Sedimentology of the grounding zone of the Kamb Ice Stream, Siple Coast, West Antarctica

<div>The grounding line of the Siple Coast incorporates six major ice streams, which together drain around a third of the West Antarctic Ice Sheet. Previously, the ~2000 km-long feature had only been sampled and directly observed at Whillans Ice Stream. This thesis examines glaciomarine sediment and processes operating at the presently stagnant Kamb Ice Stream (KIS) grounding zone ~3.3 km seaward of the modern grounding line (Lat. -82.78, Long. -155.16), where the ice is 590 m thick and overlies a 30 m thick water column. KIS-GZ is the planned site for a deep drilling project in 2023. The sea floor was accessed using a hot water drill in the 2019/20 Antarctic field season. A remotely operated submersible (‘Icefin’) was deployed under the ice shelf, which provided 800 m of sea floor video toward the grounding line. A small number of short (~0.6 m) gravity cores were collected from the seafloor, one of which was examined in this study. <br></div><div><br></div><div>The Icefin video imagery was processed using Structure-from-Motion (SfM) software, enabling the identification of two previously unrecognised sea floor sedimentary facies. One is defined by ubiquitous cm-scale ripples in fine-medium sand, where the ripples are aligned with the prevailing tidal currents flowing parallel to the grounding line. Observed current speeds are too low for the ripples to be generated under the modern oceanographic regime. The second facies is defined by abundant dropstones in mediumcoarse sand. A transition zone separates the two facies. Previously unidentified decimetre-scale bedforms are present in the transition zone and near the borehole.</div><div><br></div><div>The lithological, geochemical, and microfossil properties of the gravity core were analysed. The core sediment is sandy diamicton with weak stratification defined by decimetre-scale changes in clast abundance. Mineral counts, zircon ages, Nd/Sr isotopes, and an immature composition indicate this sediment is sourced within the Kamb catchment. The core also contains reworked late Oligocene-late Miocene diatoms. Quaternary diatoms are absent. Ramped pyrolysis radiocarbon dating was attempted on 19 carbon fractions obtained from samples at four depths. Two pyrolysis fractions yielded ages of 31.5-33.2 ka, while the rest did not contain measurable radiocarbon. This likely reflects the reworking of radiocarbon-dead material into the sediment and can only be considered a maximum age for deposition.</div><div><br></div><div>Together, the sediment and video data suggest deposition of the sea floor sediment at the core site occurred subsequent to the stagnation of KIS ~160 years ago. I assume that sediment concentrations are relatively uniform along the length of the ice stream and calculate that up to ~2.7 m of diamicton was rapidly deposited at the core site as the grounding line retreated and englacial sediment melted out and settled through the water column. Accumulation in recent decades has been comparatively low. During the period of reduced sedimentation, the sea floor diamicton has been reworked to varying degrees to form ripples and winnowed lag deposits, resulting in a textural patchwork at km scale.</div>

Sedimentology of the grounding zone of the Kamb Ice Stream, Siple Coast, West Antarctica

<div>The grounding line of the Siple Coast incorporates six major ice streams, which together drain around a third of the West Antarctic Ice Sheet. Previously, the ~2000 km-long feature had only been sampled and directly observed at Whillans Ice Stream. This thesis examines glaciomarine sediment and processes operating at the presently stagnant Kamb Ice Stream (KIS) grounding zone ~3.3 km seaward of the modern grounding line (Lat. -82.78, Long. -155.16), where the ice is 590 m thick and overlies a 30 m thick water column. KIS-GZ is the planned site for a deep drilling project in 2023. The sea floor was accessed using a hot water drill in the 2019/20 Antarctic field season. A remotely operated submersible (‘Icefin’) was deployed under the ice shelf, which provided 800 m of sea floor video toward the grounding line. A small number of short (~0.6 m) gravity cores were collected from the seafloor, one of which was examined in this study. <br></div><div><br></div><div>The Icefin video imagery was processed using Structure-from-Motion (SfM) software, enabling the identification of two previously unrecognised sea floor sedimentary facies. One is defined by ubiquitous cm-scale ripples in fine-medium sand, where the ripples are aligned with the prevailing tidal currents flowing parallel to the grounding line. Observed current speeds are too low for the ripples to be generated under the modern oceanographic regime. The second facies is defined by abundant dropstones in mediumcoarse sand. A transition zone separates the two facies. Previously unidentified decimetre-scale bedforms are present in the transition zone and near the borehole.</div><div><br></div><div>The lithological, geochemical, and microfossil properties of the gravity core were analysed. The core sediment is sandy diamicton with weak stratification defined by decimetre-scale changes in clast abundance. Mineral counts, zircon ages, Nd/Sr isotopes, and an immature composition indicate this sediment is sourced within the Kamb catchment. The core also contains reworked late Oligocene-late Miocene diatoms. Quaternary diatoms are absent. Ramped pyrolysis radiocarbon dating was attempted on 19 carbon fractions obtained from samples at four depths. Two pyrolysis fractions yielded ages of 31.5-33.2 ka, while the rest did not contain measurable radiocarbon. This likely reflects the reworking of radiocarbon-dead material into the sediment and can only be considered a maximum age for deposition.</div><div><br></div><div>Together, the sediment and video data suggest deposition of the sea floor sediment at the core site occurred subsequent to the stagnation of KIS ~160 years ago. I assume that sediment concentrations are relatively uniform along the length of the ice stream and calculate that up to ~2.7 m of diamicton was rapidly deposited at the core site as the grounding line retreated and englacial sediment melted out and settled through the water column. Accumulation in recent decades has been comparatively low. During the period of reduced sedimentation, the sea floor diamicton has been reworked to varying degrees to form ripples and winnowed lag deposits, resulting in a textural patchwork at km scale.</div>

