19. ORGANIC DEPOSITION AT A CONTINENTAL RISE: ORGANIC GEOCHEMICALINTERPRETATION AND SYNTHESIS AT DSDP SITE 397, EASTERN NORTH ATLANTIC

Chris Cornford, Programmgruppe fur Erdoel und Organische Geochemie der Kernforschungsanlage JülichGMBH, D-517 Jülich, West Germany1

ABSTRACT

The deposition of organic matter in the deep oceans is discussedand it is noted that, while particulate matter in deep sea water con-tains 20 to 50 per cent organic carbon, the accumulated sedimenttypically contains 0.3 per cent organic carbon. Differential settling isdiscussed and quantified. It is concluded that benthic activity re-duces organic carbon concentration by one order of magnitude ator immediately below the sea floor. The organic geochemical re-ports on DSDP Site 397 are reviewed and the results related to thebroader geological setting of the site. It is concluded that the rela-tively high geothermal gradient and organic maturity in the sedi-ments studied is related to high heat flow of the nearby Canary Is-lands volcanic center. On the basis of the predominance of higherplant (terrestrial) kerogen at this continental rise site, a model forthe preferential survival of the land-derived organic matter is pro-posed: planktonic matter produced by high bioproductivity inthe upwelling waters of the Tertiary is poorly preserved. Slumpingand turbidity flow are shown to be mechanisms for the emplace-ment into the deep-water rise setting of organic-rich sediments,originally deposited under oxygen-minimum conditions on the shelfedge.

INTRODUCTION

That part of the carbon cycle operating in deepocean waters and associated sediments, past andpresent, is poorly understood. Some understanding isnecessary if the organic matter of deep-sea sediments isto be related via transport and depositional mecha-nisms to its source on land or in the sea. For example,the extent of biodegradation or reworking of organicmatter by bottom fauna (benthics) and bacteria withinthe sediments of deep ocean basins is a point of discus-sion, but it will determine the extent of preservation ofeasily bio-degradable organic matter. Also poorly un-derstood is the extent of seaward transport of terres-trial organic matter. To answer these and other ques-tions, a carbon mass balance is attempted in this re-port.

The majority of the organic geochemical reports onDSDP Site 397 concern only the hydrocarbon sourcerock characteristics and maturity. This report discussesthe conclusions of the organic geochemical reports, andattempts to relate these findings to the detailed strati-graphic, sedimentological, and regional data availablefor this site. Of the Site 397 organic geochemical inves-tigators, Deroo et al. and Cornford et al. (this volume)took individual samples. The remainder of the investi-gators took portions of the 50-cm core lengths collected

1 Present address: The British National Oil Corp., 150 St. VincentSt., Glascow G2 5LJ, United Kingdom.

for the Organic Geochemistry Panel. The results fromany one set of subsamples therefore, should, be directlycomparable within the limits of sediment homogeneity.

The reports of Whelan (this volume) concerninggaseous hydrocarbons, and Baker and Palmer (thisvolume) concerning porphyrin pigments, emphasizedifferent aspects of organic geochemistry and are notconsidered in this synthesis. Insufficient background iscurrently available to interpret the data in a more gen-eral way.

ORGANIC DEPOSITION IN THE DEEPOCEAN

Relating the types of organic matter found in deepocean sediments to their sources implies a knowledgeof transport and depositional mechanisms. At present,this knowledge is in a state of qualitative speculation(Hunt, 1974; Gormley and Sackett, 1977). McCave(1975) discussed the vertical flux of particles in theocean, while Meinhold (1977) quantitatively treatedthe cycling of organic matter in the world's oceans.

