Vol. 5, No. 1 January 1995 INSIDE • 1995 GeoVentures, p. 4 • Environmental Education, p. 9 GSA TODAY • Southeastern Section Meeting, p. 15 A Publication of the Geological Society of America • North-Central–South-Central Section Meeting, p. 18 Stability or Instability of Antarctic Ice Sheets During Warm Climates of the Pliocene? James P. Kennett Marine Science Institute and Department of Geological Sciences, University of California Santa Barbara, CA 93106 David A. Hodell Department of Geology, University of Florida, Gainesville, FL 32611 ABSTRACT to the south from warmer, less nutrient- rich Subantarctic surface water. Up- During the Pliocene between welling of deep water in the circum- ~5 and 3 Ma, polar ice sheets were Antarctic links the mean chemical restricted to Antarctica, and climate composition of ocean deep water with was at times significantly warmer the atmosphere through gas exchange than now. Debate on whether the (Toggweiler and Sarmiento, 1985). Antarctic ice sheets and climate sys- The evolution of the Antarctic cryo- tem withstood this warmth with sphere-ocean system has profoundly relatively little change (stability influenced global climate, sea-level his- hypothesis) or whether much of the tory, Earth’s heat budget, atmospheric ice sheet disappeared (deglaciation composition and circulation, thermo- hypothesis) is ongoing. Paleoclimatic haline circulation, and the develop- data from high-latitude deep-sea sed- ment of Antarctic biota. iments strongly support the stability Given current concern about possi- hypothesis. Oxygen isotopic data ble global greenhouse warming, under- indicate that average sea-surface standing the history of the Antarctic temperatures in the Southern Ocean ocean-cryosphere system is important could not have increased by more for assessing future response of the Figure 1. Elevation of Antarctic ice sheets showing the continental ice sheet on East Antarctica than ~3 °C during the warmest Antarctic region to global warming. and the marine-based ice sheet on West Antarctica that is largely grounded below sea level. The Pliocene intervals. A small rise in As a result, paleoclimatologists have history and stability of these ice sheets differ in that the West Antarctic ice sheet is less stable and Southern Ocean temperatures may developed later (late Miocene) than the East Antarctic ice sheet, which is believed to have devel- turned their attention to times when have caused limited melting of the oped to its approximate present form by the middle Miocene (~14 Ma). (From The Antarctic Ice climate was warmer than today. The ice sheets and associated marine by U. Radok, copyright ©1985 by Scientific American, Inc. All rights reserved.) early Pliocene was one such interval. transgression, but maximum sea During that time (4.8 to 3.2 Ma), cli- level rise was likely less than 25 m mate was warmer than at any other above the present level. Recently dis- time within the past 7 m.y. (Kennett covered evidence from the Antarctic the cold circum-Antarctic current thermally decoupled from lower lati- and Vella, 1975; Elmstrom and Ken- dry valleys indicate relative stability (“stability hypothesis”—Shackleton tudes. By 20 Ma, during the early nett, 1986). Did this early Pliocene of the Antarctic climate-cryosphere and Kennett, 1975; Kennett, 1977; Miocene or shortly thereafter, a vigor- warmth lead to major deglaciation of system since middle Miocene time Clapperton and Sugden, 1990; Kennett ous circumpolar current had undoubt- the Antarctic ice sheets and significant (~14 Ma). and Hodell, 1993). This implies that edly been established (Kennett, 1977; warming of the Southern Ocean? the Antarctic cryosphere-ocean system Lawver et al., 1992). Today the Drake INTRODUCTION Stability Hypothesis is robust and that the ice sheet is diffi- Passage imposes a unique dynamic cult to remove because of powerful constraint on poleward transport of The Antarctic cryosphere is the Until recently, most workers thermal inertia of the Antarctic circum- warm water because persistent westerly largest accumulation of ice on Earth believed that the East Antarctic ice polar current and strong negative feed- winds in the circumpolar belt deflect and comprises some 30 × 106 km3 (Fig. sheet had grown to its approximate backs tending to maintain stability. warm surface waters northward. Also 1). If all Antarctic ice melted, sea level present form by the middle Miocene Once tectonic changes such as the the position of the Antarctic circum- would rise by ~70 m. The Antarctic ice (~14 Ma) and then remained relatively opening of the Tasmanian Seaway and polar current is fixed partly by seafloor sheets are divided at the Transantarctic stable under polar desert climate due Drake Passage permitted circumpolar Mountains