22. FLUCTUATIONS IN THE PAST RATES OF CARBONATE SOLUTION IN SITE 149:A COMPARISON WITH OTHER OCEAN BASINS AND AN INTERPRETATION

OF THEIR SIGNIFICANCE

Anthony T. S. Ramsay, Department of Geology, University College of Swansea,Swansea, South Wales, Great Britain

Nahum Schneidermann, Department of Marine Sciences, University of Puerto Rico, Mayaguez, Puerto Ricoand

J. W. Finch, Department of Geology, University College of Swansea,Swansea, South Wales, Great Britain

INTRODUCTION

The abundance and diversity of calcareous planktonspecies in suspension in the modern ocean are determined bythe influence of global temperatures on species diversityand the combined effect of atmospheric and hydrosphericcirculation on organic productivity in the photic zone. Thespecies composition and dominance of individual specieswithin deep-sea sedimentary assemblages are, however,primarily a function of the resistance of calcareous tests tosolution in the calcium carbonate undersaturated oceandeep-water. Resistance to solution is determined by thecomposition of the skeletal elements, i.e., calcitic oraragonitic, a low concentration of pores, the tendency todevelop thickened tests with age in some species ofplanktonic foraminifera (Berger and Parker, 1970; Berger,1971), and the presence or absence of a protective organicsheath in both the planktonic foraminifera and the calcar-eous nannoplankton (Berger, 1970; Mclntyre and Mclntyre,1971).

The influence of species selective dissolution and theresulting modification of recent sedimentary assemblages ofplanktonic foraminifera is already documented for parts ofthe ocean (Berger, 1970, 1971). Modern calcareous nanno-plankton assemblages which are similarly modified arerecognized by Mclntyre and Mclntyre (1971) in sedimentsfrom the Atlantic and Indian oceans and their results havebeen substantiated and extended by Schneidermann(1972).

The affects of species selective dissolution are alsoevident in fossil foraminiferal and calcareous nanno-plankton assemblages in deep-sea sediments. Depending onthe degree of solution foraminiferal-nannofossil assemblagessubjected to dissolution are characterized by a decreasingratio of planktonic to benthonic foraminiferal tests, thefragmentation of planktonic foraminiferal tests, low speciesdiversity, and/or species with more massive tests (Cita1971). Ultimately planktonic foraminiferal tests becomerare or are totally absent. The dissolution of planktonicforaminiferal tests is accompanied by the enrichment of thesediment in the more resistant calcareous nannofossils andleads to the formation of nannoplankton chalk or ooze.

Species selective dissolution in Eocene nannofossilassemblages is indicated by an increase in the percentage ofsome species of Discoaster (Ramsay, 1972a). The data

presented in this report show that Miocene and Pliocenenannofossil assemblages are similarly affected.

Solution-induced species patterns in ancient deep-seasequences provide information on past rates of calciumcarbonate solution in the oceans. Although most interpreta-tions are based on analyses of planktonic foraminiferalassemblages, calcareous nannofossils are potentially equallyuseful and possibly even superior indicators of solution atdepth. Apart from their abundance, they are also moreresistant to solution than planktonic foraminiferal tests andtherefore provide information on sediments in which theseorganisms are either rare or absent as a result of solution.

DISCUSSION OF SITE 149 AND COMPARISONWITH OTHER DSDP SITES

Site 149 which is situated in the center of theVenezuelan Basin provides an ideal section for investigatingpast rates of carbonate dissolution. Although the core hasbeen disturbed in parts by drilling, it nevertheless containsan essentially complete record of pelagic sedimentationfrom the Paleocene to the Recent. The core descriptionsalso indicate that Site 149 contains an alternating sequenceof pelagic sediments which were influenced by varyingdegrees of dissolution. Similar sequences in the Atlantic andPacific are interpreted by Hay (1971) and Ramsay (1972b)in terms of fluctuations in the calcite compensation depth,and in the rate of calcite solution.

