thathas previously only been reported from Zone NP9 ofthe upper Paleocene (Edwards and Perch-Nielsen, 1975),Hornibrookina australis, was observed within this Zone

420

Table 7. Distribution of Neogene calcareous nannofossils at Site 555.E

po

ch

ü<o

EIS

TO

_ i

Idl

Lü~z.l±JooΣ

LU

<

Lü

zLU(_)O

ΣLÜ_Ir\

MID

80)

Bukry

19

Zone

(O

kαd

α &

CN Kb-15

CN13-14α

CN10b

CN10α

CN 9

CN7-8

CN 7-8OR

CN5L>6

CN5t>6

CN5α

CN 4

CN2

1971

)Z

on

e (

Ma

rtin

i,

NN20-71

NN 19

NN12

NN 11

NN9-10

NN 9-10OR

NN 7-8

NN7-8

NN6

NN5

NN3

.tio

n c

m)

Sa

mp

le

(55

5

co

re-s

ec

1, CC

2, CC3-2, 1453-3, 803-4, 80

3-5, 803, CC

4-6, 585, CC6-4, 507, CC8, CC9. CC

10, CC11, CC12, CC

13, CC14, CC

15, CC16, CC17, CC

18, CC19. CC20. CC21, CC22. CC23. CC24-1, 10024-3, 10024-3, 135

24-4, 13524-5, 10025, CC26, CC

Ab

un

da

nc

eA

AAAA

AA

AAAAAA

AAAA

AA

AA

AA

AAAAAA

A

AAAA

Pre

se

rva

tio

nG

MGGG

GM

GM

GGMMMGMb

MMMM

MMMMMMGM

M

MPMP

top

oru

sC

alc

idis

cu

s le

p

o

o

o

o

o

•

•

+•o

•

o

+

•

•

o

•

o

o

o

o

++

Sj

I

o

oo

+

+

+

+

+

o

+

+

+

o

+o

+

+o

++

+

ipe

lag

icu

sC

oc

co

lith

us

mil

+

+

+

+•

•

o

o

3

_a

o

•

•

•

•

•

o

•

•

•

•

•

•

•

•

•

o

o

•

•

•

o

•

•

•

•

•

•

•

•

tes

ce

ns

Cór

onoc

yclu

s m

+

+

++o

+

sn

uQjlo-3a<O

Cyc

licar

golit

hiC.

flo

ridan

us

+

+

+++o

o

o

+o

•

hess

land

iD

icty

ococ

cite

s

•

•

•

•

•

D• p

erpl

exus

D. p

rodu

ctus

0

+0

•

•

•

•

•

•

•

•

0

•

•

•

o

o

0

•

•

•

•

•

imat

eus

Dis

coas

ter

adi

+

D• b

ellu

sD•

br

aaru

dii

+

D• b

rouw

eri

++

+

+

o

+

+

+

+

+

++

D• c

alca

ris

o

++

++

+

D• c

halle

nger

iD•

dec

or u

s

+

++ +

+

++

+

+D.

def

land

rei

O

+

++o

o

D•

exili

s+

+

+++

<̂

"aij

-&

+

+

+

+

+

++

++

+

o

c •- 5

| |1Cj Cj Q

+

++

+

+

-Q

D.

pse

ud

ova

ria

+

D• q

uin

quera

mu

+

+

+

+

++

D• s

ign

us

D• s

urc

ulu

s

++++

++

D.

varia

bilis

+o

o

++

+

+++

++

+

+

+++

+

Em

ilian

ia

huxl

• •>

jafa

ri

_a

"QJ

-c

5ε

+

G.

rotu

la

+

o

+

o

+

0

o

o

+

++

o

0

o

+

+

o

+

+

SP

P•

aIO

&to

5

o

•

oo

o

-B

amp

liap

erH

elic

osp

haer

a

+

+

H.

cart

eh

O

+o

+o

+o

+

+

o

o

o

0

+

o

o

o

•

o

++

+

o

o

+

"a

f01

+

H.

