Onset of Cenozoic Antarctic Glaciation

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Deep-Sea Research II 54 (2007) 22932307

Onset of Cenozoic Antarctic glaciation

Peter F. Barker a, , Bernhard Diekmann b , Carlota Escutia c

a Threshers Barn, Whitcott Keysett, Clun, Shropshire SY7 8QE, UK b Alfred Wegener Institute for Polar and Marine Research (Research Unit Potsdam), Telegrafenberg A43, 14473 Potsdam, Germany

cInstituto Andaluz de Ciencias de la Tierra (IACT), CSIC-University de Granada, Campus de Fuentenueva s/n, 18002 Granada, Spain

Accepted 24 July 2007Available online 22 October 2007

Abstract

This paper considers the wide range of evidence, both direct and indirect, for the onset of Cenozoic Antarctic glaciation.It distinguishes two useful phases of Antarctic glacial onset: an initial phase of mountain glaciation, from which ice streamsoccasionally and in isolated locations reached sea level, and a subsequent phase of full glaciation, with an ice sheet aslarge as todays, extending everywhere to sea level. According to direct evidence, generally proximal, from the continentitself or surrounding Southern Ocean, the rst of these occurred probably during the late Eocene, while the seconddeveloped at the EoceneOligocene boundary. Indirect evidence, mainly involving proxy measurements from DSDP andODP sites remote from the Southern Ocean, suggests that middle and late Eocene glaciations may have been full also (icesheets possibly even larger than todays) but short-lived, and that the E/O boundary onset differed from these mainly inproducing a stable ice sheet. In pursuing the notion of glacial onset, we examined the direct record separately for East

Antarctica, West Antarctica, and the Antarctic Peninsula, the different sub-ice topography and geographic positions of which suggest that their glacial histories could have been different. The direct record for an initial, middle or late Eocenephase is very sparse: only the rare occurrence of IRD at Southern Ocean DSDP and ODP sites suggests the possibility of early ice, and all three regions include mountains that could have hosted such ice. Although the indirect record and climatemodelling in combination suggest that full glaciation of each region was probably synchronous, we nd differences inthe available direct evidence. There is abundant evidence that East Antarctica became fully glaciated in the earliestOligocene, but certain evidence of glaciation of a similar age extending to sea level is sparse for the Antarctic Peninsula,and is not found until the late Oligocene for West Antarctica. High-resolution direct evidence is required to resolveuncertainties in glacial history.r 2007 Elsevier Ltd. All rights reserved.

Keywords: Antarctic; Glacial onset; Palaeoclimate

1. Introduction

The high latitudes receive little of their heatdirectly from the sun. The greater part is receivedindirectly, by atmospheric and oceanic transport

from lower latitudes. Thus, the high latitudesexperience the coolest climates and, provided thatmoisture is available, are the most likely toexperience glaciation. Within the Cenozoic, inwhich cooling was near-global in extent, it has beenthe Southern Hemisphere, with land over thegeographic pole, that appears to have experiencedglaciation earlier than the Northern Hemisphere

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Corresponding author.E-mail address: [email protected] (P.F. Barker).
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(although evidence for early Cenozoic bipolarglaciation is growing) and for which glaciation hasbeen more persistent. Nevertheless, Cenozoic Ant-arctic continental glaciation has been a complicatedprocess: the continent went through different phases

of glaciation, different regions of Antarctica havehad different glacial histories, and periods of deglaciation and re-glaciation may have occurred.In the face of this complexity, it is necessary to focuson an important aspect of glaciation, that has thepotential to provide a greater understanding of itscauses and effects and perhaps the reasons for agenerally lesser expression of bipolar glaciation.Here we consider glacial onset; the evidence forsubsequent variation is both sparse and controver-sial, and less obviously related to cause.

We aim to distinguish between an early phase of glacial onset, in which ice generated at highelevation may occasionally and in a very few placesdrain at sea level, and a later-stage full glaciation,with an ice sheet almost as large as todays. We areencouraged that these phases can be distinguishedby the external, essentially distal and indirect,evidence of ocean-oor geochemistry (from benthicforaminifera: e.g., oxygen isotopes, Zachos et al.,2001 and Mg/Ca ratios, Lear et al., 2000 ) andnumerical modelling (e.g., Huybrechts, 1993 ; DeConto and Pollard, 2003 ), showing that full,

stable Antarctic glaciation probably developedto an advanced state quite rapidly, around theEocene-Oligocene boundary (E/O) interval.

The possible causes of Antarctic glaciation arepoorly understood. The thermal isolation of Ant-arctica within a growing Southern Ocean, with thedevelopment of a strong, deep-reaching AntarcticCircumpolar Current (ACC) (e.g., Kennett, 1977 )has long been a preferred explanation. Thus, as anadditional contribution to an understanding of cause, evidence for the time of onset of the ACCis also considered elsewhere in this volume. How-ever, numerical modelling has led more recently tothe suggestion ( De Conto and Pollard, 2003 ) that asteady global decline in atmospheric pCO 2 couldhave triggered a rapid glaciation even without acontribution from changing ocean circulation. Thenon-linear ice volume response found by De Contoand Pollard is a feature of Antarctic ice sheetgrowth (concerning surface elevation or albedoperhaps) rather than of CO 2 involvement, as itgures also in a purely temperature-dependent icesheet model (e.g., Huybrechts, 1993 ; Barker et al.,1999a ; Fig. 1 ). Also, therefore, although not ruled

out, an event, such as the onset of an ACC, is nota requirement. Other suggested causes for global

cooling are changes in ocean circulation (e.g.,Lawver and Gahagan, 2003 ) and tectonic uplift(e.g., Raymo and Ruddiman, 1992 ) elsewhere in theworld. Here, we discuss the kinds of evidence thatbear on glacial onset, and assess the present state of knowledge for Antarctica.

