leg 28, deep sea drilling project

35. PALEOCLIMATIC SIGNIFICANCE OF DIACHRONOUS BIOGENIC FACIES,LEG 28, DEEP SEA DRILLING PROJECT

Elizabeth M. Kemp1 and Lawrence A. Frakes2, Department of Geology, Florida State University,Tallahassee, Florida

andDennis E. Hayes, Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York

ABSTRACTAt Sites 265, 266, 267, and 268, in the southeast Indian

Ocean, siliceous sediments, consisting of diatom oozes ordiatom silty clays, overlie calcareous nannoplankton oozesand chalks. The boundary between siliceous and calcareoussediments shows a pronounced diachronism, youngingtowards the north. At the southern sites, 267 and 268, the tran-sition from a calcareous to a siliceous sedimentary regime oc-curred in the interval late Oligocene to early Miocene; at Site266, the transition was gradual and commenced in the middleMiocene; at Site 265, the most northerly, the change did notoccur until probably late in the Miocene.

Back-tracking of these sites along an empirical age-depthcurve for the southeast Indian Ocean suggests that thesilica/carbonate boundary was stationary with respect to thecrest of the Southeast Indian Ridge throughout most of theMiocene, i.e., in the period 5.5-22 m.y.B.P., although alatitudinal migration of the boundary apparently occurred inconcert with the ridge system itself. In the early Pliocene,however, a rapid northward shift of the boundary with respectto the ridge crest occurred.

The distribution of ice-rafted detritus in these sites showsthe same diachronous pattern as the biogenic facies boundary.The earliest ice-rafting is evident from the early Miocene atSite 268, from the middle Miocene at Site 267, and the lateMiocene at Site 266, and is not apparent at Site 265.

Paleoclimatic interpretation of these lithofacies shifts leadsus to conclude that surface waters have been relatively coolfrom at least the late Oligocene-early Miocene; the principaleffects of this environment were felt only in near-continentregions. Continued gradual cooling persisted until after theearly Pliocene. Upwelling may be responsible for the highdiatom content of sediments at southern sites. Extensiveiceberg melting was similarly confined to extreme southernregions. In the earliest Pliocene, intensified, relatively suddenchilling resulted in a northward shift of the silica/carbonatefacies boundary and of the zone of iceberg melting.

INTRODUCTION

Sites 265, 266, 267, and 268 are situated in a generallinear arrangement (with a spread of only 8° longitudebetween the four sites), from near the crest of theSoutheast Indian Ridge in the north, to the Wilkes Landcontinental rise in the south. These sites thus provide a

'Present address: Bureau of Mineral Resources, Geology andGeophysics, Canberra, Australia

2 Present address: Department of Earth Sciences, Monash Universi-ty, Clayton, Victoria, Australia

north-south section through a region which has ex-perienced dominantly pelagic processes of sedimenta-tion throughout its history (except for Site 268, thesouthernmost site, where terrigenous sedimentation hasprevailed).

Pelagic sedimentation processes are presently con-trolled in the southeast Indian Ocean by those factorswhich determine the distribution, in surface waters, ofsediment-forming pelagic organisms and of icebergs.Oceanic circulation is the prime factor governing thisdistribution, and this is in turn dependent on therelationship between oceanic temperature structure andatmospheric circulation. The modern sedimentaryregimes generated by this complex of factors show a

909

E. M. KEMP, L. A. FRAKES, D. E. HAYES

pronounced latitudinal zonation, with belts of distinc-tive sediment type arranged in a circumpolar fashion.Latitudinally disposed oceanographic singularities suchas the Antarctic Convergence give rise to this circum-polar arrangement of sediment belts; this is reinforcedby a similar disposition of the major tectonic features.The Southeast Indian Ridge trends in an east-west direc-tion and controls the three-dimensional shape of sedi-ment bodies by acting as a generative center whichgenerates new sea floor and spreads it to progressivelygreater depths both towards the north and the south. Itfollows that a north-south section should provide an op-portunity to examine the movement of the boundaries ofsedimentary facies through time, movements which haveultimately depended on fluctuations in climaticparameters such as air and ocean temperatures, butwhich also bear some imprint of the regional tectonicpicture.

