Pleistocene cyclostratigraphy on the continental rise and abyssal plain of the western Ross Sea, Antarctica

Abstract

This thesis investigates glacimarine sedimentation processes operating on the continental margin of the western Ross Sea during the Pleistocene (˜2.5 Ma). This time period is characterised by a major global cooling step at ˜0.8 Ma, although several proposed episodes of major marine-based Antarctic Ice Sheet (AIS) retreat in warm interglacial periods are inferred to have occurred after this time. Constraining the timing and magnitude of past marine-based AIS retreat events in the Ross Sea through this time will improve our understanding of the forcing mechanisms and thresholds that drive marine-based ice sheet retreat. Identifying such mechanisms and thresholds is crucial for assisting predictive models of potential ice sheet collapse in a future world with rapidly rising atmospheric carbon dioxide (CO₂) concentrations.

Six sedimentary cores forming a north-to-south transect from the continental rise to the abyssal plain of the western Ross Sea were examined in order to identify potential sedimentary signatures of past marine-based ice sheet variability and associated oceanographic change. A lithofacies scheme and stratigraphic framework were developed, which allowed the identification of shifting sedimentary processes through time. The sediments are interpreted to have been deposited primarily under the influence of bottom currents, most likely from changing rates of dense Antarctic Bottom Water (AABW) formation over glacial-interglacial cycles. Two dominant lithofacies (laminated and bioturbated) are recognised in the Pleistocene contourite sequences. Laminated facies alongside reduced ice-rafted debris (IRD) fluxes and reduced biological productivity are interpreted to represent expanded ice sheet and sea ice margins during glacial conditions, which acted to restrict surface water ventilation resulting in less oxygenated bottom waters. Conversely, laminated facies alongside reduced IRD fluxes and increased productivity are inferred to represent a reduction of ice shelf and sea ice cover resulting in enhanced AABW formation and sediment delivery. In general, it is interpreted that bioturbated facies in combination with enhanced productivity are common during interglacial conditions, with peaks in IRD associated with ice sheet retreat events leading into interglacial conditions. However, the relationships between laminated and bioturbated facies vary between sites, and facies at most sites generally alternate on timescales exceeding that of individual glacial-interglacial cycles (<100 kyr). Nonetheless, there are clear baseline shifts in the facies distributions through time across the sites, and it is inferred these represent step-like shifts in the ice sheet volume and sea ice processes on the continental shelf and above the study sites during the Pleistocene.

This thesis also assesses and compares three independent methodologies of obtaining IRD mass accumulation rates (MARs). The three methodologies include counting clasts >2 mm in x-ray images, the sieved weight percentage of the medium-to-coarse sand fraction (250 µm-2 mm), and volumetric estimates of the > 125 µm sand fraction using a laser particle sizer. The x-ray and sieve methods produced comparable results, while the volumetric estimate, although showing comparable long-term trends, produces a lesser correlation to the other two methods.

Spectral analysis of the IRD content and the magnetic susceptibility data series reveals that during the Early Pleistocene (2.5-1.2 Ma) ice discharge into the western Ross Sea was paced by the 41 kyr and 100 kyr cycles of obliquity and eccentricity, respectively. The Mid-Pleistocene Transition (MPT;1.2-0.8 Ma) was characterised by a switch to a higher-frequency, lower-amplitude IRD flux during a long-term period of high power in eccentricity, obliquity and precession (˜23 kyr) observed in the orbital solutions, suggesting a relatively linear response to orbital forcing at this time. The colder climate state of the Late Pleistocene (0.8-0.01 Ma) is characterised by IRD fluctuations modulated primarily by the 100 kyr eccentricity forcing that became dominant by 400 ka. In the western Ross Sea, IRD fluxes show a clear response to the orbital pacing of glacial-interglacial cycles, but are equivocal in identifying the magnitude of ice sheet loss or growth through glacial-interglacial cycles.