Biogeochemical Contrasts Between Mid-Cretaceous Carbonate

8/13/2019 Biogeochemical Contrasts Between Mid-Cretaceous Carbonate

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476 Y. Iryu and T. Yamada

ment that ocean temperatures (both surface andbottom water) in the Cretaceous were significantly

warmer than the present and that there has beena lowering of temperature since the Cretaceous

with brief intervals of warming (Shackleton &Kenett 1975; Savin 1977; Miller et al. 1987). Thesedata clearly show that a climatic change from theCretaceous onwards is one of the most represen-tative transitions from the warm greenhouse tocool icehouse Earth.

This paper aims to compare biotic and abioticcarbonate production and sedimentation betweenthe warm and cool phases registered in mid-Cretaceous carbonate platforms on Allison andResolution Guyots, Mid-Pacific Mountains, coredby ODP Leg 143 (Sager et al. 1993) and upperOligocene to Pliocene reefs observed in theKita-daito-jima Borehole drilled in 1934 and

1936 (Sugiyama 1934, 1940) and cropping out atthe surface, and to discuss their significance froma biogeochemical point of view.

In this paper, limestones are basically describedaccording to the Dunham (1962) classificationmodified by Embry & Klovan (1971).

OBSERVATIONS AND RESULTS

MID-CRETACEOUS CARBONATE PLATFORMS

Site 865 (Allison Guyot)

Allison Guyot is located on the western Mid-PacificMountains at 1810-50N, 17900-50W (Fig. 1).

The guyot has a domed upper surface risingapproximately 300500 m above the platform edge.Site 865 (1826.41N, 17933.34W), at a depth of1518 m, contains deposits formed in the formerinterior lagoon (Sager etal. 1993; Van Waasbergen& Winterer 1993). Hole 865A penetrated 700m ofshallow-water carbonates (Units IIBIV) overlainby 140 m of pelagic sediments (Units I and IIA) offoraminiferal and nannofossil ooze (Fig. 2). Theshallow-water carbonate succession ranges in agefrom late Aptian (?)/early Albian (?) to late Albian(Arnaud-Vanneau & Sliter 1995; Jenkyns et al.1995; Rhl & Ogg 1996).

Shallow-water carbonate sediments of AllisonGuyot are lithologically divisible into three units(Sager et al. 1993). These units can be grouped intotwo: the lower part of the site (Unit IV) consist-ing of wackestone and packstone with varying

amounts of benthic foraminifers, molluscs and cal-careous algae, intercalating thin beds of clay; andthe upper part (Subunit IIB and Unit III) charac-terized by wackestone and packstone with gas-tropods, calcareous sponges and siliceous spongespicules.

Unit II comprises manganiferous/phosphatizedlimestone and is divisible into shallow-water car-bonates (Subunit IIB) and overlying pelagic lime-stone and cavity fills (Subunit IIA). Planktonicforaminifers and nannofossils suggest that pelagicsediments in Subunit IIA are of early to middle

Turonian age and late Santonian to Maastrichtianage (Sliter 1995). Subunit IIB, 67.6m thick, is

Fig.1 Map showing localities of Sites865 on Allison Guyot and Sites 866, 867and 868 on Resolution Guyot in the Mid-Pacific Mountains.


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phosphatized and karstified, consisting of pack-stone and wackestone, locally with requieniidrudists.

Unit III is 424.2 m thick and composed of lime-stone, which is similar in lithology to overlyinglimestone in Subunit IIB: wackestone andmudstone and rare packstone and grainstone

with molds of molluscs. Benthic foraminifers are

common to abundant throughout the unit. Twosubunits can be distinguished within Unit III.Subunit IIIA is composed chiefly of wackestoneassociated with minor packstone and grainstone.

