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Theses Doctoral

Antarctica's Geologic and Ice Sheet History from Isotopic Sedimentary Provenance Studies

Pierce, Elizabeth Lane

Within the constraints of uncertainty in the nature of erosion and transport in the sub-glacial environment, the study of glacialy-derived material from marine sediments located off the margin of East Antarctica provides a means for characterizing the sub-glacial geology obscured by the more than 98% ice cover of the continent. These insights into the geology of East Antarctica in turn provide characterization of sedimentary source areas, the knowledge of which can be applied to sediment provenance studies of ice rafted detritus (IRD) and thus about East Antarctic ice sheet history. Much of what has been learned of East Antarctica's Cenozoic ice sheet evolution has been achieved through the study of marine sediments, as ice sheets tend to erode their own history and much of what is preserved is, like the geology, obscured the ice sheet. Determining the provenance of ice-rafted detritus allows for spatial and temporal reconstructions of ice sheet behavior. Accordingly sedimentary provenance studies are key to documenting how Antarctica's ice sheet evolved through the Cenozoic. In this work, I have taken samples of marine sediments and used grain size and physical properties to separate different terrigenous components, and I have examined the sand fractions under a microscope. Following the sedimentological characterization, I have separated specific minerals from the sand fraction and employed isotopic measurements - 40Ar/39Ar on detrital hornblende and biotite grains, U-Pb on detrital zircons. I have also employed Nd isotope measurements on the terrigenous fine (< 63µm) fractions of these same samples. Chapter 2 is published in the journal Paleoceanography, and chapters 3 and 4 are to be submitted to Earth and Planetary Sciences and Paleoclimatology, Palaeoecology & Paleoceanography, respectively. In Chapter 2, I demonstrated that (1) four main sectors between the Ross Sea and Prydz Bay, separated by ice drainage divides, are distinguishable based upon the combination of 40Ar/39Ar ages of detrital hornblende and biotite grains and the εNd of the bulk fine fraction, (2) 40Ar/39Ar biotite ages can be used as a robust provenance tracer for this part of East Antarctica, and (3) sediments shed from the coastal areas of the Aurora and Wilkes sub-glacial basins can be clearly distinguished from one another based upon their isotopic fingerprints. This is particularly significant as the Aurora and Wilkes sub-glacial basins have elevations significantly below sea level, and thus are likely prone to being destabilized during warm climates. My work confirms and extends previous published evidence for episodes of massive ice rafting from these sectors. In Chapter 3, I addressed the relative merits of U-Pb zircon and the 40Ar/39Ar hornblende and biotite systems for sedimentary provenance studies. U-Pb zircon is a widely used detrital provenance tool. In polar and subpolar regions where chemical weathering is minor, hornblende and biotite are viable alternatives, and because they are more abundant in crystalline rocks it is possible to find significant populations in the relatively small samples that are available from marine sediment cores. My work has demonstrated that (1) detrital U-Pb zircon, 40Ar/39Ar hornblende and 40Ar/39Ar biotite ages all faithfully record the geologic history of East Antarctica, as expressed in their respective age populations, although different aspects may be accentuated in one or another (2) a number of previously unknown source regions have been identified (though not found on the continent yet) with this method (3) there is benefit to combining the three chronometers where possible as they are not completely redundant. Chapter 4 concerns the middle-Miocene climate transition (MMCT) on the Wilkes Land margin. In this study I combined 40Ar/39Ar with εNd of the terrigenous fine fraction across the MMCT in IODP Site U1356. The results from the two size fractions tell different stories and provide further support for the application of multiple tools. Specifically I found that (1) the hornblende ages in the MMCT of Site U1356 have a very dominant 1400-1550 Ma age population, which is not commonly found on the Wilkes land margin. I interpret these results, in the context of published geophysical interpretations of the sub-glacial geological boundaries, to require that the EAIS was greatly retreated in the Wilkes sub-glacial basin prior to and during the MMCT, and sat along the extension of the Mertz Shear Zone, at the western edge of the Wilkes Basin (2) While the hornblende evidence for provenance does not indicate large changes in iceberg sources, the εNd of the bulk fine fraction shows excursions to more radiogenic Nd values, 8 epsilon units higher than the local signal during times of elevated IRD concentrations at this site. The data reveal evidence for three events, which closely correspond with periods of glacial outburst floods responsible for carving part of the spectacular scabland topography (the Labyrinth) found in the Dry Valleys, and with provenance signals consistent with a significant sediment contribution from such a source and (3) the major pulse of dropstones to IODP Site U1356 occurs between ~14.0 and ~13.7 Ma, corresponding to the timing of the major pulse of IRD at ODP Site 1165 from near Prydz Bay, as well as to the timing of published records interpreted to indicate significant ice volume growth from oxygen isotope records and eustatic sea-level reconstructions. Collectively these observations allow tying together direct evidence for EAIS growth from proximal glacial-marine sediments and results from far field proxies. Overall, this work demonstrates the efficacy of isotopic and geochronological provenance tools for studying East Antarctica's geologic and ice sheet history. Furthermore the application of this approach to studying East Antarctic ice-sheet dynamics across the mid-Miocene Climate Transition (~14 Ma), one of the most important periods of East Antarctic ice sheet growth, highlights the powerful potential for future discoveries.

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More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Hemming, Sidney R.
Degree
Ph.D., Columbia University
Published Here
August 28, 2012
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