Retreat of Antarctica’s ice sheets under warm climates of the geological past offers insights into how ice on Antarctica will change under future climate warming. For example, during Pliocene times (~5 to 3 million years ago, Ma), global temperatures were approximately 3°C warmer than today, and sea levels oscillated and reached up to about 20m higher the present day as the result of melted ice. This much sea level rise would not only require the West Antarctic and Greenland ice sheets to melt, but also parts of the large East Antarctic ice sheet, which was previously considered to be reasonably stable. Yet there was very little direct evidence about which parts of the East Antarctic ice sheet melted back under warm conditions in the past. In this project, we examined marine sediment cores from the Southern Ocean offshore of East Antarctica to produce records of sediment provenance and interpret the history of glacial erosion, iceberg calving, and ice sheet advance and retreat in geographic sectors of East Antarctica, covering time intervals from the mid-Miocene (14 Ma) to the present.

Ice sheet history is recorded in marine sediments in the following way: When ice sheets retreat back, they calve icebergs at a faster rate than normal; Antarctic icebergs are carried westwards along the coast by ocean currents; The icebergs themselves are ephemeral, but they carry mineral grains and rock fragments scoured from Antarctic bedrock; As the icebergs melt, this iceberg-rafted debris (IRD) falls to the sea bed and is steadily buried in marine sediments to form a record of iceberg activity. In this project we developed and applied the use of isotope geochemical tracers to track the provenance of this material (rock fragments, mineral grains, and muddy sediment) to the source areas where it was scoured by ice from Antarctic bedrock.

Key science findings 

In this project, we delineated contrasting thermochronological areas (cratons and ancient orogens) of East Antarctica by analyzing over 3000 individual mineral grains in IRD deposited at about 30 sites around East Antarctica margin. We measured U-Pb geochronological ages of zircon, and ⁴⁰Ar/³⁹Ar thermochronological ages of hornblende and biotites. We also measured Nd isotopes in the fine sediment fraction. These new data extend existing studies on onshore outcrops to the hidden subglacial geology of Antarctica, and provide the foundation for provenance studies to examine ice conditions in the past.

At Integrated Ocean Drilling Program (IODP) Site U1361, offshore of Wilkes Land, fine sediment deposited during Pliocene warm intervals has a distinct Nd and Sr isotopic signature relative to the intervening colder intervals. This signature points to a provenance in the Wilkes Subglacial Basin, indicating preferential glacial erosion and ice margin retreat of several hundred km into this basin during warm climate intervals. Ice sheets lying on such subglacial basins are understood from models to be susceptible to retreat under warmer climates, and this project is one of the first to provide direct evidence that this is so. The implication is that the present ice sheets lying on these subglacial basins will become unstable under the temperature conditions expected for the end of this century.

During the mid-Miocene Climate Transition (MMCT), about 14 to 13.8 million years ago, Antarctic ice is thought to have grown from small inland ice caps to a size close to the present day ice sheet. Our data from IODP Sites U1356 and 1165 provide a direct record of repeated collapse and re-growth of ice in at least three of East Antarctica’s main drainage basins (the Wilkes and Aurora Subglacial Basins, and the Lambert Graben), at a scale not seen in the Miocene either before or after the MMCT.

We used models of iceberg drift and m...

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