Miocene to present oceanographic variability in the Scotia Sea and Antarctic Ice Sheet dynamics: Insight from revised seismic-stratigraphy following IODP Expedition 382

Scotia Sea and the Drake Passage is key towards understanding the development of modern oceanic circulation patterns and their implications for ice sheet growth and decay. The sedimentary record of the southern Scotia Sea basins documents the regional tectonic, oceanographic and climatic evolution since the Eocene. However, a lack of accurate age estimations has prevented the calibration of the reconstructed history. The upper sedimentary record of the Scotia Sea was scientifically drilled for the first time in 2019 during International Ocean Discovery Program (IODP) Expedition 382, recovering sediments down to ∼643 and 676 m below sea floor in the Dove and Pirie basins respectively. Here, we report newly acquired high resolution physical properties data and the first accurate age constraints for the seismic sequences of the upper sedimentary record of the Scotia Sea to the late Miocene. The drilled record contains four basin-wide reflectors – Reflector-c, -b, -a and -a' previously estimated to be ∼12.6 Ma, ∼6.4 Ma, ∼3.8 Ma and ∼2.6 Ma, respectively. By extrapolating our new Scotia Sea age model to previous morpho-structural and seismic-stratigraphic analyses of the wider region we found, however, that the four discontinuities drilled are much younger than previously thought. Reflector-c actually formed before 8.4 Ma, Reflector-b at ∼4.5/3.7 Ma, Reflector-a at ∼1.7 Ma, and Reflector-a' at ∼0.4 Ma. Our updated age model of these discontinuities has major implications for their correlation with regional tectonic, oceanographic and cryospheric events. According to our results, the outflow of Antarctic Bottom Water to northern latitudes controlled the Antarctic Circumpolar Current flow from late Miocene. Subsequent variability of the Antarctic ice sheets has influenced the oceanic circulation pattern linked to major global climatic changes during early Pliocene, Mid-Pleistocene and the Marine Isotope Stage 11

Ancient marine sediment DNA reveals diatom transition in Antarctica

Antarctica is one of the most vulnerable regions to climate change on Earth and studying the past and present responses of this polar marine ecosystem to environmental change is a matter of urgency. Sedimentary ancient DNA (sedaDNA) analysis can provide such insights into past ecosystem-wide changes. Here we present authenticated (through extensive contamination control and sedaDNA damage analysis) metagenomic marine eukaryote sedaDNA from the Scotia Sea region acquired during IODP Expedition 382. We also provide a marine eukaryote sedaDNA record of ~1 Mio. years and diatom and chlorophyte sedaDNA dating back to ~540 ka (using taxonomic marker genes SSU, LSU, psbO). We find evidence of warm phases being associated with high relative diatom abundance, and a marked transition from diatoms comprising <10% of all eukaryotes prior to ~14.5 ka, to ~50% after this time, i.e., following Meltwater Pulse 1A, alongside a composition change from sea-ice to open-ocean species. Our study demonstrates that sedaDNA tools can be expanded to hundreds of thousands of years, opening the pathway to the study of ecosystem-wide marine shifts and paleo-productivity phases throughout multiple glacial-interglacial cycles

Antiphased dust deposition and productivity in the Antarctic Zone over 1.5 million years

The Southern Ocean paleoceanography provides key insights into how iron fertilization and oceanic productivity developed through Pleistocene ice-ages and their role in influencing the carbon cycle. We report a high-resolution record of dust deposition and ocean productivity for the Antarctic Zone, close to the main dust source, Patagonia. Our deep-ocean records cover the last 1.5 Ma, thus doubling that from Antarctic ice-cores. We find a 5 to 15-fold increase in dust deposition during glacials and a 2 to 5-fold increase in biogenic silica deposition, reflecting higher ocean productivity during interglacials. This antiphasing persisted throughout the last 25 glacial cycles. Dust deposition became more pronounced across the Mid-Pleistocene Transition (MPT) in the Southern Hemisphere, with an abrupt shift suggesting more severe glaciations since ~0.9 Ma. Productivity was intermediate pre-MPT, lowest during the MPT and highest since 0.4 Ma. Generally, glacials experienced extended sea-ice cover, reduced bottom-water export and Weddell Gyre dynamics, which helped lower atmospheric CO2 levels

Millennial-scale Variability of a Major East Antarctic Outlet Glacier during the Last Glaciation