The role of carbonate dissolution in delineating theboundary between calcareous and siliceous biogenicsediments on the sea floor cannot be evaluated at pre-sent because of a lack of dissolution studies on cal-careous organisms in this region. For the moment, weassume that the observed lithologic changes result fromdeposition of pelagic debris on either side of a more orless vertical boundary separating different water masseswhich contain differing suites of microorganisms.

MODERN SEDIMENT DISTRIBUTION,SOUTHEAST INDIAN OCEAN

The circumpolar belts evident in maps of modernsediment distribution (Lisitzin, 1962, 1972; Hays, 1967)consist of: (1) a southern, near-continent zone of glacialmarine sediments—mostly pebbly muds with somebiogenic detritus—which extends across the continentalshelf of Antarctica to proximal portions of the abyssalplain; (2) a more northerly belt of siliceous ooze whichextends up the southern flank of the mid-oceanic ridge,and (3) further north, a belt of calcareous, largelyforaminiferal oozes, with clays and muds in sea-floorareas below the calcium carbonate compensation depth.

The broad belt of siliceous oozes, chiefly composed ofdiatom frustules, with some radiolarian and silico-flagellate tests, reflects the surface water zone of highbiologic productivity which lies between the AntarcticDivergence to the south, at approximately 62°-65°Slatitude, and the Antarctic Convergence, or Polar FrontZone, to the north at 52°-55°S latitude. The highlyproductive nature of this zone depends on the upwellingof relatively warm, nutrient-rich water, the CircumpolarDeep Water, in the region of the divergence, where itreplenishes Antarctic Surface Water, and flows in ageneral northerly direction to sink in the Polar FrontZone.

The position of the northern boundary of the siliceousooze zone, roughly coincident with the Polar Front, isgoverned by the present production of cold water inAntarctic regions, which in turn governs the intensity ofvertical circulation patterns in the Southern Ocean. Azone of mixed siliceous-calcareous oozes that presentlyoccurs in a 600-1000 km wide belt in the mid-oceanic

ridge region (Connolly and Payne, 1972), reflects theshort-term fluctuations in the position of the northernboundary of the siliceous ooze zone. The initiation ofthe Polar Front Zone at some time in the past must havebeen marked by, or followed by, the initiation of the beltof high diatom productivity. This paper is concernedwith the possible motion of this boundary and its effectson siliceous and calcareous sedimentation.

SEDIMENT DISTRIBUTION IN LEG 28 SITESIn each of the sites examined, a downhole transition

from siliceous to calcareous sediments is evident. Thetransition is most clearly observable in those sites wherepelagic sedimentation processes have dominated. Inthese sites, the transition is manifested as a change fromdiatom oozes in the upper part of the sequence tocalcareous nannoplankton oozes below. In the sitesnearer the continent, viz, Sites 267 and 268, the influenceof terrigenous sediment is stronger, and the transitionappears as a change from diatom-bearing or diatom-richsilts and clays above, to silts and clays with nan-noplankton below.

The changes in sediment type are illustrated in Figure1. In this diagram, the percentages of the main sedimentcomponents, classified here as the diatom, calcareousnannoplankton and detrital (=terrigenous) fractions,have been averaged for each core and are illustrated inbar form on the right-hand side of the general strati-graphic columns. Although the figures on which theaverages are based were derived from shipboard ex-amination of smear slides, they are considered to be con-sistent enough to show gross changes in the proportionsof the main sediment fractions.

At Site 265, high on the southern flank of the mid-oceanic ridge at a water depth of 3582 meters, sedimen-tation has been wholly pelagic. Calcareous nan-noplankton oozes and chalks constitute the main sedi-ment type up to and including Core 15. The highest nan-noplankton ooze (wherein nannoplankton are the majorsedimentary component, in excess of 50% of the totalsediment) occurs in Core 15, Section 2, and is of middleMiocene age. All of the overlying sediments, includingCore 14, are diatom oozes, without appreciable car-bonate content. The oldest diatom ooze, in Core 14, Sec-tion 6, is of early Pliocene (Gilbert Magnetic Epoch)age. The change from deposition of calcareous tosiliceous oozes occurred somewhere between the middleMiocene and the early Pliocene. The diatom zones iden-tified at this site suggest that a marked time break is pre-sent between Cores 14 and 15, which represent diatomZones 6 and 9, respectively. In terms of millions ofyears, these intervals, respectively, represent sedimentslaid down some 2.8-4.0 m.y.B.P., and 9-12 m.y.B.P. (seebiostratigraphic correlation chart, Chapter 1, thisvolume). The radiolarian zones, which have poorerresolution in this interval, do not indicate the presenceof such a time break.

