glacial eustasy during the cenozoic: sequence stratigraphic implications

8/17/2019 Glacial Eustasy during the Cenozoic: Sequence Stratigraphic Implications

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ABSTRACT

A newly constructed composite oxygen isotoperecord is a proxy for eustasy that extends back tothe Cretaceous–Tertiary boundary and provides anindependent test of sequence stratigraphic–basedeustatic curves. The isotope record shows severaleustatic episodes that are consistent with the geo-logical record of ice-sheet evolution.

The first evidence for the existence of an ice sheetin East Antarctica occurs near the lower–middleEocene boundary (base of the Lutetian stage).There is no evidence for a large ice sheet on Antarctica prior to this time; however, strata of thisage are lacking over most of the continent. The iso-tope curve also indicates that the ice sheet experi-enced phases of growth during the late Eocene andmiddle Oligocene, followed by a decrease in vol-ume in the early Miocene.

The Ross Sea stratigraphic record indicates initialevolution of the West Antarctica ice sheet duringthe early Miocene. By the middle Miocene, the ice

sheet spread across the Ross Sea, Weddell Sea, and Antarc tic Peninsula continental she lves. ThePliocene–Pleistocene record of glaciation in Antarctica includes numerous glacial erosion sur-faces on the continental shelf, indicating repeatedadvance and retreat of both East and West Antarctica ice sheets. These volume changes in the Antarctica ice sheet were in response to the riseand fall of sea level caused by expanding and con-tracting Northern Hemisphere ice sheets.

There is a reasonable correlation between eusic curves derived from sequence stratigraphic sies and the composite oxygen isotope record sithe middle Eocene. This correlation indicates tglacial eustasy has been the principal factor reguing stratal stacking patterns on a global scale sithe middle Eocene.

INTRODUCTION

A significant portion of the Earth’s stratigraprecord bears evidence for continental glaciatiMajor glacial episodes occurred during the lPrecambrian, early Paleozoic (Saharan Glaciatiolate Paleozoic (Gondwana Glaciation), and Cozoic. Each glacial episode was different becausthe distribution of continents and, therefore, ofsheets. Of particular importance is the extenterrestrial ice sheets in lower latitudes; subpolatemperate ice sheets are more responsive to hfrequency climatic changes, hence causing hifrequency sea level oscillations. These glacepisodes are manifested in the global stratigraprecord by strong cyclicity and by complex strgraphic and facies relationships.

A reasonable generalizat ion in most basin tings is that subsidence rates are slower than rate of glacial eustatic oscillation. Thus, glaeustasy can predominantly regulate stratigraparchitecture (i.e., creation of accommodatspace). This assumption implies that sequenceslikely to be global in scale during glacial episodIn contrast, stratal stacking patterns in nonglaintervals may be more strongly regulated by otfactors, such as climate and tectonism, which

more regional in scale. Thus, an improved recof glaciation provides a stronger basis for glosequence stratigraphic interpretations.

Nearly 20 yr have passed since Vail et al. (19published their sequence stratigraphic results icating third-order sea level fluctuations extendback in time far be yond when ice sheets wthought to exist. Since 1977, more detailed sequestratigraphic studies have revealed third-or

1

©Copyright 1998. The American Association of Petroleum Geologists. All

rights reserved.1Manuscript received October 8, 1996; revised manuscript received May

9, 1997; final acceptance February 5, 1998.2Rice University, Department of Geology and Geophysics, Houston,

Texas 77005-1892.We would like to express our gratitude to Peter Vail, Jan Hardenbol,

Geoffrey Haddad, and Andre Droxler for suggestions and discussions duringthe preparation of this paper. Special thanks to Emoke Vakarcs, StephanieShipp, Gabor Vakarcs, and Gerald Baum for reviewing early versions of themanuscript. Thanks also to Nicholas Christie-Blick, whose review greatlyimproved the original work.

Glacial Eustasy During the Cenozoic: SequenceStratigraphic Implications1

Vitor S. Abreu and John B. Anderson2

AAPG Bulletin, V. 82, No. 7 (July 1998), P. 1385–1400.


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eustatic cycles dating well into the earlier Cenozoic (Haq et al., 1987). An important applica-tion of these studies has been the establishment of global stratigraphic correlation charts. The under-lying assumption in these charts is that observedsequence stratigraphic events are global in scale,implying a glacio-eustatic control on stratigraphicpackaging. The validity of the Haq et al. (1987)

curve and similar cycle charts has raised consider-able debate over the past decade. In light of thisdebate, it is important to view other lines of evi-dence for major volumetric changes in ice sheetsthat could have influenced the stratigraphic archi-tecture of basins on a global scale. Also, knowl-edge of the rate and magnitude of eustatic changeis necessary for predicting the thickness, extent,and facies architecture of coastal and marine depo-sitional systems. In this paper, we provide a brief overview of what is known about ice-sheet evolu-tion since the Cretaceous. We focus mainly on Antarctica’s glacial history because ice sheets onthe Northern Hemisphere date only to the late

Miocene.

OXYGEN ISOTOPE RECORD

Carbonate precipitated organically or inorgani-cally in the ocean records the seawater isotopiccomposition. If the ocean is enriched in 18Obecause of the growth of large ice sheets, theδ18O of calcite is heavy. If the ocean is depleted in 18Obecause of the melting of major ice sheets, theδ18O of calcite is light. By observing downcore variations in the δ18O of diagenetically unalteredcalcite, variations in ice volume can be inferred.Caution must be taken in making this interpreta-tion, because the δ18O composition of calcite isalso dependent on water temperature and salinity.Thus, oxygen isotopes provide an independentgauge to ice-sheet/ocean-volume changes, which should correlate with the sequence stratigraphicrecord of eustasy.

Several composite isotope records based onplanktonic and benthonic foraminifera generatedsince the 1970s aimed at understanding the cli-mate evolution from the Cretaceous to the pres-ent. Representative records are shown in Figure 1.Some previous composite isotope records wereconstructed mainly using low-resolution data setsfrom specific sites (e.g., Douglas and Savin, 1975;Shackleton and Kennett, 1975). Douglas andSavin (1975) compiled isotopic values for theCretaceous and Cenozoic from several DSDP(Deep-Sea Drilling Project) sites. They suggested a warming trend in the Early Cretaceous, a thermalmaximum in the Albian, and a cooling trend in theLate Cretaceous, with a drop in marine tempera-

tures from the Campanian to the Paleocene.Shackleton and Kennett (1975) constructed twocomposite isotope records for the Cenozoic basedon Southern Ocean DSDP sites 277, 278, and 281(Figure 1). They interpreted a strong positive oxy-gen isotope shift during the middle Miocene as anindication of significant ice-sheet accumulation in Antarctica.

