Changes in Climate, Ocean and Ice-Sheet Conditions in the Ross Embayment, Antarctica, at 6 Ka

jitnai.' o!''Ginrioiogr 27 1998 t;i Imrmarional Glaciological Society

Changes in climate, ocean and ice-sheet conditions in the Ross embayment, Antarctica, at 6 ka

ERicJ. STEIG,' CHARLES P. HART,' JAMES W C. WHITE,' WENDY L. CUNNINGHAM : . MATHEW D. DAVIV ERIC S, SALTZMA^' 'Inxilutfaf Antic and Alpine Research, University of Colorado, Boulder, CO 80309, VS 4 tut School of Marine and Atmospheric Science, University oJMiam, Miami, FL 33J49, U.S.A.

ABSTR.A.CT. Evidence from the Ross embayment, Antarctica, suggests an abruot cooling and a concomitant uiereasein sea-ice cover at about 6000 BP (6 ka? Stable-isotope |*D, concentrat^ tn theTaylor Dome ice core, at. the wcsterncdge of theRoss «n££ mem, decline rapidly after 6 ka, and continue to decline through, the late Holoce.ne Methanesulfomc acid concentrations at Taylor Dome show opposite trends to SD Sedi- ment cores from the western Ross Sea show a percentage minimum for the sea-ice diatom Fngtbtnejau curia- between 9 and 6 ka, whenTayk>r Dome SD values arc highest followed by an increase through the late Holocene, Radiocarbon dates. from raised beach deposits indicate that the retreat of ice shelves in the .Ross embayment ceased at about 6.k a coincident with the environmental changes inferred :from the sediment and ice-core records The similarity in timing suggests an important role for climate in controlling the evolution of ice-shelf margins following the end of the last glaciation.

INTRODUCTION independent control on marine environmental conditions ir: coastal Antarctica, and suggests that changes in climate in The laic-Wisconsin and HolQcenegl.aciiirhistQ.ryof the Ross the Ross cmbayment had an important influence on Hit Sea embavnicm. Antarctica (Fig. I), has been the subject of evolution of ice-shelf martri us. rxienskr ivxrarrh since i he early part of this century (Scott,, IW.i: Hollin. 19ti2: Mercer, 1968; Demon and others, 1991). Glacial geologic records show chat the Ross Ice Shelf once EVIDENCE FROM THE TAYLOR DOME ICE CORE extended virnr 2000 km further north than it does today, Taylor Dome is located just inland of the Transaniarctic ami thai ;-i grounded Ross Sea ice sheet extended into the Mountains across McMurdo Sound from Ross Island, Ant- Hrv Yidir'YK reirea? of the ice-sheet grounding line began arctica. Taylor Dome is a local ice-accu.miiJatio.n area thai bv uhfmi 14 000 BP (H ka) (Stuiver and others, 1981; Denton and others. 1989: Licht and others. 1996). It seems clear from Ulizow-Holm the timing of grounding-line retreat — coincident with the Bay melting of Northern Hemisphere ice sheets — that rising L sea level played a dominant role in the initiation of deglacia- tion in the R»ss cmbaymeni (Thomas and Bcntley, 1978; Drntnn and others, 1986), Less well understood is the relationship between regional climate and continued evolution of thr in-shH f margin after the end of the last glaciation. .In particular, chanaes in ocean temperatures, which control it-r-shelf basal inching rates, could have played an impor- U.TII rule in determining the rate and timing of ice-shelf manrin retreat '"naomas and others, 1984; MacAyeal and ™..hrr.v 1986; Ron and others, 1996). In this paper, we present new data from the Taylor Dome Koss ire core, ai die western margin of the Ross embayment, IctSMf" Ross . x/ .Stab|?-i.sotopr arid met.hanesulfonic acid concentrations in Sea ihf Taylor Dome core indicate that warm conditions pre- vailed in the ra.rly Holocene followed by rapid cooling at abou! 6 ka and continued climatic deterioration through Fig. 1. Map of 'Antarctica, skoioing locatiaiK mentiomd in the the late Hotocenc. Inferred mid- to late-Holocene cooling UxL Shaded areas .an ice shttoa. Ross -Sea st&meat CTTO toe K corroborated by preliminary results from Ross Sea :se.d5- mcjii rores, which show an increase in percentages of sea- KC37) and Star (J

