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Rapid retreat of the southwestern Laurentide Ice Sheet during the Bølling-Allerød interval
The timing of Laurentide Ice Sheet deglaciation along its southwestern margin controlled the evolution of large glacial lakes and has implications for human migration into the Americas. Accurate reconstruction of the ice sheet’s retreat also constrains glacial isostatic adjustment models and is important for understanding ice-sheet sensitivity to climate forcing. Despite its significance, retreat of the southwestern Laurentide Ice Sheet (SWLIS) is poorly constrained by minimum-limiting 14C data. We present 26 new cosmogenic 10Be exposure ages spanning the western Interior Plains, Canada. Using a Bayesian framework, we combine these data with geomorphic mapping, 10Be, and high-quality minimum-limiting 14C ages to provide an updated chronology. This dataset presents an internally consistent retreat record and indicates that the initial detachment of the SWLIS from its convergence with the Cordilleran Ice Sheet began by ca. 15.0 ka, concurrent with or slightly prior to the onset of the Bølling-Allerød interval (14.7–12.9 ka) and retreated >1200 km to its Younger Dryas (YD) position in ~2500 yr. Ice-sheet stabilization at the Cree Lake Moraine facilitated a meltwater drainage route to the Arctic from glacial Lake Agassiz within the YD, but not necessarily at the beginning. Our record of deglaciation and new YD constraints demonstrate deglaciation of the Interior Plains was ~60% faster than suggested by minimum 14C constraints alone. Numerical modeling of this rapid retreat estimates a loss of ~3.7 m of sea-level equivalent from the SWLIS during the Bølling-Allerød interval.
<p>The Antarctic Ice Sheet is a significant component of the Earth System, modulating Earth‘s sea level and climate. Present day and projected ice mass losses from Antarctica are of paramount concern to human populations in low-lying communities around the world. Ocean freshening from future ice discharge events also has the potential to destabilise global climate patterns. Over 40 years of satellite observations have tracked changes in ice mass, extent and thickness in Antarctica. However, ice sheets respond on timescales that range from annual to millennial, and a geologic perspective is needed to fully understand ice sheet response on timescales longer than a few decades. This research seeks to provide an improved understanding of Antarcticas future by constraining its past. I focus on one of the largest outlet glaciers in Antarctica, the David Glacier/Drygalski Ice Tongue system which drains the East Antarctic Ice Sheet, dissects the Transantarctic Mountains and discharges into the Ross Sea. I seek to answer two questions; (1) what is the timing and nature of David Glacier thinning since the Last Glacial Maximum approximately 20,000 years ago, and (2) what physical processes were responsible for the observed thinning? I answer these questions by mapping the terrestrial and marine geomorphology along the former margins and seaward extension of David Glacier, and by using surface exposure dating of bedrock and glacial erratics to constrain the timing of glacier thinning. I then use a numerical flowline model to identify the processes that drove glacier thinning and retreat. Surface exposure ages from bedrock and glacial erratics at field sites both upstream and downstream of the modern grounding line reveal that David Glacier thinned for two millennia during the mid-Holocene. Near the coast, this thinning occurred at ∼6.5 kya at a rapid rate of up to 2 m/yr. Upstream from the grounding line, the thinning was more gradual but occurred simultaneously with thinning downstream. The timing of glacial thinning at David Glacier correlates with thinning events at other glaciers in the region and is consistent with offshore marine geological records. To identify the mechanisms responsible for the observed thinning of David Glacier, I conduct numerical model sensitivity experiments along a 1,600 km flowline, extending from the ice sheet interior to the continental shelf edge in the western Ross Sea. Offshore, the glacier flowline follows the Drygalski Trough, where it crosses numerous grounding zone wedges of various sizes. The flowline and prescribed ice shelf width is guided by the orientation and distribution of mega-scale glacial lineations as well as overall sea floor bathymetry. I explore the response of a stable, expanded David Glacier to the effects of increasing sub-ice shelf melt rates, and decreasing lateral buttressing which may have occurred as grounded ice in the Ross Sea migrated southward of the David Glacier. These forcings were also combined to explore potential feedbacks associated with Marine Ice Sheet Instability. This modelling demonstrates that David Glacier likely