Matthews and Poore (1980) presented a general-ized δ18O record for the Cenozoic. They assumedconstant tropical sea-surface temperatures sincethe Cretaceous to evaluate ice-volume changesbased on the isotope record. According to theseauthors, an ice-free world with sea-surface tempera-ture of 28°C would yield shallow-marine δ18O val-ues of about –3‰. Their approach implied signifi-cant ice volumes since at least the late Eocene andpossibly for much of the Cretaceous.

More recently, composite isotope records havebeen constructed through mathematical smoothingof data sets from several sites (e.g., Miller et al.,1987; Prentice and Matthews, 1988). Miller et al.

(1987) constructed two composite isotope recordsfor the Tertiary. They interpreted synchronouschanges in both benthonic and planktonic δ18O val-ues as related to ice growth and decay during theOligocene and Miocene. According to theseauthors, δ18O values lighter than 1.8‰ in benthon-ic foraminifera ( Cibicidoides ) could possibly berelated to an ice-free world (Figure 1). The y assumed substantially ice-free conditions duringthe Paleocene and Eocene, with glacio-eustaticchanges restricted to the Oligocene and Neogene,and concluded that glacio-eustasy was responsiblefor producing sequence boundaries on passive mar-gins since the Oligocene. Prentice and Matthews(1988) assumed that the planktonic isotopic recordreflected ice-volume changes and that the benthon-ic record was more likely related to bottom-water temperature changes. They concluded that an icesheet had existed on Antarctica since the middleEocene (Figure 1).

The effect of smoothing on isotope records or of using low-resolution data has been to remove or reduce the amplitude of higher frequency events.The composite oxygen isotope record for theCenozoic presented here aims at preserving thechronostratigraphic position and amplitude of higher frequency (of about 1 m.y.) positive isotopeevents. We prefer this approach for comparing theisotope record with the third-order sequencestratigraphic record (Haq et al., 1987; Hardenbol etal., in press). The i sotope record shows events

with a 1.5 m.y. frequency from the Paleocene tothe Miocene, which is similar to the frequency of Haq et al. (1987) sequences, although the frequency observed in the pre-upper Miocene δ18O recordmay be affected by the DSDP/ODP (Ocean Drilling

1386 Glacial Eustasy and Sequence Stratigraphy


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Program) site sample densities. Abreu and Haddad(in press) constructed a composite smoothed iso-tope record for the Oligocene–Holocene. Weexpanded the composite record of Abreu andHaddad (in press) to the Cretaceous–Tertiary boundary using DSDP/ODP isotope records fromsites with detailed isotope data in the Paleoceneand Eocene. The sites used in this work are pre-sented in Table 1.

This work builds on recent publications thatdefined positive oxygen isotope events during theOligocene and Miocene (Miller et al., 1991, 1996; Wright and Miller, 1992; Abreu and Savini, 1994; Abreu and Haddad, in press). The chronostrati-graphic position, oxygen isotope value, and ampli-tude of each event were defined in the sites with more well-represented sections (Table 2). Weattempted to use, as far as possible, sites situatedin middle latitudes and intermediate paleowater depths. We propose six new positive isotopeevents for the middle and late Eocene based onsites 689 and 690 (Table 2). These sites are locatedin high latitudes, but present the best resolutionfor the Eocene. The standard isotope records fromsites 522 (Miller et al., 1988) and 608 (Wright etal., 1992) are the reference sections for theOligocene and Miocene records, respectively. ThePliocene isotope record, based on sites 502 and704, was smoothed to keep the 100 k.y. cycle andlonger cycles. We have used the filtered isotopecurve of Haddad and Vail (1992) for the

Pleistocene–Holocene sections; this curve is atered version (low pass, 1/66–1/45 k.y.) of stacked benthonic isotopic records from sites and 677 (Raymo et al., 1990).

Oxygen isotope records have been among most widely used proxy indicators of glaciationnumber of records have been generated frDSDP/ODP dril l sites (i.e. , Shackleton aKennett, 1975; Matthews and Poore, 19Shackleton, 1986; Miller et al., 1987; Prentice Matthews, 1988; Stott et al., 1990; Wise et 1992; Robert and Kennett, 1994; Flower aKennett, 1994). The interpretations of threcords with regard to ice-sheet evolution Antarct ica differ considerably (Figure 2), whserves to emphasize the need to interpret oxyisotope records in light of other lines of evidefor ice-sheet evolution.

GEOLOGICAL RECORD OF GLACIATION

The following discussion will examine the glogic evidence for ice-sheet evolution on Eaduring the Cenozoic. The focus is mainly Antarctica; the Antarctica ice sheet is the sourcglacial eustatic changes for most of the CenozDiscrepancies in the published record of Antarctiglacial history primarily result from differenceinterpretation of the deep sea “proxy” rec(Figure 2) and different emphasis being placed

Abreu and Anderson 1

Figure 1—Comparison of the composite isotope record constructed for this study with earlier composite record

CHRONO-

STRATI.- 10123

δ18 O ( 0 / 00 )

Shackleton and Kennett, 1975

4 - 10123

δ18 O ( 0 / 00 )

Miller et al., 1987

- 101

C E N O Z O I C

P A L E O -

C E N E

E O C E N E

O L I G O C E N E

M I O C E N E

P L I O -

P L E I S T .

l o w e

r

m i d d l e u p p e r

l o w e r

u p p e r

m i d d l e

u .

l o w e r

23

δ18 O ( 0 / 00 )

Prentice and Matthews, 1988

4 01234

δ18 O ( 0 / 00 )

This work


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the proxy record vs. the terrestrial and continentalmargin records. For example, interpretation of thefirst occurrence of a large ice sheet on Antarcticabased on results from the two most recent ODP legsin the region (Legs 113 and 119) differ, with one setof results dating the ice sheet in the middle Eocene(Barron et al., 1991) and the second set of results dat-ing the ice sheet in the late Miocene (Kennett andBarker, 1990). Our summary attempts to integratethe deep-sea record with the terrestrial and continen-tal margin records, although we emphasize the ter-restrial and continental margin record, which is aprincipal difference between our synthesis and thatof some other workers (e.g., Kennett and Barker,1990; Miller et al., 1991; Flower and Kennett, 1994).Our summary has benefited from several recently

published reviews on the subject (Denton et al.,1991; Moriwaki et al., 1992; Wise et al., 1992;Hambrey and Barrett, 1993; Barrett, 1996).