gra

nu

lata

0

o

0

o

o

0

0

+

oo

o

o

+

+

+

++

a"5

QJ

-Qj

c

+

cè

++

+

H. s

elli

i

•

•

0

o

+

++

attentu

aP

onto

spha

era

++

+

+

+

++

++

P• j

ap

on

ica

+

+

P. j

ones

i

+

++

+

+

P•

mut

tipor

a

++

++

++

++

a

ia

lacu

noP

seud

oem

ilian

++

o•

rmos

usP

yroc

yclu

s h

e

+

+

+

+

+

++

++

+

+

P

inve

rsus

+

+

P• o

rang

ensi

s

+

UJ

1a

Ret

icul

ofen

esf

o

o

o

o

0

•

1

o

•

o

•

o

o

•

+

R. m

inut

a

>m

•

•o

o

•

•

•

0

+

o

•

o

o

+

o

+••

o

•

o

•

o

o

o

+

o

_a3̂a

+

o

0

o

+o

ica

-3ε

1aQj

C L

•

•

•

•

•

•

+

•

•

•o

o

•

•

•

•

•

•

•

•

+

o

+

+

<o

Scy

ph

osp

hae

n

+

+

+

+

+

+

+

+

bele

mno

sS

ph

en

olit

hu

s

++

+

IO

&

oε

εQ J

+

+

S.

mor

iform

is

+

+

+

++

+

++

o

o

+o

0

o

S• n

eoab

ies

+

0

+

++

+

+

+

+

++

éIO

Syr

acos

phae

ra

o

++

+

+

3IO

lulu

s ru

gcT

rique

fror

hab

+

+

+

+

+0

o

+

+

Note: Filled circles = abundance > 10% of total assemblage, open circles = abundance 1-10% of total assemblage, crosses = abundance < 1% of total assemblage.

Table 8. Distribution of Paleogene calcareous nannofossils at Site 555.

Ep

oc

hE

AR

LY

EO

CE

NE

LA

TE

P

ALE

OC

EN

E

Zo

ne

(Okα

dα 8

Buk

ry,

1980

)CPX)

CP9b

CP9α

->

CP8

Zo

ne

(Ma

rtin

i, 19

71)

NP12

NPΠ

NP10

NP9

Sa

mp

le

(555

co

re-s

ec

tio

n,

cm)

27-1, 527-2,13227 CC28-1, 6430-2. 631 -2. 3331. CC32-1.4432. CC33-2.8833. CC/41. CC42. CC43-2.10243, CC44-1, 4544-1, 13044-2. 5644-2. 9644. CC45-1. 9145-2. 9345. CC

46-1, 70/55. CC56-2. 3256-5.9556. CC57. CC58-3. 3258. CC59-4,7459. CC60-3. 13960. CC61 - 2 . 13061, CC62-4, 13264. CC66. CC67-3. 9868-2. 35/83-3. 3084. CC

85-1.6086-2.7686-3,9687-3.11188-1.92/88-2,9588-4.2088-4.4888-4.6588-4. 8588-4.11188. CC/93, CC

Ab

un

dan

ceP

rese

rva

tio

n

F MR M

BARREN

F M

R P

F P

F M

F M

F M

R P

BARREN

R P

R P

R M

R M

R P

R P

R P

F M

R P

R P

R P

BARREN

R P

R P

R P

R P

R P

R P

R P

R P

R P

R P

R P

R P

R P

BARREN

R P

R P

BASALT

BARREN

R P

R P

R P

R P

BARREN

R P

R P

R P

R P

R P

BARREN

Bra

arud

osp

haer

a s

pp

.C

ampy

losp

haer

a e

od

ela

Ch

iasm

olit

hu

s b

ide

ns

C.