Pliocene-Pleistocene Orbital Cyclostratigraphy and Glacial Evolution of the East Antarctic Ice Sheet from Continental Rise IODP Site U1361, Wilkes Land Margin, East Antarctica

<p>Stability of the East Antarctic Ice Sheet (EAIS), in response to the orbitally-paced cooling climate of the Late Neogene, is largely unknown. The Wilkes Land margin of East Antarctica, largely grounded below sea level, has previously been proposed to respond dynamically during the warmer climate of the Pliocene, similarly to other marine based sectors of Antarctica (i.e. West Antarctica). Sediment deposition on the Wilkes Land continental rise, recovered in Integrated Ocean Drilling Program U1361A drillcore provides a distal but continuous record of EAIS fluctuations. Changes in sedimentary depositional environments at U1361A core site, were determined through analysis of lithofacies and physical property logs: natural gamma-ray (NGR), gamma-ray attenuation bulk density (GRA), magnetic susceptibility (MS) and L* colour reflectance. NGR primarily reflected biogenic content and a synchronous relationship between NGR, GRA and MS was used to identify interglacial and glacial phases, whereby decreased NGR, GRA and MS values indicated an increase in biogenic material. L* colour reflectance was more variable through time, displayed higher frequency fluctuations and a changing relationship with the other physical property logs down core. Two depositional models, based on facies interpretations and the defined physical property relationships, were produced for the Middle Late Pleistocene (last ~550 kyr; model A) and mid-Pliocene (~4.2-3.6 Ma; model B), which represent end members. Depositional processes common to both models occurred in the intervening core, spanning the Late Pliocene-Early Pleistocene (3-1 Ma). Model A, applied to the Middle Late Pleistocene, shows that alternating diatom-rich clays to silty clays in the upper 9 m of core U1361A, reflect the large amplitude ~100 kyr paced glacial-interglacial cycles, which is confirmed by spectral analysis of the physical properties for this interval. Model B, applied to the Early Pliocene, suggest that the depositional processes recorded by facies may have been less sensitive to EAIS fluctuations, probably due to the fact that the ice margin was generally more distal to the core site during glacial-interglacial cycles of advance and retreat. Nevertheless, these more subtle changes in lithology were characterised by variations in the physical property logs, and spectral analysis of these time series implied orbital pacing was still influential on depositional processes at this time (displaying power in precession and obliquity frequencies). Spectral analysis of the physical property logs and visual correlations to the benthic δ18O stack, confirmed the 4.2-1 Ma interval was paced by ~40 kyr and implies obliquity-paced oscillations of the margin of the EAIS. Precession periodicities, significant in spectra throughout the 4.2 Myr record, are proposed to be the response of phytoplankton productivity in response to seasonal insolation controlling sea-ice extent.</p>

Environmental Controls on Coccolithophore Blooms in the Southwest Pacific Ocean during Marine Isotope Stages 5e (125 ka) and 7a (210 ka)