Cretaceous Glacial Record

Matthews and Poore’s (1980) interpretation of the composite oxygen isotope record suggests ice

buildup in Antarctica during the Cretaceous.Geological evidence for widespread glaciation onthe continent at that time is lacking; however, noLower Cretaceous exposures exist on Antarctica,and Upper Cretaceous strata are restricted to the Antarctic Peninsula region. Thus, the continent’sglacial and climatic setting during the Cretaceous isincomplete. Upper Cretaceous deposits in the Antarc tic Peninsul a occur on James Ross andSeymour islands (Figure 3) and include the Latady and Fossil Bluff formations. The deposits contain arich pollen and spore assemblage, includingconifers, cycads, ginkgos, and some angiosperms.These indicate that a conifer-dominated rainforestinhabited the region at that time (Askin, 1992). Theadjacent marine record from ODP Leg 113 off the

Queen Maud Land margin (Figure 3) penetratedUpper Cretaceous deposits. Studies of oxygen iso-tope concentrations, clay mineralogy, and plank-tonic microfossil assemblages indicate temperate tocool subtropical climatic conditions in the region(Kennett and Barker, 1990).

Some insight into Antarctica’s climate during theCretaceous is afforded by examining the stratigraphicrecords of neighboring Gondwana continents,


Table 1. Sites Used to Construct the Composite Isotope Record

Site Location Sample Type Paleowater Depth References

502 Caribbean P. wuellerstorfi Middle bathyal Oppo et al., 1995552 North Atlantic P. wuellerstorfi Middle bathyal Keigwin, 1987704 South Atlantic Cibicides Middle bathyal Hodell and Venz, 1992

mid-latitudes607 North Atlantic Cibicidoides Lower bathyal Raymo et al., 1990677 Equatorial Pacific Uvigerina Lower bathyal Raymo et al., 1990846 Equatorial Pacific Uvigerina and Lower bathyal Shackleton et al., 1995

P. wuellerstorfi 608 North Atlantic P. wuellerstorfi Lower bathyal Wright et al., 1992

mid-latitudes and Cibicidoides563 North Atlantic P. wuellerstorfi Lower bathyal Miller and Fairbanks,

mid-latitudes and Cibicidoides 1985; Wright et al.,1992

747 Indian Ocean P. wuellerstorfi Upper bathyal Wright and Miller, 1992mid-latitudes and Cibicidoides

Well A South Atlantic Bulk rock Upper bathyal Abreu and Savina, 1994;low latitudes Azevedo et al., 1993

522 South Atlantic Cibicidoides Lower bathyal Miller et al., 1988mid-latitudes

529 South Atlantic Cibicidoides Middle bathyal Miller et al., 1987

mid-latitudes803 Equatorial Pacific Cibicidoides Middle bathyal Barrera et al., 1993689 Southern Ocean Cibicidoides Upper bathyal Kennett and Stott, 1990690 Southern Ocean Cibicidoides Middle bathyal Kennett and Stott, 1990865 Equatorial Pacific Cibicidoides and Upper bathyal Bralower et al., 1995

G. beccariformis527 South Atlantic Cibicidoides Lower bathyal Shackleton et al., 1984577 Pacific Cibicidoides Upper bathyal Browning et al., 1996

mid-latitudes215 Southern Ocean N. truempyi Lower bathyal Hovan and Rea, 1992


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particularly those still attached to Antarctica duringthe Cretaceous. Cretaceous deposits from Australia, Africa, and South America yield sparse evidence of glaciation. A piston core into Lower Cretaceousdeposits on the continental shelf off Wilkes Land yielded a rich pollen and spore assemblage indicat-ing a relatively warm temperate climate and no ice-rafted material (Domack et al., 1980).

In summary, evidence suggests temperate or sub-tropical climate in Antarctica during the Cretaceous,but Cretaceous and Paleocene exposures are restrict-ed to the northern Antarctic Peninsula region. Although evidence does not favor the existence of a

large ice sheet in Antarctica during the Cretaceoice caps may have been present at times in the ctral portions of the continent.

Paleocene Glacial Record

Stott et al. (1990) suggested that the Paleoce

Eocene boundary has been possibly the warmtime during the Cenozoic, based on oxygen isotdata. In contrast, Denton et al. (1991) interpretedδ18O record as consistent with the existence of sigicant ice on Antarctica during the Paleocene. OLeg 113 sites in the Weddell Sea sampled Paleocstrata with diverse calcareous planktonic and bthonic microfossil assemblages and clay mineralindicative of warm climatic conditions (Kennett Barker, 1990). The oxygen isotope data from thsites also indicate relatively warm surface and bott waters in the Weddell Sea throughout most of Paleocene (Robert and Kennett, 1994).

Paleocene strata are confined to the north

An tarctic Peninsu la region, and none of thdeposits contain ice-rafted material; rather, palylogical data from exposures on James Ross Islindicate that Late Cretaceous–Paleocene climain the Antarctic Peninsula region were warmcool temperate, with high rainfall (Askin, 199These observations are not inconsistent with existence of an ice cap in the interior portion Antarctica during the Paleocene.

Correlation between the isotopic and sequence stratigraphic records for the Paleocenpoor (Figure 4). In the upper Paleocene and lowEocene, the isotope record shows the most netive values of the Cenozoic. The relatively light ogen isotopes values indicate periods of warmer btom waters or the presence of only small volumof ice on Antarctica. In summary, there is little dence for significant ice cover on Antarctica ding the Paleocene, but the lack of outcrops in irior East Antarctica lends to uncertainty abthe existence of ice caps in the region during Paleocene.

Eocene Glacial Record

Shackleton and Boersma (1981) argued tpolar surface waters were 10 to 15°C warmer tthe present during the early Eocene. Prentice Matthews (1988) and Poore and Matthews (19discussed that the overall positive oxygen isotshifts in the Eocene indicate a significant buildupice on Antarctica. In contrast, Kennett and S(1990) observed a steplike increase in δ18O in upper part of the Eocene at site 689 on Maud Rand attributed this increase to a temperat


Table 2. Positive Oxygen Isotope Events for the MiddleEocene–Pliocene*

Age δ18O Amplit.Event Site Chron. (Ma) (‰) (‰)

PGi-2 502 C2r.1r 2.1 3.0 1.2PGi-1 502 C2r.2r 2.5 2.8 0.8PPi-2 704 C2An.1n 2.8 3.5 0.8PPi-1 704 C2An.2r 3.3 3.2 0.8PZi-3 704 C2Ar 4.0 2.9 0.4PZi-2 704 C3n.3n 4.8 3.0 0.9PZi-1 552 C3n.4n 5.2 2.7 0.9MMi-2 608 C3r 5.7 2.4 0.5MMi-1 608 C3Ar 6.7 2.1 0.3MTi-4 608 C4n.2n 7.7 2.2 0.3MTi-3 608 C4r.3r 8.6 2.1 0.2MTi-2 608 C4Ar.2r 9.4 2.2 0.3MTi-1 608 C5n.1n 9.8 2.3 0.4MSi-4 608 C5r.3r 11.7 2.3 0.9MSi-3 608 C5Ar.2n 12.7 1.8 0.8MSi-2 608 C5ABr 13.6 1.6 1.0MSi-1 747 C5ADn 14.6 1.5 0.8MLi-1 608 C5Br 16.1 1.5 0.6