supplies ice to Taylor Glacier to the southeast and to -the Skelton Neve to the southwest. A 554m ice core retrieved at Taylor Dome in early 1994 yields a climate record through marine isotope- stage 5e (Grootes and others, 1994). Glacio- logical reconstructions, constrained by the glacial-geologic record at the margins ofTayior Glacier, show that the height of Taylor Dome has changed by at most 160 m -in the last I Ma,' and by far less in the last 100 ka (Marchantand others, 19941 Thus, the isotope profile at. Taylor Dome should not be significantly affected by elevation, change, Taylor Dome differs in this respect from many other long ire cores in Antarctica, including Vbstok (central East Ant- arctica - and Byrd Station {central West Antarctica^ which may have experienced significant elevation change since the Last Glacial Maximum (Jensscn. 1983; Grootes and SetuVer, 19861 We measured deuterium isotope concentrations (S"D, the DTI ratio-normalized to a value-of 5D\ss»ffi)w ~ 0%oj an" metbanesulfonic acid concentrations (GH^SOsH or MSA.) at ~C).5rn resolution through the Wisconsin—Holocenc transition at about 375m in the Taylor Dome ice core. Ana- lyses for SD were performed at the University of Colorado.. according 10 the methods discussed by Vaughn (1994). MSA concentrations were measured at the University -of Miami, according to methods described by Whang and others 1994). The time-scale for the Taylor Dome core is based on a finite-element flow model, constrained by radar profiling and surface velocity data, and accumulation .-rates from Be concentrations, as discussed by Steig (!996), Morse (1997) and Steig and others (1998). We estimate that this time-scale is accurate to about 10% during the Hoioecne, as indicated by an independent comparison .with the GISP2 ice core an (he basis of high-resolution trace-gas fCH^. a.n.d•§ O -of'-Ojt) measurements, figure 2 shows SD and MSA at'Taylor Dame during the Hoiocene., with Vbstok SD (Jouzel and others, 1993) for comparison. Figure 2 also shows sea-ice diatom percentages from western Ross Sea sediment cores, discussed in the following section. Clearly, isotopic -changes at Taylor Dome during the Hoiocene are much larger in .magnitude than at Vbstok, and air anricorrelated with MSA. The. "early Holo- renc warm period", as inferred from relatively high <5D values, ends by about 8 ka a.t Vostok, as at other sites in central East Antarctica {Ciais and others, .1992). At Taylor Age (ka) Dome, high <5D values last until about 6.ka. Between about 9 and 6 ka, (5D values are higher than .at any other time Fig. 2. Tap pa»el: siable-imtffpe conctnimtim ss "J'tyiei since the previous imerglacial.period. At 6ka, SD,drops by Dome (tipper) ma I'asto*. (latasrj. A/tMe pasd: MSA a full 10%a, and then declines more gradually, towards-the cemeenirat3tms -at Taylor Dame. SettompoauL abrndimut t>f present by another 10%c. Fragilariopsis curta in sediment com from the western The <5:D concentration in polar precipitation is widely Ross Sm: plus signs, c,mt .KRF35(a~iiC3l: cmm. cm accepted as a proxy for temperature (Dansgaard and. XBPS50J-JCC3Z others, 1973). The most straightforward interpretation, of-; the SD profile at Taylor Dome core is that it reflects rapid '