Ongoing retreat of Antarctica’s marine-based glaciers is associated with warm (~2° C) modified Circumpolar Deep Water intrusion onto the continental shelf, suggesting that Southern Ocean temperatures may influence Antarctic ice sheet stability. Understanding past cryosphere response to environmental forcing is crucial to modeling future ice sheet behavior. Of particular interest is the response of the East Antarctic Ice Sheet (EAIS), which stands to contribute ~20 m to global sea level. However, marine sediment sequences recording timing and variability of EAIS fluctuations through the last major climate shift, the Last Glacial Maximum (LGM), are either missing from the margin or have poor chronological control. Here we present three marine sediment cores that contain a record of pre-LGM fluctuations of the marine-based Lambert Glacier-Amery Ice Shelf (LG-AIS) system into Prydz Channel, East Antarctica. Analyses of core lithology, physical properties, cosmogenic nuclide concentration and diatom assemblage demonstrate that Prydz Channel was characterized by alternating open-marine and sub-shelf deposition, implying repeated LG-AIS fluctuations through the LGM. Our radiocarbon chronology demonstrates that LG-AIS fluctuations occurred on millennial timescales. Our record corroborates regional marine and terrestrial records, which demonstrate millennial scale variability in Antarctic Circumpolar Current strength, ice-rafted debris deposition, sea ice extent, Antarctic atmospheric temperature, and Southern Ocean sea surface temperature. This evidence suggests that the EAIS was sensitive to sub-orbital climate forcing in the past, and has implications for modeling future EAIS behavior

Ocean Forcing of Quaternary East Antarctic Ice Sheet Evolution: An Ice-Proximal Sedimentary Perspective

Antarctica and the Southern Ocean play a critical role in Earth’s climate system. Antarctica’s ice sheets contain enough ice to raise global sea level by ~58 m, and the Southern Ocean distributes climate signals and nutrients to the major ocean basins and the deep ocean. Antarctica’s largest ice sheet, the East Antarctic Ice Sheet (EAIS), was considered stable compared to those in West Antarctica and the Antarctic Peninsula because it was thought to be grounded above sea level. However, subglacial topography now reveals vast submarine basins and measurements of ice velocity in the Pacific sector indicate marine-terminating outlet glacier thinning and retreat over the last four decades associated with warm ocean water presence at depth in some locations over East Antarctica’s continental shelves. Modern observations provide the impetus for investigating past EAIS response to climate change, particularly to ocean thermal forcing. To understand past EAIS response to ocean thermal forcing, marine geologic investigations of sediments recovered from Antarctica’s continental shelves and the Southern Ocean are required, particularly from intervals of warmer-than-modern conditions. This dissertation explores the Quaternary (last 2.6 Ma) behavior of the EAIS by reconstructing outlet glacier behavior using well-dated millennial- to orbital-resolution sedimentary sequences collected from East Antarctica’s continental shelves and the Southern Ocean. Each chapter targets a specific time period, focusing on past warm periods and climate transitions. To establish the timing of glacial advance and retreat, and to reconstruct depositional environment and upper ocean temperature, I utilize the Ramped PyrOx (RPO) technique and radiocarbon (14C) analyses, sedimentary beryllium-10 concentration, and the TetraEther IndeX of 86 carbons (TEX86) paleothermometer. Results yield new insights into the Quaternary evolution of East Antarctica’s marine-terminating glaciers and the role of ocean heat in that evolution. Chapter two investigates the response of an EAIS outlet glacier system, the Lambert Glacier-Amery Ice Shelf (LG AIS), to ocean perturbations (e.g., sea level rise and ocean heat) since the last deglacial-Holocene (15–0 ka). To constrain the timing of deglaciation of Svenner Channel, a glacially carved trough in eastern Prydz Bay, through which warm modified Circumpolar Deep Water currently flows towards the LG-AIS grounding line, I generated bulk acid-insoluble organic matter 