Dodd, Justin P.
M.S. (Master of Science)
Department of Earth, Atmosphere and Environment
The West Antarctic Ice Sheet (WAIS) plays a significant role in Earth’s climate. WAIS is currently retreating due to changes in wind-driven oceanic currents resulting in an increasing flux of warm waters to ice margins. Brine rejection in the Ross Sea is also major source of High Salinity Shelf water (HSSW) and Antarctica Bottom Waters (AABW), which creates a stratified ocean and plays a significant role in regulating Earth’s climate. Currently, there are very limited proxies that can quantify the variable input of HSSW formation on geologic time scales. Here we provide a 18Odiatom and biogenic silica (BSi) record which supports a brine rejection mechanism and demonstrated influences from orbitally-paced climate cycles on local geochemical proxies.Coupled climate and ice sheet models show that ocean heat delivered to the Ross Sea via Circumpolar Deep Water (CDW) has likely played a key role in controlling WAIS ice sheet variability since the Miocene. International Ocean Discovery Program (IODP) Expedition 374 collected marine sediment cores on the outermost continental shelf in the Ross Sea. IODP Site U1523 from this expedition was selected for study to correlate with previously drilled inner continental shelf stratigraphic records of past ice sheet dynamics such as ANDRILL (AND-1B) and ODP 1090/849. The primary objective of this study was to test the hypothesis that 18Odiatom and biogenic silica (BSi) values reflect orbitally-paced climate cycles during the Neogene to early Quaternary. A secondary objective was to assess if oxygen isotope values (18O and 17O) derived from diatom silica could be used as a proxy for cryogenic brine production and the formation of Ross Sea Bottom Waters (RSBW) on glacial-interglacial time scales. The Late Miocene (8.78 – 10.44 Ma) 18Odiatom values (21.6‰ to 31.8‰) and BSi record is complicated by reworked sediments, diagenetic alteration of 18Odiatom, and dissolution of diatom frustules (BSi wt. %). The primary oceanographic data recoded by the 18Odiatom has likely been overprinted by interactions with the diagenetic fluid; however, the triple oxygen (17O) and 18Odiatom values appear to correlate with a low 18O, non-marine fluid, which demonstrates the isotopic influence from the RSBW on the pore waters. 18Odiatom values during Plio-Pleistocene (2.84 – 2.52 Ma) record orbitally-paced climate cycles. The driving mechanism if the Plio-Pleistocene 18Odiatom values may be seasonal sea-ice/polynya formation in the Ross Sea, which would have enhance brine formation and the generation of RSBW. BSi wt.% is controlled by temperature and upwelling of the modified circumpolar deep water (mCDW). U1523 18Odiatom values (30.1 to 37.2 ‰) are comparable to values from an inner-shelf site, AND-1B (32.6 to 37.6 ‰). 17O indicates an isotopic influence from the RSBW during the Plio-Pleistocene. Here I show influences of high-salinity RSBW on 18Odiatom values and BSi productivity during transitional periods between warm and cold periods are driven by polynya formation. This effect is superimposed on the broader climate and ice-volume effects recorded by 18Odiatom.
Mclennan, Donald James, "Miocene-Pleistocene Diatom Oxygen Isotope and Biogenic Silica Record From IODP Site U1523, Ross Sea, Antarctica" (2021). Graduate Research Theses & Dissertations. 7423.
Northern Illinois University
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