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

<p>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.</p>

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

<p>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.</p>

Sea ice deformation and thickness in the Western Ross Sea

&lt;p&gt;Sea ice cover is arguably the longest and best observed climate variable from space, with over four decades of highly reliable daily records of extent in both hemispheres. In Antarctica, a slight positive decadal trend in sea ice cover is driven by changes in the western Ross Sea, where a variation in weather patterns over the wider region forced a change in meridional winds. The distinguishing wind driven sea ice process in the western Ross Sea is the regular occurrence of the Ross Sea, McMurdo Sound, and Terra Nova Bay polynyas. Trends in sea ice volume and mass in this area unknown, because ice thickness and dynamics are particularly hard to measure.&lt;/p&gt;&lt;p&gt;Here we present the first comprehensive and direct assessment of large-scale sea-ice thickness distribution in the western Ross Sea. Using an airborne electromagnetic induction (AEM) ice thickness sensor towed by a fixed wing aircraft (Basler BT-67), we observed in November 2017 over a distance of 800 km significantly thicker ice than expected from thermodynamic growth alone. By means of time series of satellite images and wind data we relate the observed thickness distribution to satellite derived ice dynamics and wind data. Strong southerly winds with speeds of up to 25 ms&lt;sup&gt;-1&lt;/sup&gt; in early October deformed the pack ice, which was surveyed more than a month later.&lt;/p&gt;&lt;p&gt;We found strongly deformed ice with a mean and maximum thickness of 2.0 and 15.6 m, respectively. Sea-ice thickness gradients are highest within 100-200 km of polynyas, where the mean thickness of the thickest 10% of ice is 7.6 m. From comparison with aerial photographs and satellite images we conclude that ice preferentially grows in deformational ridges; about 43% of the sea ice volume in the area between McMurdo Sound and Terra Nova Bay is concentrated in more than 3 m thick ridges which cover about 15% of the surveyed area. Overall, 80% of the ice was found to be heavily deformed and concentrated in ridges up to 11.8 m thick.&lt;/p&gt;&lt;p&gt;Our observations hold a link between wind driven ice dynamics and the ice mass exported from the western Ross Sea. The sea ice statistics highlighted in this contribution forms a basis for improved satellite derived mass balance assessments and the evaluation of sea ice simulations.&lt;/p&gt;

Influence of Late Holocene climate on Lake Eggers hydrology, McMurdo Sound

Ice-covered lakes in Antarctica preserve records of regional hydroclimate and harbour extreme ecosystems that may serve as terrestrial analogues for exobiotic environments. Here, we examine the impacts of hydroclimate and landscape on the formation history of Lake Eggers, a small ice-sealed lake, located in the coastal polar desert of McMurdo Sound, Antarctica (78°S). Using ground penetrating radar surveys and three lake ice cores we characterize the ice morphology and chemistry. Lake ice geochemistry indicates that Lake Eggers is fed primarily from local snowmelt that accreted onto the lake surface during runoff events. Radiocarbon ages of ice-encased algae suggest basal ice formed at least 735 ± 20 calibrated years before present (1215 C.E.). Persisting through the Late Holocene, Lake Eggers alternated between periods of ice accumulation and sublimation driven by regional climate variability in the western Ross Sea. For example, particulate organic matter displayed varying δ15N ratios with depth, corresponding to sea ice fluctuations in the western Ross Sea during the Late Holocene. These results suggest a strong climatic control on the hydrologic regime shifts shaping ice formation at Lake Eggers.

The role of tides in bottom water export from the western Ross Sea

Approximately 25% of Antarctic Bottom Water has its origin as dense water exiting the western Ross Sea, but little is known about what controls the release of dense water plumes from the Drygalski Trough. We deployed two moorings on the slope to investigate the water properties of the bottom water exiting the region at Cape Adare. Salinity of the bottom water has increased in 2018 from the previous measurements in 2008–2010, consistent with the observed salinity increase in the Ross Sea. We find High Salinity Shelf Water from the Drygalski Trough contributes to two pulses of dense water at Cape Adare. The timing and magnitude of the pulses is largely explained by an inverse relationship with the tidal velocity in the Ross Sea. We suggest that the diurnal and low frequency tides in the western Ross Sea may control the magnitude and timing of the dense water outflow.