2. The present ice sheet

The modern Antarctic ice sheet covers the entirecontinent, with only 0.3% of the land area ice free(e.g., Lythe et al., 2001 and Fig. 2 ). It reaches athickness exceeding 4500 m in places and has a totalvolume of 25.4 million km 3 (equivalent to a sea-levelrise of 57 m). It acquires volume by snowfall and,being cold, loses volume largely (up to 90% Paterson, 1994 ) by calving ice streams. The meanresidence time of ice within the ice sheet exceeds 0.5Myr in places close to the centre, but is less than20 kyr near the edge ( Paterson, 1994 ), wheresnowfall is both greater and warmer (leading togreater basal melting and faster ow). The snowof which the ice sheet is formed is isotopicallylighter than the oceans, because of fractionation

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Fig. 1. Ice sheet volume plotted against the rise in mean annualtemperature at sea level around Antarctica, compared withmodern temperatures ( Huybrechts, 1993 ). A non-linear relation-ship between temperature and ice volume is clearly seen.

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during evaporation and precipitation, but by anamount that may have varied with time. It isimportant to appreciate that the relationshipbetween ice volume and ice temperature is un-certain: cold ice is stiffer than warm ice, so ows lesseasily, and there is less melting; but colder air carriesless moisture, suggesting lower snowfall. Thus, acolder ice sheet may not be larger. The presentregime is interglacial, in which the groundingline of the Antarctic ice-sheet is well away fromthe continental shelf edge, mainly because thereare no major Northern Hemisphere ice sheets,that lower sea level during glacial maxima so asto permit grounding line advance (but, mostprobably, only a minor increase in Antarctic ice-sheet volume, as the continental shelves are largelyoored by low-viscosity till, permitting rapid iceow). A signicantly larger ice sheet than this in thepast is difcult to imagine. Many workers distin-guish between a cold and a temperate ice sheet,and describe todays as cold. An Oligocene ice sheetis generally considered to have been warmer thantodays.

3. Nature and validity of evidence of onset of glaciation

3.1. Indirect evidence

Some of the distal or indirect evidence has alreadybeen mentioned. Oxygen isotopic measurements onbenthic foraminifera ( Zachos et al., 2001 , provideda useful recent compilation) are ambiguous, aschanges in isotopic ratio may result from changes of seawater temperature or of mean seawater composi-tion (which is affected mainly by the volume, butalso by the isotopic chemistry, of sequestered ice:e.g., Mix and Ruddiman, 1984 ). Mg/Ca measure-ments, also on benthic foraminifera, (e.g., Learet al., 2000 ) are insensitive to ice volume/chemistry,so have the ability to resolve the isotopic ambiguitybetween ice volume and water temperature ( Fig. 3 ):the data base of reliable measurements is growingbut some interpretational uncertainties remain, asthe ratio may be sensitive to other factors. Globalsea-level change is sensitive to grounded ice volume(Browning et al., 1996 ; Pekar et al., 2005 ), but is not

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Fig. 2. Surface of the present ice sheet (modied from Drewry, 1983 ). Ice ow directions are drawn perpendicular to surface slopes, and donot show ice stream locations. Thin dashed lines show ice divides and thick dashed lines divide East from West Antarctica and theAntarctic Peninsula. Numbered black locations are existing DSDP and ODP drill sites close to the continent. Western Ross Sea drill sitesCR Cape Roberts, MS MSSTS, CI CIROS 1. Labelled red locations are ice core sites V Vostok, B Byrd, C Dome C, S Siple, T TaylorDome. GaM Gamburtsev Subglacial Mountains.



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widely used as a determinant at present because of difculties in separating global and regional effects(but see Pekar et al., 2002 ; Miller et al., 2005 foruseful recent comparisons that suggest greaterfuture use; Miller et al., 2005 , propose intermittentsubstantial glaciation throughout the past 100 Ma,to account for global sea-level changes). Theindirect data are mainly volume effects, so (at thepresent level of understanding) can indicate onlymajor changes, such as the inferred dramatic growth

in ice-sheet volume in the E/O interval. Theinterpretation of other oxygen isotopic changes of similar magnitude, as within the late Oligocene andearly Miocene, is less certain. Perhaps signicantalso, in view of speculation that short-lived (un-stable?) Eocene glaciations and the initial earliestOligocene Oi-1 isotopic peak infer ice volumeslarger than the present Antarctic ice volume, is thesuggestion of Toggweiler and Bjornsson (2000) thatone effect of an ACC would have been to enhancethe climatic difference between Northern andSouthern Hemispheres. Before ACC onset there-fore, climate in high latitudes of the two hemi-spheres could have been more equal, with signicantice volumes developing in the north.

An additional problem is distinguishing betweenorbital forcing of ice-sheet volume and the longer-term changes that are of interest here. Orbitalvariation in ice sheet volume is very evident from allhigh-resolution oxygen isotopic studies, and inseveral (e.g., Paul et al., 2000 ) it has been suggestedthat orbital variation triggered major ice-sheetvolume change. However, it is necessary to separatethe two time scales, as our main concern here is with

the longer-term, irreversible changes in earthresponse. The model of De Conto and Pollard(2003) suggested that ice-sheet volume variationover an orbital cycle could be quite large, providedthat a basal sliding mechanism (such as a renewablebasal till) was always and everywhere available.Barker et al. (1999a) speculated that orbital varia-tion in ice-sheet volume could be large in the initialphases of glaciation, before ice-sheet extent hadbecome constrained by the continental shelf edge

(rather like the unconstrained Pleistocene NorthernHemisphere ice sheets). However, uctuations inoxygen isotopic measurements at single sites (e.g.,Kennett and Stott, 1990 ; Zachos et al., 1996 ) place alimit on such volume changes (although Wade andPa like, 2004 , argue for larger amplitude orbitalvariation, from measurements at equatorial Site1218), suggesting that, at the lower time resolutionwe can employ (i.e. limited by onshore geology), weshould be able to identify an advanced full glacialonset without confusion from glacialinterglacialvariation. An additional consideration, not ad-dressed by numerical models, concerns ice-streamdrainage, which could have distorted the simplepicture produced by models by providing evidenceof glaciation at the coast well in advance of the mainice-sheet margin.

Measurements on ODP samples far from theSouthern Ocean have pointed to additional com-plexity, for which more proximal and directevidence is required. One such circumstance, al-ready mentioned, is the combination of the benthicoxygen isotopic compilation of Zachos et al. (2001)and measurements of Mg/Ca ratio in benthic

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Fig. 3. Zachos et al (2001) compilation of benthic oxygen isotopic data (raw data blue, 0.2 Ma RMS red) for the Cenozoic (modied fromBarker and Camerlenghi, 2002 ), showing oxygen isotopic events Mi-1 and Oi-1 and hypothesised short-lived Eocene glaciations, andcomparison of Zachos et al. (2001) data with Lear et al. (2000) measurements of Mg/Ca ratio in benthic foraminifera from DSDP Site 522(Walvis Ridge, SE Atlantic) across the E/O interval.