At Site 266, the change from calcareous chalks andoozes below to siliceous oozes above is not so abrupt asat Site 265. The highest calcareous ooze occurs in Core10, Section 1, in what are probably late Miocene

910

PALEOCLIMATIC SIGNIFICANCE OF DIACHRONOUS BIOGENIC FACIES

sediments; this interval is, however, overlapped by thelowest diatom ooze, downhole in Core 12, Section 2.Diatom-rich sediments occur in fact as far down as Core16, so that probably the whole of the middle and lateMiocene, as represented at this site, constitutes a transi-tion zone of mixed calcareous and siliceous sediments.Above the base of Core 9, which is of early Pliocene tolatest Miocene age (4-6 m.y.), on the basis of diatoms,radiolarians and silicoflagellates, sediments aredominantly pure diatom oozes, with little admixture ofother biogenic components or of detrital material.Below Core 16, which is earliest middle Miocene in age(14-15.5 m.y.) they are dominantly calcareous nanno-plankton oozes, with some clay-rich intervals.

Three holes were drilled at Site 267, situated on thesouth flank of the Southeast Indian Ridge at the edge ofthe deep basin, and the transition from calcareous tosiliceous sedimentary regimes is more difficult torecognize because the influence of terrigenous sedimentis stronger here than at the sites further north. Never-theless, at the combined Site 267-267A, a basal intervalof calcareous sedimentation is clear; calcareous oozesoccur as high in the sequence as Core 4, Section 1 (withan estimated nannofossil content in excess of 75% of thetotal sediment). The youngest of these oozes is of lateOligocene age, based on nannofossil data. Above thislevel, in Core 3, of early Miocene age, diatoms are thedominant biogenic fraction, although the sediment islargely detrital. True diatom oozes, with diatoms in ex-cess of 50% of the total sediment, occur in Core 3A, Sec-tion 2. The basic change between calcareous andsiliceous biogenic regimes, however, can be broadlyascertained as having occurred in the interval lateOligocene-early Miocene, i.e., at an older date than atSite 266.

In Hole 267B, middle Miocene sediments are similarlydominated by a diatomaceous biogenic component. Thelate Eocene or early Oligocene sediments at the base ofthe sequence are nannofossil chalks; the middle and lateOligocene section is missing.

Site 268, the southernmost in the line of sites betweenthe crest of the Southeast Indian Ridge and the Ant-arctic continental margin, has been influenced byterrigenous sedimentation to an even more markeddegree than was Site 267. Sediments throughout the se-quence penetrated are mainly silty clays, but the ad-mixture of biogenic debris is sufficient to enable achange to be detected from a lower calcareous biogenicregime to a siliceous one in the upper part of the section.Calcareous nannofossil oozes occur in sediments asyoung as early Miocene (in Core 8, Section 2) althoughthe proportion of nannofossils in the cherty sequence inthe deeper parts of the hole is extremely low. The upperlimit of calcareous nannofossil deposition at this site liesbroadly within the same time interval (late Oligocene-early Miocene) as that observed at Site 267. However,the presence of a substantial time break between theyoungest calcareous sediments and the oldest diatomooze (early Pliocene, in Core 7, Section 2) at Site 268precludes precise identification of the time at which thechange in biogenic sediment type occurred.

FACIES GEOMETRY

An overview of the sites on this north-south sectionsuggests that, within the relatively broad limits ofpaleontological control, the boundary betweencalcareous and siliceous biogenic facies is diachronous,being older in the southern sites than in those to thenorth. At Sites 267 and 268 the change from nannofossilooze deposition to a diatom depositional regime occurswithin the late Oligocene-early Miocene interval; at Site266, some 300 km to the north of 267, the transitionbegins in the middle Miocene and is complete by theearliest Pliocene. At Site 265, still further north, meagerevidence suggests that the late Miocene is a time of full-scale carbonate deposition; again, however, by the earlyPliocene, the change to a siliceous sedimentary regimewas complete at this site.