Possible mechanisms for the transport of organicmatter are represented in Figure 1. These are eoliantransport followed by settling through the water body(Milliman, 1977; Simoneit and Eglinton, 1977); fromthe autochthonous biomass (zooplankton and phyto-plankton) produced in near-surface waters, followedby settling (Menzel, 1974); from allochthonous sus-pended organic particles introduced by rivers from aterrestrial source (e.g., Gardner and Menzel, 1974;Gadel and Ragot, 1974); by turbidity current flow or

503

C. CORNFORD

EOLIAN DUST

SEDIMENTARY-.ACTIVITY

SHELF SLOPE RISE ABYSSAL PLAIN

Figure 1. Factors effecting the organic matter content of deep-sea sediments.

slumping (Site Chapter, this volume; Jacobi, 1976);and settling from suspension in ocean currents, or bysettling from the nepheloid layer produced by deepwater bottom currents (Biscaye and Eittreim, 1977).Exchange between dissolved and particulate organiccarbon is also a possiblity (Gormly and Sackett, 1977).

Some extreme cases of accumulation of organic-richsediments under oxygen deficient conditions have beendiscussed in the literature (Richards, 1970; Schlangerand Jenkins, 1976; Ryan and Cita, 1977) but few dis-cussions are found of the general case (Gormly andSackett, 1977).

Near-surface waters (e.g., 0 to 400 m) of openoceans contain 25 to 750 µg/1 of suspended particulatematter, which includes 30 to 300 µg/1 of particulateorganic carbon (Table 1). Below about 400 meters, thetotal suspended particulate matter falls in the range 25to 100 µg/1, and may show some lateral and verticalanomalies (Brewer et al., 1976). A minimum ("theclear water minimum") has been described between2000 and 4000 meters in the Atlantic (Biscaye andEittreim, 1977). Near the sea floor, the nepheloid layeroccurs (McMaster et al., 1977) which can contain up toone order of magnitude more suspended particulatematter (Table 1).

Particulate organic matter below about 400 metersis found at a relatively constant concentration in therange 5 to 75 µg/1 (Table 1); a value of 10 µg/1 canbe taken as typical (Menzel, 1974). It has also beennoted that the particulate organic matter at depth is in-dependent of the surface bioproductivity (Menzel,1974), most carbon being recycled in the upper watercolumn. The opposite appears true for total particulateconcentrations (Biscaye and Eittreim, 1977). The ma-jority of organic carbon that sinks below this upperlayer is contained in fecal pellets. As shown in Table 1,the waters of the Cariaco Trench contain anomalouslyhigh particulate organic matter concentrations at depth,

TABLE 1Total Particulate Matter and Particulate Organic Carbon Variation

With Depth: A Compilation of Literature Data (bracketedvalues = ranges; unbracketed = means)

ParticulateOrganic Carbon (µg/1)

Total ParticulateMatter (µg/1)

Surface waters(0 to 400 m)

30(15-52)d50h0.5 20-80a

(140-420)e 12(05-145)J 140 (16-670)d

(100-300)g(100-250)k (250-750)e

Middle and deep waters 5-10b 4 (0. 5-33)J 20 (12-25)a

(>400 m from surface to 10 (3-10)f (20)k 3.5C

> 1000 m from sea floor) 60 (50-75)h (16-50)1

Nepheloid layer(<lOOOm from deep (1.6-3.0)Jsea floor)

20-200a

150c

(320)d

Deep-sea sediments

Sediment accumulationand lithification

(0.3% C-org)m

Sources: (a) Brewer/et al., 1976 (Atlantic); (b) Menzel and Ryther,1970 (Pacific and Atlantic); (c) Biscaye and Eittreim, 1977 (At-lantic); (d) McMaster et al., 1977 (Atlantic, W. Africa coast); (e)Milliman, 1977 (Atlantic West Africa coast); (f) Menzel, 1974(world mean); (g) Huntsman and Barber, 1977 (Atlantic, WestAfrica coast); (h) Karl et al., 1977, (Cariaco Trench); (j) Gor-don, 1977 (NW Atlantic); (k) Parsons and Seki, 1970; (1) Riley,1970 (review, North Atlantic); (m) Mclver, 1975 (worldwide).

but these occur in the anoxic zone below 500 meters(Karl et al., 1977). Dissolved organic carbon in deepsea water is typically between 350 and 700 µg/1 (Men-zel and Ryther, 1970).

