Upper miocene history of the West Antarctic ice sheet inferred from sequence stratigraphy, clay mineralogy, and paleoecology of the Andrill 1B core

Advisors: Ross D. Powell.Committee members: Eric E. Hiatt; Reed Scherer.Includes illustrations and maps.Includes bibliographical references.The Upper Miocene succession (Motif 3, ~758-1073 meters below sea floor) of the ANDRILL 1B core was recovered from below the McMurdo Ice Shelf, in the flexural moat around the volcanic Ross Island, Antarctica. Clay mineral assemblages and microfossils of the succession have the potential to be used as paleoenvironmental and provenance indicators, which when placed in a glacial sequence stratigraphic framework, may be used to help constrain past dynamics of the West Antarctic Ice Sheet (WAIS) under warm paleoclimatic conditions. Smectite and illite clay mineral assemblages dominate Motif 3 and chlorite (+kaolinite) is a minor component. High relative smectite percentages may be directly related to volcanic units of the McMurdo Volcanic Complex, located locally to the drill site. Higher relative percentages of illite, combined with lower smectite proportions, may be sourced from weathered schists, amphibolites and gneisses of the Koettlitz Group, granitoids of the Granite Harbor Intrusives, and other basement rocks along the coast to the south and west of the drill site within the Transantarctic Mountains. That clay assemblage is considered an indicator of transport by the WAIS from the south and that assemblage was then mixed with reworked, locally derived sediment (the volcaniclastic smectite clays). Clays of the mudstones showed little excess Si, which is used to infer that low numbers of diatoms were present during deposition, even in open-water settings. A paucity of diatoms in this interval had been attributed to either loss by diagenesis or a lack of productivity in brackish surface waters rich in suspended muds coming from temperate-polythermal ice. Data presented here indicate the latter option is most likely, and agree with other facies indicators. Diagenesis and authigenesis may also be expressed in the recorded clay signatures. Post-depositional alteration can occur within sedimentary successions and it is often difficult to discriminate between smectites occurring from primary detrital volcanic grains, and those grown in situ either by authigenic growth from hydrothermal fluids, or by alteration during diagenetic reactions within the original parent sedimentary rock. Understanding the sediment provenance established using clay mineralogy in the Upper Miocene may provide a better understanding of ice dynamics by helping constrain the inferred glacial sequences that are used to interpret past WAIS dynamics. This pilot study has shown the way forward, but more detailed sampling and more advanced analytical techniques should be performed in the future to fully develop the concept. These data along with more detailed microfossil analyses should allow for the development of an enhanced and perhaps more reliable sequence stratigraphic model, which could lead to better-constrained interpretations of WAIS dynamics under warmer climatic conditions. Understanding how the ice reacted to past major cooling and warming events will allow for better model predictions for the future.M.S. (Master of Science