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foraminifera from Site 522 in the SE Atlantic(Fig. 3 ), which is used to argue that the entireoxygen isotopic shift at the E/O interval is caused byice volume change, with no change in deep andbottom-water temperature. This is of concern, as it

implies that another process, now perhaps inopera-tive, caused pre-glacial high-latitude creation,through cooling, of deep and bottom water, andthat the (glacially induced) processes that nowoperate, namely brine production beneath forming(fresh-water) sea ice followed by supercoolingbeneath cold ice shelves ( Foster and Middleton,1980 ; Foldvik and Gammelsrd, 1988 ) have beenless important in the past. An alternative, perhaps,is to investigate the possibility of additionalinuences, not as yet determined for the Site 522data, on measured Mg/Ca ratios (e.g., Lear et al.,2004 ), or to question the value of compilations of isotopic data from a range of sites, lest there beinter-site differences in bottom-water source or indating. Closely related are two additional questions,of the large amplitude of the initial (early Oligocene)Oi-1 oxygen isotopic excursion (implying an icevolume larger than that of the present day, if conned to the Antarctic and having the sameaverage isotopic composition) and its rapid devel-opment, and of the proposed occurrence of earlier(middle and late Eocene) glaciations, short-lived

(ca. 500 ka) but again larger in amplitude than thepresent Antarctic ice volume and considered inpart to represent Northern Hemisphere glaciation(Tripati et al., 2005 ). In both cases, the data set isdiverse, comprising oxygen isotopic and Mg/Cadata and carbonate concentrations used to deter-mine variations in carbonate compensation depth(CCD), mainly from the equatorial Pacic (ODPSites 1218 and 1219). Support for Northern Hemi-sphere glaciation (which does not concern us here) issaid to be Oligocene NADW formation and thepresence of IRD within the Eocene of the onlyArctic Ocean section extant, from the LomonosovRidge ( Shipboard Scientic Party, 2005 ). SouthernHemisphere support is cited as Eocene glacialsediments from McMurdo Sound ( Ehrmann,1998 ), and uctuations in Antarctic-derived clayminerals (kaolinite, smectite, chlorite and illite) onMaud Rise and the Kerguelen Plateau ( Ehrmannand Mackensen, 1992 ; Robert and Kennett, 1992 ).

In all the instances described above, the assump-tion is made that atmospheric pCO 2 is the dominantinuence on glaciation. It should be pointed outalso that, according to the model of De Conto and

Pollard (2003) , ice volume changes in the earliestphase of development could be rapid (though notquite as rapid as required by Coxall et al., 2005 ), sothat short-lived ice sheets are mechanically feasible.

3.2. Direct evidence

A wealth and a wide range of direct evidencebears on the history of East Antarctic glaciation,much less on West Antarctic and Antarctic Penin-sular glaciation but sufcient to suggest that thethree regions should be treated separately. The EastAntarctic ice-sheet is today by far the largest, withthe greatest scope for minor variation (although theWest Antarctic ice sheet is considered the leaststable): changes inferred from indirect evidencecannot be assumed to reect changes in anyparticular region of Antarctica alone. Also, it wouldbe unwise to rely overmuch on numerical models:among the problems with many of the mostinteresting numerical models of ice-sheet develop-ment currently available are that they do not treatof ice grounded well below present sea level (i.e.much of the West Antarctic ice sheet), of ice-sheetdrainage by ice streams (affecting all regions) or of sea ice capable of moving on the ocean surface(perhaps relevant to the Antarctic Peninsula).

Ice-rafted debris (IRD) is an important direct

indicator of glaciation. It may be released frommelting ice grounded below sea level both proxi-mally (e.g., in the continental shelf facies residualglacial marine of Anderson et al., 1980 ) anddistally, after transport in icebergs, within ocean-oor sediments. There is a limit to the extent towhich variations in IRD at a particular site may beused simply to infer variations in glaciation, sincemelting is also affected by storminess and watertemperature along iceberg paths, by changing oceancirculation, and by the location of debris within theiceberg. In particular, debris may fall onto thesurface of a valley glacier, and wind-blown terrige-nous material (usually therefore of a more restrictedsize range) may accumulate on sea ice that thenmelts or moves. Both of these are typical of the earlyshallow phases of glaciation, while thicker-ice-sheet regimes are dominated by basally transporteddebris within and beneath ice streams. The largestice streams, such as those within the Filchner Ronne ice shelf today, carry no IRD to the icebergcalving zone because of prior basal melting. Insummary, IRD onset may be used to infer the start,or existence, of a phase of glaciation in which

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grounded ice reaches sea level at the continentalmargin, or sea ice formed there, but more-detailedinterpretation, involving changes in IRD concentra-tion, is more difcult.

Observations of cold surface water and sea ice

around Antarctica in the past, determined usingmicrofossil assemblages for example, provide aninference of conditions onshore, but not a clearindication of the phase of glaciation. Because of uncertainties over the existence of inter-oceanconnections at a particular time, such an inferencecan be identied with one particular region of Antarctica only if the ocean off other regions ismarkedly different.

Clay mineralogy (e.g., Ehrmann and Mackensen,1992 ; Robert and Chamley, 1992 ) at offshore siteshas been used to distinguish a cold climate on aneighbouring landmass (with dominantly physicalweathering of continental rock exposure, producingmore chlorite or well-crystallised illite) from a warmwet climate (dominantly chemical weathering, pro-ducing more kaolinite and smectite). It is necessaryto know the geographic origin of the clays, and tobeware changes in the composition of the erodingrocks, as might easily occur with continued erosion.For example, physical re-erosion of older, chemi-cally weathered sediment or older marine sedimentis a possibility. Also, an authigenic component may

mislead (e.g., Diekmann et al., 2004 ), as may achange in ocean circulation if samples are from adistal site.