Deep-sea sediments have a mean organic carboncontent of 0.3 per cent, with a median value of 0.1 percent (Mclver, 1975). Hunt (1974) has shown similarvalues when considering the variance with lithology ofdeep-sea organic carbon contents. Simple addition oftypical deep seawater values, e.g., 10 to 25 µg/1 of to-tal particulate matter and 5 to 10 µg/1 of particulate

504

ORGANIC DEPOSITION

organic carbon, would suggest that the resulting sedi-ments should contain 20 to 50 per cent organic carbon.The particulate matter in deep sea water has been esti-mated to contain between 25 and 50 per cent organicmatter (McCave, 1975). Sedimentation is a dynamicprocess and, hence, the long-term accumulation rate isdependent on concentrations at the "clear water" mini-mum. Since deep water samples are taken instantan-eously (i.e., they represent the standing concentration,not taking rates of sedimentation into account), differ-ential settling must be considered. Settling rates for or-ganic (Riley, 1970) and inorganic particles, Table 2,show a maximum of one order of magnitude differencebetween experimentally determined values for organicaggregates, a non-ideal case, and maximum theoreticalvalues for perfect spheres of quartz or calcite, vitrinite,and fusinite obtained by the application of Stokes law(Galehouse, 1971). In practice, microfossils and nanno-fossils would be expected to settle at a slower rate be-cause of lower bulk densities and external protuberances.It appears from Table 2 that differential settling mightincrease the relative concentration of organic carbon byone order of magnitude as a maximum. After such a cor-rection, the sediment accumulating on the sea floorwould still be expected to contain at least 10 times moreorganic carbon (e.g., 2 to 5%) than found in most deep-sea sediments.

This rough calculation shows agreement with the or-ganic carbon content of the near sea-floor nepheloidlayer deduced from Table 1, where an average organiccarbon content of 2 µg/1 is associated with averageparticulate contents of about 100 µg/1.

A discrepancy of one order of magnitude remains. Itcan be accounted for by two possible mechanisms: bydilution of the settling matter by bottom-transportedorganic-free mineral matter in a ratio of at least 10:1, orby a high level of bottom faunal or bacterial reworkingof the accumulated sediments. Known values for sedi-mentation rates preclude a major contribution from theformer mechanism, as well as the lack of a source forthe mineral matter. Therefore, this analysis providesstrong evidence for a high level of benthic or bacterialactivity, which reworks the organic matter in most deep-

TABLE 2Settling Velocities of Spheroidal Particles of 10 µm Diameter

Spheroidal Particles

Settling Time to SettleVelocity Through 4000 m of(m/sec) Water (yr)

10 µm quartz or calcitea 62 X 10"6 2.0(density 2.6 g/cm^)10µmfusinitea 23 X 10~6 5.4(density 1.6 g/cm3)lOµm vitrinitea 12 X 10~6 10.9(density =1.3 g/cm•*)5 to 15 µm organic 3 to 6 X 10"6 22.0aggregates^3

Calculated from Stokes law.^Experimental values (Riley, 1970).

sea sediments. The broad geographic base of the datain Table 1 suggests that extensive organic activity is thenorm in bottom sediments of all the major oceans.

It follows that, as in shallower water, preferential re-moval of the more biodegradable organic matter is tobe expected. The faster settling rates for higher densityparticles will increase their relative abundance in thesediments. Organic matter attached to or absorbed oninorganic particles will also experience accelerated set-tling and, hence, be enriched. Evidence for incorpora-tion of dissolved organic matter in sediments based oncarbon isotopes has been proposed (Gormly and Sack-ett, 1977). Perturbation of the system could occur byexchange between particulate and dissolved organiccarbon. In deep waters, about 30 times more organicmatter exists in solution than in the particulate form(Menzel and Ryther, 1970). Bacteria have been shownto produce particulate from dissolved organic carbon(Parsons and Seki, 1970).

A CONCENSUS OF RESULTS

The sediments of Site 397 can be subdivided usingthe previously defined sedimentological units (Site Re-port, this volume). The relationships of these units tothe shipboard organic geochemical studies are discussedin the organic geochemistry section of the Site Report(this volume). Briefly, these units are as follows:

Unit 1: 0 to 300 meters, Quaternary to Pliocene inage. Fluctuating organic-rich and organic-poor siliceousand carbonate oozes.

