Pliocene-Pleistocene Orbital Cyclostratigraphy and Glacial Evolution of the East Antarctic Ice Sheet from Continental Rise IODP Site U1361, Wilkes Land Margin, East Antarctica

摘要

Stability of the East Antarctic Ice Sheet (EAIS), in response to the orbitally-paced cooling climate of the Late Neogene, is largely unknown. The Wilkes Land margin of East Antarctica, largely grounded below sea level, has previously been proposed to respond dynamically during the warmer climate of the Pliocene, similarly to other marine based sectors of Antarctica (i.e. West Antarctica). Sediment deposition on the Wilkes Land continental rise, recovered in Integrated Ocean Drilling Program U1361A drillcore provides a distal but continuous record of EAIS fluctuations. Changes in sedimentary depositional environments at U1361A core site, were determined through analysis of lithofacies and physical property logs: natural gamma-ray (NGR), gamma-ray attenuation bulk density (GRA), magnetic susceptibility (MS) and L* colour reflectance. NGR primarily reflected biogenic content and a synchronous relationship between NGR, GRA and MS was used to identify interglacial and glacial phases, whereby decreased NGR, GRA and MS values indicated an increase in biogenic material. L* colour reflectance was more variable through time, displayed higher frequency fluctuations and a changing relationship with the other physical property logs down core. Two depositional models, based on facies interpretations and the defined physical property relationships, were produced for the Middle Late Pleistocene (last ~550 kyr; model A) and mid-Pliocene (~4.2-3.6 Ma; model B), which represent end members. Depositional processes common to both models occurred in the intervening core, spanning the Late Pliocene-Early Pleistocene (3-1 Ma). Model A, applied to the Middle Late Pleistocene, shows that alternating diatom-rich clays to silty clays in the upper 9 m of core U1361A, reflect the large amplitude ~100 kyr paced glacial-interglacial cycles, which is confirmed by spectral analysis of the physical properties for this interval. Model B, applied to the Early Pliocene, suggest that the depositional processes recorded by facies may have been less sensitive to EAIS fluctuations, probably due to the fact that the ice margin was generally more distal to the core site during glacial-interglacial cycles of advance and retreat. Nevertheless, these more subtle changes in lithology were characterised by variations in the physical property logs, and spectral analysis of these time series implied orbital pacing was still influential on depositional processes at this time (displaying power in precession and obliquity frequencies).Spectral analysis of the physical property logs and visual correlations to the benthic δ18O stack, confirmed the 4.2-1 Ma interval was paced by ~40 kyr and implies obliquity-paced oscillations of the margin of the EAIS. Precession periodicities, significant in spectra throughout the 4.2 Myr record, are proposed to be the response of phytoplankton productivity in response to seasonal insolation controlling sea-ice extent.