cσlif

orn

icu

sC

co

nsu

etu

s

a

a

π m

m π π

π a

π p

paP P

π p

a aa aπ

a a aπ P

D D

P P

aD

•aπ aa P

D

D

a

Daa

a

a

a

C. e

og

ran

dis

C

gra

nd

isC

hia

smo

lith

us

spp

(s

mal

l)C

occo

lithu

s cr

ibe

llum

C• f

orm

osu

s

m ap

P P D

a

•

D

P

P

P

a

π

p

Q

Pa

an

a

a

C.

pe

lag

icu

s

Dis

coas

ter

bar

bad

ien

sis

D• b

inod

osus

D• d

iast

ypu

sD•

ele

gans

m aa

m a

m

• D D

• P

P

•P P

P

πa

D

D

D

π

p

P P

p

P P

an

a

aπ

aa

πσ

a

D

p

p

D

P

D

D•

falc

atu

s0

ku

ep

pe

riD

le

ntic

ula

ris

D• lo

do

en

sis

D• m

edio

sus

π ap

p

a

p

D P

P P

P

D P

P

a

aa

D.

mo

hle

riD,

m

ulti

rad

iafu

sεi

lipso

lith

us

dis

tich

us

ε

mac

ellu

sFa

scic

ulith

us

fym

pani

form

is

D

P P

D D

Da

D Dπ a

P D

a p

a

aP

P P

p

D

DP

ΠP

a

aa

aa

a

pa a

p p

a

o

a

Fasc

icul

ithus

sp

p.

He

lioiit

hu

s ri

ed

elii

Hom

ibro

okin

a au

stra

lisIm

pe

rias

ter

ob

scu

rus

Mar

kaliu

s sp

p

P P

p

aa

••

•

•a

P

PG

a I a

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NP10 interval. However, the range of H. australis, asshown by Edwards and Perch-Nielsen from Site 277,partly overlaps with that of D. diastypus, and the spe-cies disappears shortly before E. macellus; it possiblyalso overlaps in range with T. orthostylus. The range ofH. australis at Site 555 thus cannot be regarded asanomalous.

It is noteworthy that Rhomboaster cuspis also ispresent in Cores 555-42 through 555-45.

There is no evidence available that can be used to re-fer the interval between Cores 555-56 and 555-68 (belowthe lower barren interval and above the basalt/hyalo-clastite pile) to any biostratigraphic zone. The assem-blage in this interval is nevertheless considered to be in-digenous because of its distinct character from the un-derlying nannofossil bearing sediment; see, for example,micrantholiths. Unfortunately, depositional rates do notprovide any clues as to the placement of this interval. Per-haps the best biostratigraphic indication available is theabsence of fasciculiths, which disappear close to theZone NP9/Zone NP10 boundary, but within Zone NP9(Shackleton, in press). However, the adverse depositonalenvironment of this interval, with a water depth repre-senting a shallow shelf environment (Murray, this vol-ume), suggests that absence of certain taxa may be dueto environmental causes rather than reflecting biostrati-graphic relationships.

Fasciculiths are "common" from the top of Core555-85 to Sample 555-88-4, 111 cm. Well-developedspecimens of D. multiradiatus indicate that this intervalmay be confidently referred to Zone NP9. One speci-men of D. mohleri was observed in Sample 555-88-4, 85cm. The interval also shows a co-occurrence of Toweiuseminens and T. occultatus, together with transitionalforms between the two. The former species is not presentin any other Leg 81 material.

CORRELATION OF THE PALEOGENE:SW ROCKALL AREA (LEG 48 AND LEG 81)

One of the principal objectives of Leg 81 was to gath-er evidence and create a conceptual framework of therifting history between Greenland and the Rockall Pla-teau and of the early phases of seafloor spreading be-tween these blocks of continental crust. A first step to-ward the achievement of these objectives is to establishbiostratigraphic correlations between the retrieved se-quences. The next step involves the interpretation ofthese correlations in terms of chronology (see Backmanand others, this volume).

