Abstract
Ice-ocean-atmosphere interactions play an important role in governing the stability of the Antarctic ice sheets at their marine margins. In this thesis, I describe a body of work that places modern oceanographic changes in the Ross Sea, Antarctica’s largest ice drainage basin, in the context of the current Holocene interglacial period (11,700 years ago to the present). The results presented here contribute new insights into how the Ross Sea responds to meltwater discharge associated with ice sheet retreat and centennial-scale trends in external forcing and dynamic modes of climate variability.
The main aims of this research are to (i) reconstruct Holocene Ross Sea oceanography as the Antarctic ice sheets retreated to their modern configuration and (ii) characterize natural variability in the Ross Sea prior to anthropogenic influence. Multi-proxy reconstructions from five Holocene marine sediment cores collected from the Victoria Land coast are integrated with modern observations and paleoclimate model simulations to investigate the interplay between oceanographic change and ice sheet stability in the western Ross Sea. New insight into the linkages between global and Antarctic climate using Earth system models and paleoclimate data assimilation places Holocene reconstructions in a broader context. Novel radiocarbon dating techniques (Ramped PyrOx, RPO, and pyrolysis gas chromatography mass spectrometry, Py-GC-MS) are adopted to improve the chronologies of Antarctic marine sedimentary records, which are difficult to accurately radiocarbon date. These novel methods are applied to develop a robust chronology for a high-resolution reconstruction of relative paleointensity (RPI). The prominent features of the RPI record will serve as independent stratigraphic correlation and dating tool for future Holocene marine and terrestrial sediment sequences from the Antarctic.
Analysis of the oxygen isotopic composition of fossil diatoms, a proxy for ice sheet meltwater discharge, establishes that modern freshening in the Ross Sea is outside the range of natural variability during the last 5,500 years and is consistent with a period of sustained ice sheet contraction. Intervals of enhanced glacial discharge occur between 6 and 5.5 ka, a period when terrestrial records of glacial lowering and marine sediment cores indicate that the Antarctic ice sheets underwent a rapid phase of retreat in the Ross Embayment, and since the mid-20th century.
Marine sedimentary records indicate that modern-style oceanographic conditions in the Ross Sea were not established until the ice sheets reached their modern configuration in the Ross Embayment ca. 4 ka. I propose that evidence of persistent ice cover along the Victoria Land coastal margin during the mid-Holocene climate optimum (6-4 ka) is related to the local impact of supercooled meltwater and tabular icebergs associated with ongoing ice sheet retreat. Based on these observations, I hypothesize that mid-Holocene ice sheet discharge may have altered polynya dynamics in the Ross Sea and water mass exchange at the shelf break.
Following the formation of the modern Ross Ice Shelf ca. 4-3 ka, oceanographic conditions in the western Ross Sea were relatively stable during the late Holocene prior to a dynamic Common Era. Inferred Common Era trends in polynya activity, CDW intrusion, primary productivity, and sea ice duration in the western Ross Sea are synchronous with other regions along the Antarctic margin. Analysis of paleoclimate model simulations and reanalysis products suggests that these Common Era oceanographic changes are linked to global cooling during the Little Ice Age and mean state changes in the Southern Annular Mode and El Niño-Southern Oscillation.