Our interpretation of Site 149 is based on a comparisonbetween vertical changes in the abundance of solution-resistant species of Discoaster (see Table 1) in the nanno-fossil fraction and variations in the calcium carbonatecontent of the sediment. Although the samples which wereinvestigated for their carbonate content do not alwayscoincide with the sample which we examined, a comparisonof these analyses shows a distinct parallelism in the trendsof both parameters. The data presented in Figure 1 show anegative correlation (R = —0.725 at 53 df with a signifi-cance level of 0.001) between changes in the percentage ofDiscoaster in the nannofossil assemblage and changes in thecarbonate content of the sediment. This comparison indi-cates that the dissolution of calcium carbonate was accom-panied by a simultaneous increase in both the noncarbonatefraction of the sediment and the solution-resistant elementsof the nannofossil assemblage. In Late Miocene andPliocene samples the elements most resistant to solution are

805

A. T. S. RAMSAY, N. SCHNEIDERMANN, J. W. FINCH

TABLE 1Variations in the Abundance ofDiscoaster in the Nannofossil

Fraction of the Sedimentsof Site 194

TABLE 1 - Continued

Core

999999

1010

11111111111111

12121212

1313131313

141414

15151515151515151515151515

161616161616161616

171717

2323

24

25

Section

555666

44

1223446

3346

23366

344

1122333344455

122334455

245

55

1

3

Top ofInterval(cm)

77859342292

7580

13320551215256141

7696100128

55371105255

94118144

15979810525398299221011071236

744010525998212925110

11910658

25100

98

99

% Discoaster

8655924098

180

800770000

91433540

462245620

4800

005835203416502918222119

403833292560464726

264125

128

14

9

Core

26

2727

28

29

30

3131

32323232

3333

3434

35353535353535

36

373737

3838

414141414141

424242

Section

1

33

2

2

2

12

1344

12

13

1223456

1

134

22

122446

334

Top ofInterval(cm)

98

2599

25

25

25

4719

13111984145

131113

99120

964987776118136

119

1101457

3876

5911014669121148

75148143

% Discoaster

19

1715

11

17

1

8790

8969450

055

4544

3140437523419

0

142035

250

516632532820

482642

members of the species Discoaster brouweri which in onesample (149-9-6, 92-93) makes up 93% of the nannofossilassemblage. In Late Eocene sediments, elements of thespecies Discoaster barbadiensis and D. saipanensis showmaximum resistance to solution, and in Sample15-149-3102 (19-20 cm) constitute 64% and 25% of thenannofossils respectively. Forms assigned to Discoasterbarbadiensis dominate Middle Eocene nannofossil assem-blages which were affected by dissolution.

The vertical changes in the calcium carbonate andDiscoaster content of Site 149 sediments are illustrated inFigure 2. The close correspondence between the oscillationswhich occur in both parameters is in our opinion notfortuitous. Although all the sediments which we examinedshow some evidence of dissolution, nannoplankton assem-

806

FLUCTUATIONS IN RATES OF CARBONATE SOLUTION IN SITE 149

10 20 30 i.0 50

% Cα CO

60 70 80 90 100

Figure 1. The relationship between Discoaster and calciumcarbonate in the sediments of Site 149. The vertical scalerepresents the percentage of Discoaster in the nanno-fossil fraction.

blages which contain a high percentage of Discoaster and alow carbonate content, or sediments which contain nocarbonate, very possibly reflect deposition during intervalswhich were characterized by elevated rates of carbonatedissolution. In Site 149 (see Figure 2) there is evidence fornine intervals of increased solution; three Paleogene inter-vals in which biogenic siliceous sediments predominate, andsix intervals in the Middle Miocene to Late Pliocenesequence which are represented by marls or clays. Thecarbonate and Discoaster analyses for the Oligocene sedi-ments indicate a reduced rate of dissolution for theduration of this epoch.

The alternation of sediments characterized by varyingdegrees of carbonate dissolution in Site 149 could beproduced by one of three mechanisms:

1) By vertical movements of the sea floor through awater column in which the rate of carbonate dissolutionand consequently the calcite compensation depth haveremained in a steady state.

2) The redeposition of carbonates deposited on theflanks of the Venezuelan Basin onto the floor of a basinwhich was situated below the calcite compensation depth.

3) Changes in the surface productivity of calcareousorganisms, and associated temporal fluctuations in thecalcite compensation depth and other solution levels.

We have rejected vertical tectonics and redeposition aspossible mechanisms for producing the alternation ofdifferent dissolution facies recorded at Site 149 for thefollowing reasons: a) this phenomenon is widely distributedin the pelagic sequences of other ocean basins (see Figure 2,and Ramsay, 1972b); b) parts of the alternating sequenceof different dissolution facies in Site 149 are synchronouswith similar sequences recovered from less complete sec-tions at other DSDP sites (see Figure 3); c) because there is

no geophysical or structural evidence for vertical tectonics,or sedimentological evidence for redeposition at Site 149.