It may be pertinent in this context to explain whysuch correlations are not presented for the Neogene se-quences. Hole 552A provides the best material premisesfor study of late Cenozoic paleoenvironments in theRockall area. The other Neogene sequences drilled dur-ing Leg 48 and Leg 81, many of which are spot coredand severely disturbed during drilling, add less crucialinsights about the evolution of Neogene paleoceanogra-phy and paleoclimatology. In contrast to this, all Paleo-gene sequences are of critical importance in order to es-tablish a comprehensive understanding of the Green-land-Rockall syn-rift and post-rift development.

Sites 552, 553, and 555 are investigated by a singleworker. Consistent taxonomic concepts thus are appliedin the study of these sequences. The correlations be-tween these sites therefore are presented alone (Fig. 7).The sites in Figure 7 are not arranged in accord withtheir locations on a west-east transect (see Figure 1). In-stead, the correlations between Sites 553 and 555 areemphasized. Apart from biostratigraphic indications,three magnetic normals (RA, RB, and RC; see Krum-siek, this volume) are shown in Figure 7, together withthe base of the glauconitic horizon (Morton et al., thisvolume). Figure 8 is constructed from Müller's (1979)and the present data. The sites are arranged accordingto their positions on a west-east transect.

It appears from Figures 7 and 8 that no lines of cor-relation cross each other, indicating that reworking hasnot affected the biostratigraphic order. Sites 404 and 552show a good representation of Zones NP11 throughNP13. This biostratigraphic interval is short and marredby hiatuses at Sites 403, 553, 554, and 555.

The base of the glauconitic unit represents a majortransgression over the area. The first appearance of Tri-brachiatus orthostylus and the last occurrence of Ellip-solithus macellus occur at or slightly above this level atall sites, indicating that sedimentation commenced vir-tually synchronously after the transgressive event at alldrilled locations.

E. macellus survives T. orthostylus at Site 404. Thedata of Okada and Theirstein (1979) from Site 386 sug-gest that E. macellus survives T. orthostylus, but not towhat extent, since there is a 10-m unsampled interval intheir material between the last occurrence level of E.macellus and the first appearance level of Discoaster lo-doensis. Nevertheless, Okada and Thierstein's observa-tions imply that Sites 403, 553, and 555 represent dis-continuous records at the exit/entrance level of E. ma-cellus and T. orthostylus. It follows that the first appear-ance levels of T. orthostylus at Sites 403, 553, and 555must represent a younger age than the age of its firstevolutionary appearance. This conclusion is supportedby magnetostratigraphic evidence. At Sites 553 and 555T. orthostylus has its first appearance within a normalperiod interpreted as Anomaly 24B (Krumsiek, this vol-ume). Lowrie et al. (1982) suggested that T. orthostylushas its first appearance immediately below Anomaly24B, thus supporting the idea of a late arrival of T.orthostylus at Sites 553 and 555. At Site 403 the entireinterval, less one sample, from Core 403-25 to Core 403-41 is reversed (Hailwood, 1979), thus preventing anybio- and magnetostratigraphic conclusions. Site 404 isthe only site where the stratigraphic relationship be-tween E. macellus and T. orthostylus agrees with thatderived by Okada and Thierstein from Site 386. Not sur-prisingly, Site 404 also agrees with Lowrie and other'sdata. That is, T. orthostylus appears immediately belowAnomaly 24B.

Site 552 shows normal polarity at Core 552-21 (sedi-ment/basalt contact). The base of the glauconitic unit isnot present at this site. Moreover, the last occurrencelevel of Toweius magnicrassus and the first appearancelevel of Tribrachiatus orthostylus coincide at Site 552,

423

J. BACKMAN

NP11

i First occurrenceI Last occurrence

Dlod = D. lodoensisEmac = E. macellusRdic = /?. dictyodaTmag = T. magnicrassusTocc = T. occultatusTort = T. orthostylus© = Base glauconitic unit

RARBRC

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Basalt

Basalt

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PresenceD. mu/tiradiatus

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89

91

7

7

Figure 7. Stratigraphic correlations between Paleogene sequences of Hole 552, Hole 553A, and Hole 555. The data are discussed in thetext. Notice that the Hole 555 sequence is divided into two columns.