Wackestone contains abundant requieniid rudists.Many of the rudists have articulated valves. Coralsoccur locally; high-spired gastropods are commonto abundant throughout the subunit. UnderlyingSubunit IIIB is richer in lime mud than SubunitIIIA and includes some mudstone as well aspackstone. Dasycladacean algae, ostracods,sponge spicules and sponges are characteristic;

gastropods and rudists are less abundant than inSubunit IIIA. Dasycladacean algae are scatteredand are generally very poorly preserved. Someepisodic subaerial exposures are indicated duringdeposition of this unit by the occurrences of brec-ciation of well-indurated wackestones, reddishstaining and, in some places, erosional discontinu-ities within the sediment.

Unit IV, more than 249.0 m thick, is representedby clayey limestone/claystone, extending to thebase of the hole where it has been intruded byseveral basaltic sills. Four subunits are recog-nized within this unit. Subunit IVA consists of

wackestone and packstone with claystone laminae(approximately 2mm thick). Bioclasts includebenthic foraminifers, ostracods, gastropods, bi-

valves, solitary corals, sponges, sponge spiculesand dasycladacean algae. Pebble-sized lithoclastsof dark gray mudstone (named black pebbles inthis paper) are contained. The facies of Subunit

Mid-Cretaceous carbonate platforms and Cenozoic reefs 477

Fig.2 Stratigraphic column of Hole 865 A. Note that there exists a possibility that the lower part of the carbonate succession is late Aptian in age(Arnaud-Vanneau & Sliter 1995).


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IVB is basically the same as that in Subunit IVA.This unit, however, has black pebbles in moreabundance and commonly dolomitized, locally

with fine-grained pyrite. Possible desiccationcracks occur, indicating subaerial exposures.Benthic foraminifers are abundant throughout.Subunit IVC distinctively includes dispersed finecarbonaceous fragments that are more abundantdownhole. This unit is also characterized by inter-calating beds of carbonaceous mudstone andintensely bioturbated and stylolitized clayey lime-stone similar to those described in the upper sub-units. Dolomitization is pervasive. Black pebblesare locally abundant. Pyrite is commonly found.This subunit yields roots in growth position andcoalified woody material. Oyster fragments areobserved in several horizons. Subunit IVD com-prises clayey limestone and carbonaceous mud-

stone intruded by basaltic sills. The facies of thissubunit is similar to that of Subunit IVC but differsin having large quantities of oyster and otherbivalve shells. Wood fragments and clusters ofgranular pyrite are clearly recognized. No distinctbase for this sedimentary unit was recovered fromHole 865 A.

Site 866 (Resolution Guyot)

Resolution Guyot is located in the western Mid-Pacific Mountains (Fig.1) at 2112-22N, 17410-

30W. The semicircular guyot has a rather flatsurface at approximately 13001400 m depth. Site

866, at a depth of 1362 m, was drilled near thenorthern edge. Hole 866 A (2119.95N, 17418.8W;Fig. 3) penetrated a 1600-m-thick Hauterivian toupper Albian (Arnaud-Vanneau & Sliter 1995;Jenkyns et al. 1995; Rhl & Ogg 1996) shallow-

water carbonate platform (Units IIIVIII) over-lain by 24 m of pelagic carbonate ooze (Unit I)and Cretaceous (?) limestone encrusted by man-ganese, the detailed nature of which is unknown(Unit II).

Unit III is 414.9 m thick and composed domi-nantly of wackestone with mudstone, packstoneand grainstone. Bioclasts include gastropods,bivalves, benthic foraminifers, echinoids, ostra-cods, sponges and sponge spicules and dasy-cladacean algae and much less abundant corals,serpulids and bryozoans. Most fossils havedissolved to leave moldic porosity. Intraclasts ofmudstone are commonly found and oncoids occa-sionally occur. Peloids are distributed in a micriticmatrix but in many places are concentratedas geopetal fills in burrows. Brown, laminated

calcrete crusts or evidence for incipient calcretiza-tion is recognized in this unit, becoming morecommon in the lower subunit (Subunit IIIC); thisindicates repeated episodic subaerial exposuresduring the deposition of this unit. Keystone vugs,desiccation cracks and fenestrae are identifiedat some levels. The degree of porosity and theabundance of calcrete varies among horizons

within Unit III, which leads to distinguishingthree subunits despite the similarity of the generallithologies.