underwent rapid thinning over a period of ∼500 years as the grounding line retreated to a prominent sill at the mouth of David Fjord. After a period of ∼ 5 ka of stability, a second period of grounding line retreat in the model leads to the glacier reaching its modern configuration. This simulated two-phase grounding line retreat compares well with onshore geologically constrained thinning events at two sites (Mt. Kring and Hughes Bluff), both in terms of timing and rates of past glacier thinning. This retreat pattern can be forced by either increased ice shelf melting or reduced buttressing, but when combined, lower melt rates and less lateral buttressing is required to match onshore geologic constraints. Together, the findings in this thesis provide new data to constrain the past behaviour of a significant portion of the East Antarctic Ice Sheet and critical insights into the mechanisms that control ice sheet thinning and retreat. Incorporation of these constraints and improved understanding of the underlying mechanisms driving glacier thinning and grounding line retreat will ultimately improve continental scale ice sheet models which are used to project the future behaviour of the Antarctic Ice Sheet and its influence on global sea level.</p>
<p>Terrestrial cosmogenic exposure studies are an established and rapidly evolving tool for landscapes in both polar and non-polar regions. This thesis takes a multifaceted approach to utilizing and enhancing terrestrial cosmogenic methods. The three main components of this work address method development, reconstructing surface-elevation-changes in two large Antarctic outlet glaciers, and evaluating bedrock erosion rates in Victoria Land, Antarctica. Each facet of this work is intended to enhance its respective field, as well as benefit the other sections of this thesis. Quartz purification is a necessary and critical step to producing robust and reproducible results in terrestrial cosmogenic nuclide studies. Previous quartz purification work has centred on relatively coarse sample material (1 mm-500 μm) and is effective down to 125 μm. However, sample material finer than that poses significant purification challenges and this material is usually discarded. The new purification procedure outlined in this thesis shows that very fine sand size material (125-63 μm) can be reliably cleaned for use in terrestrial cosmogenic nuclide studies. The results below show that 35% mass loss in very fine-grained quartz is sufficient to remove major elements (Al, Ti, Na, K, Fe, Mg, Ca, Mn,) and trace elements (9Be, and 10B) along with meteoric 10Be. Insufficient leaching is most detrimental to Al concentration, however errors up to 27% in exposure age and up to 29% in erosion rate are possible if meteoric 10Be is not fully removed from quartz during the HF leaching stages. Outlet glaciers have been well observed since the beginning of the satellite era, approximately 60 years ago. However, we do not currently know how these important glaciers, which drain a significant portion of the Antarctic Ice Sheet, have behaved on centennial to millennial timescales. Dating glacial erratics deposited by a thinning outlet glacier provides a window into the long-term outlet glacier and ice sheet response to climatic forcing. New results in this thesis constrain the thinning history of Mawson and Tucker glaciers over the last several thousand years. Mawson Glacier undergoes rapid thinning from at least ~6.5 kya to ~4.9 kya then transitions to slower thinning until ~1 kya, with a minimum of 250 m of ice-surface-lowering. While Tucker Glacier ~450 km north undergoes gradual thinning from ~19 kya to ~5 kya with ~300 m of ice-surface-lowering. The results of this work show that either the Tucker Glacier was not significantly affected by the Ross Ice Shelf grounding line, or that Antarctic mountain glaciers respond differently to the outlet glaciers connected to the Easty Antarctic Ice Sheet. The style, rate, magnitude, and duration of thinning is unique to each outlet glacier, even with similar climate forcing. The results of this work shed light on the style and duration of outlet glacier thinning and retreat that is possible following a climate perturbation. Antarctica’s average bedrock erosion rate is consistently lower than 4.5 m/Myr, the lowest bedrock erosion rates for any region on Earth. Therefore, many cosmogenic dating studies assume zero erosion when calculating exposure ages. However, previous erosion rate work in Antarctica is biased to arid high-elevation inland sites (~60% of work) and the hyperarid ice-free McMurdo Dry Valleys (~40% of work). These studies do not capture the effects of coastal maritime climates, where many outlet glacier studies are conducted, on the rate of bedrock erosion. New results presented in this thesis show that the Northern Victoria Land