‘The Middle Man’ – Nd Isotope Signatures of Marine Sediments as a Provenance Tool to Reconstruct Oligocene Ice Sheet Instability in the Wilkes Subglacial Basin, East Antarctica
Abstract:High-amplitude variability in the global benthic oxygen isotope record during the Oligocene has been interpreted to indicate large fluctuations in sea level due to a waxing and waning Antarctic ice sheet. Although variability in ice-sheet volume is inferred from as early as ~40 Ma using proxies such as Mg/Ca ratios of benthic foraminifer, the location and spatial extent of Antarctic ice-sheet lobes cannot be deduced from such data. The use of neodymium (Nd) isotopes in bulk marine sediments as a provenance tracer can however provide insight into these questions, serving as a connection between the marine and terrestrial realms.
We present new bulk-sediment Nd isotopic data from a well-dated Eocene-to-Oligocene section recovered from Integrated Ocean Drilling Program Expedition 318 Site U1356, located proximal to the Antarctic continent on the East Antarctic margin. Sediment at this location is predominantly terrigenous material, and bulk Nd analysis reveals the provenance of sediment and hence the geological location of the most active ice erosion. A total of 90 samples were analysed that span the Eocene and Oligocene sections at Site U1356, with 45 of these samples targeting a ~500kyr window to study orbitally-paced changes in ice dynamics (<10kyr resolution).
Low-resolution Eocene and Oligocene Nd isotope results reveal (i) two distinct baseline Nd isotopic values separated by two epsilon units (-14.1 and -12.0), and (ii) an abrupt shift between these values at the transition from the late Eocene to the early Oligocene. While this change is likely related to a provenance change associated with continental scale ice expansion on Antarctica, short-term fluctuations during the Oligocene point to an alternation of the same two source areas at precessional (~20kyr) and long-eccentricity (400kyr) periodicities. These results indicate that systematic Nd isotope variations in detrital marine sediments from the Antarctic margin can be used as a tool to link proximal records of ice sheet dynamics with the global record of climate change from the deep sea.