Unit IV consists of 232.1 m of bioturbatedpeloidal packstone, wackestone and grainstone

with gastropods and foraminifers. Also includedare small shell fragments of bivalves and gas-tropods and dasycladacean algae. Dark, organic-rich laminated wackestone, packstone andmudstone intervals occur at some levels, increas-

ing in abundance downhole. These limestones arecommonly organized into recurring meter-scalepackets. Each packet generally commences withlaminated organic-rich mudstones grading upwardinto bioturbated, less organic-rich packstones andgrainstones, overlain by white wackestones withbenthic foraminifers and gastropod molds. Desic-cation cracks are observed in a single horizon.Lignitic fragments and large, vertically oriented,coalified plant fragments are contained.

Unit V, 115 m thick, consists of massive ooliticgrainstone (Fig.4a). It is locally cross-laminated

and generally well-sorted. Keystone vugs arerecognized at several horizons. Other componentsinclude peloids, aggregated grains, intraclasts,rare oncoids and bioclasts of bivalve, echinoids,foraminifers and dasycladacean algae and lessabundant corals and gastropods. Although grain-stone is the main constituent of this unit, rudstoneand packstone also occur. In some levels, peloidsare more dominant than ooids. The ooids domi-nantly show radial internal structures but they arepoorly to heavily micritized or primarily micritic,displaying a peloid-like appearance. Limited bio-

turbation is observed.Unit VI consists of 412m of packstone, wacke-stone, and algal laminites (Fig. 4c), intercalatingcentimeter-thick intervals of clay/organic-richmudstone (Sager et al. 1993; Fig. 25 on page 207).This unit is distinguished by lithologies in cyclesand the occurrence of laminoid-fenestral fabrics,keystone vugs, tepee structures and desiccationcracks. The occurrence of a thick bed abounding inrudists and their debris (biostrome) and pervasivedolomitization below the bed enables subdivisionof the unit into three subunits. Subunit VIA mainly


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Fig.3 Stratigraphic column of Hole 866 A.


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comprises meter-scale cycles which begin withcentimeter-thick layers of clay/organic-rich mud-stone that grade up into peloidal packstone-

wackestone and, finally, into algal laminites.The bioclasts within the packstone/wackestoneinclude gastropods, bivalves, small-sized benthicforaminifers and dasycladacean algae. The algallaminites and clay/organic-rich mudstone locallycontain abundant ostracods. Fenestrae, tepeestructures and desiccation features occur withinthe algal laminites. Subunit VIB, 57.8 m thick,is composed mainly of peloidal packstone/

wackestone and grainstone/rudstone with abun-dant caprinid rudists and their debris. Therudist biostrome constitutes most of the subunit:other components include oysters and benthicforaminifers. The lithologies in Subunit VIC areconsiderably similar to that in Subunit VIA butdiffer from the latter by the occurrence ofdolomite. The dolomite is patchily distributedthroughout the subunit, being more commondownhole.

Unit VII consists of 196.3 m of dolomitizedoolitic/peloidal grainstone, oncoidal wackestone

and algal laminites with clay/organic-rich inter-vals (see Sager et al. 1993, fig.25 on p. 207). Theunit is distinguished by its pervasive sucrosicdolomitization and the increased contribution ofoncoids. Birds-eye and keystone vugs are dis-cernible in several intervals. The constituents areoncoids, peloids, bivalve shells, serpulid wormtubes, echinoids, rudists, corals and nerineid gas-tropods. Three subunits can be distinguished

within this unit. Subunit VIIA is composed ofdolomitized wackestone to grainstone. The upperpart of this subunit is characterized by the same

small-scale sequences as the lower part of SubunitVIC. Peloidal oolitic packstone/grainstone disap-pears downhole and the sequence is dominated byfacies inter/supratidal deposits that are generallydolomitized. Subunit VIIB consists of whitesucrosic to, in part, light-red-stained dolomitethat has replaced the original peloidal grainst-one to a varying degree. Keystone vugs can beobserved. The organization and interrelationshipof facies in this subunit are unknown because ofits extensive dolomitization. Subunit VIIC consistsof variably dolomitized peloidal grainstone, locally

