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Did the Antarctic Ice Sheets Expand During the Early Pliocene?

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Did the Antarctic Ice Sheets Expand During the Early Pliocene? Did the Antarctic ice sheets expand during the early Pliocene? P.J. Bart Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA ABSTRACT Seismic data show that glacial unconformities are located within lower Pliocene strata on the Antarctic continental shelves. The glacial unconformities are signi®cant because they provide direct evidence that the Antarctic ice sheets advanced despite the generally warmer climates and elevated sea levels that characterized most of the early Pliocene. The magnitudes of peak eustatic lowstands and 18O enrichments indicate that the ice volume on Antarctica may have exceeded today's ice volume by approximately 18%, which sug- gests that the ice-sheet grounding events on the shelves probably were associated with larger than present ice volumes on two to three occasions during the early Pliocene. Keywords: Antarctica, ice sheet, global warming, Pliocene, seismic stratigraphy. INTRODUCTION tion seismic studies have been previously con- Table 1 shows the depth and two-way trav- On the basis of marine data from the south- ducted (the eastern Ross Sea, Alonso et al., eltime ranges of the lower Pliocene strata from ern high latitudes, Kennett and Hodell (1993) 1992; the Antarctic Peninsula, Bart and An- the DSDP-ODP drill sites. At Sites 271 and argued that early Pliocene temperatures of derson, 1995; Prydz Bay, Cooper et al., 1991). 1097, I used a velocity of 2000 m/s to convert Antarctic surface waters increased by a max- These studies were selected because the re- the depths to two-way traveltime. At Site 739, imum of ;3 8C and that the volume of the sults are directly correlated to DSDP-ODP a geophysical survey indicates a velocity of Antarctic ice sheets ¯uctuated within narrow sites where lower Pliocene strata were sam- 2125 m/s between the sea¯oor and 130 m be- limits, resulting in a maximum sea-level rise pled and because the data-acquisition design low the sea¯oor (mbsf), whereas below 130 of ;25 m. Despite the evidence for ice-vol- for the seismic surveys was similar. In addi- mbsf, a velocity of 2625 m/s is indicated. ume reductions, the large magnitudes of eu- tion, these continental shelves received drain- Theoretically, a thick ice sheet could thin static lowstands (Haq et al., 1987) and 18O age from the three primary components of the and advance across the continental shelf with enrichments (Shackleton et al., 1995) suggest Antarctic cryosphere, the West Antarctic ice a minimal change in its overall ice volume. To that ice volumes may have also expanded to sheet, Antarctic Peninsula ice sheet, and the evaluate if ice-sheet advances were also as- considerably larger than present levels during East Antarctic ice sheet (Fig. 1). sociated with signi®cant ice-volume increases, the early Pliocene. If these lowstands and 18O enrichments were due to extreme expansions of the Antarctic ice sheets, direct evidence of such should exist on the Antarctic continental shelves in the form of glacial unconformities. Thus far, lower Pliocene strata have been sam- pled at Deep Sea Drilling Project±Ocean Dril- ling Program (DSDP-ODP) sites on the east- ern Ross Sea (Hayes et al., 1975), Antarctic Peninsula (Barker et al., 1999), and Prydz Bay (Barron et al., 1989) continental shelves (Fig. 1). In this study, I evaluate the seismic stra- tigraphy of lower Pliocene strata on the Ant- arctic shelves to address these questions: Did the Antarctic ice sheets advance across the shelf during the early Pliocene, and if so, were the advances indicative of large increases in the overall ice volume? Given the current global warming and the potential effect of ice- sheet ¯uctuations on sea level, understanding the behavior of the Antarctic ice sheets in the early Pliocene, a time of warmer than present climatic conditions, is a fundamentally impor- tant issue. Figure 1. Location map showing East Antarctic ice sheet (EAIS), West Antarctic ice sheet METHODS (WAIS), and Antarctic Peninsula ice sheet (APIS). Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites and seismic pro®les referred to in text are indicated, as This investigation was con®ned to three are elevation contours (white dashed lines), ice ¯ow lines (thin white solid lines) and ice- continental shelf areas for which high-resolu- drainage divides (thick white solid lines). q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; January 2001; v. 29; no. 1; p. 67±70; 3 ®gures; 1 table. 