Rock exposure onshore or on the continentalshelf is a potential source of climatic information,provided it is representative. We should perhapsbear in mind that, given the erosion common at thebase of grounded ice and, particularly, glacialoverdeepening at the continental margin, onshorepreservation of a geological record of an interveningwarmer period is unlikely. In fact, continental rockoutcrop relating to Cenozoic climate is sparse. Somedoes occur, and there are indications of continentalgeology also in glacial erratics exposed onshore, indeep-sea sediments off drainage basins, and in drillholes. Useful indicators include marine macro- andmicrofossils, pollen and spores. The former reectconditions inshore in shallow water, the latteronshore vegetation, therefore non-glacial condi-tions, and constraints on glaciation are providedby their declining diversity or geographic distribu-tion, and disappearance. In circumstances of rapidand perhaps reversing climate change, as within awell-developed orbital cycle, the preferential pre-

servation of representatives of one part of a changecan lead to misunderstanding.

There is a particular interest in the ora, in thecontext of glacialinterglacial variation. SouthernHemisphere (including Oligocene Antarctic) ora

usually include Nothofagus , for which it is generallyaccepted that even moderate seaways present abarrier to dispersal ( Hill and Dettmann, 1996 ). Wetherefore may assume perhaps, despite the greaterOligocene proximity of East Antarctica and Aus-tralia, or the Antarctic Peninsula and SouthAmerica, that re-colonisation after Oligocene glacialmaxima had to be from Antarctic refugia ratherthan more distant sources. This would limit theareal coverage of a maximal Oligocene ice sheet, incontrast to conclusions based on indirect data.

Antarctic continental shelves are commonly deepand inward-sloping, having been eroded by moving,grounded ice. Many shelves have been extended byprograding wedges of diamictunsorted terrige-nous material deposited by or from ice (mostcommonly by the rapidly moving ice streams,grounded across much or all of the presentcontinental shelf around glacial maxima). Thewedges display a range of geometries, and aredifcult to recover by drilling, owing to the lack of sorting and poor consolidation of the sediments,exposure of the drill ship to ocean swell etc. (see

Hayes et al., 1975 ; Barker et al., 1999b ; Cooper andOBrien, 2004 for results of drilling the wedges).Dates from the wedges probably indicate theexistence of an ice sheet of signicant size at thatpart of the continental margin at that time. It hasbeen argued, on the basis of inverse modelling of exure and sediment compaction, that glacial over-deepening of the shelves is preceded by a phase of lesser glaciation, with shallow, outward-slopingcontinental shelves (e.g., De Santis et al., 1999 ;Camerlenghi et al., 2002 ). A proposal to combinedrill data from different parts of the continentalmargin, where the prograded wedge and derivedupper rise drift reect different phases of Antarcticglaciation in different regions, was generated byANTOSTRAT, an alliance of Antarctic marinegeoscientists formalised within the Scientic Com-mittee for Antarctic Research (SCARsee Barkeret al., 1998 ). Directed by a model of glacialevolution adduced from numerical modelling(Huybrechts, 1993 ; Barker et al., 1999a ) it was onlypartly carried out. Recovery, though poor in places,was sufcient to provide useful information aboutthe onset and recent (last 9 myr) phases of glaciation



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(Barker and Camerlenghi, 2002 ; Cooper andOBrien, 2004 ), and an extant proposal for drillingthe shelf and upper rise off Wilkes Land ( Escutiaet al., 1997, 2005 ) seems capable of elucidating theintervening phases.

Along with the classical model of a glaciationcaused by thermal isolation of the continent by anocean current, goes the glacial production of cold,saline bottom water (at present by sinking of a brineconcentrated by persistent sea ice formation,followed by supercooling beneath oating iceshelves), invigorating ocean thermohaline circula-tion. Under this scheme, which operates today,evidence of invigorated thermohaline circulationcould be used as an indicator of the existence of Antarctic glaciation. However, the validity of thisindicator is now in question: attribution of virtuallyall of the E/O interval isotopic shift to ice sheetgrowth ( Lear et al., 2000 ) implies that intermediateand bottom waters of the world oceans were alreadycooling in the absence of an Antarctic ice sheet, anddid not cool dramatically when an ice sheet wasformed.

4. Regional pattern of glaciation

We choose to treat separately East and WestAntarctica and the Antarctic Peninsula, partly on

account of the existing evidence, but also because of clear differences in sub-ice topography and positionbetween them. For example, the South Pole hasbeen located within East Antarctica since theEarly Cretaceous ( DiVenere et al., 1994 ); the othertwo have lain farther north. East Antarctic ice ismostly grounded above (pre-ice-sheet) sea level,whereas most West Antarctic ice, including thethickest parts, is grounded below sea level. TheAntarctic Peninsula is the farthest north, buthas a narrow axis at high elevation, which wouldhave acted as a nucleus for early ice formation.The existence at onset times of an ACC or similar,to reduce sea-surface temperature variationaround the Antarctic margin or to extend theimplications of marine-derived observations toother regions of Antarctica, is uncertain (see Barkeret al., 2007 ).

5. East Antarctica

The isostatically compensated sub-ice topographyof East Antarctica is mostly above sea level, and thecomparatively large size of the present East

Antarctic Ice Sheet (52 m sea-level rise on melting Lythe et al., 2001 ) is generally taken to imply thatit was the earliest to form. This impression isstrongly supported by numerical modelling (e.g.,Huybrechts, 1993 ; De Conto and Pollard, 2003 ).

The models suggest that an ice sheet developed rston mountain ranges (the Sr Rondane and Gam-burtsev Mountains in particular, and the Transan-tarctic Mountains if they were then elevated), thenspread relatively rapidly (i.e. non-linearly in termsof response to external variables) to form an icesheet extending everywhere to sea level. It is possibleto identify Wilkes Land (in particular the Wilkesand Aurora Subglacial basins) as the part of EastAntarctica likely to have become glaciated last.However, as already mentioned, such numericalmodels do not include ice-stream drainage, whichcould have transported potential IRD material tothe coast well in advance of the ice-sheet margin.A comparison of benthic oxygen isotopic and Mg/Ca ratio data ( Zachos et al., 2001 ; Lear et al., 2000 )also supports the view that a large ice sheet (at leastas large as todays) developed rapidly within theE/O interval.