Units 2 and 3: 300 to 690 meters, Pliocene to lateMiocene in age. Organic-lean hemipelagic oozes andchalks.

Unit 4: 690 to 1300 meters Miocene in age. Alloch-thonous slumped and turbiditic clays, silts, and sands,with high organic carbon values in the finer units. Sedi-ments of Units 2 and 3 type continue as a minor com-ponent (background sedimentation, Facies F5) inter-spersed with the allochthonous sediments of Unit 4.

Unit 5: 1300 to 1452 meters, Hauterivian (EarlyCretaceous) in age. Organic-rich silty claystones withsideritic lamellae.Maturity

All authors described the entire section as immaturethat is. in an early stage of organic genesis. Their matu-ration data, from techniques as diverse as the tempera-ture of maximum volatile evolution during pyrolysis(Deroo et al., this volume), vitrinite reflectance (Corn-ford et al.; Pearson et al.; Kendrick et al., all this vol-ume), thermal alteration index (Johnson et al., this vol-ume), and extract quantity and type, are in reasonableagreement with the measured down-hole temperatures,which extrapolate to about 65 °C at 1500 meters. Iftime and temperature are considered, a high sedimen-tation rate ('VδO m/m.y.) in the upper part of the hole,would indicate that the duration of heating (effectiveheating time) is short. For example, the Cretaceoussediments at 1450 meters have been subjected to tem-peratures over the 10°C interval from 55 to 65°C for

505

C. CORNFORD

only 3 million years. This short effective heating timewould be predicted to give a lower maturation stagefor a given temperature.

Type of Organic Matter

To extract useful paleogeographic information fromthe reports cited earlier, it is necessary to compare thevocabularies used by the various authors to describethe organic matter. Table 3 contains a list of approxi-mate equivalents, the terms being taken mainly fromthe tabulated results. In the left-hand column, these arerelated to the origin (provenance) of the organic mat-ter (a genetic classification), while the petroleum po-tential is indicated in the right-hand column.

The samples studied are listed in Table 4 and re-lated to the lithostratigraphic units and lithofacies de-scribed in the Site Report (this volume). Also given forcomparison are the organic carbon and extract yieldsdetermined by the various authors. While the variabil-ity in organic carbon (a well-standardized procedure)is probably due to sediment inhomogeneity (e.g., seeErdman and Schorno, 1976), the variation in extractquantities must also be related to the extractionmethod and the solvent used. On the basis of organiccarbon content, the Organic Geochemical Panel (OGP)

samples from Sections 397-95-4, 397A-12-2, and397A-16-4 are heterogeneous, which is expected forthe slump sediments of Lithofacies F4a (Site Report,this volume). Sections 397A-23-4 (Lithofacies F3) and397A-51-4 (Cretaceous) show little variability in or-ganic carbon values, and are presumably homoge-neous.

The results obtained by the various authors concern-ing the type of organic matter encountered at Site 397are tabulated in Table 5. These results can be relatedto the source of the organic matter via Table 3. Each li-thostratigraphic unit is considered in turn.

Unit 1: 0 to 300 Meters (mean organic carbon of0.45%; Site Report, this volume)

This unit is described by Johnson et al. (this vol-ume) as containing a predominantly woody and coalykerogen with perhaps some indication of reworking; byCornford et al. and Kendrick et al. (this volume) ascontaining a predominantly higher plant suite of parti-cles such as vitrinite, inertinite and spores, pollen, andcuticle of the liptinite group. They also report a minorcontent of amorphous or algal fluorescing debris.