14C ages via RPO from a 17 m long sediment core and integrated the new ages with existing carbonate 14C ages. My 14C-based chronology indicates regional deglaciation occurs at 15±0.5 ka, coincident, within chronological constrains, with the Meltwater Pulse 1A sea level rise event. Upper ocean temperatures reconstructed using the archaeal lipid-based paleothermometer TEX86 indicate relatively warm ocean waters exist in Svenner Channel in the early to middle Holocene (11–7 ka) and cool significantly at 7 ka. This record indicates that current temperatures are cooler than those in the early Holocene and suggests that the eastern Prydz Bay marine environment may evolve with continued warming. Chapter three examines the response of the LG-AIS system to climate and ocean change over the last ~40 ka. Three sedimentary sequences recovered from western Prydz Channel contain a sequence of alternating diatom-rich muds, silts, and sands/diamicts hypothesized to reflect advance and retreat of the LG-AIS system. Using bulk sediment RPO 14C dating, three episodes of LG-AIS retreat are documented over the last ~40 ka that coincide with millennial-scale warm events observed in Antarctic ice cores. This is the first direct record of outlet glacier variability from Antarctica’s continental shelves on millennial timescales during the last glaciation. Sedimentary beryllium-10 confirms that diatom-rich muds preserved in Prydz Channel are deposited in open marine conditions. Results are the first to directly link millennial-scale records of ice rafted debris from Southern Ocean sediments with outlet glacier behavior inferred from ice-proximal sediment records. Chapter four seeks to understand EAIS response to ocean forcing on orbital timescales during the early Pleistocene (1.5–1.0 Ma). This period partially encompasses the Mid-Pleistocene Transition (1.25–0.7 Ma) wherein 41-kyr pacing of glacial cycles in the late Pliocene/early Pleistocene transitions to 100-kyr pacing in the late Pleistocene. To assess relationships between upper ocean temperature and ice sheet mass balance, I generated TEX86-based temperature records from high-resolution sedimentary sequences recovered from the Scotia Sea during the International Ocean Discovery Program Expedition 382 and compared these records with sedimentary physical properties proxies for terrigenous and biogenic input. Results indicate that relatively cool upper ocean temperatures coincide with intervals of greater diatomaceous input. I observe relatively warm upper ocean temperatures between 1.55 and 1.28 Ma, followed by a cool period centered on 1.25 Ma, then a return to relatively warm temperatures between 1.22 and 0.99 Ma. The Scotia Sea paleotemperature record demonstrates an increasing temperature trend between 1.55 and 1.25 Ma, but no discernable trend after 1.25 Ma. I observe an average 42-kyr pacing of Scotia Sea upper ocean temperatures proximal to Antarctica between 1.55 and 1.25 Ma, implying that obliquity plays a major role in modulating Southern Ocean temperatures in the early Pleistocene. The obliquity-paced temperature fluctuations are not apparent after 1.25 Ma, suggesting that the transition to cooler temperatures represents some transition in Southern Ocean response to obliquity. The research presented herein demonstrates that ice-proximal studies are critically required for identifying the source regions of ice mass loss, shed light on magnitudes and rates of glacier retreat, and assess EAIS vulnerability to climate forcing. This dissertation utilizes bulk sedimentary geochemistry in Southern Ocean and Antarctic continental shelf sediments to constrain changes in upper ocean temperature and depositional environment, providing a new perspective to existing paleoenvironmental studies. The ability of ice-proximal paleoceanographic reconstructions to capture EAIS dynamics will be crucial in forecasting future ice mass balance. Because ice-proximal paleoceanographic records provide direct evidence for the timing of glacier retreat, changes in depositional environment, and upper ocean temperature evolution, they can be used to constrain the timing and response of EAIS marine-based glaciers to a range of climate scenarios that models aim to predict, thereby constraining future sea level rise scenarios

