Diatom and Geochemical Constraints on Pliocene Sea Surface Conditions on the Wilkes Land Margin, East Antarctica

Abstract:

The mid-Pliocene is the most recent interval in Earth’s history to sustain global temperatures within the range of warming predicted for the 21^st century, providing an appealing analog with which to examine the changes we might encounter in the coming century. Diatom-based Southern Ocean sea surface and sea ice reconstructions by the USGS Pliocene Research Interpretations and Synoptic Mapping (PRISM) Group suggest an average +2° summer SST anomaly during the 3.3-3.0 Ma interval relative to modern.

Here, we present a reconstruction of Pliocene sea surface conditions from a marine sediment core collected at IODP Site U1361, on the continental rise of the Wilkes Land margin. U1361 biogenic silica concentrations document the alternation of diatom-rich and diatom-poor lithologies; we interpret 8 diatom-rich mudstones within this sequence to record interglacial conditions between 3.8 and 2.8 Ma, across the transition from obliquity control to precession control on East Antarctic ice volumes. This progression also preserves 3 packages of interglacial sediments within the 3.3-3.0 PRISM interval, providing an opportunity for direct comparison to proximal PRISM site Eltanin 50-28. Diatom assemblages in both cores are characterized by Fragilariopsis barronii and Rouxia antarctica, extinct species with an inferred ecological preference for waters south of the polar front. However F. weaveri, an extinct diatom with inferred preference for more northerly waters and moderate abundance in E50-28, has not been identified at U1361. This may indicate that the polar frontal zone migrated across E50-28 (62° 54’S) but remained north of U1361 (64° 25’S) during the mid-Pliocene. This interpretation is bolstered by the low abundance of extant polar front species (e.g., Thalassiosira oliverana, T. lentiginosa) at U1361; these diatoms dominate the E50-28 assemblage. In contrast, the U1361 assemblage includes a number of extant sea ice indicators (F. sublinearis, F. curta, Chaetoceros resting spores) that are rare or absent in E50-28. Ongoing work seeks to place the U1361 PRISM interval in the context of a longer record, to examine whether the 3.3 Ma onset of cooling in the Ross Sea, identified from ANDRILL sediments, is also recorded on the East Antarctic margin.