We consider that the vertical changes in the carbonateand Discoaster content of Site 149 sediments and thealternation of different dissolution facies in Tertiary mid-latitude sequences at other DSDP sites are best explained interms of temporal fluctuations in the calcite compensationdepth and other solution levels, and that these fluctuationswere associated with changes in the surface productivity ofcalcareous organisms through the Tertiary. Variations in thesurface productivity of the calcareous plankton wereprobably related to changes in global temperature. Thisview is based on the similarity between the oxygen (δO18)isotope palaeotemperature curve from the Paleocene to theQuaternary, compiled from data published by Devereaux(1967) and Douglas and Savin (1971), and the proposedcurve for fluctuations in the calcite compensation depth forthe same time interval (see Figure 3), compiled by Ramsay(1972b); and the somewhat cruder correlation betweenthese curves and the Tertiary record of phytoplanktonproductivity and abundance compiled by Tappan (1968).During warm periods enhanced phytoplankton productioncorresponds with an elevated calcite compensation depth.In cooler periods both phytoplankton productivity and thecalcite compensation depth are depressed.

If our interpretation of sequences which are charac-terized by alternations of different dissolution facies iscorrect, then Site 149 contains the most complete record oftemporal fluctuations in the calcite compensation depth,and of surface plankton production recovered by DSDP forthe Tertiary of the Atlantic/Caribbean area. A comparisonof Site 149 with other DSDP sites (see Figure 3) demon-strates both the correspondence in sedimentary eventswhich was caused by simultaneous fluctuations in thecalcite compensation depths of different ocean basins (e.g.,the Pliocene deposits of Sites 149, 29, 6/7, 9, 10, 13, and2) and the variation in the history of sedimentation of thesesites. This variation is due to the various depths ofdeposition at each site through the Tertiary and to theposition of the calcite compensation depth in the watercolumn, which, whether elevated or depressed, probablydiffered within and for each ocean basin. The sedimentaryhistory of the Atlantic sites is also complicated by the factthat these deposits accumulated on the descending flanks ofthe spreading mid-Atlantic Ridge. Whereas some sites (e.g.,Sites 6,7, and 9) were situated below the calcite compensa-tion depth through much of the Tertiary, others such asSite 20 were probably carried within the range of thefluctuating compensation depth on the descending oceaniccrust.

AN INTERPRETATION OF FLUCTUATIONS IN THECALCITE COMPENSATION DEPTH

In modern oceans, away from the continental margins,the position of the calcite compensation depth in the watercolumn is very much a product of the rate of supply ofcalcium carbonate to the ocean bottom and its rate ofsolution at depth (Lisitzin, 1971). The record of afluctuating calcite compensation depth in Atlantic, Carib-bean, and Pacific (Ramsay, 1972b) Tertiary mid-latitudesections suggest that these parameters have also varied.

807

A. T. S. RAMSAY, N. SCHNEIDERMANN, J. W. FINCH

Ld

LU

O

LU

11 -

17 H

CORE

11

20

21

24

25

26

27

28

29

30

31

32

33

34

35

36

37

40

42

HOLE149

%CαCθ3 1QQo/oDiscoαster

ICHALK

| CLAY

| MARL

| RAD. OOZE

ICALC/I RAD. OOZE

I CHERT

Figure 2. A comparison between the nature of Site 149 sediments and their Discoaster content. Time scale afterBerggren in Maxwell et al. (1970).

808

FLUCTUATIONS IN RATES OF CARBONATE SOLUTION IN SITE 149

PERIOD/SERIES

PACIFICJOIDES SITES COMPENSATION DEPTH Km

QUATERNARY

OLIGOCENE

PALAEOCENE

LITHOLOGICAL SYMBOLS

O Forαm. Ooze

EE I Nαnno. Clay

E3 Nanno Marl

π~π Nanno. Ooze

• i Red Clay

Rad Ooze

O Rad Nanno Ooze

Chert

CRETACEOUS

Figure 3. Schematic representation of the vertical fluctuations in the calcite compensation depth for the Pacific plottedagainst time. Time scale after Berggren in Maxwell et al. (1970) and Harland et al. (1964).