Erdman et al. (this volume) report heavy (marine)carbon isotope values for the lipids. Nevertheless, the

TABLE 3Comparison of Termsa Used to Describe Kerogen and Their Use as Indicators of

Sediment Provenance and Petroleum Potential, DSDP Site 397

Provenance

.a /

^N

. A

quat

/

Sub

-aer

ial

(ter

rest

rial

)

Johnson,Mclver,Rogers

Algal

Amorphous

Herbaceous

Woody

Coaly

Kendricks,Hood,

Castano

Amor- /phous /

Liptinite

Vitrinite

/ ^

Inertinite

Pearson,Dow

Amorphous

Humic

Deroo,Herbin,

Roucaché,Tissot

Type I

Type II

/

Type III

Erdman,Schorno

δ13CUpid

<28.5

δ13CUpid

>32.5

Cornford,Rullkötter,

Welte

Unstruc-tured y

Liptinite

Structured

Vitrinite

\×^Inertinite

PetroleumPotential

mai

nly

LIQ

UID

GA

S +

\ L

IQU

ID

s<SO

ו

NO

NE

aTerms taken mainly from tables of results of the various authors (this volume).

506

ORGANIC DEPOSITION

TABLE 4Lithologies of DSDP Site 397 Samples for Organic Geochemical Analysis and Comparison of Organic Carbon

Content and Extract Yield of the Various Authors (this volume)

Sample(Interval in cm)

2-5, 125-150a

6-3, 120-150a

10-6,0-50a

18-5, 100-150a

29-3, 100-150a

32-2, 125-150a

45-3, 120-150a

52-3, 100-150a

68-2, 120-150a

95-4, 100-150a

12A-2, 0-50a

15A-6, 6-1016A-4, 130-150a

16A-5, 81-9017A-1, 144-150

21A-2, 120-150a

23A-4, O-35a

24A-4, 60-6628A-2, 112-11735A-3, 98-108

46A-3, 127-13748A-1, 92-10551A-4, 100-15052A-1, 104-117

Solvent for extraction:

Depth ±(m)

165193

168269

297423498648908

10321066107310751079

11281158117812421309

1395141114441449

Unit/Lithofacies

11111

12 + 32 + 32 + 34/F4

4/F4-F34/F34/F44/F44/F3

4/F1-F44/F34/F34/F25

5555

Pearson and Dow

Corg Extract(%) (ppm)

_ _- _— _- _-_ _- __ __ _-

1.33 39.9— _

2.03 40.6- --_ _

1.29 129- _— _-_ _- _

0.99 39.6-

Benzene-methanolAzeo trope

Deroc>, HerbinRoucaché, Tissot

org(%)

____-____-

4.56-

3.594.78

__

2.922.140.56

0.530.66

_0.92

Extract(PPm)

____-____-_

1100

_1100

___

700100___100

HCC13

Johnson, Mclver,and

Corg

__

0.320.45

-

0.23_

0.12_

0.36

1.14_

1.36--

0.251.24_

-_

0.83-

"or

Rogers

Extract(ppm)

_

256480

-

382_

5 83—

239

417_

540_-

286431

_-__

834-

ganicsolvent"

Kendricks, Hood,and Castano

Co rσ Extract(%) (ppm)

_ _

_ —_ __ —-

_ __ -_ __ —-

3.74_ —

1.84- --

_1.24

_ —_ —-_ _

_ —0.88

-

Erdman and Schorno

Corg(%)

0.710.850.440.320.16

0.280.260.310.370.21

0.80—

0.87--

0.441.21

_

-_

_0.98

-

Extract(ppm)

616440

76

4012157

15

16-37--

48180

--

-

_-

248-

CH 2 C1 2

aOrganic Geochemistry Panel Sample.

TABLE 5Classification of Organic Matter as to Source for the Various Lithologic Units, DSDP Site 397

Authors

Pearson,Dow

Deroo,Herbin,Roucaché,Tissot

Johnson,Mclver,Rogers

Kendricks,Hood,Castano

Cornford,Rullkötter,Welte

Maturity

Immature

Immature

Immature

Immature

Immature

Organic Matter (Kerogen) Type

Unit ia

-

-

woody+

coaly

vitrinitic

higherplant

Unit 2+

Unit 3

amorph. +secondaryhumic

-

woody+

coaly

-

higher plant+ minoralgal matter

Unit 4

Lithofacies

2

-

-

-

-

higherplant

3 4a

amorphous

Type II

algal+woody÷herbaceous

terrestrial

higherplant

woody+

algal

amorphous

algal÷higherplant

4b+l

-

-

-

-

barren

Unit 5

humic

Type III

woody+

herbaceous

vitrinitic

higherplant

aLithostratigraphic units as defined in the site chapter, this volume.