coast has the highest known erosion rate in Antarctica. Two sample sites were selected, one coastal and one in the interior. The coastal bedrock erosion rates are 8.86±0.78 m/Myr and 7.15±0.6 m/Myr while the interior bedrock erosion rates are 1.07±0.08 m/Myr and 0.42±0.03 m/Myr. The coastal erosion rates are average for non-polar cold climates while the inland sites are below average for polar erosion rates. The results suggest a strong gradient in the rate of erosion is present from the Antarctic coastline inland. If exposure ages are not calculated with an appropriate erosion rate the apparent age may under-estimate the actual age by as much as 12%, which is thousands of years for Holocene thinning histories like those found in this thesis.</p>
<p>Terrestrial cosmogenic exposure studies are an established and rapidly evolving tool for landscapes in both polar and non-polar regions. This thesis takes a multifaceted approach to utilizing and enhancing terrestrial cosmogenic methods. The three main components of this work address method development, reconstructing surface-elevation-changes in two large Antarctic outlet glaciers, and evaluating bedrock erosion rates in Victoria Land, Antarctica. Each facet of this work is intended to enhance its respective field, as well as benefit the other sections of this thesis. Quartz purification is a necessary and critical step to producing robust and reproducible results in terrestrial cosmogenic nuclide studies. Previous quartz purification work has centred on relatively coarse sample material (1 mm-500 μm) and is effective down to 125 μm. However, sample material finer than that poses significant purification challenges and this material is usually discarded. The new purification procedure outlined in this thesis shows that very fine sand size material (125-63 μm) can be reliably cleaned for use in terrestrial cosmogenic nuclide studies. The results below show that 35% mass loss in very fine-grained quartz is sufficient to remove major elements (Al, Ti, Na, K, Fe, Mg, Ca, Mn,) and trace elements (9Be, and 10B) along with meteoric 10Be. Insufficient leaching is most detrimental to Al concentration, however errors up to 27% in exposure age and up to 29% in erosion rate are possible if meteoric 10Be is not fully removed from quartz during the HF leaching stages. Outlet glaciers have been well observed since the beginning of the satellite era, approximately 60 years ago. However, we do not currently know how these important glaciers, which drain a significant portion of the Antarctic Ice Sheet, have behaved on centennial to millennial timescales. Dating glacial erratics deposited by a thinning outlet glacier provides a window into the long-term outlet glacier and ice sheet response to climatic forcing. New results in this thesis constrain the thinning history of Mawson and Tucker glaciers over the last several thousand years. Mawson Glacier undergoes rapid thinning from at least ~6.5 kya to ~4.9 kya then transitions to slower thinning until ~1 kya, with a minimum of 250 m of ice-surface-lowering. While Tucker Glacier ~450 km north undergoes gradual thinning from ~19 kya to ~5 kya with ~300 m of ice-surface-lowering. The results of this work show that either the Tucker Glacier was not significantly affected by the Ross Ice Shelf grounding line, or that Antarctic mountain glaciers respond differently to the outlet glaciers connected to the Easty Antarctic Ice Sheet. The style, rate, magnitude, and duration of thinning is unique to each outlet glacier, even with similar climate forcing. The results of this work shed light on the style and duration of outlet glacier thinning and retreat that is possible following a climate perturbation. Antarctica’s average bedrock erosion rate is consistently lower than 4.5 m/Myr, the lowest bedrock erosion rates for any region on Earth. Therefore, many cosmogenic dating studies assume zero erosion when calculating exposure ages. However, previous erosion rate work in Antarctica is biased to arid high-elevation inland sites (~60% of work) and the hyperarid ice-free McMurdo Dry Valleys (~40% of work). These studies do not capture the effects of coastal maritime climates, where many outlet glacier studies are conducted, on the rate of bedrock erosion. New results presented in this thesis show that the Northern Victoria Land coast has the highest known erosion rate in Antarctica. Two sample sites were selected, one coastal and one in the interior. The coastal bedrock erosion rates are 8.86±0.78 m/Myr and 7.15±0.6 m/Myr while the interior bedrock erosion rates are 1.07±0.08 m/Myr and 0.42±0.03 m/Myr. The coastal erosion rates are average for non-polar cold climates while the inland sites are below average for polar erosion rates. The results suggest a strong gradient in the rate of erosion is present from the Antarctic coastline inland. If exposure ages are not calculated with an appropriate erosion rate the apparent age may under-estimate the actual age by as much as 12%, which is thousands of years for Holocene thinning histories like those found in this thesis.</p>