Thus the depressed nature of the calcite compensationdepth during cool intervals may have resulted from anincrease in the rate of supply of calcium carbonate to theocean floor together with a decrease in its rate of solutionat depth. In the absence of adequate data for Tertiary highlatitude sequences it is not possible to resolve whether thissituation could have applied to the whole Tertiary ocean oronly to its mid-latitudes.

A comparison between the Tertiary and Quaternaryrecords of carbonate sedimentation in mid-latitudesindicates that they are analogous in that they both bear theimprint of climatic variations.

The Quaternary mid-latitude sequences are characterizedby carbonate cycles (Arrhenius, 1952; Hays et al., 1969;Olausson, 1971) which indicate maximum carbonate depo-sition during glacial periods and reduced carbonate sedi-mentation during interglacial episodes. In the Pacific andIndian ocenas, the alternations of glacial carbonate or

siliceous carbonate ooze with interglacial siliceous ooze incores obtained close to the modern calcite compensationdepth suggest that the compensation depth was elevatedduring warm intervals and depressed during cold episodes.The increase in numbers of solution-resistant species ofplanktonic foraminifera and coccoliths contained in car-bonates deposited above the calcite compensation depthduring interglacial cycles (Olausson, 1971; Zobel in Berger,1971; Schneidermann, 1971) suggest that the solution rateof calcite at depth was also elevated during interglacialepisodes.

The history of carbonate sedimentation in the highlatitudes of the Quaternary North Atlantic basin is thecomplete reverse of the equatorial and mid-latitude record.Here the glacial episodes are represented by carbonateminima and the interglacial intervals by carbonate maxima(Olausson, 1971; Mclntyre et al., 1972). Although Olausson(1971) attributes this record to enhanced solution during

809

A. T. S. RAMSAY, N. SCHNEIDERMANN, J. W. FINCH

AGEMY

O•

1O•

20-

30-

40-

50-

60-

7O-

80-

9 0 -

IΛΛ.

110-

120-

130-

140-

150-

160-

170-

180-

190-

PERIOD/

SERIES

QUATERNARY

MIOCENE

OLIGOCENE

EOCENE

PALAEOCENE

MAESTRICHT1AN

CAMPANIAN

SANTONIAN

CONIACIAN

TURONIAN

CENOMANIAN

ALBIAN

APTIAN

BARREMIAN

HAUTERIVIAN

VALENGINIAN

RYAZANIAN

PURBECKIAN

PORTLANDIAN

KIMMERIDGIAN

OXFORDIAN

CALLOVIAN

BATHONIAN

BAJOCIAN

TOARCIAN

PLIENSBACHIAN

SINEMURIAN

HETTANGIAN

CARIBBEAN

313369

t• h h

1 1 1

1 I 1

! 1 1

146/1493972

i i ij i ii i »

> » ii \ i

> > i

294247

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tiù

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NORTHATLANTIC

4/55361

i i ii i i

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itiiiiin

U IAI

6/75185

M M

94965

104697

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L13

4585

SOUTHATLANTIC2 0

4518

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1 1 1[ i 1

15 AL3927

t• h \•

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it

E

CαCθ3 COMPENSATION

DEPTH

.PS5 4 Km

c

1 N

)

r

i

I

5

A )

TEMPERATURE

10° 15° 20°

>

i

77/

1

^zr1

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Figure 4. Schematic representation of vertical fluctuations in the calcite compensation depth for the Atlantic and Caribbean,plotted against time and compared with a graph of Jurassic-Recent temperatures. Time scale after Berggren in Maxwell etal. (1970) and Harland et al. (1964).

glacial cycles, the data presented by Mclntyre et al. (1972)indicate that the carbonate minima accompany periods ofreduced carbonate production which were associated withthe southward penetration of cold polar water duringglacial intervals.

If the North Atlantic record is typical of high-latitudesequences in other ocean basins, then it is possible that thefluctuations of the calcite compensation depth in mid-latitudes during the Quaternary was controlled by theproductivity of carbonate organisms in high latitudes.During warm episodes the increased production and sedi-mentation of calcareous organisms in high latitudes wasaccompanied and compensated for by the elevation of thecalcite compensation depth in low and mid-latitudes.

During glacial intervals the reduced surface production andsedimentation of calcareous organisms in high latitudes isreflected by a depressed calcite compensation depth, andpossibly a decreased rate of carbonate solution at depth, inmid- and low latitudes.

