Quaternary Antarctic Ice Sheet Dynamics

Abstract

The Antarctic Ice Sheet (AIS) plays a major role in the evolution of Quaternary glacial interglacial cycles and in the global climate system in general. By a variety of ice ocean and ice atmosphere feedback processes, changes in the dynamics of the southern ice giant are felt throughout the globe. Ice cores drilled down to the bedrock of the East Antarctic Ice Sheet provide a glimpse into the climate history of the past one million years via water stable isotopes and trapped gasses conserved in the slow flowing ice (climate proxies). Dramatic changes in ice volume and extent characterize the evolution of the AIS during the last 130.000 years, affecting both Southern Hemisphere and global climate. A central protagonist in this history of the AIS is the West Antarctic Ice Sheet (WAIS), due to its unique setting extending into several ocean basins and thus being prone to destabilization triggered by warming of the Southern Ocean. The objective of this thesis is to shed light on the glacial interglacial dynamics of the AIS by means of 3D ice sheet modeling. The dynamic evolution of the WAIS during the Last Interglacial (LIG) and in the future is investigated and potential climate thresholds for an marine ice sheet collapse identified. Special attention is given to the role of basal melting underneath the ice shelves in the growth and decay of the WAIS. An ocean warming range of 2o 3 C is found to be sufficient to trigger an irreversible retreat of the marine ice sheet culminating in its complete collapse and a global Antarctic sea level contribution of up to 5m during the last interglacial as well as within the next millennia. It is found that the collapse depends on a complex interplay of precipitation patterns, bedrock topography, ocean bathymetry and ultimately Southern Ocean warming. Intrinsic timescales defined by the topographic features of the WAIS are found leading to a characteristic two phase collapse of the ice sheet. This thesis, for the first time, provides an estimate of LIG climate conditions required for a WAIS collapse within the range of proxy data for LIG Southern Ocean temperatures on the basis of 3D ice sheet modeling. Peak LIG sea levels of 7 o 9m are corroborated and the Antarctic contribution to the latter identified. Future Southern Hemisphere warming as projected by the IPCC would set the WAIS on a path of long term retreat culminating in several meter sea level rise during the next millennia. Rapid warming however, triggering a fast collapse of the Antarctic ice shelves within centuries, would set the stage for a multi centennial collapse of the WAIS with grave consequences for low lying coastal settlements. The full last glacial cycle is simulated with a transient climate forcing incorporating proxy data as well as results from a global circulation model. The processes behind the timing and extent of the last glaciation in Antarctica are investigated in sensitivity studies, illuminating the role of ice shelf ocean dynamics en route to the Last Glacial Maximum. Flat grounded ice sheets are found to have covered the Weddell and Ross Sea at least partially, while grounding lines advanced considerably in the Bellinghausen and Amundsen Sea as well as around the Antarctic Peninsula. Strong sensitivities of this ice sheet advance with respect to small changes in basal shelf melting are found, as well as teleconnections between dynamics in geographically separated ocean basins. Further, the ultimate retreat leading to the Holocene AIS, is simulated focussing specifically on marine ice sheet dynamics as well. A passive tracer advection scheme is utilized to simulate ice cores which are then compared to the deep Antarctic ice cores (e.g. Dome C, EDML, Vostok), providing valuable insights and constraints into ice evolution and the hydrological cycle during the last interglacial. Finally, ice sheet modeling excursions into the Pliocene and Miocene provide a broader context of the AIS behavior in the deeper past. This thesis extends the current knowledge of Antarctic Ice Sheet dynamics during the last glacial cycle and beyond as well as shedding light on potential future contributions to global sea level.