A first examination of such a situation might suggestthat the diachronous form of the facies boundary is aresult of a south to north migration of some factorwhich controlled the type of sediment accumulation onthe sea floor. Such a boundary might be envisioned as akind of "ancient Antarctic Convergence" migratingnorthward in response to climatic cooling. However, in-terpretation in these relatively simple terms would havevalidity only in relation to a stable, immobile sea floor.The same geometric situation, that of a facies boundarybecoming older and deeper away from an active mid-oceanic ridge, could arise by the horizontal and/or ver-tical passage of the sea floor itself through an oceano-graphic boundary which had remained relatively fixedthrough time (Frakes and Kemp, 1972).

In order to determine whether the observed dia-chronism can be ascribed to movement of an oceano-graphic boundary or alternatively, to sea-floor-spreading motions with respect to a stationary bound-ary, it is necessary to account for and remove the effectsof a spreading sea floor. Any apparent displacementremaining must reflect relative motion between theboundary and the ridge. Spreading effects can be re-moved by replotting each of the sites in its geographic ordepth position for any particular point in time, that is,by "back-tracking" (Berger, 1972) the sites along the ap-propriate empirical age-depth curve and thus depictingthe sites in their respective positions at that time.

The construction of such a curve is made possible bythe general correlation between sea-floor age and depthin widely separated oceanic areas (Sclater et al., 1971;Hays and Pitman, 1973). In Figures 2-5, Sites 265 to 267have been plotted on an age-depth curve constructed us-ing data from the published bathymetry (Hayes andConolly, 1972), magnetic lineation ages (Weissel andHayes, 1972), sediment isopachs (Houtz and Markl,1972) and the Leg 28 geophysical data and site results.Data for the longitudinal corridor from 105°E to 115°Ewere combined in arriving at the empirical curve ofbasement age versus depth for both the north and southflanks of the Southeast Indian Ridge and as shown inFigures 2-5. The basement depths are shown with es-timates of the error bars at locations corresponding towell-identified and dated sea-floor magnetic lineations.

911

265

DIATOMS

g NANNOPLANKTON

] ] OETRITAL

^ MISCELLANEOUS

• LOWEST ICE RAFTED

MIC ARB - BEARINGDIATOM OO2E,

NAN NO-andFORAM BEARING,

ly CLAY-BEARINGICARB-RICH

268

COLUMN DESCRIPTION

2

3

CLAY, SILTY CLAY,SAND AND DIATOM

4 OOZE

5

s7

θCLAY, SILTY CLAYAND CLAY NANNOOOZE

9

0 * ?

SILTY CLAYJami-2 noted SILTY CLAY

and CLAYEY SILT,CLAYEY SILT ond

B CHERT

14

5

6

7

9

y ,1

1

Uü

raft

ing

*

?*

Figure 1. Stratigraphic and sedimentological record of sites making up a ridge flank to continental rise profile in the 105°-110°Eportion of various sediment types is shown on the right of each column (see text for further explanation).

IWjtH D pttl 3529 ml

longitudinal corridor. The pro-

PALEOCLIMATIC SIGNIFICANCE OF DISCHRONOUS BIOGENIC FACIES

A smooth curve was then fitted by eye to the entire dataset. The empirical curve is in general agreement withthat derived by Hayes and Weissel (in preparation) us-ing a more rigorous mathematical approach, but itdiffers significantly from the average global age-depthrelationship determined by Sclater et al. (1971).

In Figures 2-5, the present positions of Sites 265, 266,and 267 have been plotted in accordance with the age ofthe basal sediment. Age determinations based onmagnetic lineations are all slightly older. Site 268 is notshown because paleontological control is very poor; thissite is expected to occupy a position on the curve near 50m.y., from magnetic lineation data. For Sites 265 and266, the expected depth, the "A depth" of Berger (1972),as determined from this curve, is somewhat less than thereal one, probably because of regional conditions. AtSite 267, for instance, strong local basement relief is evi-dent. Additionally, the error in terms of dating the sedi-ment immediately above basement is not well known.