424

Hole 554A Hole 403

| First occurrenceLast occurrence

Dlod = D. lodoensisEmac • E. macellusRdic = R. dictyodaTocc = T. occultatusTort = T. orthostylus(G) = Base glauconitic unit

salt

is)

T. D.

Figure 8. Stratigraphic correlations between Paleogene sequences of Hole 554A (Leg 81), Site 403 (Leg 48), Hole 553A (Leg 81), Site 404 (Leg 48), and Hole 552 (Leg 81). See Müller (1979) for Leg48 data. Two possible levels for the first appearance of D. lodoensis at Site 404 are indicated. The data are further discussed in the text.

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J. BACKMAN

but not at Sites 553 and 555 where there is a short over-lap in ranges of the two species. Since the disappearancelevel of Toweius magnicrassus obviously occurs abovethe base of the glauconitic unit (Sites 553 and 555), onecannot expect to find the base of this unit in the 26-cminterval separating the sediment/basalt contact and thedisappearance level of T. magnicrassus at Site 552. It ap-pears likely that indigenous deposition began at the low-ermost level containing Tribrachiatus orthostylus, whichwould explain the absence of this species in a sample 5cm above the sediment/basalt contact.

Abundant Cyclolithella spp. characterize two sam-ples in Core 552-21. Sample 404-14,CC shows a peakabundance of a species that Müller (1979) referred to asEricsonia subpertusa. It is not unlikely that these peakabundances of E. subpertusa and Cyclolithella spp.may represent a single event, which thus provides a pos-sible correlation. If so, this indicates that the D. lodoen-sis correlation between Samples 552-18-3 and 404-16-1is erroneous. Müller observed only a "trace" of D. lo-doensis in Sample 404-16-1. The species becomes a reg-ular member of the assemblages in Sample 404-11,CC,and was not recorded between Sample 404-11,CC andSection 404-16-1. The correlation between E. subpertu-sa (Sample 404-14,CC) and Cyclolithella spp. (Section552-21-1) appears correct assuming that D. lodoensishas its first appearance in Sample 404-11,CC and thatthe "trace" of this species at Section 404-16-1 is due tocontamination. This alternative has the advantage thatthe lines of correlation between the very closely locatedSites 404 and 552 become approximately parallel, as-suming that the two sequences should exhibit similardepositional histories. Sample 404-11,CC shows normalpolarity (Hailwood, 1979) and may represent Anomaly24A, which would agree with the data of Lowrie et al.(1982). Unfortunately, the minimal recovery in the criti-cal intervals at both sites neutralizes any serious attemptto estimate how much of the lower part of Zone NP11may be missing in the sedimentary record at Site 552.But with reference to the Site 404 sequence, placing D.lodoensis in Sample 404-11,CC and using T. orthostylus(at Site 404) and the E. subpertusa/Cyclolithella spp.correlation, as much as 45% of the lower part of ZoneNP11 at Site 552 may be missing.

In conclusion, Site 555 exhibits the most expandedsedimentary Zone NP10 interval of all sites drilled dur-ing Leg 48 and Leg 81. Good Zone NP10 intervals alsoare preserved at Sites 403 and 553. These three sitesshow short and discontinuous lower Eocene records aboveZone NP10. Zones NP11 through NP13 (and ZoneNP14 at Site 552) are well represented at Sites 404 and552, although the recovery is poor at both sites. Site 554shows a very short lower Eocene sequence.

From a biostratigraphic point of view, the lower ex-tension of Zone NP10 is vaguely determined at Sites403, 404, 553, and 555. The dinoflagellate stratigraphyis possibly even more ambiguous concerning the place-ment of the Paleocene/Eocene boundary (see Brownand Downie, this volume). On the other hand, the nan-nofossil biostratigraphic control in the lower Eoceneand upper Paleocene is surprisingly good, considering

the paleoecological and depositional environments—theextremely shallow or even brackish waters, the high ter-restrial sedimentary input, and the substantial deposi-tion of volcanic ash and lapilli.