MBi-3 747 C5Cr 17.2 1.3 0.3MBi-2 608 C5Dr 18.0 1.5 0.4MBi-1 747 C6n 19.4 1.7 0.3MAi-3 608 C6An.2n 21.1 1.5 0.4MAi-2 608 C6AAr.1r 22.2 1.4 0.3MAi-1 529 C6Cn2n 23.8 2.0 0.8OCi-3 529 C7r 25.2 1.6 0.4OCi-2 529 C9n 27.3 2.1 0.8OCi-1 529 C10n.1n 28.4 2.0 0.5ORi-3 529 C10r 29.4 1.8 0.3ORi-2a 522 C11r 30.2 2.0 0.7ORi-2 522 C12r 31.9 1.7 0.6ORi-1 522 C13n 33.5 2.2 1.0EPi-2 689 C15n 34.7 2.1 0.6EPi-1 689 C17n.1n 37.0 2.0 0.6

EBi-1 689 C18n.1n 39.9 1.7 0.8ELi-4 689 C19r 42.3 1.3 0.5ELi-3 690 C20n 43.7 1.0 0.3ELi-2 689 C21n 46.3 1.0 0.6ELi-1 690 C21r 48.2 0.5 0.5

*The isotope events were named according to the stage in which theevent occurs and its relative position within the stage. For example, the ORi-1event represents the oldest isotopic event within the Rupelian stage. Thesenames correspond to the names used in Figure 2.


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decrease and not an ice-volume increase in Antarctica. A pronounced positive shift of the oxygen

isotopes occurs at the base of the Lutetian stage (baseof the middle Eocene). The average value for the iso-tope record in the upper part of the Lutetian stagereached 1‰ (Figure 4). Another significant positiveisotopic shift occurred near the Lutetian–Bartonianstage boundary. The upper Eocene (Bartonian andPriabonian) is marked by isotope values around 2‰and has high-amplitude fluctuations (Figure 4). TheEocene isotope record indicates cooling of marine

waters, as well as an ice-volume increase toward thelate Eocene. The more positive values during the late

Eocene, relative to the middle Eocene and the early Oligocene (Figure 4), correlate with lower sea levelduring the late Eocene (Haq et al., 1987).

The most compelling evidence for glaciation inEast Antarctica during the Eocene comes from ODPLeg 119 drill sites on the continental shelf in PrydzBay (Figure 3). Site 742 in Prydz Bay (Figure 3)recovered middle–upper Eocene massive diamic-tons, interpreted as water-lain till ( Barron et al.,


Figure 2—Differences in interpretation of key deep-sea proxy indicators (oxygen isotopes, ice-rafting, and deep sea hiatuses) in Southern Ocean drill sites, compared with the smoothed oxygen isotope record. Solid bars in the iso-tope records indicate the existence of ice caps according to different workers, and the dashed lines represent possi-

ble existence of ice caps. Under isotope record: 1 = Matthews and Poore (1980), 2 = Prentice and Matthews (1988), 3= Miller et al. (1987), 4 = Shackleton and Kennett (1975). The solid lines in the ice-rafted debris (IRD) record indi-cate the first occurrence of significant IRD in DSDP/ODP sites, and the dashed lines indicate small occurrence or

uncertainties in age. Major hiatuses in the deep-sea record (according to Wright and Miller, 1993) are represented by horizontal wavy lines. The solid lines represent the rock record in each DSDP/ODP site. These hiatuses have been interpreted as representing episodes of strong bottom-current circulation related to more severe glacial condi-tions on the continent (Wright and Miller, 1993). The smoothed oxygen isotope recorded generated in this study isshown in the last column.

60

55

40

35

45

50

30

25

10

5

15

20

SMOOTHED OXYGEN

ISOTOPE CURVE ( 0 / 00 )

QUAT

ISOTOPE RECORD ICE- RAFTED DEBRIS HIATUS

PACIFIC ATLANTIC INDIAN

21 4

WEST ANTARCTICA

?

322

?

325

693

695

696

EAST ANTARCTICA

THIS

WORK

697

?

269

?

268

744

738

746

736

?

690

690

689

693

696

699

744

738

737

E O C E N E

P A L E O C E N E

M I O C E N E

P L I O

O L I G O C E N E

S E R I E S

T I M E ( M a )

T I M E ( M a )

3 0 1 2 3 4282

281

279

277

266

266

267

267

323

323

324

325

274

274

701

704

748

694

689

747

5

40

35

45

30

25

10

15

20

60

55

50


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1991; Hambrey et al., 1991). Prydz Bay is presently the focus of ice discharge from 22% of the East Antarctic cont inent, so the presence of the ti lloffers strong evidence for an ice sheet on East Antarc tica at thi s time (Barron et al. , 1991;Hambrey et al., 1991). The occurrence of till on thecontinental shelf corresponds to the first signifi-cant occurrence of ice-rafted detritus at Leg 119site 738 on the Kerguelen Plateau (Ehrmann,1991). On the Wilkes Land continental margin(Figure 3), the oldest major unconformity on theshelf, interpreted to be a glacial erosion surface, isinferred to be middle–late Eocene in age, based onextrapolation to DSDP site 269 on the adjacent con-tinental rise (Eittreim et al., 1995).

Evidence also exists for alpine glaciation in West Antarctica during the middle Eocene. Glacial-marinesediments recovered at CIROS-1 in the western RossSea contain middle Eocene dinoflagellates (Hannah,1994); however, there is no evidence that the West

Antarctica ice sheet advanced across the contineshelf during the Eocene (Cooper et al., 1991;Santis et al., 1995). Lower–middle Eocene gladeposits occur in small outcrops on King GeoIsland (South Shetland Islands). These deporecord an episode of glaciation in the north Antarctic Peninsula region that has been designathe Krakow Glaciation (Birkenmajer, 1991). Tage of these deposits, younger than early Eocenderived from radiometric dating of basalts tunderlie them. Coccoliths within a unit just abthe basalt indicate a late Paleocene–early Eocage (Birkenmajer, 1991).

The Krakow Glaciation was followed by Arc towski In terglacial (middle Eocene–eaOligocene), when a relatively warm, moist climand diverse vascular plant community existedthe South Shetland Islands. Eocene florasSeymour Island, located to the south of the SoShetland Islands (Figure 3), also indicate relativ


Figure 3—Geographic map of Antarctica and locations of DSDand ODP drill sites and outcropin the Antarctic region.