The hypothesis outlined above is attractive in that itaccounts for the synchronism in the fluctuations of thecalcite compensation depth in mid-latitude sequences ofdifferent ocean basins during the Quaternary. It is possiblethat the Tertiary fluctuations in the calcite compensationdepth were also controlled by the influence of climaticvariation on the surface production of the calcareousplankton in high latitudes. Drilling by DSDP in theselatitudes should provide the necessary information to test

810

FLUCTUATIONS IN RATES OF CARBONATE SOLUTION IN SITE 149

this hypothesis. On the basis of this theory one wouldexpect that in Tertiary high latitude sequences, which weredeposited above the calcite compensation depth, coolintervals like the Oligocene and Lower Miocene will becharacterized by carbonate minima, while the warmerPaleocene and Middle Miocene intervals will be charac-terized by carbonate maxima.

REFERENCES

Arrhenius, G. O. S., 1952. Sediment cores from the eastPacific. Repts. Swedish Deep-Sea Exped. 5, (1). 227 p.

Berger, W. H., 1970. Planktonic foraminifera: selectivesolution and the lysocline. Marine Geol. 8, 111.

, 1971. Sedimentation of planktonic foraminifera.Marine Geol. 11,325.

Berger, W. H. and Parker, F. L., 1970. Diversity ofplanktonic foraminifera in deep-sea sediments. Science.168, 1345.

Cita, M. B., 1971. Palaeoenvironmental aspects of DSDPLegs I-IV. Proc. 2nd. Planktonic Conf., Roma. 251.

Devereux, I., 1967. Oxygen isotope measurements of NewZealand Tertiary fossils. New Zealand J. Sci. 10, 988.

Douglas, R. G. and Savin, S. M., 1971. Isotopic analysis ofplanktonic foraminifera from the Cenozoic of thenorthwest Pacific, Leg 6. In Fischer, A. G. et al., 1971.Initial Reports of the Deep Sea Drilling Project, VolumeVI. Washington (U. S. Government Printing Office).1123.

Harland, W. B., Smith, A. G. and Wilcock, B. (Eds.), 1964.The Phanerzoic time-scale. Quart. J. Geol. Soc. London.1205,260.

Hay, W. W., 1971. Leg 4 of the Deep Sea Drilling Project.Science. 172, 1197.

Hays, J. D., Saito, T., Opdyke, N. D. and Burckle, L. H.,1969. Pliocene-Pleistocene sediments of the Equatorial

Pacific: Their palaeomagnetic, biostratigraphic, andclimatic record. Bull. Geol. Soc. Am. 80, 1481.

Lisitzin, A. P., 1971. Distribution of carbonate microfossilsin suspension and in bottom sediments. In Funnell,B. M. and Riedel, W. R. (Eds.) The Micropalaeontologyof Oceans. Cambridge (Cambridge University Press).197.

Maxwell, A. E., et al., 1970. Initial Reports of the Deep SeaDrilling Project, Volume III. Washington (U. S. Govern-ment Printing Office). 806 p.

Mclntyre, A. and Mclntyre, R., 1971. Coccolith concentra-tions and differential solution in oceanic sediments. InFunnell, B. M. and Riedel, W. R. (Eds.), The Micro-palaeontology of Oceans. Cambridge (Cambridge Univ.Press). 253.

Mclntyre, A., Ruddiman, W. F. and Jantzen, R., 1972.Southward penetrations of the North Atlantic PolarFront: faunal and floral evidence of large-scale surfacewater mass movements over the last 225,000 years. DeepSea Res. 19, 61.

Olausson, E., 1971. Quaternary correlations and the geo-chemistry of oozes. In Funnell, B. M. and Riedel, W. R.(Eds.), The Micropalaeontology of Oceans. Cambridge(Cambridge Univ. Press). 375.

Ramsay, A. T. S., 1972a. Aspects of the distribution offossil species of calcareous nannoplankton in NorthAtlantic and Caribbean sediments. Nature. 236, 67.

., 1972b. The distribution of calcium carbonate indeep-sea sediments. In Geologic History of Oceans, Hay,W. W. (Ed.). Soc. Econ. Paleontol. Mineral. Spec. Publ.

Schneidermann, N., 1972. Selective dissolution of RecentCoccoliths in the Atlantic Ocean. Unpublished Ph.D.Thesis.

Tappan, H., 1968. Primary production, isotopes, extinc-tions and the atmosphere. Paleogeog., Paleoclim.,Paleoecol. 4, 187.

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