Abundant fossiliferous ice-transported fragmentsfound onshore in the southern McMurdo Soundarea of the Ross Sea reect a cool temperate middleto late Eocene coastal climate ( Harwood and Levy,

2000 ), adjacent to the rising Transantarctic Moun-tains. No Eocene glacial facies were found, suggest-ing there was no ice at sea level, whereas Oligocenediamictites were found.

Drilling in the Ross Sea region, mostly close toEast Antarctica and the Transantarctic Mountains,has been a source of information about glacialhistory. Early Oligocene ice-rafting may be seen atDSDP Site 274 ( Hayes et al., 1975 ), but otherDSDP Leg 28 sites (including Sites 267 and 268 onthe abyssal plain and continental rise off WilkesLand) show only early Miocene or later ice rafting.Late Oligocene (27 Ma) glacial sediments weredescribed from the MSSTS-1 borehole, and earliestOligocene diamicton and IRD were seen in theCIROS-1 core ( Barrett, 1986, 1989 ). The age of thelowest part of the CIROS-1 core is uncertain (earlyOligocene to middle Eocene), because of the longranges of the microfossils described, but diamictonpersists to the base. Drilling at Cape Roberts ( CapeRoberts Science Team, 2000, 2001 ) recovered a longsection of early Oligocene (and possiblylatest Eocene) age, its base estimated as 33.5 35.0 Ma. Low-diversity fauna in its upper part,



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and occasional striated clasts within all but thelowest few tens of metres of this section, attest to itsglacial nature, despite the dominant conglomerate/debris-ow lithology that reects rapid subsidenceand proximity to a rising Transantarctic Mountains.

Strontium- and oxygen-isotopic measurements onmarine bivalves from the upper part of the sectionsuggest early Oligocene shelf bottom-water tem-peratures (depending on the ice volume and salinitymodels) of 7.6 and 5.2 1 C, with a seasonal range of between 1.2 and 4.9 1 C. Temperatures were signi-cantly warmer and had a much greater range thanthose of today. Unfortunately, an E/O transitionwas not found conclusively.

It should be noted that palynomorphs and plantfragments occur in sediment interbedded withOligocene diamictites in the CIROS-1 core, suggest-ing interglacial expansion of vegetation from refugia(Barrett, 1989 ), and palynomorphs at Cape Robertshave been described from lower Oligocene sedi-ments ( Raine and Askin, 2001 ). A very different,more diverse palynomorph fauna was found at thebase of the hole, suggesting an Eocene age and awarmer environment.

The southern Indian Ocean has been well studied,with drilling in Prydz Bay at the East Antarcticcontinental margin (ODP Legs 119, 188) and fartheroffshore on the Kerguelen Plateau (ODP Legs 119,

120, 183) opposite Prydz Bay. On the continentalshelf in Prydz Bay, diamictite occurs at the base of holes at Sites 739 and 742 (ODP Leg 119: Barronet al., 1991 ), but dating (early Oligocene andpossibly older) is uncertain. From grounding linelocations, the Oligocene ice sheet is inferred to havebeen slightly larger than todays. ODP Leg 188drilling (Site 1166: OBrien et al., 2001 ; Cooper andOBrien, 2004 ) recovered upper-middle Eocenecarbonaceous sands in Prydz Bay, with palyno-morphs and with sand-grain surface textures thatsuggested mountain glaciation to the south. Aroundthe E/O interval, these sands were overlain by sand-clay layers and, above an erosion surface, byglaciomarine diamictites containing dinoagellates,diatoms and lonestones. Leg 188 did not recoverwell-dated lower Oligocene sediments in Prydz Bay(Cooper and OBrien, 2004 ). Evidence of Antarcticglacial development is provided also by drilling onthe Kerguelen Plateau (ODP Legs 119, 120, 183).Possible IRD was seen in middle Eocene sedimentsat Sites 738 and 744 (Leg 119: Barron et al., 1991 )on the southern Plateau, and a change in claymineralogy (a decrease in smectite and increase in

kaolinite from glacial weathering on Antarctica) inthe late Eocene was attributed to glacial onset. Thiswas followed in the early Oligocene by unequi-vocal IRD at these sites and farther north at Site748 (Leg 120: Wise et al., 1992 ), by a dramatic

decrease in smectite and increase in illite concentra-tion within clays, reecting a change to physicalweathering onshore, and by coeval biogenic changes(increases in opal and decreases in calcium carbo-nate, changes to radiolarian, nannofossil andforaminiferal assemblages) suggesting cooling of surface and deep waters. The principal objectives of ODP Leg 183 ( Frey et al., 2003 ) concerned aspectsof the Mesozoic volcanic evolution of the KerguelenPlateau, but it found that cooling waters frommiddle Eocene time onward affected radiolarianand foraminifer preservation, nannofossil assem-blages, discoaster and sphenolith abundance anddiversity.

ODP Leg 113 drilled in the Weddell Sea. At Site693, on the East Antarctic margin, middle lowerOligocene sediments contained sparse gravel- andpebble-sized IRD ( Grobe et al., 1990 ). IRD was notseen in sediments of the same age on Maud Rise,but a siliceous biofacies was rst seen there in thelatest Eoceneearliest Oligocene.

Pre-glacial sediments from the Wilkes Landmargin have been recovered by dredging the eroded

anks of the Mertz-Ninnis Glacial Trough.Dredged rocks included in situ Paleogene palyno-morphs ( Escutia et al., 2005 ).

6. Antarctic Peninsula

The Antarctic Peninsula is a long, narrowdissected plateau, giving the impression of apeneplain (rising from 900 m at its northern end to17502000m in the south). The plateau is broader inthe south, and total width (between Weddell Seaand Southeast Pacic shelf edges) ranges from200 km in the north to 600 km in the south. Duringglacial maxima, an ice sheet probably extended tothe continental shelf edge. Snowfall is heavy, but thepresent Antarctic Peninsular ice sheet is a fewhundred metres thick at most, and its climaticregime has been characterised as small-volume,high-throughput, rapid-response ( Barker andCamerlenghi, 1999 ).

