Antarctic

The Antarctic Ice Sheets are an important component of the Earth system. They interact with ocean, atmosphere and land affecting, among others, global sea level, global ocean circulation and mean isotopic composition of sea-water. The history and behaviour of Antarctic ice sheets over millions of years is increasingly of interest as rising CO2 levels even now exceed any in the ice core record of the last 0.4 Ma, and yet these ancient ice sheets came about through changes and events whose causes are not well understood. The southern continent and its surrounding ocean basins have been the target of several science drilling efforts, all focused on acquiring sediment cores to decipher various stages and aspects of Antarctica´s ice cover and its effects on ocean circulation and paleoclimates. The ocean basin records have clearly documented the long-term cooling of climates over the past 50 m.y. and the large variability in the last 3-5 m.y. They also show events that either abrupt or brief (i.e., the Paleocene warming event with a duration of less than 1 m.y.), or are marked by a distinct shift in the rate at which long-term changes occur (i.e., middle-Miocene increased cooling trend). The explanation for these events include changes in atmospheric gas concentrations (e.g., carbon dioxide and methane), opening of gateways with enhanced ocean circulation, peaks in orbital forcing resulting from Milankovitch cyclicities, interactions with northern hemisphere glaciations and others.

Figure 1: Antarctic average sea ice cover maximum and minimum (August and February), to show better, longer access and in-ice swell reduction resulting from use of ice-capable drill rig. From a USGS CD-ROM DDS-27 by Schweitzer (1995).

Scientific drilling on the Antarctic continental shelf and upper slope have had significant problems with recovery (<20% in diamicton) using current ODP techniques. Consequently, the linkages between Antarctic continental shelf and abyssal ocean basin are not well established and the basic problem of ice sheet history remains unsolved. Proxy measurements (particularly oxygen isotopes) provide general details, but initiation, growth and extend of the ice sheets are still debated. These issues can be addressed only by drilling the Antarctic margin sediments. Numerical models of ice sheet behaviour provide a way of combining data from different regions of the margin, and help to understand the significance of events in a particular region for the history of the entire ice sheet. Initial nucleation would probably have taken place around Gamburtsev Mountains in the central part of the continent. Further expansion created an East Antarctic ice sheet reaching firstly Prydz Bay, then southernmost Ross and Weddell Sea margins, and then covered all of East Antarctica, including Wilkes Land. As the East Antarctic ice sheet culminated, the West Antarctic ice sheet developed from highland nuclei in Marie Byrd Land and the Antarctic Peninsula, and drained into the eastern Ross and Weddell Seas. This part of the margin is likely to be most sensitive to changes when the ice sheet arrives at the margin. To receive the complete history of ice sheet development the temporal and spatial variability of the margin environments of several margin sectors is needed.

Figure 2 : Antarctic Ice Sheet surface contours and flow lines, DSDP/ODP sites and proposed regions of margin drilling transects - Wilkes Land (WL) and Eastern Ross Sea (ERS). Adapted from Barker et al (1998)

Additionally, significant gaps exist in circum-Antarctic paleoenvironmental history for Late-Jurassic to Early Cretaceous, late Cretaceous, and the Cretaceous-Tertiary boundary. Only few cores are available for these intervals, and are inadequate to estimate, for example the extent and impact of Early Cretaceous anoxic events recorded by the "black shales" recovered in the Weddel Sea; the pan-Antarctic Late Cretaceous paleoclimate; and the patterns of faunal changes and mass extinctions, if any, across the Cretaceous-Tertiary boundary.
Significant benthic faunal restructuring occurred in association with the oceanic Paleocene-Eocene thermal event. Furthermore its influence on water mass circulation, continental climate and ice-sheet formation is unknown.