In order to evaluate any movement of the silica/car-bonate facies boundary through time, several discreteintervals of time have been selected, and the sites havebeen back-tracked along the ridge profile to their paleo-depositional depths for those particular intervals. Ex-

amination of the type of sediment being deposited atthose points in space and time should then indicate therelative position of the facies boundary with respect topoints on the ridge flank. The time intervals selectedwere based on radiolarian Zones 5, 6, and 8, which cor-respond, approximately, to time intervals of 3.5-5.5m.y.B.P., 5.5-12 m.y.B.P., and 13-14 m.y.B.P., respec-tively. In addition to the time intervals represented bythese zones, the sites were also back-tracked for apaleontologically less well-defined early Miocene inter-val, i.e., 16-22 m.y.B.P. Time intervals defined byradiolarian zones were selected in preference to thosedefined by other fossil groups because contiguous zonescould be recognized at all three sites plotted.

When back-tracking of Sites 267 and 266 is carriedout for the oldest time intervals considered, i.e., for theperiods 16-22 m.y. and 13-14 m.y. (Site 265 was not inexistence at this time), it becomes evident that the boun-dary separating siliceous and calcareous ooze depositionintersects the ridge flank between the back-trackedpositions of the two sites in each case. Taking the mid-point of the depth ranges of the back-tracked sites as anarbitrary estimate of the limits of the lithological boun-dary position, then the depth range of the facies boun-

3000

4000

5000

6000

BACKTRACKED

SITE 266

,Nanno ooze and chalk

EARLY MIOCENE

(16-22 m.y.)Δ Sclater, et al. (data - SE Ind.)

D Sclater, et al. (empirical - Pac.)

4 Hayes (Connolly data - SE Ind.)

•-Leg 28 drill sites

BACKTRACKED

SITE 267

Diatoms dominant

Depth range of

intersection of boundary

with seafloor, as estimated

by midpoints of backtracked

depths (3360-4150 m).

267

Correction for underlying sediments

(base of early Miocene)

266 - no correction

267 - 90 m (between Cores 35 and

20 30 40 50AGE (m.y.)

Figure 2. Age-depth curve for the Southeast Indian Ocean, with drill sites back-tracked to their paleodepth on the ridge flankfor their early Miocene, middle Miocene, late to late middle Miocene, and early Pliocene positions. Depth corrections arebased on distance of interval under consideration above basement. (See text for derivation of curve and explanation ofback-tracking process).

913


3000

4000

5000

6000

MIDDLE MIOCENE (13-14 m.y.RADIOLARIAN ZONE 8

BACKTRACKEDSITE 266Nanno ooze

Δ S c l a t e r , e t a l . (data - SE I n d . )D S c l a t e r , e t a l . (empir ica l - Pac.| Hayes (Connolly data - SE I n d . )•-Leg 28 d r i l l s i t e s

BACKTRACKEDSITE 267Diatoms dominant

\Depth range ofintersection of boundarywith seafloor, as estimatedby midpoints of backtrackeddepths (3620-4420 m).

267

Correction for underlying sediments(base of rad. zone 8)

266 - 173 m (bottom of Core 12)277 - 137 m (bottom of Core 8)

20AGE (m.y.)

30 40 50


dary lies between 3360 and 4250 meters for the olderpositions (Figure 2, spanning a time interval of some 6million years), and 3620 and 4420 meters for theyounger interval (Figure 3).

When Sites 265, 266, and 267 are back-tracked totheir range of position for the interval 5.5-12 m.y.B.P.(Figure 4, radiolarian Zone 6), it is evident that thefades boundary is still positioned between Sites 267 and266. The time interval at Site 266 is represented by atransition zone of mixed diatom and nannofossil deposi-tion, so it is possible that there were small fluctuationsof the boundary across the site during that period—thatis, Site 266 lay near the boundary. For the boundary, thepossible depths on the ridge flank range from 3980meters (the back-tracked depth of the youngest nan-nofossil ooze) to 4380 meters. Considered together withthe depth ranges for the older time intervals, thissuggests that the boundary was positioned over a bot-tom depth of about 3980 meters, and about 300 kmsouth of the ridge crest. Little movement of the bound-ary with respect to the ridge crest appears to have takenplace within the interval 5.5-22 m.y.B.P., although thisobservation implies an absolute northward migration ofthe boundary of about 450 km.