SOUTH

AMERICA

AUSTRALIA

0°

30°E

120°E

90°W

120°W

PACIFIC OCEAN

INDIAN

OCEAN

ATLANTIC OCEAN

Ross Ice

Shelf

Queen Maud Land

Prydz Bay

West Antarctica

Ice Sheet (WAIS)

Weddell Sea

James Ross Island

MarieByrd Land

•

•280

•281

•279

•277

••

275

276

278

270

282

•269

•

Ross

Sea

• •

•

••

272

271

273

•274

324

323

•

•

•322325

326

••••

511

327

329

330

••• • •

•

691-2

693694

697

695

•• •

• •• •

701702

700

699

698

328

512

••

704

703

•689690

•

•

252

251

•••••739-743

• •

•• ••

• •

•

738

744

745-6

749

748

751

750

747737

736

•

••

268

267

266

265

•

•

•Kerguelen

Plateau

•

A n t a r c t i c P e n i n s u l a

V i c t o r i a L

a n d

OUTCROP

LAND

ICE SHELF

274 WELL SITE

Seymour Island

East Antarctica

Ice Sheet (EAIS)

T r a n s a n t a r c t i c M o u n t a i n

s

W i l k

e s L a n

d

696

80°S

S o u t h

S h

e t l a

n d

I s l a

n d s


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60

55

45

50

40

35

30

25

15

20

10

5

0200

EUSTATIC CURVE

(meters)

SMOOTHED COMPOSITE ISOTOPE

RECORD - δ18O ( 0 / 00 )

GLACIOEUSTATICEVENT

PZi-2

PZi-3

PPi-1PPi-2

PGi-1

PZi-1

0 1 2 3

PGi-2

4

SEA LEVEL CURVES

LANGHIAN

B U R D I -

G A L I A N

A Q U I -

T A N I A N

C H A T T I A N

R U P E L I A N

S E R R A -

V A L L I A N

ZANCLEAN

PIACENZIAN

T O R T O -

N I A N

MESSI-NIAN

GELASIAN

D A N I A N

S E L A N -

D I A N

T H A N E T I A N

Y P R E S I A N

B A R T O N I A N

L U T E T I A N

P R I A B O -

N I A N

SEQUENCE

STRATIGRAPHY

MAGNITUDE - MFS

MAJORMODER.MINOR

100-k.y. CYCLES

M I O C E N E

O L I G O C E N E

E O C E N E

CHRONO-

STRAT.

T I M E ( M a )

P A L E O C E N E

QUATER-

NARY

P L I O C E N E

U P P E R

L O W E R

U P P E R

M I D D L E

L O W E R

U P P E R

L O W E R

U P P E

R

L O W E R

M I D D L E

GLACIAL

HISTORY

EAIS

WAIS

NHIS

?

MMi-1

MMi-2

MTi-4

MTi-3

MTi-2

MTi-1

MSi-4

MSi-3

MSi-2

MSi-1

MLi-1

MBi-3

MBi-2

MBi-1

MAi-3

MAi-2

MAi-1

OCi-3

OCi-2

OCi-1

ORi-3

ORi-2a

ORi-2

ORi-1

EPi-2

EPi-1

EBi-1

ELi-1

ELi-2

ELi-3

ELi-4


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warm conditions, with a subsequent shift toward Nothofagus-dominated flora indicating cooling(Askin, 1992). No sedimentological evidence of glaciation exists in the form of ice-rafted clasts inEocene deposits on Seymour Island (Zinsmeister,1982), nor is there any indication of glacial erosionon the adjacent continental shelf at that time(Anderson et al., 1992; Sloan et al., 1995). This is animportant observation because it tells us that largeice sheets could have existed in East Antarcticaprior to the Eocene without leaving a geologicalrecord of their existence in the Antarctic Peninsula

region, where the only known Cretaceous andPaleogene outcrops occur.

In summary, there is evidence for glaciation inEast Antarctica since the middle Eocene (Figure 2).This evidence is supported by the presence of glacial sediments and glacial erosion surfaces onthe continental shelf, as well as by the reasonablecorrelation between the isotopic and sequencestratigraphic records for the middle and upper Eocene (Figure 4).

Lower Oligocene Glacial Record

At the Eocene–Oligocene boundary, a major shiftin δ18O occurs at sites on Campbell Plateau(Shackleton and Kennett, 1975) and at some lower latitude locations (Douglas and Savin, 1973; Wrightand Miller, 1993). Zachos et al. (1992) interpretedthe lower Oligocene oxygen isotope record of ODPsites from the Kerguelen Plateau to indicate theexistence of ice sheets similar in volume to thepresent Antarctica ice sheet. Miller et al. (1987),Denton et al. (1991), and Wright and Miller (1992,1993) also indicate the existence of significant ice volumes on Antarctica during the Oligocene. TheRupelian–Chattian stage boundary is marked by apronounced positive isotope event, which coin-cides with the middle Oligocene sequence bound-ary (30 Ma of Haq et al., 1987) and sequenceboundary Ch-1 (Hardenbol et al., in press; Vakarcset al., in press).

Deep-sea hiatuses mark the Eocene–Oligoceneboundary (Figure 2) in portions of the South At lantic (Wright and Miller, 1993), but it is notclear how these hiatuses relate to glaciation on the

continent. Since the late Eocene, significantrafting was occurring at ODP Sites 738, 7(Ehrmann, 1991; Wise et al., 1992), and 748 (Wet al., 1992) on the Kerguelen Plateau (Figure 3)Site 744 the increase in ice-rafted debris (IRoccurs in conjunction with a change from smecdominated clay assemblages to illite- and chlordominated assemblages, an increase in biosilicesedimentation, and an increase in the percentagcold-water calcareous nannofossils, interpretedevidence for the existence of the East Antarcice sheet (Wise et al., 1992). Site 739 (Figure 3

Prydz Bay penetrated lower Oligocene diamicinterpreted as till (Hambrey et al., 1991). The pence of till indicates that a grounded ice shextended to the continental shelf break (Barronal., 1991; Hambrey et al., 1991).

Some evidence exists for glaci ation in W Ant arctic a dur ing the ea rly Oli gocene. LowOligocene glacial-marine sediments were recoveat CIROS-1 in the Ross Sea (Hambrey et al., 198but these deposits may record alpine glaciatiBirkenmajer (1991) described till and glacmarine deposits of the Polonez Cove FormatiSouth Shetland Islands, and concluded that t were deposited by an ice cap that extended acrthe northern Antarctic Peninsula. Basalts, wK/Ar ages of 32.8 and 30 Ma, overlie these glasediments. Birkenmajer (1991) referred to tglaciation as the Polonez Glaciation. Seismrecords from the Antarctic Peninsula continenshelf show no evidence for glaciation (glacial sion surfaces or geomorphic features) prior to Miocene (Bart and Anderson, 1995).