<p>The Antarctic Ice Sheet is a significant component of the Earth System, modulating Earth‘s sea level and climate. Present day and projected ice mass losses from Antarctica are of paramount concern to human populations in low-lying communities around the world. Ocean freshening from future ice discharge events also has the potential to destabilise global climate patterns. Over 40 years of satellite observations have tracked changes in ice mass, extent and thickness in Antarctica. However, ice sheets respond on timescales that range from annual to millennial, and a geologic perspective is needed to fully understand ice sheet response on timescales longer than a few decades. This research seeks to provide an improved understanding of Antarcticas future by constraining its past. I focus on one of the largest outlet glaciers in Antarctica, the David Glacier/Drygalski Ice Tongue system which drains the East Antarctic Ice Sheet, dissects the Transantarctic Mountains and discharges into the Ross Sea. I seek to answer two questions; (1) what is the timing and nature of David Glacier thinning since the Last Glacial Maximum approximately 20,000 years ago, and (2) what physical processes were responsible for the observed thinning? I answer these questions by mapping the terrestrial and marine geomorphology along the former margins and seaward extension of David Glacier, and by using surface exposure dating of bedrock and glacial erratics to constrain the timing of glacier thinning. I then use a numerical flowline model to identify the processes that drove glacier thinning and retreat. Surface exposure ages from bedrock and glacial erratics at field sites both upstream and downstream of the modern grounding line reveal that David Glacier thinned for two millennia during the mid-Holocene. Near the coast, this thinning occurred at ∼6.5 kya at a rapid rate of up to 2 m/yr. Upstream from the grounding line, the thinning was more gradual but occurred simultaneously with thinning downstream. The timing of glacial thinning at David Glacier correlates with thinning events at other glaciers in the region and is consistent with offshore marine geological records. To identify the mechanisms responsible for the observed thinning of David Glacier, I conduct numerical model sensitivity experiments along a 1,600 km flowline, extending from the ice sheet interior to the continental shelf edge in the western Ross Sea. Offshore, the glacier flowline follows the Drygalski Trough, where it crosses numerous grounding zone wedges of various sizes. The flowline and prescribed ice shelf width is guided by the orientation and distribution of mega-scale glacial lineations as well as overall sea floor bathymetry. I explore the response of a stable, expanded David Glacier to the effects of increasing sub-ice shelf melt rates, and decreasing lateral buttressing which may have occurred as grounded ice in the Ross Sea migrated southward of the David Glacier. These forcings were also combined to explore potential feedbacks associated with Marine Ice Sheet Instability. This modelling demonstrates that David Glacier likely underwent rapid thinning over a period of ∼500 years as the grounding line retreated to a prominent sill at the mouth of David Fjord. After a period of ∼ 5 ka of stability, a second period of grounding line retreat in the model leads to the glacier reaching its modern configuration. This simulated two-phase grounding line retreat compares well with onshore geologically constrained thinning events at two sites (Mt. Kring and Hughes Bluff), both in terms of timing and rates of past glacier thinning. This retreat pattern can be forced by either increased ice shelf melting or reduced buttressing, but when combined, lower melt rates and less lateral buttressing is required to match onshore geologic constraints. Together, the findings in this thesis provide new data to constrain the past behaviour of a significant portion of the East Antarctic Ice Sheet and critical insights into the mechanisms that control ice sheet thinning and retreat. Incorporation of these constraints and improved understanding of the underlying mechanisms driving glacier thinning and grounding line retreat will ultimately improve continental scale ice sheet models which are used to project the future behaviour of the Antarctic Ice Sheet and its influence on global sea level.</p>