When the drill sites are back-tracked along the ridgeprofile to their early Pliocene positions (Figure 5, radio-larian Zone 5, 3.5-5.5 m.y.), then clearly, the silica/car-bonate facies boundary lies to the north of Site 265, asdiatom oozes are being deposited at all three sites duringthis interval. The boundary lies between the ridge crestat 3000 meters (or even to the north of it), and 3360meters above Site 265 on the ridge flank. This positionrepresents a significant motion of the boundary withrespect to the ridge crest, when compared with positionsfor the earlier time intervals. Because all sedimentsyounger than early Pliocene are dominated by their dia-tom fraction, it appears that there has been no sig-nificant regression of this facies boundary in a southerlydirection since that date—at least not in this particularsector of the southeast Indian Ocean.

All of the boundary motion discussed above refers tomovement of the boundary with respect to the ridgecrest only. A further complication remains, in that theSoutheast Indian Ridge has almost certainly beenmigrating in a northerly direction, away from Ant-arctica (Weissel and Hayes, 1972). Available evidencesuggests that Antarctica has remained relatively fixedwith respect to the earth's spin axis since the Late

914


3000

4000

5000 -

6000 -

^ i BACKTRACKED[%T SITE 265^ k Nanno ooze

i '*** Depth range of

intersection of boundarywith seafloor, as estimatedby youngest nanno ooze (266)and midpoint of backtracked

- range (276B) - 3980-4480 m.

-

l 1

LATE TO LATE MIDDLE MIOCENE(5.5 - 12 m.y.)

RADIOLARIAN ZONE 6

BACKTRACKEDSITE 266Mixed nanno-diatom oozes(youngest nanno ooze)

Zbb ^ • ~ - ^ I ^ ^ ^

i i i i

Sclater, et al. (data - SE Ind.)D Sclater, et al. (empirical - Pac.)^ Hayes (Connolly data - SE Ind.)-•-Leg 28 drill sites

BACKTRACKEDHOLE 267BDiatom ooze

267 ~ "i

Correction for underlying sediments(base of rad. zone 6)

265 - 44 m (top Core 15)266 - 178 m (top Core 12)267 - 94 m (bottom Core 6)

1 1 1 110 20 30 40 50

AGE (m.y.)


Cretaceous (McElhinney, 1973); Lowrie and Hayes, thisvolume), and thus Australia has migrated steadilynorthward since the early Tertiary. The ridge systemitself probably maintained a near median position withrespect to the Australian and Antarctic continents; itsmigration speed thus equaling approximately half therate of separation of those continents, or about 2.5-3 cmper year. This being the case, it seems likely that thesilica/carbonate facies boundary, which appears, on theevidence of back-tracking, to have maintained a cons-tant depth position with respect to the ridge crest duringthe Miocene (5.5-22 m.y.B.P.), must have undergone asteady northward migration during that interval, mov-ing at a rate comparable to that of the entire ridgesystem (~2-3 cm/yr). In the early Pliocene thenorthward movement of the facies boundary appears tohave accelerated abruptly with respect to the ridge; therelative northward displacement of the boundaryprobably exceeded 300 km. The Polar Front Zone nowcoincides roughly with the ridge crest, whereas duringthe Miocene its ancient counterpart apparently layabout 300 km south of it.

PALEOCLIMATIC INTERPRETATIONInterpretation of the shift of the facies boundary in

terms of climatic changes affecting the southeast Indian

Ocean must be undertaken with caution in view both ofthe scarcity of data points offered by the present studyand inadequacies in our understanding of the physicalfactors presently controlling sedimentation in highsouthern latitudes. While it is tempting to equate thefactors which have in the past determined the silice-ous/calcareous sediment boundary with the presentPolar Front or Antarctic Convergence, the parallel isnot exact, since the modern oceanic boundary separatesdiatomaceous ooze from calcareous foraminiferal ooze;through much of the Neogene, however, the ancientboundary separated diatomaceous sediment and cal-careous nannoplankton ooze with only a fewforaminifera.