There is a discrepancy between onshore andoffshore information on glacial history: offshoreocean drilling during ODP Leg 178 ( Barker andCamerlenghi, 2002 ) detected a late Miocene (9 Ma)



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to present ice sheet (grounded regularly to the shelf edge), extending back speculatively to about 15 Mausing related seismic reection studies (e.g., Rebescoet al., 1997, 2002 ). Ice-sheet volume was found to beinsensitive to climate change, over the past 9 myr.

Earlier drilling (DSDP Leg 35 Hollister et al.,1976 ) was ambiguous because coring was discontin-uous, but very sparse dropstones were reportedfrom 15 to 16Ma-aged sediment at two sites.Onshore exposures of shallow glacial marine sedi-ments on the South Shetland Is (separated from thenorthern Antarctic Peninsula by the Plio-Pleisto-cene opening of Branseld Strait), described byTroedson and Smellie (2002) and Troedson andRiding (2002) , have been dated by Sr-isotopicmeasurements ( Dingle and Lavelle, 1998 ) on in-cluded bivalves and brachiopods, and 40 Ar/ 39 Ardeterminations ( Troedson and Smellie, 2002 ) forinterbedded lavas, as middle and late Oligocene,and earliest Miocene. These ages supersede olderKAr dates reported by Birkenmajer et al. (1987) ,which had led to Eocene and other glaciations thatwere difcult to understand. The sediments con-tained clasts of rocks now exposed only in theTransantarctic and Ellsworth mountains, which wasexplained in terms of a glaciation in those regions,draining into a Weddell Sea that was experiencing aclockwise circulation, as today. Intervening non-

glacial sediments were recorded, suggesting aglacial/interglacial cyclicity operating at the marginsof glaciation. The sparseness of the surviving recorddid not permit of a precise age for Oligocene glacialonset, but Barker and Camerlenghi (2002) inferredan Oligocene glaciation to sea level along theAntarctic Peninsula, assisted by the clockwisecirculation of sea ice and icebergs within theWeddell Sea, followed (perhaps) by deglaciation,and renewed glaciation at sea level in the middleMiocene. More recently, Birkenmajer et al. (2005)have reported evidence of mountain glaciation onKing George Island that they propose, on the basisof scattered KAr dates, is of middle Eocene age.Both the scatter and the vulnerability to othereffects of KAr dates make this occurrence un-certain evidence of early glaciation.

Eocene sediments (of the shallow-water LaMeseta Formation) have been examined on Sey-mour Island, on the Weddell Sea continental shelf atthe northern end of the Antarctic Peninsula, andnumerous climate-related studies undertaken. Thesedimentary section extends up into the latestEocene. Most recently, Dingle et al. (1998) de-

scribed cooling effects on clay mineralogy andsediment maturity, in the late middle and lateEocene, and Dutton et al. (2002) interpretedmeasured oxygen, carbon and strontium isotopicratios on bivalves, to show cooling of surface waters

from ca. 14 to 101

C in the middle Eocene (assumingice-free conditions), remaining at that temp-erature until ca. 34 Ma. Other studies (e.g. onpalynomorphs, Askin, 1997 ; bryozoa, Hara, 1997 ;mollusca, Stillwell and Zinsmeister, 1992 ; landfauna, Reguero et al., 2002 ) show a matchingdecrease in diversity. No late Eocene glacialsediments have been identied, but Ivany et al.(2006) have reported an E/O interval occurrence of glaciation at sea level from directly above the LaMeseta Fm., with dating based on dinoagellatesand clast lithologies, and on Sr 87 /Sr 86 ratios inbivalves. Younger sediments than these (i.e. Oligo-cene and younger) could be accessible offshore.

Eocene sediments have been sampled farther easton the South Orkney microcontinental block bydrilling, and within the Scotia Sea. ODP Site 696, onthe former, provided a shallow-water late middle tolate Eocene and possibly early Oligocene sectionbeneath a (presumed break-up) unconformity.Mohr (1990) suggested a slightly warmer environ-ment than on Seymour Island on the basis of palynomorphs, and for ferns suggested mean annual

temperatures of 9111

C for the late Eocene and57 1 C for the early Oligocene ( Mohr, 2001 ).A dredge site in the Scotia Sea yielded a middleEocene (4546 Ma) age ( Toker et al., 1991 ) and fernpalynomorphs indicated mean annual temperaturesonshore of 1620 1 C (Mohr, 2001 ). These dataare incompatible with prolonged pre-Oligoceneglaciation.

7. West Antarctica

West Antarctica comprises the elevated andrugged provinces of Marie Byrd Land and theJones Mountains along the Pacic margin and theEllsworth-Whitmore Mountains closer to EastAntarctica, and an inner, much lower province thatis partly smooth (close to the Ross Sea) but haselsewhere considerable sub-ice topographic relief,including the Byrd Subglacial Basin and BentleySubglacial Trough, which are at present up to2500m below sea level ( Lythe et al., 2001 ). The deepregions now contain the thickest ice, but there islittle doubt that West Antarctic glacial onset wouldhave involved ice-sheet nucleation on the more



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elevated regions. The West Antarctic ice sheet isgrounded largely below sea level and is thereforeconsidered by some to be unstable: melting wouldproduce a sea-level rise of about 5 m, one-tenth of the result of East Antarctic ice-sheet melting. The

model of Huybrechts (1993) suggests that onset of aWest Antarctic ice sheet (i.e. nucleation on eleva-tions) would have occurred at a lower mean annualSST (around the margin, compared with todays)than for East Antarctica.

The glacial history of Marie Byrd Land has beenreviewed recently by LeMasurier and Rocchi (2005) .The record of glaciation is partial and ambiguous,occurring mainly within hyaloclastites accompany-ing volcanic eruptions, usually at considerablepresent-day elevation (e.g. 2927 Ma 40 Ar/ 39 Ar agesfor the base of section at 2700 m on Mt. Petras Wilch and McIntosh, 2000). A Marie Byrd LandDome is inferred from progressive uplift and blockfaulting, over the past 34 myr or more, of a LateCretaceous (West Antarctic-New Zealand separa-tion) West Antarctic Erosion Surface (most prob-ably, originally near sea level), that accompaniedvolcanic activity. Included marine microfossils,theoretically capable not only of dating these eventsbut also of demonstrating that glaciation occurredat sea level, are rare: in their absence, the palaeo-elevation of the site is in question. The detection of

open-ocean forms among these microfossils is usedto infer episodes of deglaciation, but entanglementwith the Pliocene deglaciation controversy ( Webband Harwood, 1991 ; Sugden et al., 1993 ; Stroeven,1997 ; Gersonde et al., 1997 ), which concerns theorigins of microfossils in glacial sediment, rendersthese interpretations uncertain.

