Scherer, Reed P.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
Subglacial lakes--Antarctica; Ice sheets--Antarctica; Micropaleontology--Antarctica
In January of 2013 and 2015 the WISSARD (Whillans Ice Stream Subglacial Access Research Drilling) science team collected sediment cores from Subglacial Lake Whillans (SLW), and the Whillans Grounding Zone (WGZ) which are both part of the Whillans Ice Stream (WIS) in West Antarctica. These sediment cores along with sediment samples from the California Institute of Technology (Caltech) Glaciology group, which include sediments from Kamb Ice Stream (KIS) and Bindschadler Ice Stream (BIS) are compared with published micropaleontological data from the WIS Upstream B camp (UpB), the Crary Ice Rise (CIR) and the Ross Ice Shelf Project (RISP), using observations on preservation of microfossils in these deposits. Diatoms and other microfossils provide biostratigraphic and paleoenvironmental constraints on past marine deposition in West Antarctica interior basins, as well as inferences regarding ice stream erosion, particulate provenance and glacial mixing. Most subglacial samples contain a mixture of eroded diatoms that reflect initial deposition throughout the Cenozoic. Pleistocene diatoms are widespread but never abundant, reflecting erosion of marine sediments deposited in the West Antarctic marine basin during Quaternary ice sheet retreat events. Geologic drilling near Ross Island (the ANDRILL McMurdo Ice Shelf Project) provided abundant evidence for Pliocene and early Pleistocene retreat of the West Antarctic Ice Sheet, yet Pliocene diatoms are rare in sediments recovered from beneath grounded ice, which suggests erosion of Pliocene deposits in the Ross Embayment. Miocene age diatoms are dominant in subglacial and sub-ice shelf deposits, reflecting extensive Miocene deposition in the basin prior to entering a dominantly glacial phase. Additionally, Paleogene fossils, both marine and non-marine, occur widely, reflecting deeper erosion and providing insights into earlier basin history. SLW contains Upper Miocene fossils with a mix of younger and older taxa. These ages and taxa are consistent with previously published results from samples recovered ~200 km upstream from the UpB sites, suggesting a connection between the sites with little evidence of new subglacial erosion of material. Diatom abundance is on average lower than UpB in samples, suggesting sediments at SLW have experienced additional cumulative shearing and transport. WGZ cores exhibit stratigraphic variation in microfossil abundance as well as a transition in age dominance of taxa. Four lithostratigraphic units were observed in grounding zone cores, but a fifth unit is recognized based on microfossil data. Unit I is thought to be rain out from the base of the debris rich ice, which has a dominant Upper Miocene assemblage. Unit II is described as subglacial and is dominated by long ranging taxa, with a mix of younger material. Unit III is thought to be sub-ice shelf, which is consistent with diatomite microclasts (silt-sized aggregates) and a radiolarian that has a Late Pleistocene diatom assemblage. Unit IV is described as subglacial, based on the lithostratigraphy, however, microfossils are well-preserved, allowing definition of subunits, Unit IV-B and Unit IV-A. The stratigraphic variability at the grounding zone indicates changes in sediment provenance, indicating a variable glaciological regime, which suggesting a dynamic grounding line. Higher overall diatom abundance at WGZ indicates less cumulative glacial shear strain within the sediments than in SLW and UpB tills. KIS is one of the ice streams on the Siple Coast that has been shut down for the last 150 years. Most KIS sediments contain Upper Miocene fossils that are relatively unmixed and well-preserved, containing an order to two orders of magnitude higher diatom abundance than for all WIS samples. An exception is one sample from the KIS sticky spot (SS) that has low abundance and poor preservation indicating high cumulative shear strain. The unmixed Upper Miocene assemblage and preservation of the rest of these samples suggests close proximity to Miocene source rocks at this site. BIS is the northern-most active ice stream from this study in the Siple Coast area. These deposits contain no microfossils younger than Oligocene, suggesting tectonic processes, subglacial processes or change in source for this area. The age transition is also observed in other microfossil groups, which likely indicates a different source of sediments for BIS. Paleogene microfossils at this site are well preserved and relatively abundant. Biostratigraphic characterization of the Ross Embayment using age-specific diatoms help constrain basin scale productivity events that are linked to warm interglacial periods. Assessing when these events happened and diatom fragmentation patterns from different subglacial environments adds new insights into sediment-ice interactions and modern subglacial processes.
Coenen, Jason James, "Inferring West Antarctic subglacial basin history and ice stream processes using siliceous microfossils" (2016). Graduate Research Theses & Dissertations. 3563.
xii, 195 pages
Northern Illinois University
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