507

C. CORNFORD

chromatogram of the n-heptane soluble lipids shows an«-alkane distribution maximizing in the C2 7 to C2 9 re-gion with a large odd chain length predominance. Thisdistribution is consistent with derivation from landplant waxes. Thus, most of the evidence from the fewcores investigated in this unit suggests that the sedi-ments contain moderate amounts of organic matter ofterrestrial origin.

Units 2 and 3: 300 to 690 Meters (mean organiccarbon of 0.17%; Site Report, this volume)

Sediments from these units are lean in organic mate-rial. They are described by Johnson et al. (this volume)as woody and coaly, while Erdman et al. (this volume)report lipid δ13C values in the marine to intermediaterange and show chromatograms of the w-heptane solu-ble lipids with maxima in the C27-C31 and C1 9 ranges,the former predominating. Cornford et al. (this vol-ume) found these units to contain vitrinite, little iner-tinite, but considerable fluorescent (liptinitic) matterwhich was a mixture of higher plant (pollen, cuticle)and amorphous, filamentous, and unicellular algal par-ticles.

This interval contains a mixed type of organic mat-ter with a predominant contribution from terrestrialhigher plants, but some evidence of minor input fromplanktonic sources.

Unit 4: 690 to 1300 Meters (mean organic carbon of1.39%; Site Report, this volume)

Unit 4 consists of five major lithofacies (Site Reportsedimentology section); three were investigated by or-ganic geochemists. These lithofacies can be grouped inthe following way:

1) Lithofacies F2 and F3; green, well-sorted some-times graded silts and clays of turbiditic origin.

2) Lithofacies F4a; a poorly-sorted coarse shell"hash" probably emplaced by a mass slumping mech-anism.

3) Lithofacies F4b and Fl; coarse-grained beds ofvolcanoclastic and quartz sand composition, respec-tively.

Since OGP sections are not lithologically describedby the shipboard party, the lithofacies designation ofthese samples can only be inferred by interpolation be-tween the lithologies above and below. The inferred li-thotypes of the OGP sections and those known for De-roo et al. (this volume) are given in Table 2. Those ofCornford et al. are given in their report (this volume).

Considerable variation exists in the descriptions ofthe type of organic matter found in this unit, and thisvariation is not obviated by comparing only data froma single lithofacies. Cornford et al. (this volume), onevidence from microscopy, concluded that LithofaciesF2 and F3 contain a predominance of higher plantmatter, while Lithofacies F4a has a higher amorphousor algal input, a result that is confirmed by the study ofKendrick et al. (this volume). In contrast, Pearson etaL and Deroo et al. (this volume) found the sedimentsof Lithofacies F2, F3, and F4 to be similar and to con-tain a predominantly amorphous or Type II marine al-

gal organic matter (see Table 3), while Johnson et al.(this volume) report a mixed algal and higher plantkerogen, but with the Lithofacies F3 sample being themost algal rich. This latter conclusion is supported bythe δ13C analysis of Erdman et al. (this volume), whofind the lipid fractions of the Lithofacies F3 sample tobe isotopically heaviest (most marine), while some ofthe Lithofacies F4a samples give the lightest values inthe hole.

From analysis of the w-alkane distributions reportedfor this unit, Deroo et al. (this volume) show a mark-edly terrestrial distribution for F3 samples and analgal distribution for F2 samples. Cornford et al. (thisvolume) show bimodal w-alkane distributions for F3samples indicating both algal and higher plant waxcontributions, and F2 sample in which «-alkanes wereonly minor components and with a high percentage ofsteranes, sterenes, and triterpane.