Glacial history may be inferred from the nature of erosion, under the assumption that it is sensitive toboth elevation and climatic change. The anomalousunroong of a 34 Ma ( 40 Ar/ 39 Ar Rocchi et al.,2006 ) gabbro is here used to imply an early, warmerphase of glaciation, and the contrast of cirqueerosion of an early Miocene volcano with minimaldissection since ca. 14 Ma is taken to mark thetransition from a warmer to a colder, less erosiveregime, perhaps in the period 1715 Ma. These maytranslate into West Antarctic glacial onset in theOligocene, and a signicant change at 17-15 Ma,but there is no unequivocal indication of the phaseof glacial onset that each represents, or of any longintervening non-glacial period. The likely contribu-tion of uplift to glaciation in Marie Byrd Land addsuncertainty.

During DSDP Leg 28, drilling at Site 270 in thecentral part of the Ross Sea recovered 1 m of pre-glacial Oligocene (26 MaK/Ar age) glauconiticsand, directly overlain by Oligocene to earlyMiocene glacial marine silty claystone with sparse

erratics distributed throughout ( Barrett, 1974a, b ).No source region was identied, but (perhapsbecause of the telling absence of Trans-AntarcticMountains clasts except for one brief interval in themiddle Miocene) a source within the lower-relief part of West Antarctica may be inferred. This is aclear indication of West Antarctic glaciation,possibly glacial onset, at or close to sea level,shortly after 26 Ma.

Piston cores are a useful supplement to drilling inthe Southern Ocean, when they sample oldersediments. Wei (1992) has rened the age of Eltaninpiston cores from the South Pacic, reported byMargolis and Kennett (1971) to contain IRD(quartz grains), as early and middle Eocene,suggesting that some ice occurred at sea levelaround Antarctica at that time. The closest regionis West Antarctica, and the presence of IRD at morenortherly locations is justied by the likelihood of different iceberg paths at a time when there were nointer-ocean pathways. A contribution from otherregions (the Antarctic Peninsula, or East Antarcticavia the Ross Sea?) is possible, but perhaps unlikely.

8. Discussion and conclusions

We have described above the direct and indirectpublished evidence of development of the Antarcticice sheet. In general, it is important to have thebenet of both kinds of evidence before drawingrm conclusions. For example, use of the combina-tion of benthic oxygen isotopic and Mg/Ca data(assuming that all such data are valid) to reveal icevolume should not be pushed too far. The isotopiceffect of an ice sheet is a combination of volume andisotopic ratio of the sequestered water, whichdepends partly on such factors as the prior historyof precipitation and the source of the accumulatedsnow. These may have changed signicantly be-tween the Oligocene and now (particularly if theoceans were less well connected then), leadingperhaps to a misplaced appreciation (from proxydata) of the nature of the substantial Oligoceneglaciation. Also, if the Oligocene ice sheet waswarmer than todays, the balance between snowinput and output (involving softer ice and perhaps



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additional processes such as ablation), would havebeen different.

We consider that glaciation included the Antarc-tic Peninsula and West Antarctica to some extent, aswell as East Antarctica. Despite the lack of direct

evidence, might it also have included a WestAntarctic and Antarctic Peninsular glaciation assubstantial as todays? There is direct evidence forlate Oligocene and subsequent West Antarcticglaciation, but to infer an earlier glaciation (i.e.E/O interval onset) at sea level in West Antarcticawithout unequivocal direct evidence is to go too far.Also, it is premature to interpret the oxygen isotopicpeak Oi-1 as an entirely Antarctic glacial volume.

More generally, the involvement of an orbitalvariation of insolation in ice-sheet volume changesis certain, in the Oligocene as in the Pleistocene,perhaps also in the Eocene, but it is difcult to drawrm conclusions about its function as a trigger forany specic phase of onset, or the nature of anyshort-lived glaciation: we have not attempted suchstudies here. The possible existence of refugia forplant species would limit the extent of Oligoceneglacial maxima.

One problem concerning some of the directevidence for glacial onset within the E/O interval,particularly onshore and in the older publishedrecord, and unavoidable at times, is the use for

dating of low-latitude microfossil assemblages thathave an age range extending into both the earlyOligocene and the late Eocene. It is clear that, in orclose to Antarctica, late Eocene and early Oligoceneenvironments were, or could have been, dramati-cally different, so that high-latitude faunal assem-blages could have been different too. At lowerlatitudes, however, such differences across the E/Ointerval could have been muted, or absent entirely.We hesitate to insist on such an explanation for thecommon disagreements, particularly in earlier work,over the age of the base of glacial sections duringthis period, but consider that it has played a part.

Direct evidence for an early phase of glacial onsethas been published, mainly (but not entirely) in theinitial occurrence of IRD (striated quartz, minorgravel, etc.), and in changes in clay mineralogy, indeep-water sediments offshore. Based on NorthernHemisphere occurrences, it is difcult to distinguishbetween material from a dry, cold but non-glacialenvironment, wind-blown onto sea ice, and materialwithin mountain glaciers that owed long distancesto sea level and beyond, unless the grain size istoo large for wind transport. Given a cooling

environment, either occurrence is likely to be thepreserved indication of the earliest (mountain)phase of glaciation.

Such indications are seen in the late and middleEocene in Prydz Bay, on the southern Kerguelen

Plateau and in the South Pacic. At the same time,waters around the Antarctic margin (Prydz Bay, theRoss Sea, Seymour Island and the Scotia Sea),though cooling, appear too warm in most cases foreven seasonal sea ice to form. Thus, we supportconclusions that an early phase, with mountainglaciers reaching the sea at intervals and in a fewplaces, developed through the middle and lateEocene. We cannot see trends (although the lateEocene was colder than the middle Eocene) orspecify locations. Although neither numerical mod-els nor proximal evidence show it, the Pacic Oceanpiston cores suggest that an early ice sheet may havedeveloped on the coastal mountains of WestAntarctica or the southern Antarctic Peninsula, aswell as within East Antarctica, possibly because of abundant snowfall. Few data points are availableand inter-ocean connections are unlikely.

