Abstract
This thesis investigates environmental, nutrient, and circulation changes that occurred during the late Pliocene to early Pleistocene (3.94-1.67 Ma) along the Wilkes Land continental rise and how those local changes relate to major climate shifts of the time. Studying past climate conditions provides a framework for understanding the underlying sensitivity of the Antarctic environment and the response of inter-hemispheric climate networks to small changes in global temperatures and CO2 concentration. The main aims of this research was to answer two questions: 1) is there is a notable shift in environmental and geochemical conditions after the intensification of Northern Hemisphere Glaciation and 2) what was the sensitivity of primary production and nutrient consumption to global cooling during the Pliocene-Pleistocene transition? Five proxy data sets were generated from Integrated Ocean Drilling Program Site U1361 sediment: weight percent biogenic silica (wt% BSi), diatom assemblage, bulk sediment carbon isotopes (δ13C), bulk sediment nitrogen isotopes (δ15N), and diatom-bound δ15N. In addition to Site U1361, bulk and diatom-bound δ15N records were generated for IODP Site U1524 along the Ross Sea continental rise targeting the Mid-Piacenzian Warm Period (~ 3.3-3.1 Ma) as a part of a larger collaboration with other Expedition 374 scientists.
Site U1361 is well positioned for studying the impact of EAIS glaciation and deglacation while also investigating changes in the position and chemistry of Southern Ocean water masses and circulation features. The results and conclusions from Site U1361 examine the entire Pliocene-Pleistocene transition (2-3 Ma) in detail which current published records do not do. The diatom assemblage from Site U1361 is interpreted to show that sea-ice became a dominant environmental factor after the intensification of NHG, and as Southern Ocean fronts shifted north and south, the extent of sea ice over Site U1361 shifted as well. While studying the diatom assemblage, two previously undocumented diatom taxa are proposed from Site U1361: Rouxia raggatensis Duke et Riesselman sp. nov. and Fragilariopsis clava Duke sp. nov. Diatom-bound δ15N is a pristine record of surface water nutrient consumption and availability because the biogenic silica skeleton of a diatom shields the isotopic signal from secondary fractionation in the water column or sediment interface. Using the weight percent of biogenic silica (wt% BSi) in the bulk sediment as a proxy for primary productivity, diatom-bound δ15N at Site U1361 reveals how nutrient consumption depended on the ratio of nitrate utilization to nitrate supply after the Pliocene began cool around 3.6 Ma. The implication of the results is that the efficiency of the biological pump is reduced when a site is proximal to strong upwelling by diverging surface waters. Comparing diatom-bound δ15N to bulk δ15N at Site U13524 reveals that nutrient consumption was high during the Mid-Piacenzian Warm Period (~3.3-3.1 Ma), often an analogue for future warming, along the Ross Sea continental rise similar to the Wilkes Land margin.