Abstract:
Today the West Antarctic Ice Sheet (WAIS) is grounded mostly below sea level on a landward-sloping continental shelf, making it sensitive to oceanic temperature and circulation changes. However, recent reconstructions of the Cenozoic bedrock topographic evolution of West Antarctica have suggested that the WAIS may have first formed as a terrestrial ice sheet at the Eocene-Oligocene boundary (34 Ma), when there was up to 20% more land area in West Antarctica. At some point during the Oligocene to mid-Miocene (34-14.5 Ma) vast areas of West Antarctica became an over-deepened marine continental shelf, as is observed today. Atmospheric CO₂ also fluctuated between 600 and 200 parts per million (ppm) during this time period. Constraining the development of a marine-based WAIS during these climates with significant CO₂ fluctuations is critical in the context of understanding the sensitivity of ice sheet systems to environmental change.
In order to better constrain the development of the WAIS this study re-examined the Oligocene to early Miocene aged sediment cores recovered from the central Ross Sea, a principal drainage area of the WAIS, at Deep Sea Drilling Project Site 270 (77° 26.48’ S, 178° 30.19’ W). Using high-resolution visual core descriptions, as well as grainsize analysis to identify changes in ice-rafted debris and characterise the background sedimentation, six lithofacies were recognised. By incorporating new geochemical and magnetic susceptibility data, as well as existing information of the palynology and foraminifera, with the facies, six lithostratigraphic units were recognised. Together with the existing Ross Sea seismic framework the lithostratigraphic units were used to reconstruct the glacial history of the central Ross Sea during the Oligocene to early Miocene.
The late Oligocene was examined in detail, because the decreasing or invariant atmospheric CO₂ values appear to contradict the contemporaneous δ¹⁸O records which imply a climatic warming and/or ice volume loss. This study shows that marine terminating ice masses of substantial size were present in West Antarctica for much of the late Oligocene, and displayed at least one significant advance. These ice masses were likely centred on the Central High and Marie Byrd Land based on the seismic stratigraphy and ice-rafted debris provenance. Only during the latest Oligocene (~23.4 Ma) do the sediments at DSDP 270 indicate a (local) reduction in ice volume in the central, and possibly eastern, Ross Sea. Plausible causal factors of this retreat include tectonic subsidence, and/or the culmination of a climatic warming. Either way, this deglaciation may have contributed to the decrease δ¹⁸O shift during the latest Oligocene recorded in many far-field sites.
Furthermore, the Mi-1 glaciation has previously been suggested to represent the first significant marine-based advance of the WAIS, and has been linked to ocean and climate cooling. At DSDP 270, the Mi-1 appears to have terminated the aforementioned period of a (local) glacial minimum. However, the lack of ice overriding evidence in the core and the seismic data suggests that the WAIS did not extend as far north as DSDP Site 270. This indicates that the WAIS did not advance to the continental shelf edge during Mi-1.
Finally, spectral analyses of the late Oligocene aged sediments at DSDP 270 revealed the presence of all three orbital cycles, and a switch from eccentricity to obliquity dominated cyclicity. This is congruent with coeval near and far-field records, and represents the first evidence of an orbital forcing of West Antarctic ice volumes during the Oligocene.