cooling at 6 ka fallowed by continued climatic deterioration Dome core, this .indicates a cooling of about 2l'C between through the late Hoiocene. This cooling was evidently -re- 6.fca and the present,-in good agreement with results fran stricted to coastal regions, since it does not appear in the borehole :paJ.eotherm0inetry (Waddiagtoa.and Cicm; S5a Vbstok record i Ciais and others, 1992). Although recent The importance oCseasjnal changes in moisture sonrm work in central Greenland shows that the slope of'the iso- an.d in the timing of precipkatioa in determining nwsn tope-temperature relationship cannot be -assumed to be annual isotope values has been emphasized recently in sk constant at all temporal scales, it: may -be reasonable to literature (Charles and-others. 1994; Steig and others. !9W: assume that a linear relationship appEes For the Hoiocene Br^eett ;Smd others, 1997). Chaqges in <5D 'may not be strictly (Caffey and others, 1995). For central East Antarctica, related to temperature. At Taylor Dome, only 150km from Jcmzcl and others (1996) use a dSD/dT gradient of seasonally open water, <5D may be particularly scasitivc to y%o"C '. Applying the same relationship to the Taylor the seasonal influence of sea-ice cover. According to model 306 •^« and other,: Changes in climate, ocean andice^a cendrtimi w /&.« mbwmcn, ai 6ke estimates, a ! %odD largely reflects the increased distance to the Sea shelf sediments (Han and others, 1996k all ages mua be moisture source during the period of maximum precipka- considered, maximum ages. There are, bowevw, no age re- lion ;mostly in winter:. Increased sea ice, either locally in versals during the Holocene portions of the cores, and cal- ihr R'K< embaymem or Antarctica-wide, may additionally culated sedimentation rates are essentially constant, giving delay summer warming {Hanna, 1996). resulting in a us confidence in the chronologies. A correction factor of tit-crease in (he relative proportion of summer (high 5.D) 1200 years is applied to account for Antarctic marine MC snowfaJi in the annual average. reservoir effects (Gordon and Harkness. 1992). \'Ve converted The MSA profile in the Taylor Dome ice core (Fig, 21 the reservoir-corrected dates to calendar age using the casi- supports the inference that the dD decrease since 6ka in bratioH of Stutver and Reimer (1993); for corrected ages pan reflects an increase in sea-iee cover. MSA is an oxida- greater than 9000 radiocarbon years BP we use the U-Th tion product of dime.thylsulfide (DMS). which is produced calibration ofBardand others '1,9931 by certain marine algae, especially in polar waters (sec The bottom panel in Figure 2 shows FraglJariofKu atria SaltzmaiL 1995, for a review). A positive correlation a:bundance (percentage by number of diatoms counted; vs between sea-ice cover and MSA deposition at the ice-sheet age for cares 31 and 37. Between 9 and 6 ka, F. atria percen- surface is postulated to result from enhanced productivity of tages are low, generally below 20%. After 6 ka. thin- DMS-preducJng algae during high-sea-ice years (see increase steadily to 40 65%. Surveys of modern Ross Sea l,r.grand. 1995, for art-view). In particular, Phaeocystts is an diatom assemblages show that F. curia is common in bath ahundan! component of pack-ke algal assemblages (Garri- fast ice and pack ice and may be considered an indicator of siin and fiihcrs, 1987) and is a major producer of DMS seasonal sea-ice cover (e.g. Truesdale and Kellogg, 1.975); Gibson and others. 1989). Although the environmental Lev-enter and Dtinbar. 1996). Thus, the F. curia profiles in ianors determining the distribution and abundance of both of these western Ross Sea cores suggest increased sea- PktttcfStu are not fully understood (Leventer and Dtinbar, ice cover since about 6ka, in excellent agreement with our 1996, Pharnnyhs blooms are probably favored by stabiliza- inference, from the Taylor Dome ice-con- data. Increased tion of the water column resulting from sea-ice melt-out sea-ice cover may have been lirciitt-d to coastal areas: /*' curia Smith and Nelson, 1985; DiTullio and Smith, 1995). percentages in core. NBP95Q1-KC39, From the central Ross A positive correlation between sea-ice cover and MSA Sea, do not change appreciably throughout die entire Holo- rnnrrnt.rac.ion has been observed .for at least two Antarctic cene (Cunningham. 1997). ice rores Peel and Mul vancy, 1992; Welch and others. 1993). \Vckh and others :1993) report that the correlation between iea icr and MSA is not statistically significant at Taylor MAGROFOSSIL EVIDENCE FROM RAISED BEACH Dome: however, the period of comparison is less than DEPOSITS 20 year; and the changes in MSA concentration are small. For the Holocenc, MSA and SD atTaylor Dome are strongly Assemblages of molluscs an isastaticaUy -uplifted beach de- antkorrdai ed: MSA values are lowest between 9 and 6 ka, posits are known, from a number of locations in coastal Easi where AD values HIT highest, while MSA increases rapidly Antarctica, including the .Ross embaymeut (see Bcrktnan, ai 6 ka and continues to rise throughout the Holocene, with 1992, for a -summary-}. These .assemblages are found as both a total change of more than a factor of five. Taken together, in situ placements and wave-reworked shell accumulations. the MSA and t>D data in the Taylor Dome core both .suggest A compilation of radiocarbon-dates from these assemblages, r.ooier temperatures and increased sea ice since 6 ka in the and From ice-shelf uplifted macrofossils in saudicrn Rms rmhnvmrnt. McMurdo Sound (Kellogg and others., 1990s, suggests thai the environmental -changes recorded in the Taylor Dorm- ice core and Ross Sea sediment cores may coincide with thr M1CROFOSSIL EVIDENCE FROM ROSS SEA SEDI- last phase of ice-shelf retreat in t heRoss -ernbayment. The chronology of degiaciatkin in Ae Ross cmhaynseni MENT CORES is based largely on radiocarbon dates from terminal and independent evidence for colder conditions and/or marine glacial deposits (e.g. Stuiver and others, 1981s. Uiufi such .data, D.enton and others (1989) and ticht and orhrre increased sea-ice ewer si nee 6.ka comes from the microfossil fl.996} showed that grounded ice had retreated from record in the Ross Sea. Burckle (1972) and Kellogg and. McMurdo Sound by 7 ka: this date gives-.a .minimum esti- Tnicsdale (1S79) used diatom assemblages from sediment mate for ice-sheet grounding-line retreat. A chronology re- cores 10 infer both cooling and increased sea-ice extent stricted .to fossil samples collected from ice shelve? and sround Antarctica during the late Holocene. We reporrhere raised beach deposits oaa be used to-datethe establishment results from cores obtained from cruise 9501 of the .R/V of full marine conditions in a particular locale. This reflects . \aiiiemtl B. Palmer in the Ross Sea which support, this earlier more directly the existence of ice shelves or sea ice, as op- research. Because this work is still in progress, we report posed to:-an ice-sheet grounding line .(Demon and others. data only for kasten cores NBP95Q1-KC31, NBP9501-KC3? 1991). Because withdrawal of ice shelves is not a prensquiate and NBP9501-KG39, obtained respectively from basins at for colonization of an area by benthk invertebrates >Haa) wiser depths of 879, 924 and 557 m. Alsa, we report data and Mclles, 19M1. such a chronology should yield a maxi- only for the dominant diatom species in these cores, Fmgda?- i age for ice-shelf rctreat. iaftui airta. bistafframs of ladi'Qcarbon dates com- Chronologies were obtained by interpolation of ten. piled,for (1) caastaJ.vatic, East Antarcticaa exdudimcvclucttng; ibe radiocarbon dates fone every -1000 years) on bulk organic '~sheei