Finally, it may be of interest to give some attention toHole 117A of Leg 12 which was drilled on the easternflank of the Hatton-Rockall Basin. Laughton, Berg-gren, et al. (1972) suggest that sediment of late Paleo-cene age was recovered in that sequence. However, Mor-ton et al. (1983) demonstrate that the lowermost nanno-fossil-bearing sediment belongs to Zone NP10 of thelower Eocene. The short Hole 117A sequence correlateswell with a part of the neighboring Site 555 sequence.Hornibrookina australis and Rhomboaster cuspis havelimited and virtually identical ranges in both sequences,where the former reaches slightly higher than R. cuspis.Furthermore, a barren interval begins shortly (2 m) be-low the mutual first appearance level of H. australis andR. cuspis at Hole 117A, which probably compares withthe lower barren interval at Site 555. This barren inter-val encompasses two cores at Hole 117A. The core be-low contains basalt. Comparing the two sequences, it isclear that the upper part of Zone NP10 is shorter at Site117 than at Site 555.

NOTES ON TAXONOMIC USAGE

The reader who is interested in taxonomic nomencla-ture used in this chapter is referred to Loeblich and Tap-pan^ "Annotated index and bibliography of the calcar-eous nannoplankton, I-VII," and to the INA Newslet-ter, Proceedings of the International NannoplanktonAssociation, Vol. 1-4 (S.E. van Heck, ed.). Nevertheless,a few points need explanation.

Braarudosphaera spp. incorporates specimens thatare too poorly preserved to make determinations at thespecies level. Such groupings are established for thesame reason in the following cases: Cyclolithella spp.,Fasciculithus spp., Micrantholithus spp., Neochiastozy-gus spp., and Syracosphaera spp.

The forms referred to as "Chiasmolithus spp.(small)" at Site 555 have a morphology similar to Chias-molithus sp. 1 of Okada and Thierstein (1979).

Dictyococcites perplexus was described by Burns(1975), and occurs in great numbers around the middle/upper Miocene boundary in the Rockall area.

As explained in the text, gephyrocapsid specimensobserved need extensive morphometric studies beforeconsistent species concepts can be applied.

Two forms are incorporated in the group Markaliusspp.: M. astroporus and M. apertus. Intermediate formsbetween the two were observed. Typical specimens of Mapertus were not observed above Zone NP11 of thelower Eocene. Perch-Nielsen (1979) observed this spe-cies in lower Paleocene strata from Denmark, whereasOkada and Thierstein (1979) recorded it from the upperPaleocene (northwestern Atlantic Ocean). The latter au-thors referred to M. apertus as Markalius sp. 1.

Bramlette and Sullivan's (1961) concept of Pontos-phaera aff. P. pulchra is applied.

Distinction has not been made between Sphenolithusprimus and 5. moriformis.

426


The scyphosphaerids are not separated at the specieslevel because, at present, the group offers little strati-graphic information, and intermediate forms of severalspecies are common.

ACKNOWLEDGMENTS

I am grateful to Katharina Perch-Nielsen for help with the determi-nation of Hornibrookina australis. Discussions with N. J. Shackletonon the stratigraphy around the Paleocene/Eocene boundary helpedclarify some concepts. Comments on linguistical and other items of afirst draft of this paper by D. G. Roberts and A. C. Morton are appre-ciated. This study was made possible through support from the U.S.National Science Foundation and the Swedish Natural Science Re-search Council. The paper was reviewed by Katharina Perch-Nielsenand Ton Romein; I gratefully acknowledge their constructive criticismwhich improved the manuscript.

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Date of Acceptance: June 8, 1983

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Deep Sea Drilling Project Initial Reports Volume 81...CENOZOIC CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY FROM THE NORTH-EASTERN ATLANTIC OCEAN—DEEP SEA DRILLING PROJECT LEG 811 Jan