U.S. Geological Survey and The National Academies; USGS OF-2007-1047, Extended Abstract 037 Insights into the East Antarctic Ice Sheet, 3.5 to 19 Ma, inferred from iceberg provenance T. Williams, 1 T. van de Flierdt, 1 E. Chung, 2 M. Roy, 3 S. R. Hemming, 1 S. L. Goldstein, 1 M. Abrahamowicz, 4 and B. Tremblay 4 1 Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, USA (trevor@, tina@, sidney@, steveg@ldeo.columbia.edu) 2 University of Maryland, College Park, MD 20742, USA (elchung@umd.edu) 3 Universite de Quebec a Montreal, Montreal, QC H3C 3P8, Canada (roy.martin@uqam.ca) 4 McGill University, Montreal, QC H3A 2K6, Canada (maria@meteo.mcgill.ca, bruno.tremblay@mcgill.ca) Summary The past behavior of the Antarctic ice sheets under different climate conditions to today is a topic that bears directly on the stability of the ice sheets under future global warming. While global ice volume history is broadly known from δ 18 O in marine sediment records, uncertainties remain, and changes in the geographic extent of the Antarctic ice sheet are not well known. Here we address this deficiency by determining the provenance of Ice-Rafted Debris (IRD) and hence the provenance of the icebergs that carried it, using the isotope geochemical signature of the IRD. ODP Site 1165, 400 km offshore of Prydz Bay, Antarctica, covers the last 20 Ma and is well located to record changes in iceberg provenance over this time. The potential sources of icebergs, Prydz Bay and the coast to the east, have been found to have distinct neodymium and argon isotopic signatures (Roy et al., submitted, van de Flierdt et al., in press). A major provenance change in IRD at Site 1165 is observed between 14 and 7 Ma: before 14 Ma, IRD is locally sourced from the Prydz Bay sector, whereas after 7 Ma, roughly half the IRD comes from the Wilkes Land, Adelie Land, and George V Land. This is likely associated with ice expansion on East Antarctica after the mid-Miocene transition. Additionally, an iceberg drift model has been developed to assess the ocean and climate conditions required to produce the IRD provenance observations. The combination of geochemical provenance analyses and iceberg drift modeling shows promise to be a powerful tool for interpretation of IRD records around Antarctica. Citation: T. Williams, T. van de Flierdt, E. Chung, M Roy, S.R. Hemming, S.L. Goldstein, M. Abrahamowicz, and B. Tremblay (2007), Insights into the East Antarctic Ice Sheet, 3.5 to 19 Ma, Inferred from Iceberg Provenance. In Antarctica: A Keystone in a Changing World – Online Proceedings of the 10 th ISAES X, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1017, Extended Abstract 037, 4 p. Introduction Under some of the global warming scenarios in the 2007 IPCC report, we may face a future where ice cover on Antarctica could not be sustained, and even the relatively stable East Antarctic ice sheet could become unstable. This would not be the first time Antarctica has lost some or all of its ice cover, and to anticipate such changes in the future it is important to understand the history, volume, and geographic extent of past Antarctic glaciation, as well as the interaction of the ice sheet with atmospheric and oceanic circulation patterns. The broad history of ice on Antarctica is known from the oxygen isotope ratio (δ 18 O) in foraminifera tests and from ice-rafted debris (IRD) in marine sediments. However, there is only limited geographic understanding of which parts of Antarctica glaciated at what times. Proximal records of ice-sheet extent, while valuable, are often erased by subsequent ice advances. The δ 18 O record is affected by both global ice volume and local temperature. Thus there is uncertainty in the ice volume, for example in the early Miocene, Pekar and de Conto (2006) infer that ice reached 50-100% of present volume, while Zachos et al. (2001) classify the Antarctic ice sheet as partial or ephemeral. Hence there is a need for alternative data on past ice extent, of the kind presented here. The provenance data recorded in the isotopic signature of ice-rafted debris provides valuable information on the coastal sectors that are producing icebergs, and on the ocean currents that carry the icebergs. ODP Site 1165 The aim of this project was to find changes in the source of the IRD at ODP Site 1165, offshore Prydz Bay, over time, and interpret this in terms of the geographic extent of the ice sheet. Site 1165 was drilled during ODP Leg 188 and covers the last 20 Myr in a 999m hole (Figs. 1, 2) (Cooper and O’Brien, 2004). Icebergs from Prydz Bay (Lambert Glacier) drift westward over the site, carried by the polar current, and release their IRD as they melt. For this study, eight samples provide snapshots of ice sheet behavior from 3.5 to 19 Ma (Fig. 1). The time range covers the mid-Miocene transition Figure 1. Summary of age, lithology, IRD content of the sediments at Site 1165, with the ages and depths of the samples used in this study.