The presence of nearly pure, deep-water carbonatesediments in high southern latitudes does indicate,however, that much warmer temperatures than those ofthe present day prevailed in these regions from at leastthe late Eocene until perhaps sometime in the middleMiocene. The presence of nannofossil chalks of lateEocene age in the base of Hole 267B and of earlyMiocene nanno oozes at Site 268 provides evidence forthis.

At Site 268, the presence of ice-rafted sediment isfirmly established back into the early Miocene (Piper,this volume); there is a possibility that it may be even

915


3000

4000

PTH

(m)

Lü

a

5000

6000

y×Z\. BACKTRACKED^ ^ \ . SITE 2657yyy/ ^H. 1 Diatom ooze

[) ‰Depth range ofintersection of boundarywith seafloor, as estimatedby midpoints of backtracked

~ depths (<3660 m).

-

-

I 1 1

EARLY PLIOCENE (3.5 - 5.5 m.y.)RADIOLARIAN ZONE 5

BACKTRACKEDSITE 266Diatom ooze

266 ^ - - - ^ 1

i i i

Δ Sclater, et al. (data - SE Ind.)

D Sclater, et al. (empirical - Pac.)

+ Hayes (Connolly data - SE Ind.)

-•-Leg 28 drill sites

BACKTRACKED

SITE 267

Diatom ooze

267

i

a ~~

Correction for underlying sediments

(base of rad. zone 5)

265 - 89 m (bottom Core 1A)

266 - 94 m (top Core 12)

267 - 112 m (bottom Core 3)

1 1 1 1

10 20 30 40 50

AGE ( m . y . )


older, extending back into the late Oligocene. Farthernorth, at Site 267, the presence of ice-rafted sand hasbeen established in the middle Miocene. At Site 266, thepresence of sand in wholly pelagic sequences testifies toice-rafting processes which are evident only from thelate Miocene onwards. The distribution of ice-raftedsediment in this sector thus shows the same diachron-ism as does the silica/carbonate boundary, with bothbecoming significantly younger towards the north. Inthe three sites mentioned, it is significant that ice-raftingbegins in sediments in which a calcareous biogenic com-ponent is dominant; icebergs appear to have been pres-ent in the area and dropping sediment loads before thechange to a siliceous sedimentary regime occurred.

The overall picture, then, appears to be one in whichoceanic conditions were warm enough, up till the earlyto middle Miocene, to allow deposition of calcareousnannofossils at Sites 267 and 268, and in which,simultaneously, the earliest icebergs were melting andrafting debris. The change to deposition of diatom oozesor diatom-dominated sediments probably reflects achange to a cooler water regime, as evinced by fairlyrapid changes in the probable early Miocene at Site 267,and by more gradual transition beginning in the middle

Miocene at Site 266. This cooling followed after the in-itiation of iceberg sediment-rafting at each site.

Back-tracking of sites along an empirical age-depthcurve suggests that the effects of this cooling were ex-panding outward from the continent at a relatively cons-tant rate, from before the latest Miocene to the earliestPliocene. The silica/carbonate boundary appears tohave been maintained in a relatively constant positionwith respect to the ridge crest, while undergoing asystematic northward migration along with the ridgesystem itself. The distribution of ice-rafted debrissuggests that iceberg melting first took place in a near-shore zone; this zone migrated steadily northward fromthe Antarctic continent before the late Miocene. Con-ditions even after the initiation of a siliceous biogenicregime at the southern sites appear to have been warmenough to allow melting of icebergs, with release of theirsediment loads, before they reached localities as farnorth as Site 266.

A relatively rapid motion of the diatom ooze/nanno-plankton ooze boundary appears to have taken placesometime in the interval 3.5-5.5 m.y.B.P. This dating isas precise as present paleontological studies on Leg 28sites permit. Whether or not this rapid northward boun-

916


dary movement in fact represents the initiation of theAntarctic Convergence in its present form is unknown.Evidence bearing on the history of the boundarybetween diatom oozes and foraminiferal oozes awaitsfurther drilling in areas to the north of Leg 28 drill sites.

