Doctor of Philosophy (Ph.D.)
Degree Granting Department
Amelia E. Shevenell, Ph.D.
Brad E. Rosenheim, Ph.D.
Kristen N. Buck, Ph.D.
John M. Jaeger, Ph.D.
Michaeal P. Meredith, Ph.D.
Antarctica, paleoceanography, Quaternary, sediments
Antarctica and the Southern Ocean play a critical role in Earth’s climate system. Antarctica’s ice sheets contain enough ice to raise global sea level by ~58 m, and the Southern Ocean distributes climate signals and nutrients to the major ocean basins and the deep ocean. Antarctica’s largest ice sheet, the East Antarctic Ice Sheet (EAIS), was considered stable compared to those in West Antarctica and the Antarctic Peninsula because it was thought to be grounded above sea level. However, subglacial topography now reveals vast submarine basins and measurements of ice velocity in the Pacific sector indicate marine-terminating outlet glacier thinning and retreat over the last four decades associated with warm ocean water presence at depth in some locations over East Antarctica’s continental shelves. Modern observations provide the impetus for investigating past EAIS response to climate change, particularly to ocean thermal forcing. To understand past EAIS response to ocean thermal forcing, marine geologic investigations of sediments recovered from Antarctica’s continental shelves and the Southern Ocean are required, particularly from intervals of warmer-than-modern conditions.
This dissertation explores the Quaternary (last 2.6 Ma) behavior of the EAIS by reconstructing outlet glacier behavior using well-dated millennial- to orbital-resolution sedimentary sequences collected from East Antarctica’s continental shelves and the Southern Ocean. Each chapter targets a specific time period, focusing on past warm periods and climate transitions. To establish the timing of glacial advance and retreat, and to reconstruct depositional environment and upper ocean temperature, I utilize the Ramped PyrOx (RPO) technique and radiocarbon (14C) analyses, sedimentary beryllium-10 concentration, and the TetraEther IndeX of 86 carbons (TEX86) paleothermometer. Results yield new insights into the Quaternary evolution of East Antarctica’s marine-terminating glaciers and the role of ocean heat in that evolution.
Chapter two investigates the response of an EAIS outlet glacier system, the Lambert Glacier-Amery Ice Shelf (LG AIS), to ocean perturbations (e.g., sea level rise and ocean heat) since the last deglacial-Holocene (15–0 ka). To constrain the timing of deglaciation of Svenner Channel, a glacially carved trough in eastern Prydz Bay, through which warm modified Circumpolar Deep Water currently flows towards the LG-AIS grounding line, I generated bulk acid-insoluble organic matter 14C ages via RPO from a 17 m long sediment core and integrated the new ages with existing carbonate 14C ages. My 14C-based chronology indicates regional deglaciation occurs at 15±0.5 ka, coincident, within chronological constrains, with the Meltwater Pulse 1A sea level rise event. Upper ocean temperatures reconstructed using the archaeal lipid-based paleothermometer TEX86 indicate relatively warm ocean waters exist in Svenner Channel in the early to middle Holocene (11–7 ka) and cool significantly at 7 ka. This record indicates that current temperatures are cooler than those in the early Holocene and suggests that the eastern Prydz Bay marine environment may evolve with continued warming.
Chapter three examines the response of the LG-AIS system to climate and ocean change over the last ~40 ka. Three sedimentary sequences recovered from western Prydz Channel contain a sequence of alternating diatom-rich muds, silts, and sands/diamicts hypothesized to reflect advance and retreat of the LG-AIS system. Using bulk sediment RPO 14C dating, three episodes of LG-AIS retreat are documented over the last ~40 ka that coincide with millennial-scale warm events observed in Antarctic ice cores. This is the first direct record of outlet glacier variability from Antarctica’s continental shelves on millennial timescales during the last glaciation. Sedimentary beryllium-10 confirms that diatom-rich muds preserved in Prydz Channel are deposited in open marine conditions. Results are the first to directly link millennial-scale records of ice rafted debris from Southern Ocean sediments with outlet glacier behavior inferred from ice-proximal sediment records.
Chapter four seeks to understand EAIS response to ocean forcing on orbital timescales during the early Pleistocene (1.5–1.0 Ma). This period partially encompasses the Mid-Pleistocene Transition (1.25–0.7 Ma) wherein 41-kyr pacing of glacial cycles in the late Pliocene/early Pleistocene transitions to 100-kyr pacing in the late Pleistocene. To assess relationships between upper ocean temperature and ice sheet mass balance, I generated TEX86-based temperature records from high-resolution sedimentary sequences recovered from the Scotia Sea during the International Ocean Discovery Program Expedition 382 and compared these records with sedimentary physical properties proxies for terrigenous and biogenic input. Results indicate that relatively cool upper ocean temperatures coincide with intervals of greater diatomaceous input. I observe relatively warm upper ocean temperatures between 1.55 and 1.28 Ma, followed by a cool period centered on 1.25 Ma, then a return to relatively warm temperatures between 1.22 and 0.99 Ma. The Scotia Sea paleotemperature record demonstrates an increasing temperature trend between 1.55 and 1.25 Ma, but no discernable trend after 1.25 Ma. I observe an average 42-kyr pacing of Scotia Sea upper ocean temperatures proximal to Antarctica between 1.55 and 1.25 Ma, implying that obliquity plays a major role in modulating Southern Ocean temperatures in the early Pleistocene. The obliquity-paced temperature fluctuations are not apparent after 1.25 Ma, suggesting that the transition to cooler temperatures represents some transition in Southern Ocean response to obliquity.
The research presented herein demonstrates that ice-proximal studies are critically required for identifying the source regions of ice mass loss, shed light on magnitudes and rates of glacier retreat, and assess EAIS vulnerability to climate forcing. This dissertation utilizes bulk sedimentary geochemistry in Southern Ocean and Antarctic continental shelf sediments to constrain changes in upper ocean temperature and depositional environment, providing a new perspective to existing paleoenvironmental studies. The ability of ice-proximal paleoceanographic reconstructions to capture EAIS dynamics will be crucial in forecasting future ice mass balance. Because ice-proximal paleoceanographic records provide direct evidence for the timing of glacier retreat, changes in depositional environment, and upper ocean temperature evolution, they can be used to constrain the timing and response of EAIS marine-based glaciers to a range of climate scenarios that models aim to predict, thereby constraining future sea level rise scenarios.
Scholar Commons Citation
Guitard, Michelle E., "Ocean Forcing of Quaternary East Antarctic Ice Sheet Evolution: An Ice-Proximal Sedimentary Perspective" (2015). USF Tampa Graduate Theses and Dissertations.