EVIDENCE OF PLIOCENE WEST ANTARCTIC ICE SHEET COLLAPSES UNDER CONDITIONS SIMILAR TO FUTURE WARMING SCENARIOS

The West Antarctic Ice Sheet (WAIS) is potentially vulnerable to global warming because it is grounded on the sea floor rather than on land: however, its future behavior has poor predictability at present. By targeting records from the ‘warmer than-present’ early-Pliocene (~5–3Myr ago), the ANDRILL Program (AND-1B core from beneath the northwest Ross Ice Shelf) has demonstrated a 40kyr cyclic variation in WAIS extent linked to cycles in solar insolation. Our data provide direct evidence for orbitally-induced oscillations and periodic WAIS collapse, resulting in open waters in the Ross Sea. These collapses occurred when planetary temperatures were up to ~3^oC warmer than today and atmospheric CO₂ partial pressure was as high as 400ppmv; both values are within the range predicted for Earth’s near future.

Sedimentary facies changes are consistent with variability in response-time of the ice sheet to different Milankovitch forcing. Fast, short-lived ice sheet advances to less than maximum ice sheet extent, followed by rapid retreats contrast with longer-lived, more extensive and larger ice sheet advances. This geological evidence for Pliocene Antarctic Ice Sheet variability is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7m (above today) equivalent sea level associated with the loss of the WAIS and up to +3m in equivalent sea level from the East Antarctic Ice Sheet, in response to oceanic melting. We suggest that 40kyr orbital cycles may regulate southward export and upwelling of Circumpolar Deep Water with consequences for melt rates at ice sheet grounding lines. The low abundance of sea-ice-associated diatoms (<5%) in the Early Pliocene interglacial, high-productivity diatomite intervals, suggests that sea surface and air temperatures may have been above freezing for a significant part the austral summer and infer an additional influence of surface melting under conditions of elevated CO₂.