67 TABLE 1. DEPTHS AND TWO-WAY TRAVELTIMES OF LOWER PLIOCENE STRATA suring the vertical-elevation change of coastal Ross Sea Site 271* Antarctic Peninsula Site 1097² Prydz Bay Site 739§ onlap from the end of the highstand to the depth time depth time depth time beginning of the lowstand. The Haq et al. (mbsf) (msbsf) (mbsf) (msbsf) (mbsf) (msbsf) (1987) global-cycle chart indicates three eu- 204 204 150 150 100 94 static lowstands in the early Pliocene (Fig. 251 251 436 436 150 137 3A). The sea-level elevations are approxi- *Deep Sea Drilling Program, Eastern Basin Ross Sea shelf (Hayes and Frakes, 1975) mately 215 m, 160 m, and 220 m relative ²Ocean Drilling Program, Antarctic Peninsula Paci®c shelf (Barker et al. 1999) to present-day sea level. Greenlee and Moore §Deep Sea Drilling Program, Prydz Bay shelf (Barron et al., 1989) (1988) recognized four eustatic lowstands (Fig. 3B). In their most conservative estimate (i.e., 100 m paleo±water depth model), the the seismic evidence from the Antarctic ocene strata. On the Antarctic Peninsula con- lowstand sea-level elevations are 238 m, shelves was interpreted in light of two eustatic tinental shelf, Bart and Anderson (1995) 235 m, 24m,and21 m relative to present- records (Haq et al., 1987; Greenlee and found seismic evidence for at least 31 glacial day sea level. Moore, 1988) and two d18O records (Hodell unconformities. At least six glacial unconfor- and Venz, 1992; Shackleton et al., 1995). mities, numbers 23, 24, 25, 26, 27, and 28, 18 are located within the lower Pliocene strata O ENRICHMENTS DEPOSITION AND EROSION ON THE (Fig. 2B). On the Prydz Bay shelf, an area Shackleton et al. (1995) constructed a high- 18 ANTARCTIC SHELVES receiving drainage from the East Antarctic ice resolution d O record from deep-sea benthic In the current interglacial, the Antarctic ice sheet, Cooper et al. (1991) subdivided the foraminifers at ODP Site 846 (Fig. 3C), which sheets terminate near the coast or on the inner stratigraphic section into four stratigraphic is located in the eastern equatorial Paci®c shelf, and the outer continental shelves are es- units (PS.1±PS.4). At least one topset surface Ocean. The sampling interval was 10 cm, sentially sediment starved (Anderson, 1999). is located in the lower Pliocene strata. Thus which corresponds to a temporal resolution of During glacial periods, ice sheets advanced far, no detailed mapping of this topset surface ;2500 yr. At this site, there are three pro- 18 18 well into the marine realm. At these times, the has been conducted to determine whether it is nounced O enrichments (Fig. 3C). The d O 18 inner shelves became a zone of net erosion a glacial unconformity (Fig. 2C). values of these O enrichments are ;3.3½, (ten Brink et al., 1995), and sediments eroded ;3.55½, and ;3.29½, respectively. The raw from the continent and inner shelves were in- EUSTATIC LOWSTANDS d18O data from Site 846 show that each 18O corporated into basal debris zones (Alley, The magnitudes of eustatic lowstands were enrichment is supported by more than one 1989). On the shelves, the most rapid erosion estimated primarily from seismic data by mea- data point. For example, a tight cluster of ®ve and sediment transport occurred beneath ice streams, which are wide (several tens of ki- lometers) zones of fast-¯owing ice contained within slow-moving ice. At the mouths of ice streams, subglacial sediments were released as sediment gravity ¯ows that were deposited as low-angle prograding foresets. If the ice sheet advanced to the shelf edge, poorly sorted ter- rigenous sediments were supplied to upper slope depocenters. Within these depocenters, strata typically have an overall topset and foreset geometry. On seismic pro®les, topset surfaces that exhibit regional extent (several tens of kilometers) and broad glacial-trough topography are interpreted as glacial uncon- formities (Anderson, 1999). ANTARCTIC STRATIGRAPHY Interpretations of seismic pro®les from three Antarctic shelves are shown in Figure 2. The seismic-stratigraphic interpretations are from previous studies of the eastern Ross Sea (Alonso et al., 1992), Prydz Bay (Cooper et al., 1991), and the Antarctic Peninsula (Bart and Anderson, 1995). The gray-shaded areas on the pro®les are my correlations of the low- er Pliocene strata (Fig. 2). On the eastern Ross Sea outer continental shelf, an area receiving drainage from the West Antarctic ice sheet, Alonso et al. (1992) identi®ed eight glacial unconformities (labeled Figure 2. Interpreted line drawings of seismic pro®les from (A) eastern Ross Sea (after 1±8 in Fig. 2A). At least one glacial uncon- Alonso et al., 1992), (B) Antarctic Peninsula (after Bart and Anderson, 1995), and (C) Prydz formity, number 5, is located within lower Pli- Bay (after Cooper et al., 1991). Gray shading indicates lower Pliocene strata. 68 GEOLOGY, January 2001 sphere ice sheets did not develop until the late Pliocene (Berggren, 1972), the solid vertical line at 17 m in Figure 3 (A and B) corre- sponds to the expected sea-level elevation if the Greenland ice sheet did not exist and the Antarctic ice sheets existed in their present con®guration.
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