Ocean Forcing of Quaternary East Antarctic Ice Sheet Evolution: An Ice-Proximal Sedimentary Perspective

Antarctica and the Southern Ocean play a critical role in Earth’s climate system. Antarctica’s ice sheets contain enough ice to raise global sea level by ~58 m, and the Southern Ocean distributes climate signals and nutrients to the major ocean basins and the deep ocean. Antarctica’s largest ice sheet, the East Antarctic Ice Sheet (EAIS), was considered stable compared to those in West Antarctica and the Antarctic Peninsula because it was thought to be grounded above sea level. However, subglacial topography now reveals vast submarine basins and measurements of ice velocity in the Pacific sector indicate marine-terminating outlet glacier thinning and retreat over the last four decades associated with warm ocean water presence at depth in some locations over East Antarctica’s continental shelves. Modern observations provide the impetus for investigating past EAIS response to climate change, particularly to ocean thermal forcing. To understand past EAIS response to ocean thermal forcing, marine geologic investigations of sediments recovered from Antarctica’s continental shelves and the Southern Ocean are required, particularly from intervals of warmer-than-modern conditions. This dissertation explores the Quaternary (last 2.6 Ma) behavior of the EAIS by reconstructing outlet glacier behavior using well-dated millennial- to orbital-resolution sedimentary sequences collected from East Antarctica’s continental shelves and the Southern Ocean. Each chapter targets a specific time period, focusing on past warm periods and climate transitions. To establish the timing of glacial advance and retreat, and to reconstruct depositional environment and upper ocean temperature, I utilize the Ramped PyrOx (RPO) technique and radiocarbon (14C) analyses, sedimentary beryllium-10 concentration, and the TetraEther IndeX of 86 carbons (TEX86) paleothermometer. Results yield new insights into the Quaternary evolution of East Antarctica’s marine-terminating glaciers and the role of ocean heat in that evolution. Chapter two investigates the response of an EAIS outlet glacier system, the Lambert Glacier-Amery Ice Shelf (LG AIS), to ocean perturbations (e.g., sea level rise and ocean heat) since the last deglacial-Holocene (15–0 ka). To constrain the timing of deglaciation of Svenner Channel, a glacially carved trough in eastern Prydz Bay, through which warm modified Circumpolar Deep Water currently flows towards the LG-AIS grounding line, I generated bulk acid-insoluble organic matter 14C ages via RPO from a 17 m long sediment core and integrated the new ages with existing carbonate 14C ages. My 14C-based chronology indicates regional deglaciation occurs at 15±0.5 ka, coincident, within chronological constrains, with the Meltwater Pulse 1A sea level rise event. Upper ocean temperatures reconstructed using the archaeal lipid-based paleothermometer TEX86 indicate relatively warm ocean waters exist in Svenner Channel in the early to middle Holocene (11–7 ka) and cool significantly at 7 ka. This record indicates that current temperatures are cooler than those in the early Holocene and suggests that the eastern Prydz Bay marine environment may evolve with continued warming. Chapter three examines the response of the LG-AIS system to climate and ocean change over the last ~40 ka. Three sedimentary sequences recovered from western Prydz Channel contain a sequence of alternating diatom-rich muds, silts, and sands/diamicts hypothesized to reflect advance and retreat of the LG-AIS system. Using bulk sediment RPO 14C dating, three episodes of LG-AIS retreat are documented over the last ~40 ka that coincide with millennial-scale warm events observed in Antarctic ice cores. This is the first direct record of outlet glacier variability from Antarctica’s continental shelves on millennial timescales during the last glaciation. Sedimentary beryllium-10 confirms that diatom-rich muds preserved in Prydz Channel are deposited in open marine conditions. Results are the first to directly link millennial-scale records of ice rafted debris from Southern Ocean sediments with outlet glacier behavior inferred from ice-proximal sediment records. Chapter four seeks to understand EAIS response to ocean forcing on orbital timescales during the early Pleistocene (1.5–1.0 Ma). This period partially encompasses the Mid-Pleistocene Transition (1.25–0.7 Ma) wherein 41-kyr pacing of glacial cycles in the late Pliocene/early Pleistocene transitions to 100-kyr pacing in the late Pleistocene. To assess relationships between upper ocean temperature and ice sheet mass balance, I generated TEX86-based temperature records from high-resolution sedimentary sequences recovered from the Scotia Sea during the International Ocean Discovery Program Expedition 382 and compared these records with sedimentary physical properties proxies for terrigenous and biogenic input. Results indicate that relatively cool upper ocean temperatures coincide with intervals of greater diatomaceous input. I observe relatively warm upper ocean temperatures between 1.55 and 1.28 Ma, followed by a cool period centered on 1.25 Ma, then a return to relatively warm temperatures between 1.22 and 0.99 Ma. The Scotia Sea paleotemperature record demonstrates an increasing temperature trend between 1.55 and 1.25 Ma, but no discernable trend after 1.25 Ma. I observe an average 42-kyr pacing of Scotia Sea upper ocean temperatures proximal to Antarctica between 1.55 and 1.25 Ma, implying that obliquity plays a major role in modulating Southern Ocean temperatures in the early Pleistocene. The obliquity-paced temperature fluctuations are not apparent after 1.25 Ma, suggesting that the transition to cooler temperatures represents some transition in Southern Ocean response to obliquity. The research presented herein demonstrates that ice-proximal studies are critically required for identifying the source regions of ice mass loss, shed light on magnitudes and rates of glacier retreat, and assess EAIS vulnerability to climate forcing. This dissertation utilizes bulk sedimentary geochemistry in Southern Ocean and Antarctic continental shelf sediments to constrain changes in upper ocean temperature and depositional environment, providing a new perspective to existing paleoenvironmental studies. The ability of ice-proximal paleoceanographic reconstructions to capture EAIS dynamics will be crucial in forecasting future ice mass balance. Because ice-proximal paleoceanographic records provide direct evidence for the timing of glacier retreat, changes in depositional environment, and upper ocean temperature evolution, they can be used to constrain the timing and response of EAIS marine-based glaciers to a range of climate scenarios that models aim to predict, thereby constraining future sea level rise scenarios

Mega-Scale Glacial Lineations and Grounding-Zone Wedges in Prydz Channel, East Antarctica

The Prydz Bay continental shelf was sculpted by the Lambert Glacier–Amery Ice Shelf system, a large outlet glacier that drains 16% of the East Antarctic Ice Sheet (Allison 1979; Fig. 1a). Prydz Channel (71–73° E; Fig. 1b) is a NW-trending cross-shelf trough (500–700 m deep) in western Prydz Bay that formed in the Pliocene, when the Lambert-Amery system first developed a fast-flowing ice stream (Fig. 1b; Cooper & O\u27Brien 2004). In Prydz Channel mega-scale glacial lineations (MSGLs) delineate the direction and orientation of past ice flow, and large grounding-zone wedges (GZWs; Batchelor & Dowdeswell 2015) in the inner channel mark the limit of the Last Glacial Maximum (LGM) ice advance about 250 km from the shelf edge (Fig. 1c, d, e)