Fig.4 Thin-section photomicrographsof carbonate rocks from Resolution Guyot(a, b, c, ODP Leg 143 Hole 866 A) andKita-daito-jima (d, e, Kita-daito-jimaBorehole). (a) Oolitic grainstone of UnitV (Sample ODP Leg 143 Hole 866A Core79R Section 1, Interval 1721 cm). (b)Slightly dolomitized oncoidal grainstonewith peloids of Subunit VIIIA (SampleODP Leg 143 Hole 866 A Core 156RSection 2, Interval 2327cm). (c) Algal

laminite of Subunit VIC (Sample ODPLeg 143 Hole 866 A Core 102R Section2, Interval 2932 cm). (d) Coral frame-stone of Unit C1 (Kita-daito-jima Bore-hole Sample 41, 7.96 mbgs). Note coral(C), encrusting foraminifers (F) and non-geniculate coralline algae (A). (e) Bio-clastic grainstone of Unit C3(Kita-daito-jima Borehole Sample 711,136.64 mbgs). Note bioclasts of mol-luscs (M), benthic foraminifer (arrowed)and coralline algae (C).


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probable life position. Benthic foraminifers andostracods are locally abundant; dasycladaceanalgae are scattered. Algal laminites and birds-eye

vugs are present. Subunit IIC, a probable timeequivalent of Subunit IIB, is characterized by thepresence of several intervals of boundstone(bafflestone). Different types of sponges, majorbafflers, occur in growth position overlying theerosional surface to form thickets up to 20 cm high.Sponge tubes are commonly covered by encrust-ing foraminifers and cyanobacterial filaments.Boundstone is interbedded with floatstone andminor grainstone rich in requiniid and caprinidrudists, gastropods, sponge fragments andoysters. These bioclastic grains are commonlybound and/or coated with cyanobacterial filaments.Possible keystone vugs occur.

CENOZOIC REEFS

Kita-daito-jima

Kita-daito-jima is located in the Philippine Seaat 2555.657.6N, 13116.719.6E (Fig. 6),approximately 360 km east of Okinawa-jima. It isnear the northwestern periphery of the DaitoRidge extending approximately 600 km in a west-northwesteast-southeast direction (Fig. 6). It isalso located on a lithospheric forebulge of thePhilippine Sea Plate subducting beneath theEurasian Plate. The island is semitriangular inshape, pointed to the south, approximately 4 kmfrom north to south and 5km from east to west.There exists a central lowland encircled by anelevated area along the coastal periphery of the

Fig.6 Map showing locality of Kita-daito-jima (modified from Ota & Omura1992) and a site of borehole on theisland. OT and RT, Okinawa Trough andRyukyu trench, respectively.


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island. The latter is 12 km wide, approximately 50m in elevation (up to 74 m) and consists of innerand outer ridges, both of which are arrangedmore-or-less parallel to the shore.

The Kita-daito-jima Borehole was drilled at anelevation of 2.5 m near the center of the island in1934 and 1936 (Sugiyama 1934; 1940). It pene-trated 431.67 m of shallow-water carbonates. Fourlithologic units can be distinguished within theborehole (Fig.7). Carbonates cropping out at thesurface are lithologically divisible into three units:Units S2, S1, and S0 in descending order (Fig. 7).The lowest, Unit S0, at the surface, is correlativeto the uppermost, Unit C1, in the borehole.

Unit S2 is very limited in its distribution. Thisunit occurs abutting on the cliff of the easternshore at elevations less than 10 m. It consists offramestone abounded with in-situ hermatypic

corals, such as Porites spp. and non-geniculatecoralline algae. Coral debris and up to boulder-sized limestones derived from underlying Unit S1also occur.