Upper Oligocene–Lower Miocene Glacial Record

By the late Oligocene, a reorganization of platonic foraminiferal biogeographic provinoccurred in the Southern Ocean, indicating sigcant cooling (Kennett, 1978). DSDP sites on continental rise and abyssal floor off Victoria Land Wilkes Land (sites 267, 268, and 274) recthe first significant occurrence of ice-rafted matal at these sites in the upper Oligocene (Hayes Frakes, 1975). Seismic reflection profiles for


Figure 4—Composite smoothed δ18O record for the Cenozoic based on the isotope events identified in DSDP/Oand PETROBRÁS well A oxygen isotope records compared with Antarctica and Northern Hemisphere glacial hiry, the sequence boundaries of Hardenbol et al. (in press), and the eustatic curves of Haq et al. (1987) and Mitchet al. (1994). The time scale used is from Berggren et al. (1995). Solid bars in the glacial history column indicstrong evidence for ice-sheet existence, and dashed lines indicate early phases of ice-sheet development. EAIS =

Antarctica ice sheet, WAIS = West Antarctica ice sheet, NHIS = Northern Hemisphere ice sheet. Event names onisotope record correspond to names used in Table 2.


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East Antarctica continental slope and rise off PrydzBay and Wilkes Land show a prominent unconfor-mity separating more distal, hemipelagic seismicfacies below from more proximal fan depositsabove. Offshore Wilkes Land, this unconformity spans much of the Oligocene and early Miocene(Eittreim et al., 1995), and offshore Prydz Bay theunconformity spans the early Oligocene–late

Miocene (Kuvaas and Leitchenkov, 1992). The dra-matic changes in continental margin sedimentationrecorded in these regions are interpreted as the cul-mination of glacial erosion and mass wastingon these continental margins (Kuvaas andLeitchenkov, 1992; Eittreim et al., 1995).

A major unconformity in the CIROS-1 drill site in we stern Ross Sea (Figur e 3) separates lowe r Oligocene from upper Oligocene–lower Miocenestrata. The core and seismic records through thissite show seven grounding events indicated by glacial erosion surfaces and tills (Hambrey andBarrett, 1993; Bartek et al., 1996). Upper Oligoceneglacio-marine deposits at CIROS-1 alternate with

interglacial units that include fluvial to deep-water mudstone facies (Hambrey and Barrett, 1993). Thefluvial deposits, in conjunction with the occur-rence of a temperate ( Nothofagus ) po llen andspore assemblage in the mudstones, reflect temper-ate interglacial conditions (Hambrey and Barrett,1993). The oldest glacial deposits drilled in centralRoss Sea during DSDP Leg 28 are late Oligocene inage (site 270) (Hayes and Frakes, 1975). Based on arecent analysis of an extensive seismic reflectiondata set in Ross Sea, De Santis et al. (1995) demon-strated that late Oligocene glaciation in Ross Sea was characterized by localized ice caps centeredover banks and islands and that lower Miocenediamictons at DSDP site 270 record this more local-ized glaciation.

In the South Shetland Islands, the late OligoceneLegru Glaciation is recorded by glacial deposits(Birkenmajer, 1991). Fossiliferous glacial-marinestrata record an upper Oligocene to lower Mioceneglacial episode, the Melville Glaciation (Birken-majer, 1991; Abreu et al., 1992). No equivalent agedeposits exist on Seymour and James Ross islands,so the extent of the Legru Glaciation is uncertain.By the Oligocene, vegetation in Antarctica wasreduced to a sparse Nothofagus -fern assemblage(Kemp, 1972; Askin, 1992).

The upper Rupelian and lower Chattian stages arecharacterized by heavy isotope values that can berelated to low sea level, which do not entirely agree with the Haq et al. (1987) eustatic curve (Figure 4);however, the isotopic and sequence stratigraphicrecords are consistent with a period of high sea levelduring the early Miocene, with a possible decrease inice volume, followed by a pronounced sea level fallduring the middle Miocene (Figure 4).

Middle–Upper Miocene Glacial Record

A pronounced δ18O enrichment occurred duringthe middle Miocene (Figure 4). Shackleton andKennett (1975), Savin et al. (1975), Woodruff et al.(1981), and Flower and Kennett (1994) argued thatthis enrichment indicates initiation of the Antarcticaice sheet, but the evidence for prior ice-sheet evo-

lution seems irrefutable. Miller et al. (1991) indicat-ed step-like buildups of ice in the middle Miocene.Denton et al. (1991) considered that ice volume on Antarctica has been equal to roughly one-half of thepresent volume by 15 Ma, and that ice volumes per-haps had exceeded those of the present by 12 Ma.

The results of drilling in the southeast PacificBasin (DSDP Leg 35) led to the conclusion that sig-nificant glaciation occurred in West Antarctica by early–middle Miocene, based largely on the first sig-nificant occurrence of IRD at Site 325 on theBellingshausen continental rise (Tucholke et al.,1976). This conclusion is supported by results froma seismic stratigraphic investigation on the adjacent

Antarctic Peninsula continental shelf. The oldest widespread glacial erosion surface on the shelf isinterpreted to be of middle Miocene age and evi-dence exists for several grounding events on theshelf during the middle–late Miocene (Bart and Anderson, 1995).

A shelf-wide middle Miocene glacial unconformi-ty in Ross Sea (Savage and Ciesielski, 1983; Anderson and Bartek, 1992; De Santis et al., 1995)indicates that both the East and West Antarctica icesheets grounded on the continental shelf at thattime. Glacial-marine deposits dominate the upper Miocene section that rests on top of the unconfor-mity (Hayes and Frakes, 1975; Savage andCiesielski, 1983; Anderson and Bartek, 1992).Upper Miocene glacial and glacial-marine stratarecovered at DSDP sites on the Ross Sea continen-tal shelf and at CIROS-1 drill sites in westernmostRoss Sea are interbedded with meltwater deposits, which indicate temperate interglacial climatesthroughout the Miocene, and diatomaceous oozes.Seismic reflection profiles from Ross Sea show pre-dominantly glacial-marine seismic facies and nomajor glacial unconformities in the upper Miocenesection (Anderson and Bartek, 1992).

Detailed sequence stratigraphic analyses of Crary Fan in eastern Weddell Sea (Moons et al., 1992) led

to the recognition of a minimum of five channel-levee units of Oligocene–middle Miocene age. DeBatist et al. (1994) argued that episodes of fandevelopment correlate with ice-sheet groundingevents and conclude that at least five long-termglacial expansions onto the continental shelf andno fewer than fourteen smaller scale expansionsoccurred in the Weddell Sea region since the mid-dle Miocene.



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The continental record of glaciation in West Antarctica during the Miocene includes subglacially erupted volcanics at the head of Scott Glacier;radiometric dates provide ages of 9–15 Ma (Stump,1980). Rutford et al. (1972) demonstrated thatglaciation in the Jones Mountains had occurredprior to 7 Ma.

In Prydz Bay, major hiatuses span the late

Oligocene–middle Miocene. A major middle Mio-cene(?) unconformity, attributed to ice-sheet ground-ing on the shelf, also occurred on the Wilkes Landcontinental shelf (Eittreim et al., 1995). During thelate Miocene–middle Pliocene, the ice sheet retreat-ed from Prydz Bay, and glacial-marine sedimentationdominated (Hambrey et al., 1991).