<p>Dirt. It is more important than one might think. Soil, along with its bedrock-derived components, provides a nexus in the earth system for energy, nutrient, and atmospheric control; yet it is a finite resource. Soils are consumed, transported, and replenished by natural and anthropogenic forces. Weathering—both physical and chemical—is the key process breaking down and regenerating the ions and mineral constituents of soils, facilitating the pathways from solid bedrock to soil to the rest of the global ecosystem. Yet our understanding of weathering is incomplete and the available methods to investigate these processes are limited. Here, the fundamental processes of weathering are questioned by studying them at their origins, the rock surface. New techniques were developed in pursuit of quantifying weathering at small scales in-situ, to obtain the highest resolution measurements possible. These were carried out in the proglacial regions of two New Zealand glaciers, Brewster Glacier and Franz Josef Glacier. Proglacial bedrock environments provided a clean-slate model from which to measure incipient weathering at increasing exposure ages. To mitigate error, a holistic approach encompassing weathering signals from multiple angles was taken. Spatial characterisation was completed through the capture of structure-from-motion photogrammetry (SFM) at multiple scales of observation. The resultant three dimensional surface models had an average error of 1.06x10-1 mm. The models were characterised for weathering using roughness as a novel multi-point analysis of surface features, through two separate novel methods utilising global polynomial interpolation filtering and continuous wavelet transform analysis. Physical samples were collected from the field for cosmogenic radionuclide surface exposure age dating. Compositional analysis was performed through X-ray fluorescence, as well as electron microprobe analysis (EPMA). Nano-scale structural and compositional trends were investigated through optical analysis of backscatter electron imaging and secondary electron imaging. Non-directional roughness and volumetric analysis patterns present compelling information to support negligible weathering occurring on bedrock surfaces in proglacial environments. Lithologic variation was identified as a strong influence on the results. Compositional analysis demonstrated insignificant levels of chemical alteration between sites, corroborating the spatial modelling results. The lack of surficial weathering in highly productive weathering environments necessitates the role of additional weathering factors. Deep subsurface weathering was investigated and presents the strongest case as a major contributor to chemical denudation. Validating the presence of deep weathering in many environments critically alters the knowledge required to evaluate and predict patterns of landscape evolution. By establishing a better understanding of how bedrock weathers in-situ, the groundwork is laid for making more accurate and educated forecasts on how the earth system will respond to changes in the future.</p>
Abstract. Quantitative satellite observations only provide an assessment of ice sheet mass loss over the last four decades. To assess long-term drivers of ice sheet change, geological records are needed. Here we present the first millennial-scale reconstruction of David Glacier, the largest East Antarctic outlet glacier in Victoria Land. To reconstruct changes in ice thickness, we use surface exposure ages of glacial erratics deposited on nunataks adjacent to fast-flowing sections of David Glacier. We then use numerical modelling experiments to determine the drivers of glacial thinning. Thinning profiles derived from 45 10Be and 3He surface exposure ages show David Glacier experienced rapid thinning of up to 2 m/yr during the mid-Holocene (∼ 6.5 ka). Thinning slowed at 6 ka, suggesting the initial formation of the Drygalski Ice Tongue at this time. Our work, along with ice thinning records from adjacent glaciers, shows simultaneous glacier thinning in this sector of the Transantarctic Mountains occurred 4–7 kyr after the peak period of ice thinning indicated in a suite of published ice sheet models. The timing and rapidity of the reconstructed thinning at David Glacier is similar to reconstructions in the Amundsen and Weddell embayments. To identify the drivers of glacier thinning along the David Glacier, we use a glacier flowline model designed for calving glaciers and compare modelled results against our geological data. We show that glacier thinning and marine-based grounding-line retreat are controlled by either enhanced sub-ice-shelf melting, reduced lateral buttressing or a combination of the two, leading to marine ice sheet instability. Such rapid glacier thinning events during the mid-Holocene are not fully captured in continental- or catchment-scale numerical modelling reconstructions. Together, our chronology and modelling identify and constrain the drivers of a ∼ 2000-year period of dynamic glacier thinning in the recent geological past.