Unit S1 extends on the island except the centrallowland. This unit comprises reef/fore-reef sedi-ments and its laterally equivalent lagoonaldeposits (Fig. 7). Its thickness is estimated to bemore than 70m. The reef/fore-reef sedimentsare represented by coral framestone and coralbafflestone, both with abundant in-situ her-matypic corals and non-geniculate coralline algae.

The framestone contains massive (Porites spp. andFaviidae gen. et sp. indet.), tabular (Acroporaspp.), and encrusting forms of corals, while thebafflestone includes branching (Acropora spp.)forms. The bafflestone is very limited in its distri-bution and occurs on the coastal peripheries withinthe coral facies. The lagoonal deposits compriserudstone with algal-encrusted coral branches,

Halimeda wackestone/packstone and bioclasticpackstone/grainstone. This unit is extensivelydolomitized, with the exception of the western partof the island.

Unit S0 crops out in the central lowland. Thisunit is composed mainly of framestone withautochthonous hermatypic corals and less abun-dant rudstone containing coral debris. Dolomitiza-tion is pervasive in this unit.

Unit C1 mainly comprises pervasively dolomi-tized coral framestone with occasional grainstoneand packstone (Fig. 4d). It extends to 49.72mbelow ground surface (mbgs) within the borehole.Possibly in-situ hermatypic corals and non-geniculate coralline algae occur throughout theunit. Bioclasts include benthic foraminifers,

coralline algae and echinoids. This unit uncon-formably overlies Unit C2, judging from probableindications of episodic subaerial exposures andsubordinate karstification occurring in the upper9 m of Unit C2: brecciation of well-indurated lime-stone (packstone [?]) and a reddish staining.

Unit C2 is 52.05m thick and consists mainlyof bafflestone and less dominant framestone.Dolomitization is pervasive. Branching corals(Acropora spp.) are abundant in the bafflestone.The branches encrusted by non-geniculatecoralline algae abundantly occur in the lower 11 mof the unit (approximately 91102 mbgs). Bioclastscomprise benthic foraminifers, coralline algae,echinoids and less commonly molluscs.

Unit C3, 107.39m thick, comprises locallydolomitized rudstone associated mainly withgrainstone and packstone (Fig. 4e). Rudstone

characteristically includes possible in-situ coralsand much more abundant coral debris associatedwith non-geniculate coralline algae. Lowerrudstone/packstone occasionally contains coralbranches encrusted by coralline algae. Benthicforaminifers are abundant throughout the unit.Based on lithology, d13C and d18O values, andSr isotope ages, this unit is divided into threesubunits (Inagaki & Iryu 1998). They are,in descending order, Subunits C3a (103.38122.56mbgs), C3b (122.56173.35 mbgs), and C3c(173.35209.26 mbgs).

Unit C4 extends from 209.26 mbgs to the base(431.67 mbgs) within the borehole. Recoveredmaterial is represented mostly by coarse-grainedsand- to granule-sized drilling slimes. The originalconstituents of this unit appear to be packstoneabounded with benthic foraminifers, associated

with bioclasts of corals, non-geniculate corallinealgae, echinoids and molluscs. Carbonates in thisunit are not dolomitized at all.

Omura et al. (1991) obtained alpha-spectromet-ric 230Th/234 U ages of corals from Unit S2.Their ages range from 113 6 to 133 6 ka, imply-

ing that the unit was formed during the lastinterglacial stage (the oxygen isotope stage5e). Benthic foraminiferal biostratigraphy andSr-isotope stratigraphy show that Units C1 (=S0),C2, C3 and C4 are the Pliocene, late Miocene,middle Miocene (10.9, 15.5 and 16.1 Ma forSubunits C3a, C3b and C3c, respectively) andthe late Oligocene to early Miocene (18.824.3 Ma),respectively, and that dolomitization of Units C1and C2 occurred approximately 2.0 and 5.5 Ma,respectively (Hanzawa 1941; Ohde & Elderfield1992).