<p>Coccolithophores play a key role in the ocean carbon cycle, regulating the uptake and release of CO2. Satellite observations over the past few decades show ocean change in a warming world is accompanied by changes in the latitudinal distribution of coccolithophore blooms. Despite their importance in the carbon cycle, knowledge of the causes of coccolithophore blooms, and how they may respond to future climate change is limited. In this study evidence from marine sedimentary cores is used to derive longer, more complete records of past coccolithophore productivity, and the factors that potentially caused enhanced coccolithophore productivity in previous interglacials. Carbonate-rich marine cores; subtropical P71 from north of New Zealand (33°51.3‟S, 174°41.6‟E) and subantarctic Ocean Drilling Project (ODP) 1120 from the Campbell Plateau (50°3.803‟S, 173°22.300‟E) show abrupt changes between foraminiferal-rich sediments during glacials to coccolith-rich sediments during interglacials. Both cores encompass the last two complete interglacial cycles, Marine Isotope Stage (MIS) 5 (71-130ka) and MIS 7 (191- 243ka). While MIS 5 has been well-studied in the Southwest Pacific Ocean, research on MIS 7 is limited. From the literature, and data from this study, new insights are presented into the climatic and oceanographic conditions during MIS 7. Sea surface temperatures in the subtropical Tasman Inflow were comparable to present during MIS 7a (191-222ka), but were cooler in MIS 7c (235-243ka), implying a change in flow regime potentially related to the dynamics of the South Pacific Gyre. During MIS 7a and 7c the temperature gradient across the Subtropical Front (STF), which separates subtropical and subantarctic waters, was greater than present on the Chatham Rise, at >2°C per 1° latitude. In the Tasman Sea, the STF moved northwards by ~2° latitude. This thesis employs grain size data and scanning electron microscope images to show that significant coccolithophore blooms occurred during MIS 7a at subtropical core P71, but not during interglacial peak MIS 5e (117-130ka), whilst the reverse is true at subantarctic core ODP 1120. A range of paleo-environmental proxies are used to determine the potential conditions that caused these coccolithophore blooms. This includes mass accumulation rates of CaCO3 and % of <20μm grain size that texturally identifies coccoliths, to determine relative rates of coccolithophore productivity. Oxygen isotopes (δ18O) of multiple planktic and benthic foraminifera provide age models, with the former also helping to identify upper water column stratification. Mg/Ca ratios in planktic foraminifera, Globigerinoides ruber, and Random Forest modelling of planktic foraminifera assemblages have been used to derive paleo-temperature estimates. These methods, coupled with trace element data from G. ruber as a productivity proxy, foraminifera assemblages, data on solar insolation and scanning electron microscope images, collectively determine the oceanic conditions at the time of coccolithophore blooms at each core site. The results suggest that no one factor was responsible for blooming, rather it was the combination, and interactions between different environmental processes, that were important. At P71, key factors for bloom formation in MIS 7a were high insolation, thermal stratification of the uppermost ocean, and well-mixed source waters from the Tasman Inflow. At ODP 1120, blooms in MIS 5e resulted from decreased windiness, warmer sea surface temperatures and reduced oceanic circulation over the Campbell Plateau, resulting in marked thermal stratification. It is likely that coccolithophore blooms further enhanced stratification at each core site, and restricted productivity further down the water column. At P71, modern oceanic trends suggest that conditions that caused blooms during MIS 7a will not be met in the near future, and blooming is unlikely to increase at this core site. At ODP 1120, modern trends are less clear, but future conditions are projected to be comparable to MIS 5e, suggesting that coccolithophore blooming may increase in the future in subantarctic waters.</p>

Pliocene-Pleistocene Orbital Cyclostratigraphy and Glacial Evolution of the East Antarctic Ice Sheet from Continental Rise IODP Site U1361, Wilkes Land Margin, East Antarctica

