WEAK PLIOCENE BOTTOM CURRENTS IN THE JANE BASIN, NW WEDDELL SEA BASED ON MULTIVARIATE ANALYSIS OF GRAIN SIZE SPECTRA

The Antarctic is important to study to further our understanding of global climate regulation. One of the objectives in Antarctic research is to understand how, where, and why ice sheets lose mass, a critical component of climate change. The Antarctic ice sheet consists of about 26.5 million cubic kilometers of ice, enough to raise global sea levels by an average of 60 meters (Kennicutt, 2014). Although the ice sheets were stable for the last several thousand years, the Antarctic ice sheet is now losing ice at an accelerating pace due to global climate change, attributed to increased atmospheric CO₂ levels. In the next 50 years, it is predicted that the amount of carbon dioxide in our atmosphere will reach levels that have not been achieved since before the early Pliocene (Haywood, 2004). To better understand how the Earth responded to such change during the Pliocene, sediment cores from Site 697, ODP Expedition 113 were studied. Site 697 is located in the northern Weddell Sea within the Jane Basin at 3480 meters below sea level at the distal tip of the Antarctica Peninsula.

Grain size analysis was conducted on the fine fraction (<63 microns) sediment using laser diffraction with the Malvern Mastersizer 3000. The silt sized particles in our core cannot be assumed to have all be transported as sortable silt, as ice rafting and other sediment transport mechanisms are likely active. Down-core changes in grain size volume percent were analyzed through varimax-rotated, principal component analysis (VPCA) to determine the processes that influenced the grain size distribution over time. This multivariate technique allows partitioning of the silt-sized particles into independent components that can be related to distinct process based on their spectral signatures.

Clay-size particles dominate the sediment with three major components at 0.675 um, 8.67 um, and 2.75 um, explaining 85% of the variance. The sortable-silt (8-63 um) signal was identified as component 2 and the two finer size components may represent different processes such as downslope flow and across-basin nepheloid currents. In addition to the grain size analysis, spectral measurements on the fine fraction were used to identify clay minerals in order to determine the provenance of the sediment and environmental conditions in which it formed.