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Fig.7 Schematic geologic cross-section of Kita-daito-jima with stratigraphic column of Kita-daito-jima Borehole.


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DISCUSSION

Comparison between mid-Cretaceous carbonateplatforms on Allison and Resolution Guyots andCenozoic reefs on Kita-daito-jima reveals thatthere exist significant differences in their in-habitants, constituents and topographic features(Fig. 8). The following differences can be noted.Ooids, oncoids and other microbial sediments

occur abundantly in mid-Cretaceous platforms.In contrast, they are lacking or very limited inCenozoic reefs.

Corals and non-geniculate coralline algae denselygrow to form rigid frameworks on Cenozoicreefs but they are rarely found or lacking in mid-Cretaceous platforms, where rudists and spongeconstitute smaller-scaled bioherms.

Cenozoic lagoonal sediments abound in benthic

foraminifers, codiacean alga (Halimeda), corals,

non-geniculate coralline algae and molluscanshells, whereas mid-Cretaceous platformscontain molluscs (including rudists) and dasy-cladacean algae.

Reef margins of Cenozoic reefs are characterizedby topographic highs where abundant frame-building organisms, such as hermatypic coralsand non-geniculate coralline algae, occur super-imposed. This is the site for the formation ofreef facies. Such topographic highs are absentor poorly developed in mid-Cretaceousplatforms.It should be noted that sedimentation rates of

carbonate successions on Allison and ResolutionGuyots are of the same order of magnitude as thatof Kita-daito-jima, although frame-building organ-isms scarcely inhabited mid-Cretaceous carbonateplatforms. In the case of carbonates on the guyots,

the rates are calculated based on major sequence


Fig.8 Schematic figure showing differencesof constituents between Cretaceous carbonateplatforms and Cenozoic reefs. (Drawings areafter Kauffman & Johnson 1988, Nakamori1986 and Wray 1977).


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boundaries and their ages given by Rhl & Ogg(1996) since the age of top and base of the succes-sions have not necessarily been determined. Inthe carbonates on Allison Guyot, the sequenceboundaries Alb 1 and Alb 12 are discriminated,respectively, at 793.5 and 180.4 m below seafloor(mbsf), ages of which are estimated as 111.5 and101.0 Ma. The sedimentation rate of Albian car-bonates is calculated to be 58.4 m/106years. InResolution Guyot, the sequence boundaries Ltband Alb 12 are recognized at 924.8 and 60.0 mbsf,respectively. The age of Ltb is 121.0 Ma. The

Aptian to Albian carbonates accumulated at arate of 43.2 m/106 years. The sedimentation rateof non-dolomitized lagoonal carbonates on Kita-daito-jima (Unit C4) is 38.5m/106 years, becauseSr-isotope ages at 428.8 and 217.2 mbgf are 24.3and 18.8 Ma (Ohde & Elderfield 1992), respec-

tively. Similar rates are reported from EnewetakAtoll (Saller & Koepnick 1990). These rates may beinterpreted as rates of guyot subsidence = accom-modation rates rather than as sedimentationrates and might not reflect carbonate-producingcapability of reefs/carbonate platforms. Even ifthis is the case, in reality much more carbonates

were deposited in a definite duration (106 years) onmid-Cretaceous carbonate platforms than onCenozoic atolls. This is a result of deposition ofhuge amount of ooids, oncoids and other microbialsediments on the mid-Cretaceous platforms.

Our comparative study reveals that oncoidsare among the major constituents of the mid-Cretaceous platform on Resolution Guyot. Theseoncoids are composed largely of thin-laminatedmicritic envelopes, with a distinct boundary,

when recognized as such, between the envelopesand nucleus. Such characteristics, coupled withthe remnants of cyanobacterial filaments, rarelyobserved in the micritic envelopes, suggest thatcyanobacterial calcification is a likely origin forthese oncoids. In contrast, oncoids are not foundin Cenozoic reef sediments on Kita-daito-jima.