<p>Global sea level rise is contributing to the acceleration of cliff erosion rates in New Zealand, where it surpasses rates of uplift. A significant challenge facing scientists and managers is that we have no method for reliably extracting past rates of coastal erosion along harder rock cliffs over the time-scales that significant sea level change occurs (100s-1000s of years). This gap in knowledge is limiting efforts to model and understand the relationship between sea level rise and cliff erosion rates and what form of that relationship takes. Cosmogenic Beryllium-10 analysis has been applied on two low angle shore platforms in New Zealand to produce chronologies of sea cliff retreat during the late-Holocene. Surface exposure ages were attained on a tectonically active platform at Kaikoura, Canterbury and a tectonically quiescent platform at Cape Rodney, Auckland. This is the first application of cosmogenic nuclides to a shore platform study in New Zealand and adds two new data-sets to the very small group of global shore platform chronologies. Exposure ages show New Zealand platforms have developed in the late-Holocene. Long-term platform surface erosion rates at Kaikoura (0.4mm a-1), potentially due to uplift driven positive feedback such as altered sea level position, driving up weathering rates on the tidally inundated platform. Nuclide concentrations at Okakari Point, Rodney, reveal a significant role of recent sea level fall after ~4000yrs BP, driving surface denudation (0.1mm a-1). The long-term cliff back-wearing rate at Okakari point was found to be 24.66mm a-1. Patterns in cosmogenic nuclide concentrations in New Zealand’s shallow platforms differ from global examples recorded on steeper platforms. Exploratory numerical modelling was applied with the coupled Rocky Profile CRN model (RPM_CRN) to identify process relationships between key drivers within platform coastal systems and scenarios of sea level change and active tectonics. This combined geochemical and numerical modelling study has shown that shore platforms in New Zealand have complex histories, with different potential driving forces at Kaikoura and Okakari. This highlights the local variability in platform development and cliff retreat, suggesting that estimates of future shoreline erosion will need to take local contingencies into account.</p>
<p>Global sea level rise is contributing to the acceleration of cliff erosion rates in New Zealand, where it surpasses rates of uplift. A significant challenge facing scientists and managers is that we have no method for reliably extracting past rates of coastal erosion along harder rock cliffs over the time-scales that significant sea level change occurs (100s-1000s of years). This gap in knowledge is limiting efforts to model and understand the relationship between sea level rise and cliff erosion rates and what form of that relationship takes. Cosmogenic Beryllium-10 analysis has been applied on two low angle shore platforms in New Zealand to produce chronologies of sea cliff retreat during the late-Holocene. Surface exposure ages were attained on a tectonically active platform at Kaikoura, Canterbury and a tectonically quiescent platform at Cape Rodney, Auckland. This is the first application of cosmogenic nuclides to a shore platform study in New Zealand and adds two new data-sets to the very small group of global shore platform chronologies. Exposure ages show New Zealand platforms have developed in the late-Holocene. Long-term platform surface erosion rates at Kaikoura (0.4mm a-1), potentially due to uplift driven positive feedback such as altered sea level position, driving up weathering rates on the tidally inundated platform. Nuclide concentrations at Okakari Point, Rodney, reveal a significant role of recent sea level fall after ~4000yrs BP, driving surface denudation (0.1mm a-1). The long-term cliff back-wearing rate at Okakari point was found to be 24.66mm a-1. Patterns in cosmogenic nuclide concentrations in New Zealand’s shallow platforms differ from global examples recorded on steeper platforms. Exploratory numerical modelling was applied with the coupled Rocky Profile CRN model (RPM_CRN) to identify process relationships between key drivers within platform coastal systems and scenarios of sea level change and active tectonics. This combined geochemical and numerical modelling study has shown that shore platforms in New Zealand have complex histories, with different potential driving forces at Kaikoura and Okakari. This highlights the local variability in platform development and cliff retreat, suggesting that estimates of future shoreline erosion will need to take local contingencies into account.</p>