In summary, the isotope record suggests an ice- volume increase in Antarctica during the middleMiocene (Figure 2) consistent with the developmentof the West Antarctica ice sheet. The ice-volumeincrease implied by the oxygen isotopes is consis-tent with the sea level fall shown by the Haq et al.(1987) sea level curve (Figure 4). The isotopic and

sequence stratigraphic records indicate a period of low sea level during the late Miocene, suggestingan ice-volume increase during this time.

Pliocene Glacial Record

The central issue for the Pliocene is not whether an ice sheet existed on Antarctica, but concernspossible extreme warming and associated inter-glacial events (Denton et al ., 1991; Webb andHarwood, 1991; Barrett et al., 1992). Denton et al.(1991) concluded that isotopic data for the periodbetween 6.0 and 4.4 Ma suggest high-frequency ice- volume changes with little evidence of extensivedeglaciation. In contrast, Moriwaki et al. (1992)contended that the oxygen isotope record providessupporting evidence for extreme Pliocene deglacia-tions. In general, the biogeographic record fromthe Southern Ocean lends support to a highly vari-able climatic setting in Antarctica during thePliocene (Barron, 1996).

Mayewski (1975) demonstrated that during thePliocene an ice sheet covered the TransantarcticMountains. He referred to this as the Queen MaudGlaciation. Denton et al. (1991) cited other linesof evidence for a thicker ice sheet, which deposit-ed the Sirius tillite at that time. The Sirius Groupconsists predominantly of tillite and generally occurs above the 2000 m elevation level in theTransantarctic Mountains. Ironically, the mostcontroversial record of Pliocene glaciation in Antarctica centers on the Sirius Group.

Alonso et al . (1992) recognized a number ofshelf-wide unconformities within the Pliocene–Pleistocene section of the eastern Ross Sea. Seismic

records from the Antarctic Peninsula continenshelf also indicate high-frequency grounding eveduring the Pliocene (Bart and Anderson, 1995). Tincreased frequency of grounding events in thareas relative to the Miocene is attributed to stroneustatic control on ice-sheet stability (Alonso et1992; Bart and Anderson, 1995).

The early Pliocene sea level rise shown by

eustatic curve (Haq et al., 1987) seems to be content with slightly lighter isotope values (Figureindicating a possible decrease in ice volume durthat time. From the upper Pliocene to the Holocethe isotope record shows a continuous trend of hier values with high-frequency fluctuations.

Pleistocene Glacial Record

Webb and Harwood (1991) argued that the climic cooling at the end of the Pliocene marks change from unipolar, temperate, and cyclic glations to bipolar ice sheets. Prior to the Pliocene c

ing, climates were too warm to support ice capmiddle latitudes. The latest Pliocene–earliPleistocene was a period of major cooling in Transantarctic Mountains (Mercer, 1972; Dentoal., 1986a, 1989). The Sirius Group records change as a switch from pre-Pleistocene strata csisting of interbedded tillite, colluvium, and lactrine deposits to Pleistocene tillite and interbed“polar” deposits (Denton et al., 1991). Abundant imentologic evidence (Sirius, Peleus, and ProspMesa drift deposits) and geomorphologic evideexists to indicate that the East Antarctica ice shoverrode the Transantarctic Mountains, perhapsseveral occasions, during the Pliocene–Pleistocealthough the exact timing of these events remaproblematic (Denton et al., 1991).

High- and intermediate-resolution seismic pfiles from the Ross Sea, the Weddell Sea, and Antarctic Peninsula, including Bransfield bashow that the Pleistocene section on the contintal shelf is characterized by numerous glacial esion surfaces separating till sheets (Alonso et1992; Anderson and Bartek, 1992; Anderson et1992; Banfield and Anderson, 1995; Bart a Anderson, 1995). The data indicate that both Eand West Antarctica ice sheets continuedexpand and contract even after polar climate westablished. This continued expansion and conttion of the ice sheets on the continental shelf prably was caused by rising and falling sea leveproduct of Northern Hemisphere glacial-inglacial cycles (e.g., Denton et al., 1986b).

The isotope record indicates that high-amplitsea level changes of up to at least 120 m hoccurred at 100 k.y. intervals since appromately 800 ka; however, prior to 800 ka, 41



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low-amplitude Milankovitch obliquity cycles domi-nate the δ18O record (Prell, 1982).

DISCUSSION

Figure 4 compares the composite oxygen iso-tope record compiled for this study, the combined

sea level curves of Haq et al. (1987) and Mitchumet al. (1994), the cycle chart of Hardenbol et al.(in press), and a summary of ice-sheet evolutionduring the Cenozoic. Visual inspection shows rea-sonable correlation between these data sets.Specifically, the increase in δ18O values starting inthe middle Eocene is consistent with geological evi-dence for expansion of the East Antarctica ice sheetonto the continental shelf at this time. The episodeof ice-sheet development is not recorded in theeustatic curve, although both curves indicate pro-gressive development of the ice sheet throughoutthe remainder of the Cenozoic and episodes of apparent rapid growth during the late Eocene, mid-

dle Oligocene, and middle–late Miocene. Episodesof apparent reduced ice volume occurred in thelower Oligocene, lower–middle Miocene, andlower Pliocene. Our inference of an early phase of East Antarctica ice-sheet development during thelate Paleocene–early Eocene is based on the high-frequency oscillations shown in the Hardenbol etal. (in press) cycle chart (Figure 4). We infer thatthe early phase of East Antarctica ice-sheet evolu-tion would have occurred at a time when geologi-cal data suggest a temperate climate in Antarctica,at least in coastal regions. Hence, the early ice-sheet probably expanded and contracted inresponse to high-frequency climatic oscillations.

The geological record of West Antarctica ice-sheet expansion across the continental shelf dur-ing the middle Miocene is manifested in both theoxygen isotope and sea level curves. Both curvesshow high-frequency oscillations during thePliocene–Pleistocene that mark the onset of glacia-tion in the Northern Hemisphere. These high-frequency oscillations are attributed to the fact thatthe Northern Hemisphere ice sheets were situatedin more temperate latitudes and therefore weremore sensitive to high-frequency (Milankovitch-scale) climatic changes.

Even though the similarities of the eustatic andoxygen isotopic curves will not stand up to rigor-ous statistical treatment, the case for glacial-eustaticcontrol on stratal stacking patterns during the mid-dle Eocene–Holocene is a strong one, which implies that sequence stratigraphic cycle charts for this time interval are valid. To better understandthe lack of a strong correlation between the sealevel curve and the oxygen isotope record, wemust examine the methods used to der ive these

curves and the assumptions made in convertingsequence stratigraphic data and oxygen isotopedata to sea level, and the inherent errors in both methods.