The data accord well with alternating phases ofenhanced and reduced cyanobacterial calcifica-tion proposed by Riding (1992). He stated thatcyanobacteria could provide a long-term indexof marine calcification and that the Phanerozoicrecord of marine calcified cyanobacteria showsmarked episodicity (Fig. 9). He termed thosephases when calcified cyanobacteria were com-mon cyanobacterial calcification episodes (CCE),

which are separated by reduced cyanobacterialcalcification episodes (RCCE). The MiddleTriassic to Early Cretaceous falls within CCE

whereas the Late Cretaceous to Cenozoic withinRCCE.

Ooids show a similar occurrence trend to that ofoncoids. Abundant ooids, the primary mineralogyof which was calcite (Jenkyns & Strasser 1995),occur in mid-Cretaceous carbonate successions onResolution Guyot (Site 866), in contrast to theircomplete absence from Cenozoic carbonates onKita-daito-jima. This is in good agreement withpredicted secular variations in the abundanceof calcareous ooids and in ooid mineralogy inPhanerozoic limestone sequences (Fig. 9). Wilkin-son et al. (1985) documented that the abundance ofooids varies greatly with ages and that Phanero-zoic ooids exhibit peak abundances during theLate Cambrian to Early Ordovician, Carbonifer-ous, Late Jurassic to Early Cretaceous andHolocene: Sandberg (1983) pointed out that

inferred non-skeletal carbonate (ooid and cement)mineralogy was low-Mg calcite during the Juras-sic to Cretaceous (calcite threshold) and high-Mg calcite and aragonite during the Cenozoic(aragonite threshold).

There is a significant difference in modes ofcarbonate productions between mid-Cretaceousplatforms and Cenozoic reefs. Organisms domi-nating Cenozoic lagoonal sediments, such asbenthic foraminifers, codiacean alga (Halimeda),coral and non-geniculate coralline algae, precipi-tate CaCO3 in closed or semiclosed spaces within

their bodies. On the contrary, the mid-Cretaceousplatforms contain abundant chemical (?) precipi-tates (ooids) and microbial carbonate grains/sedi-ments (oncoids and algal laminites). The latterare products of cyanobacteria which precipitateCaCO3 on their filament surface by calcificationenhanced by photosynthesis. Similarly, dasy-cladacean algae, commonly found in lagoonalmuddy facies on mid-Cretaceous platforms, formtheir skeletons on their thallus surface by biologi-cally induced calcification. Such a contrastingfeature suggests that abiotic and biotic extracellu-

lar calcification more readily occurred in mid-Cretaceous marine environments than in theCenozoic. This may reflect that concentrations ofCa2+ and HCO3- in the Cretaceous sea were muchgreater than those in the Cenozoic. The biogeo-chemical model gives a supportive output forsecular variations of HCO3- concentrations in theoceans over the past 100 million years (Lasaga etal. 1985); the concentrations around 100 Ma wereapproximately three times greater than thepresent, showing the general trend of decreasesfrom the Cretaceous onwards, although the model


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predicted that Ca2+ concentrations for the Creta-ceous period were not as high as expected.

Consequently, differences in biota and sedi-ments between mid-Cretaceous platforms andCenozoic reefs could be ascribed to differencesin water chemistry in the oceans, resulting fromdifferent modes of global biogeochemical cycles(Fig. 10). The mid-Cretaceous was a time of ex-

traordinary active global volcanism (Fig. 9) andrapid spreading rates of plates; the sea-level waselevated 100200 m higher than the present(Barron et al. 1980) and the atmospheric CO2 levelquadruplicated (Berner 1994). Higher CO2 andits induced high atmospheric temperatures mighthave increased weathering of silicates and carbon-ates on the continents despite less land area, whichresulted in high concentrations of Ca2+ (?) andHCO3- in sea waters. Such marine environmentsappear to have been favorable for abiotic precipi-tation of carbonates and biotic extracellular