In any case it seems reasonable to assume that therelatively sudden early Pliocene boundary shift to thenorth represents a sudden intensified chilling of surfacewaters. The suggestion of rapid cooling in the earlyPliocene draws support from other sources. The trunca-tion of seaward-dipping sequences in the Ross Seaappears to have occurred at about this time, probablyfrom the erosive action of a grounded Ross Ice Shelf,which extended much beyond its present limits (Hayes etal., 1973). Furthermore, from observations of thenorthward movement of the siliceous ooze/glacial-marine sediment boundary offshore from the Adelie andGeorge V coasts, and from radiolarian and silico-flagellate data, Weaver (1973) suggested that a regionalcooling occurred about 4.0 million years ago, with theinitiation of an oceanic thermal structure similar to thatof modern polar seas. Correlation of these events withthose suggested by the present study of facies dis-tributions at Sites 265-268 remains tentative in view ofthe broadness of paleontological dating.

REFERENCESBerger, W. H., 1972. Deep-sea carbonates; dissolution facies

and age-depth constancy: Nature, v. 236, p. 392-395.Connolly, J. R. and Payne, R. R., 1972. Sedimentary patterns

within a continent—mid-oceanic ridge—continent profile:Indian Ocean south of Australia. In Hayes, D. E. (Ed.),Antarctic Oceanology II, Antarctic Res. Ser., 19:Washington, (Am. Geophys. Union), p. 295-315.

Frakes, L. A. and Kemp, E. M., 1972. Generation of sedimen-tary facies on a spreading ocean ridge: Nature, v. 236, p.114-117.

Hayes, D.E., 1972. Marine geophysics of the Southeast IndianOcean. In Hayes, D. E. (Ed.), Antarctic Oceanology II, An-tarctic Res. Ser. 19: Washington (Am. Geophys. Union), p.119-124.

Hayes, D. E. and Connolly, J. R., 1972. Morphology of theSoutheast Indian Ocean. In Hayes, D. E. (Ed.), AntarcticOceanology II, Antarctic Res. Ser. 19: Washington (Am.Geophys. Union), p. 125-145.

Hayes, D. E., Frakes, L. A., Barrett, P. J., Burns, D. A., Chen,P., Ford, A. B., Kaneps, A. G., Kemp, E. M., McCollum,D. W., Piper, D. J. W., Wall, R. E., and Webb, P. N., 1973.Leg 28 deep-sea drilling in the southern ocean: Geotimes, v.18, p. 19-24.

Hays, J. D., 1967. Quaternary sediments of the AntarcticOcean. In Sears, M., (Ed.), Progress in Oceanography 4:Oxford (Pergamon Press), p. 117-131.

Hays, J. D. and Pitman, W. C, 1973. Lithospheric plate mo-tion, sea level changes and climatic and ecological conse-quences: Nature, v. 246, p. 18-22.

Houtz, R. E. and Markl, R. G., 1972. Seismic profiler databetween Antarctica and Australia. In Hayes, D. E. (Ed.),Antarctic Oceanology II, Antarctic Res. Ser. 19:Washington (Am. Geophys. Union), p. 147-164.

Lisitzin, A. P., 1962. Bottom sediments of the Antarctic. InAntarctic Research: Am. Geophys. Union, Geophys.Monogr. 7, p. 81-88.

, 1972. Sedimentation in the world ocean: Soc. Econ.Paleontol. Miner., Spec. Publ. 17.

McElhinney, M. W., 1973. Palaeomagnetism and plate tec-tonics: Cambridge (Cambridge University Press).

Sclater, J., Anderson, R., and Bell, M. L., 1971. Elevation ofridges and evolution of the central eastern Pacific: J.Geophys. Res., v. 76, p. 7888-7915.

Weaver, F., 1973. Pliocene paleoclimatic and paleoglacialhistory of East Antarctica recorded in deep-sea piston cor-es: Sedimentological Research Laboratory, Florida StateUniversity Contrib. No. 36.

Weissel, J. K. and Hayes, D. E., 1972. Magnetic anomalies inthe southeast Indian Ocean. In Hayes, D. E. (Ed.), Antarc-tic Oceanology II, Antarctic Res. Ser. 19: Washington (Am.Geophys. Union), p. 165-196.

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