Little can be concluded from these results other thanthat there appears to be inhomogeneity within the sedi-ments, discrepancies between the laboratories, and alsobetween the results from different techniques applied toa single sample within the same laboratory. Reasonsfor these discrepancies are discussed in the next section.It can be concluded that the unit contains at least someterrestrial and marine organic input, probably inhomo-geneously distributed, but that no consistent trend canbe seen from the fine-grained Lithofacies F3 to thecoarse-grained suspended flow matter of F4a. Placinggreater emphasis on representative techniques such aspyrolysis (Deroo et al., this volume), and a consensusof the microscopy results (Table 3), this unit containsmore marine derived organic matter than others at Site397.

Unit 5: 1300 to 1452 Meters (mean organic carbon of0.56%; Site Report, this volume)

There is unanimity concerning the type of the or-ganic matter in Unit 5. All authors agree that theLower Cretaceous contains a kerogen of terrestrialplant derivation. Typical plant wax w-alkane distribu-tions (Cornford et al., Pearson et al., both this volume),woody or coaly kerogens, and H/C and O/C values at-test to the terrestrial origin of these sediments. Deroo etal. and Cornford et al. (both this volume) also invoke thepossiblity of recycling or distal transport for part of theterrestrial component. Results from other DSDP holesin the Atlantic show that land-derived organic matter iscommon in Cretaceous sediments.

SYNTHESIS

Maturity

The organic maturity at Site 397, determined by di-verse techniques, is low and in agreement with the rel-atively shallow penetration. Downhole temperaturemeasurements predict a gradient of 42°C/km. This rel-atively high gradient, which gives vitrinite reflectancevalues of 0.45 at about 1500 meters (Pearson et al.;Cornford et al., both this volume) may be due to the vol-canic activity of the nearby Canary Island volcanic cen-

508

ORGANIC DEPOSITION

ter. If true, and given that the earliest volcanic episodeof the Canaries at this site is of a Miocene age, it is notreasonable to assume that the present-day geothermalgradient existed during the Cretaceous.

Thus, the discrepancies between the interpretationsof Pearson et al. and Cornford et al. (both this volume),concerning the thickness of sediment lost in the LowerCretaceous-Miocene unconformity, become academic.The differences in result (360 versus <1300 m) stemfrom differences in interpretation. Pearson et al. believethat excavated and reburied vitrinite starts to increasein reflectance (though at a lower rate than first-genera-tion material) as soon as reburial starts; Cornford et al.believe that the vitrinite must be reburied to approxi-mately its original depth before the reflectance increasessignificantly. Discussion of the paleogradient in theCretaceous must be speculative. Values of 20 to 25 °C/km are more typical of the present-day passive mar-gins, but during early opening of an ocean, highervalues might be expected.

Organic Facies

Unit 1: 0 to 300 MetersFluctuating high and low organic carbon values in

this interval could be due to at least two processes. Thefirst is a fluctuating terrestrial organic input, perhapsrelating to the series of glacial/interflacial episodes, assuggested by Diester-Haass (this volume). The secondprocess is the high productivity (and preservation) ofmarine planktonic organisms due to the developmentof deep water upwelling in the Pliocene-Pleistocene.

While the high biogenic content in Unit 1 sediments,along with the rapid sedimentation rates (Site Report,this volume), suggest the process of high planktonicbioproductivity, the fact that the organic matter inthis unit is predominantly of terrestrial origin indicatesa low survival rate of the organic matter of the plank-tonic biomass. Summerhayes et al. (1976) have shownthat little organic matter survives to be incorporated inrecent sediments deposited beneath areas of upwellingoff southeastern Brazil.

The terrigenous input to these sediments is semi-quantitatively demonstrated from the smear slide anal-ysis, which indicates an average input of about 25 percent of terrigenous components (Site Report, this vol-ume). Thus, although the majority of the sedimentaryparticles are biogenic, the survival and preservation ofthe organic matter favors the terrestrial component.

As discussed previously, differential settling will initi-ate this enrichment, while the present-day conditions ofnutrient-rich and oxygen-rich waters at depth, will fa-vor destruction by the benthic population of the easilydegradable planktonic matter, while allowing the moreresistant terrestrial matter to survive.