A second phase, the development of a large-volume ice sheet (as large as, if not as cold as,todays), in the E/O interval, is better documented.Evidence of ice at sea level at that time includesdiamictite in the Ross Sea and Prydz Bay, indicating

grounding, and abundant IRD on the Ross Seaslope, the Kerguelen Plateau (including its northernpart), and the Weddell Sea margin of EastAntarctica. Interbedded palynomorph-bearing sedi-ments at proximal sites may be taken to indicatewarmer, presumably interglacial conditions, withrecolonisation from refugia. Clay mineralogy on theKerguelen Plateau provides clear evidence of achange in Antarctic weathering mode, and biofacieschanges occur widely. This evidence applies only toEast Antarctica: the existence of a deep-waterconnection between the Pacic and Atlantic oceansat this time cannot be guaranteed (although a deep-water IndianPacic connection seems likely, and ashallow PacicAtlantic connection cannot be ruledout), so even the more distal evidence cannotcertainly be taken to apply to other parts.

For West Antarctica and the Antarctic Peninsula,unequivocal direct evidence is sparse, but that whichexists suggests a later time of full glacial onset onthe former than for East Antarctica. Central RossSea drilling suggests an onset of glaciation at sealevel for West Antarctica shortly after 26 Ma, andonshore exposures on Seymour Island indicates



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grounded ice to sea level in the early Oligocene onthe Antarctic Peninsula. While in the former case,there is no evidence of subsequent deglaciation, thepreservation of South Shetland Is. and Seymour I.evidence suggests that glaciation there did not

persist. Also, the long time gap between mountainand sea-level glaciation in West Antarctica mayreect the comparative difculty of submarinegrounding of most of the present West Antarcticice sheet.

Direct evidence for short-lived Eocene glacia-tions, as large as or larger than todays, is sparse.Reporters of the indirect, distal proxy evidence citechanges in clay mineralogy on Maud Rise andglacial sediments in the Ross Sea in support, butsuch evidence is ambiguous and poorly dated. It ispossible also that other direct evidence cited here insupport of Eocene mountain glaciation was in factproduced by the few fuller but short-lived glacia-tions inferred from the indirect evidence. We shouldconsider what is required to test such hypothesesconclusively.

9. Future directions

It is clear that the main uncertainties concerningAntarctic glacial onset are two: its causes, and thereality of the short-lived late and middle Eocene

glaciations hypothesised by such as Tripati et al.(2005) on the basis of CCD and stable isotopestudies on samples from an Equatorial ODP site,and from sea-level variations (e.g., Miller et al.,2005 ; Pekar et al., 2005 ). Of more regionalsignicance is the age of full glacial onset in WestAntarctica.

The question of cause reduces to the relativeimportance of ocean circulation (in particular,development of the ACC) and variations in atmo-spheric pCO 2 . It is difcult to determine past pCO2,and consideration of the causes of its variation andtheir response times is in its infancy. Onset of theACC is considered in detail in another paper(Barker et al., 2007 ). In essence, if it can be shownthat ACC onset was not coeval with E/O intervalglacial development, a dominant role for oceancirculation would be difcult to sustain. A widerange of times of onset are postulated, on the basisof tectonics or marine geology, and a program of (IODP) drilling close to what is considered to be astrong candidate for the nal barrier in a circum-polar deep-water path has been proposed, andappears to be the next logical step in resolving the

relative importance of ocean circulation and atmo-spheric greenhouse gases.

Consideration of possible pre-Oligocene, short-lived Antarctic glaciations is delayed by lack of high-resolution, direct data from within or close to

the continent itself. Grounded ice is erosional,usually at a level signicantly below wave base, sopreservation of continental records of glaciation isuncommon. Continuous records capable of yieldinghigh-resolution information must be sought off-shore (in deep water of the Southern Ocean, close tothe continent for an unambiguous signal), oncontinental shelves or within interior basins deeperthan the palaeo-ice base. Consideration of accessrequires that the overburden should be small.Assuming that such early glaciations were large-volume (as the indirect measurements suggest), themost likely locations are perhaps:

1. The proximal Southern Ocean. IRD indicatesglaciation, but not glacial extent (i.e. mountainglaciers vs. an ice sheet). The key would bedemonstration of coeval short-lived glaciation atseveral sites.

2. Around the East Antarctic continental margin:suitable sediments might be found off WilkesLand ( Escutia et al., 2005 ), in Prydz Bay (ODP

Legs 119 and 188: Barron et al., 1991 ; Cooperand OBrien, 2004 ), and along the East Antarcticmargin of the Weddell Sea. Tills or proximalglacial marine sediments, IRD, would be likelyindicators. The most easily accessible La MesetaFm is exposed onshore on Seymour I., AntarcticPeninsula, and appears to be continuous to theE/O interval. Ivany et al. (2006) have shown thatthe E/O interval full glaciation includedgrounded ice at sea level here, but a short-livedglaciation might not.

3. Interior Basins. The Byrd Subglacial Basin andBentley Subglacial Trough are ruled out by thickpresent ice cover and uncertain sedimentarysection, but are the major intra-Antarctic sub-marine basins. Much of the Ross Sea basinsubsided during the Oligocene, and is lled bythick sediments: basal pre-Oligocene sedimentsare most probably inaccessible. Harwood andLevy (2000) report that the potentially informa-tive source rocks of the Eocene McMurdoerratics lie beneath the Ross Ice Shelf, in an areathat could be sampled post-2008 by ANDRILL(Harwood et al., 2005 ).



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Sampling sites in and around West Antarctica,and possibly also the Antarctic Peninsula, areperhaps less likely to provide certain evidence of early, short-lived glaciations than sites in andaround East Antarctica, although sites around West

Antarctica might provide an age for subsequent fullglacial onset. And, of course, in all cases samplingwould have to be preceded by detailed reconnais-sance.

Acknowledgement

We are grateful to the convenors of the JOI-USSAC Southern Oceans Workshop, from whichthis contribution derives. Two anonymous reviewersmade very helpful suggestions which have improved

the manuscript.

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