Geophysical and Geochemical Examination of the Ice-Ocean Boundary to Determine Past and Present Glacial Conditions

• Main Author: Neuhaus, Sarah
• Other Authors: Tulaczyk, Slawek
• Format: Other/Unknown Material
• Language: English
• Published: eScholarship, University of California 2021
• Subjects: Geophysics; Antarctic; The Antarctic; West Antarctica; West Antarctic Ice Sheet; Greenland; Antarc*; Antarctica; glacier; glaciers; Ice Sheet; Ice Shelf; Iceberg*; Alaska
• Online Access: https://escholarship.org/uc/item/6wm7g4fd

Ice sheets and glaciers in contact with the ocean lose ice more rapidly than land-terminating ice masses due to melting at the ice-ocean interface and iceberg calving. Hence, ice sheets and glaciers in contact with the ocean have the biggest potential to raise sea levels in the near future. With roughly a quarter of the global population living near the coast, it is therefore critical to understand how these glaciers will respond to a warming climate. The way that we do this is by examining how they interact with the ocean presently, and how they have interacted with the ocean in the past. My dissertation consists of three studies in which I examine three types of interactions between glacier ice and the ocean, covering time periods from two million years ago to the present. First, I examine the spatial and temporal distribution of icebergs inside a present-day temperate fjord in Alaska. By comparing iceberg distributions to salinity and temperature profiles taken in the fjord, I found that the icebergs had a limited effect on the thermal budget of the fjord. Currently in Greenland, icebergs are much larger and have much longer residence times, and therefore are able to alter the circulation patterns and cool the incoming water before it reaches the terminus. But my research indicates that in a warmer climate, icebergs inside Greenland fjords may stop being able to provide this buffer, resulting in an increase in melt at the glacier terminus. Secondly, I examined the position of the grounding line (the location where grounded ice transitions to a floating ice shelf) in West Antarctica following the Last Glacial Maximum. Knowing the location of the grounding line is crucial to understanding the stability of the West Antarctic Ice Sheet (WAIS) based on the theory of marine ice sheet instability. By modelling radiocarbon concentrations in the subglacial sediments, temperature profiles through ice, and chemical concentrations of sediment porewater below the ice sheet, I found that grounding line retreat coincided with climatic warm periods, and that re-advance coincided with cooling periods. This is concerning for the stability of WAIS at present because the amount of climate warming that coincided with retreat of over 1000 km was less than 2 ºC, which is the amount of warming that is predicted by the end of the 21st Century. Finally, I created a new mechanism to explain the presence of cryogenic brines in sediment pore spaces at the margin of the Antarctic Ice Sheet. Previous ideas of cryogenic brine formation require seawater to be cut off from, which is implausible in Antarctica. Thus, I proposed that hypersaline brines form in the pore spaces of sediments which have experienced repeated cycles of ice sheet advance and retreat. To test this new mechanism, I ran an advection-diffusion model of porewater chemistry and compared the results to the two Antarctic boreholes. This process not only allowed me to verify that a subglacial mechanism for brine formation is reasonable, but it allowed me to learn about the history of grounding line activity in this area.