<p>Stability of the East Antarctic Ice Sheet (EAIS), in response to the orbitally-paced cooling climate of the Late Neogene, is largely unknown. The Wilkes Land margin of East Antarctica, largely grounded below sea level, has previously been proposed to respond dynamically during the warmer climate of the Pliocene, similarly to other marine based sectors of Antarctica (i.e. West Antarctica). Sediment deposition on the Wilkes Land continental rise, recovered in Integrated Ocean Drilling Program U1361A drillcore provides a distal but continuous record of EAIS fluctuations. Changes in sedimentary depositional environments at U1361A core site, were determined through analysis of lithofacies and physical property logs: natural gamma-ray (NGR), gamma-ray attenuation bulk density (GRA), magnetic susceptibility (MS) and L* colour reflectance. NGR primarily reflected biogenic content and a synchronous relationship between NGR, GRA and MS was used to identify interglacial and glacial phases, whereby decreased NGR, GRA and MS values indicated an increase in biogenic material. L* colour reflectance was more variable through time, displayed higher frequency fluctuations and a changing relationship with the other physical property logs down core. Two depositional models, based on facies interpretations and the defined physical property relationships, were produced for the Middle Late Pleistocene (last ~550 kyr; model A) and mid-Pliocene (~4.2-3.6 Ma; model B), which represent end members. Depositional processes common to both models occurred in the intervening core, spanning the Late Pliocene-Early Pleistocene (3-1 Ma). Model A, applied to the Middle Late Pleistocene, shows that alternating diatom-rich clays to silty clays in the upper 9 m of core U1361A, reflect the large amplitude ~100 kyr paced glacial-interglacial cycles, which is confirmed by spectral analysis of the physical properties for this interval. Model B, applied to the Early Pliocene, suggest that the depositional processes recorded by facies may have been less sensitive to EAIS fluctuations, probably due to the fact that the ice margin was generally more distal to the core site during glacial-interglacial cycles of advance and retreat. Nevertheless, these more subtle changes in lithology were characterised by variations in the physical property logs, and spectral analysis of these time series implied orbital pacing was still influential on depositional processes at this time (displaying power in precession and obliquity frequencies). Spectral analysis of the physical property logs and visual correlations to the benthic δ18O stack, confirmed the 4.2-1 Ma interval was paced by ~40 kyr and implies obliquity-paced oscillations of the margin of the EAIS. Precession periodicities, significant in spectra throughout the 4.2 Myr record, are proposed to be the response of phytoplankton productivity in response to seasonal insolation controlling sea-ice extent.</p>

Environmental Controls on Coccolithophore Blooms in the Southwest Pacific Ocean during Marine Isotope Stages 5e (125 ka) and 7a (210 ka)

<p>Coccolithophores play a key role in the ocean carbon cycle, regulating the uptake and release of CO2. Satellite observations over the past few decades show ocean change in a warming world is accompanied by changes in the latitudinal distribution of coccolithophore blooms. Despite their importance in the carbon cycle, knowledge of the causes of coccolithophore blooms, and how they may respond to future climate change is limited. In this study evidence from marine sedimentary cores is used to derive longer, more complete records of past coccolithophore productivity, and the factors that potentially caused enhanced coccolithophore productivity in previous interglacials. Carbonate-rich marine cores; subtropical P71 from north of New Zealand (33°51.3‟S, 174°41.6‟E) and subantarctic Ocean Drilling Project (ODP) 1120 from the Campbell Plateau (50°3.803‟S, 173°22.300‟E) show abrupt changes between foraminiferal-rich sediments during glacials to coccolith-rich sediments during interglacials. Both cores encompass the last two complete interglacial cycles, Marine Isotope Stage (MIS) 5 (71-130ka) and MIS 7 (191- 243ka). While MIS 5 has been well-studied in the Southwest Pacific Ocean, research on MIS 7 is limited. From the literature, and data from this study, new insights are presented into the climatic and oceanographic conditions during MIS 7. Sea surface temperatures in the subtropical Tasman Inflow were comparable to present during MIS 7a (191-222ka), but were cooler in MIS 7c (235-243ka), implying a change in flow regime potentially related to the dynamics of the South Pacific Gyre. During MIS 7a and 7c the temperature gradient across the Subtropical Front (STF), which separates subtropical and subantarctic waters, was greater than present on the Chatham Rise, at >2°C per 1° latitude. In the Tasman Sea, the STF moved northwards by ~2° latitude. This thesis employs grain size data and scanning electron microscope images to show that significant coccolithophore blooms occurred during MIS 7a at subtropical core P71, but not during interglacial peak MIS 5e (117-130ka), whilst the reverse is true at subantarctic core ODP 1120. A range of paleo-environmental proxies are used to determine the potential conditions that caused these coccolithophore blooms. This includes mass accumulation rates of CaCO3 and % of <20μm grain size that texturally identifies coccoliths, to determine relative rates of coccolithophore productivity. Oxygen isotopes (δ18O) of multiple planktic and benthic foraminifera provide age models, with the former also helping to identify upper water column stratification. Mg/Ca ratios in planktic foraminifera, Globigerinoides ruber, and Random Forest modelling of planktic foraminifera assemblages have been used to derive paleo-temperature estimates. These methods, coupled with trace element data from G. ruber as a productivity proxy, foraminifera assemblages, data on solar insolation and scanning electron microscope images, collectively determine the oceanic conditions at the time of coccolithophore blooms at each core site. The results suggest that no one factor was responsible for blooming, rather it was the combination, and interactions between different environmental processes, that were important. At P71, key factors for bloom formation in MIS 7a were high insolation, thermal stratification of the uppermost ocean, and well-mixed source waters from the Tasman Inflow. At ODP 1120, blooms in MIS 5e resulted from decreased windiness, warmer sea surface temperatures and reduced oceanic circulation over the Campbell Plateau, resulting in marked thermal stratification. It is likely that coccolithophore blooms further enhanced stratification at each core site, and restricted productivity further down the water column. At P71, modern oceanic trends suggest that conditions that caused blooms during MIS 7a will not be met in the near future, and blooming is unlikely to increase at this core site. At ODP 1120, modern trends are less clear, but future conditions are projected to be comparable to MIS 5e, suggesting that coccolithophore blooming may increase in the future in subantarctic waters.</p>