calcification. Furthermore, increased hydrother-mal submarine weathering lowered the Mg/Caratio of circulating water through magnesiumremoval, which should facilitate low-Mg calciteprecipitation rather than aragonite and high-Mg calcite (Wilkinson et al. 1985). The Cenozoic

was a time of decreased volcanism, low spreadingrates, low stands of sea level and decreased flux

of CO2 to the atmosphere, resulting in coolerclimates and reduced weathering. In those con-ditions, precipitation of non-skeletal carbonatesdecreased, marine organisms formed their skele-tons within their bodies and aragonite couldprecipitate in association with high-Mg calcite.These two modes of biogeochemical cycles forthe mid-Cretaceous and Cenozoic are basicallyin agreement with the submergent mode andoscillatory mode of MacKenzie & Pigott (1981) orthe calcite sea and aragonite sea of Wilkinsonet al. (1985).


Fig.9 Carbonate abundance (Kazmierczak et al. 1985; MacKenzie & Morse 1992), non-skeletal carbonate mineralogy (Sandberg 1983), abundance ofooids (Wilkinson et al. 1985) and enhanced and reduced cyanobacterial calcification episodes (Riding 1992) compared with sea level curves (Vail et al.1977; Hallam 1984), global abundance of volcanic rocks (Ronov 1980) and mean global temperature (Frakes et al. 1992).


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CONCLUSIONS

CARBONATE PLATFORMS

Comparative study on mid-Cretaceous carbonateplatforms on the Mid-Pacific Mountains and Ceno-zoic reefs on Kita-daito-jima led to the conclusionthat their constituents are highly different fromeach other. The mid-Cretaceous platforms arecharacterized by abundant occurrences of chemi-cal (?) precipitates (ooids) and microbial carbonategrains/sediments (oncoids and algal laminites),

whereas the Cenozoic reefs consist mainly of coraland non-geniculate coralline algae, major frame-builders, benthic foraminifers and codiacean alga

(Halimeda).

CALCIFICATION

There exists a remarkable difference in modeof calcification between the mid-Cretaceous plat-forms and Cenozoic reefs. The Cenozoic reefsabound with those organisms whose calcificationsites are within their bodies. In contrast, the mid-Cretaceous platforms contain abundant grains/sediments formed by chemical (?) precipitationsand biotic extracellular calcification.

BIOTA AND SEDIMENTS

The differences in biota and sediments betweenmid-Cretaceous platforms and Cenozoic reefsmight be related to differences in water chemistryin the oceans. Tectonically induced high CO2 levelsin the mid-Cretaceous raised atmospheric temper-atures. The high atmospheric temperatures couldhave increased weathering of silicates and carbon-ates on the continents despite less land area, whichresulted in high concentrations of Ca2+ (?) and

HCO3- in sea waters. This condition might havefacilitated abiotic precipitation of carbonates andbiotic extracellular calcification. Inverse processes

are true for the Cenozoic.

ACKNOWLEDGEMENTS

We are grateful to the Ocean Drilling Program forinviting one of the authors (Y.I.) to participateon ODP Leg 143. Thanks are also due to E.L.

Winterer, W.W. Sager, J.V. Firth and othermembers of the ODP Leg 143 shipboard scientificparty and crew for their onboard cooperation; T.Nakamori and S. Ozawa for discussion and critical

comments on stratigraphy of Kita-daito-jima; K.Ishizaki for correcting and improving the Englishtext; G. Eseller and an anonymous reviewer forproviding valuable critical reviews; and J. Nemoto,R. Takashima, S. Inagaki and K. Odawara forpreparing the manuscript.

This research was financially supported bya Grant-in-Aid for Encouragement of Young Scien-tists from the Ministry of Education, Science,Sports and Culture, the Government of Japan (to

Y.I.; 07740408) and grants from the Saito GratitudeFoundation (Sendai, Japan), the Fukada Geological

Institute (Tokyo, Japan) and the Kuribayashi IkueiGakujutu Zaidan (Sapporo, Japan).

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Biogeochemical Contrasts Between Mid-Cretaceous Carbonate