into a small (3.3 km3), to continental thermal isolation by flow, the Antarctic continent became Ice Sheets continued on p. 10 marine-based sheet in the west and a larger (26 km3), continent-based sheet to the east (Fig. 1). The West Antarctic ice sheet is grounded below sea level Figure 2. Oblique aerial view and may thus be vulnerable to small looking south across the west- ern Olympus Range toward the changes in surface temperatures of the western Asgard Range in the Southern Ocean and in sea level (Mer- dry valleys sector of the Trans- cer, 1978). In contrast, the more stable antarctic Mountains, Antarctica. East Antarctic ice sheet is largely Note detached mesas and grounded on bedrock above sea level. buttes, remnants of the upper The Antarctic ice sheets and adja- planation surface. Denton et al. cent Southern Ocean act together to (1993) suggested that these form the Antarctic ocean-cryosphere upland landscapes resemble system, representing one of the most those on the Colorado Plateau important components of Earth’s cli- and formed under similar semi- arid desert conditions. The dry mate system, by strongly influencing valley landscapes date to the global atmospheric and ocean circu- middle-to-late Miocene and lation (Cattle, 1991). The Southern exhibit remarkable slope sta- Ocean is an integral part of the Antarc- bility, indicating a hyperarid, tic environmental system because the cold desert environment since cold, circumpolar current maintains that time. This geomorphologi- thermal isolation of the continent. cal evidence argues against The ocean is bounded to the north major deglaciation and warm by the Antarctic convergence, or Antarctic climates during the Polar Front zone that separates cold, Pliocene. Photo from Denton et al. (1993, p. 171; used with nutrient-rich Antarctic surface waters permission). Ice Sheets continued from p. 1 as late Pliocene age—3.1 to 2.5 Ma; EVIDENCE FROM DEEP-SEA than today (Shackleton and Kennett, Webb and Harwood, 1991) in sedi- SEDIMENTS 1975; Hodell and Venz, 1992; Shackle- topography and by the westerly surface mentary deposits of the Sirius Group, ton et al., 1994). Different views exist, If such major warming and degla- wind stress, which is strongly depen- found at high altitudes (~2000–2500 however, as to the magnitude of tem- ciation occurred during the Pliocene, dent upon land-mass distribution m) in the Transantarctic Mountains. perature and ice reduction represented clear evidence should exist in marine (Gordon, 1988). These critical tectonic The Sirius Group consists of lodgment by the oxygen isotopic signal. At one sediments from the Southern Ocean factors were not appreciably different tills interbedded with glaciofluvial, extreme, Raymo (1992) suggested major and in glacioeustatic changes on conti- during the Pliocene than today (Lawver glaciolacustrine, and colluvial sedi- Antarctic deglaciation (50% reduction nental margins. Results from deep-sea et al., 1992). Faunal and sedimentologi- ments containing fossil plant material in ice) during the early Pliocene, drilling in the Southern Ocean have led cal data indicate strong stability in the representing evidence for remarkable whereas Kennett (1977) argued for to major advances in the understand- position of the circumpolar current warmth, even as close as 500 km from relative stability of the ice sheets. ing of climate, oceanography, and the during the late Neogene; northward the South Pole (Webb and Harwood, Hodell and Venz (1992) provided biota of the Antarctic continent and and southward migrations of the Polar 1991; Hill and Truswell, 1993). The critical constraints on estimates of the surrounding ocean (Kennett and Bar- Front zone have been minor in relation diatoms are inferred to have lived in magnitude of Pliocene ice volume and ron, 1992). Marine sedimentary evi- to the vast breadth of the Southern marine basins within the Antarctic temperature change, using high-resolu- dence presented here supports relative Ocean (Lazarus and Caulet, 1993). craton and, together with associated tion oxygen isotopic records of benthic stability of the Antarctic cryosphere- The net effect of the relatively stable basinal sediments, were carried up the and planktonic foraminifera from climate-ocean system during the late position of the Antarctic circumpolar Transantarctic Mountains by develop- Ocean Drilling Program (ODP) Site Neogene. Geomorphological evidence current has been long-term thermal ing ice sheets after ~2.5 Ma, the age of 704 in the Subantarctic sector (47°S) from the continent also indicates that insulation of the Antarctic continent the youngest diatoms in Sirius sedi- of the southeast Atlantic (Fig. 3). Com- hyperarid, cold desert conditions have and resulting stability of the Antarctic mentary