The chronostratigraphic position of eventsdefined by Haq et al. (1987) was determined essen-tially from the position of sequences in stage typeand reference sections. An extensive revision of the

Cenozoic sequence chronostratigraphic recordexpressed relative to the Berggren et al. (1995)time scale is currently in preparation (Hardenbol etal., in press).

The eustatic curve of Haq et al. (1987) was con-strained by the tectono-eustatic curve of Pitman(1978), which is based on estimates of the volumeof the oceanic ridges. The magnitude of a singleeustatic fall was acquired from seismic records,mostly by measuring the vertical distance betweenthe offlap break of the previous sequence bound-ary and the youngest coastal onlap of the followinglowstand, and the degree of onlap observed in out-crop. The age of the sequence boundaries and the

maximum flooding surfaces defined the inflectionpoints of the falling and rising sea level, respective-ly. The magnitude of the eustatic rise is related tothe amount of transgression relative to the previouscoastline. In the case of a major transgressive event,the eustatic curve would reach the position of thePitman (1978) curve.

Several workers used the Pleistocene calibrationof 0.11‰ δ18O variation per 10 m of sea level changedetermined by Fairbanks and Matthews (1978) andFairbanks (1989) to estimate the magnitude of Cenozoic sea level variations (e.g., Miller et al., 1987,1991; Williams, 1988; Haddad et al., 1993; Abreuand Savini, 1994). This is only a rough estimatebecause a significant component of the δ18O recordis expected to have been caused by decreasingocean temperatures through the Cenozoic (Savin,1977; Miller et al., 1987). Although the positive iso-tope events correlate favorably with sequenceboundaries since the middle Eocene, the isotoperecord commonly shows smaller magnitudes thanthe sequence stratigraphic record.

On average, the amplitude of the positive oxy-gen isotope events since the middle Eocene is0.6‰, corresponding to a sea level variation of about 55 m (Table 2). Approximately 60% of thesepositive events have amplitudes between 0.3 and0.6‰, which corresponds to 25 to 55 m of sealevel change. High-amplitude events (up to 1‰)occur during the Oligocene, middle Miocene,Pliocene, and Quaternary. These high-amplitude iso-tope events would correspond to sea level variationsof about 90 m.

The average eustatic amplitude in the Haq et al.(1987) chart for the middle Eocene–Holocene is70 m, approximately 30% higher than the average



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amplitude predicted by the δ18O record. Oneexplanation for this difference is that depositionalsequences are the result of relative sea level change(eustasy plus subsidence), whereas the isotopic sig-nal is related more to eustasy (ice volume).

For most of the Cenozoic, sequence boundariesseem to be generated by sea level falls in the range of 30 to 50 m. Thus, the ice-volume changes required to

generate sequences would be substantially smaller than previously thought. A sea level fall of 40 m would correspond to the melting of about 20% of thetotal ice volume accumulated during the Last GlacialMaximum, or less than the amount of ice presenttoday in the East Antarctica ice sheet (equivalent toapproximately 50 m sea level rise).

Lastly, the West Antarctica ice sheet (WAIS) contri-bution to sea level has undoubtedly decreased with time. Presently, the WAIS contains enough ice to raisesea level approximately 6 m if melted. However, themodern WAIS is mostly situated below sea level, thuslimiting its contribution to sea level change. However,there is strong evidence in drill cores and seismicrecords from West Antarctica that the continentalshelf was much shallower during the Oligocene;glacial erosion and isostasy have since then created thegreat depth of the shelf (Hayes and Frakes, 1975;Cooper et al., 1991; Anderson and Bartek, 1992; Bartand Anderson, 1995; De Santis et al., 1995). A largeportion of the early WAIS was situated above sea leveland probably contributed more to eustasy than themodern WAIS. The same can be said for the East Antarctica ice sheet (EAIS) but to a lesser degree. Themain portion of the EAIS is situated above sea level,but the continental shelf has been significantly low-ered by glacial erosion and isostasy.

CONCLUSIONS

During the past two decades, the proxy recordof sea level change derived from oxygen isotopesand the sea level records derived from sequencestratigraphic analyses have steadily improved. Therecords support the existence of large ice sheetsduring much of the Cenozoic. Concurrently, thegeologic record of glaciation on Antarctica hasimproved considerably. The result has been toextend the age of the Antarctica ice sheet farther back in time so that the geologic record of ice-sheetdevelopment is more consistent with the oxygenisotope and eustatic records.

Current evidence does not favor the existenceof a large ice sheet in Antarctica during theCretaceous–early Eocene; however, an ice cap may have occupied the central portion of the continent,far from the only Cretaceous and Paleocene sedi-mentary exposures, which are restricted to thenorthern Antarctic Peninsula region.

We found strong evidence that the East Antarcice sheet was present by the middle Eocene and it had significant volume changes during the Eocene and middle Oligocene. The West Antarcice sheet did not form until the early–midMiocene, and its early evolution was characterizedrepeated episodes of advance and retreat across continental shelves. During its early history, the W

Antarctica ice sheet was grounded at or near sea le With time, it eroded deeply into the sedimentbasins on which it rests, so that it currently is groued well below sea level. Hence, its contributionglobal sea level was diminished with time. Higherquency glacial/interglacial cycles of the PliocePleistocene heralded the onset of NorthHemisphere ice sheets that were more strongly inenced by high-frequency climatic changes (eDenton et al., 1986b).

A reasonable corre la tion exis ts be tweenquence boundaries (Hardenbol et al., in press) oxygen isotope positive events for almost entire Cenozoic (Figure 4), markedly since

early–middle Eocene boundary. Both the isotorecord from deep ocean sites and the eustacurve (Haq et al., 1987) show a similar trend sithe upper part of the middle Eocene. This evidestrongly suggests the control of glaciation upeustasy during most of the Cenozoic and provisupport for global stratigraphic correlation backthe early–middle Eocene.

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Vitor Abreu

Vitor Abreu received his B.S. andM.S. degrees at the Federal Uni- versity of Rio Grande do Sul (Brazil).He has worked for Petrobrás since1987 and was the manager of thesector of biostratigraphy and paleo-ecology for four years. Currently,

Vitor is at Rice University workingon his Ph.D., which focuses on thegeologic evolution in conjugate vol-canic passive margins in the Pelotasbasin, Brazil, and in offshore Namibia, Africa. His research interests include sequence stratigraphy in passive marginsand stable isotope stratigraphy.

John B. Anderson

John Anderson completed his B.S.degree at the University of South Alabama (1968), his M.S. degree atthe University of New Mexico in

Albuquerque (1970), and his Ph.D.at Florida State University (1972).

John is currently professor and chair-man of the Department of Geology and Geophysics at Rice University.His research interests are in

Antarctic marine geology and theQuaternary evolution of the Gulf of Mexico. He has

worked in Antarctica for over two decades, including 17scientific expeditions to the continent.

ABOUT THE AUTHORS

glacial eustasy during the cenozoic: sequence stratigraphic implications

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