Units 2 and 3: 300 to 690 MetersThe organic content of this unit is low but that mat-

ter which is present is dominated again by terrestrialparticles. These hemipelagic sediments contain a highfraction of biogenic mineral matter, so it appears againthat in this deep water continental rise setting, the al-

lochthonous terrestrial organic matter has a better sur-vival rate than the autochthonous planktonic input.

Unit 4: 690 to 1300 MetersThis unit is atypical of Miocene deep-sea sediments

of the Atlantic, both in its high organic carbon contentand its violent mode of deposition. Organic-rich Mio-cene sediments were encountered during Leg 38, onthe V0ring Plateau (Erdman and Schorno, 1976).From the organic geochemistry reports, the type of or-ganic matter present at Site 397 seems highly variable.Accepting analytical and interpretational difficulties,the kerogen type appears to fluctuate from algal-rich topredominantly terrestrial matter. This is not an unrea-sonable conclusion since these sediments contain al-lochthonous benthic and planktonic foraminiferal as-semblages, which appear to have originated in an outershelf environment (Site Report, this volume). The or-ganic matter in these sediments, therefore, could be de-rived from a variety of depositional environments.

Preservation of large quantities of organic matter istypical of anoxic environments, which suggests thatthese sediments have been transported from a positionwhere the oxygen minimum encroaches on outer shelfsediments (Figure 1). Subsequent rapid transport anddeposition by slumping or turbid flow has broughtthem to their present position. Thus, the deposition ofthese anomalous organic-rich Miocene deep-sea sedi-ments appears to be due to a combination of: (1) oxy-gen-minimum conditions on the shelf edge providing amechanism for the preservation of significant autoch-thonous marine as well as allochthonous terrestrial or-ganic matter, and (2) sediment flow allowing emplace-ment in the deep sea.

Unit 5: 1300 to 1452 MetersThis unit is composed of Lower Cretaceous cyclic

claystone and siderite sediments with terrestrial organiccontent, which were deposited during the early spread-ing stage of the central Atlantic. The sparse sedimentaryevidence suggests deposition in a distal turbidite orprodelta setting (Site Report, this volume). The pres-ence of a terrestrial kerogen and woody fragments arein agreement.

CONCLUSIONSIt is unusual for so wide a variety of organic geo-

chemical techniques to be applied to a single set ofsamples. In this case, the data are sometimes in goodagreement (e.g., Unit 5), but can also show a bewilder-ing diversity of interpretation (e.g., Unit 4). These in-consistencies highlight the need to clarify and developthe interpretation of organic geochemical data. DSDPsamples provide a unique opportunity to comparemethods and interrelate interpretations.

From the literature and the organic geochemical re-ports of Site 397, the following conclusions can bedrawn.

1) Comparison of the concentrations of paniculateorganic and inorganic matter in the oceans with deep-sea sediments indicates considerable biodegration oforganic matter at or immediately below the sea floor.

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C. CORNFORD

2) Hemipelagic continental rise sediments contain adominance of land-derived organic matter, despite adominantly planktonic input to the inorganic compo-nent of the sediment. Less refractory planktonic or-ganic matter recycled in the upper 400 meters of wateris reduced in concentration by differential settling, andis biodegraded by benthic organisms. All three mech-anisms serve to enrich the more resistant terrestrial or-ganic components.

3) Slumping and turbidity current sediment trans-port from the shelf-edge zone of low oxygen and highorganic accumulation is a mechanism for the introduc-tion of organic-rich sediments into deep-sea (rise) loca-tions. This mechanism is observed for the allochtho-nous Miocene sediments of Unit 4, which show thehighest content of marine organic matter of any of thefive units.

4) The sediments show a low level of organic dia-genesis.

ACKNOWLEDGMENTS

During preparation this synthesis has been circulated to allorganic geochemical contributors to the Site 397 Report. Ishould like to thank Drs. B. Tissot, J. Kendrick, S. Palmer, D.Pearson, and K. Schorno for detailed comments and reviewof the manuscript. I am solely responsible for the final text.

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