DISCUSSION

The geochemical and isotopk data a! TayJor -Dome-and-Ac microfossil evidence from Ross Sea serlimem core* show thai 6 ka is an important temporal boundary for climate conditions in the Ross -embay/mem. The bulkol fhe evidence points .to an extended length for the "early Hokiccne cit- matic- optimuttM", which is generally taken 10 have endrd a; about 8 ka (Ciais and others, 1992), and to lower tempera- CURS and increased sea-ice cover through the bit Bok>c«i!. Demon and others (1986) have argued thai sea kvei Fig 5. Histograms efpublished mdmtirbm dales played a dominant role in the initiation of ttefteoation in l.o calendar &ge )from raised beach deposits/Tom East Antarc- the Ross enibayiaeui following the end nf the lasi.glarktiffli. tica, excluding I'M RKK embajmimi, and from Terra .Nova Bay, They further suggest that retreat of the Ross Ice Shelf mas- and from mised beaches and ice-shelf sediments in southern gin led to increased southward advecijon of warm W-(»K McMurde Saitnd. References are given in the text. water, which contributed 10 regionally increased prccipsw- tion rates and higher temperatures, accounting For Halc.- cenc advances of alpine glaciers ia the DryVallcy's. Fsrtbf baymeni (Pickard, 1.985; Adamson and Golhoun, 1992: early Holocenc, data from the Taylor Donw icr core tmd w Hayashi and \hsbida,, 1994: Igarashi and others, 1995a. b); support this concept: temperatures mi Tkykir Dome, asinfcr- f2) Terra Nova Bay, northwestern Ross embayrnent (Baroni red from SB, are highest at about. 9 and J! fet, wdl after tbt and others, 1991); a,nd (35 southwestern Ross crabayroent/ initiation of ice-sheet retreat. Also, Strig -J997? shtmifd that M.cM.urdo Sound (Scuiver and odiers, 1981; Kellogg and the increase in accumulation raw bctvMmi the last giac»- others, 19901 Sampling- locations are shown on the map in tion and the present was greater ai Taylor Dome than .« Figure I. Radiocarbon studies on modern Antarctic inverte- Vastok and other more inland sites. brates collected prior to 1945 (and therefore uncoritarm- Our compilation of radiocarbon dates suggests thai tib nated by bomb- C) suggest that species-specific .reservoir at least roughly marks a maximum for Holocenc ice-sW corrections might be appropriate (Berkman and Forman, retreat in.the Ross embaymcnt.This agrees rather wdS.with 1996). For this 'review we use a reservoir correction of the evidence given above for warmer ocean and avnw- 1400 years for dates from the scallop Adamusfium colbicki spheric conditions until 6 ka, suggesting as important roif and a correction of 1200 years for all other analyses on car- for climate in controiJing- the timing of ke-shdf retrcau bonate shells. The corrected data have been calibrated Certainly, the recent collapse of the Larsen 1« Shelf on OK according to Stuiverand Reimer (1993), as'described above Antarctic Peninsula illustrates the importance of ocean sni- for the Ross Sea sediment cores. Face temperatures in controlling ice-shelf dynamics :.Ra» The upper histogram in Figure 3 shows that the-esta;b- and others, 1996). Thus, for the mid-Holocene. warre 308 Sieigandolktrs: Changes in climate, ocean and ice-sheet conditiarts in Ross anbavment at 6ka ocean-surfarr temperatures may have contributed to coo- tion of the radiocarbon content in Antarctic marine life: implication* ilinied evolution of ice-shelf margins, while the abrupt cool- for reservoir corrcctiom in radiocarbon dating Out! So Her in-H iae as about 6 ka stabilized the margin a.nd prevented 697-708. further retreat. Continued cooler air and sea-surface tem- Grootcs, P. M. -and M. Sutivcr. 1986. 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