Glycan Epitopes and Potential Glycoside Antagonists of DC-SIGN Involved in COVID-19: In Silico Study

Glycosylation is an important post-translational modification that affects a wide variety of physiological functions. DC-SIGN (Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin) is a protein expressed in antigen-presenting cells that recognizes a variety of glycan epitopes. Until now, the binding of DC-SIGN to SARS-CoV-2 Spike glycoprotein has been reported in various articles and is regarded to be a factor in systemic infection and cytokine storm. The mechanism of DC-SIGN recognition offers an alternative method for discovering new medication for COVID-19 treatment. Here, we discovered three potential pockets that hold different glycan epitopes by performing molecular dynamics simulations of previously reported oligosaccharides. The “EPN” motif, “NDD” motif, and Glu354 form the most critical pocket, which is known as the Core site. We proposed that the type of glycan epitopes, rather than the precise amino acid sequence, determines the recognition. Furthermore, we deduced that oligosaccharides could occupy an additional site, which adds to their higher affinity than monosaccharides. Based on our findings and previously described glycoforms on the SARS-CoV-2 Spike, we predicted the potential glycan epitopes for DC-SIGN. It suggested that glycan epitopes could be recognized at multiple sites, not just Asn234, Asn149 and Asn343. Subsequently, we found that Saikosaponin A and Liquiritin, two plant glycosides, were promising DC-SIGN antagonists in silico.

A detailed radiostratigraphic data set for the central East Antarctic Plateau spanning from the Holocene to the mid-Pleistocene

We present an ice-penetrating radar data set which consists of 26 internal reflecting horizons (IRHs) that cover the entire Dome C area of the East Antarctic plateau, the most extensive to date in the region. This data set uses radar surveys collected over the space of 10 years, starting with an airborne international collaboration in 2008 to explore the region, up to the detailed ground-based surveys in support of the Beyond EPICA – Oldest Ice (BE-OI) European Consortium. Through direct correlation with the EPICA-DC ice core, we date 19 IRHs that span the past four glacial cycles, from 10 ka, beginning of the Holocene, to over 350 ka, ranging from 10 % to 83 % of the ice thickness at the EPICA-DC ice core site. We indirectly date and provide stratigraphic information for seven older IRHs using a 1D ice flow inverse model, going back to an estimated 700 ka. Depth and age uncertainties are quantified for all IRHs and provided as part of the data set. The IRH data set presented in this study is available at the US Antarctic Program Data Center (USAP-DC) (https://doi.org/10.15784/601411, Cavitte et al., 2020) and represents a contribution to the SCAR AntArchitecture action group (AntArchitecture, 2017).

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site

The crystal orientation fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect bulk anisotropic fabric patterns by exploiting the birefringence of ice crystals at radar frequencies, with the assumption that one of the crystallographic axes is aligned in the vertical direction. In this study, we conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations near the Western Antarctic Ice Sheet (WAIS) Divide ice core site. From these measurements, we are able to quantify the azimuthal fabric asymmetry at this site to a depth of 1400 m at a bulk-averaged resolution of up to 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide ice core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to identify and conclude that the fabric orientation is depth-invariant to at least 1400 m, equivalent to 6700 years BP (years before 1950) and aligns closely with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk fabric asymmetry and orientation compares well with much more laborious sample-based COF measurements from thin ice sections. Because it is the bulk-averaged fabric that ultimately influences ice flow, polarimetric radar methods provide an opportunity for its accurate and widespread mapping and its incorporation into ice flow models.