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1. New constraints on the last deglaciation of the Cordilleran Ice Sheet in coastal Southeast Alaska

   https://doi.org/10.1017/qua.2020.32  

   Lesnek, Alia J.; Briner, Jason P.; Baichtal, James F.; Lyles, Alex S. (July 2020, Quaternary Research)

   null (Ed.)

   Abstract Understanding marine-terminating ice sheet response to past climate transitions provides valuable long-term context for observations of modern ice sheet change. Here, we reconstruct the last deglaciation of marine-terminating Cordilleran Ice Sheet (CIS) margins in Southeast Alaska and explore potential forcings of western CIS retreat. We combine 27 new cosmogenic 10 Be exposure ages, 13 recently published 10 Be ages, and 25 new 14 C ages from raised marine sediments to constrain CIS recession. Retreat from the outer coast was underway by 17 ka, and the inner fjords and sounds were ice-free by 15 ka. After 15 ka, the western margin of the CIS became primarily land-terminating and alpine glaciers disappeared from the outer coast. Isolated alpine glaciers may have persisted in high inland peaks until the early Holocene. Our results suggest that the most rapid phase of CIS retreat along the Pacific coast occurred between ~17 and 15 ka. This retreat was likely driven by processes operating at the ice-ocean interface, including sea level rise and ocean warming. CIS recession after ~15 ka occurred during a time of climatic amelioration in this region, when both ocean and air temperatures increased. These data highlight the sensitivity of marine-terminating CIS regions to deglacial climate change. 

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2. Heterogeneous Basal Thermal Conditions Underpinning the Adélie‐George V Coast, East Antarctica

   https://doi.org/10.1029/2023GL105450  

   Dawson, Eliza J.; Schroeder, Dustin M.; Chu, Winnie; Mantelli, Elisa; Seroussi, Helene (January 2024, Geophysical Research Letters)

   Abstract Adélie‐George V Land in East Antarctica, encompassing the vast Wilkes Subglacial Basin, has a configuration that could be prone to ice sheet instability: the basin's retrograde bed slope could make its marine terminating glaciers vulnerable to warm seawater intrusion and irreversible retreat under predicted climate forcing. However, future projections are uncertain, due in part to limited subglacial observations near the grounding zone. Here, we develop a novel statistical approach to characterize subglacial conditions from radar sounding observations. Our method reveals intermixed frozen and thawed bed within 100 km of the grounding‐zone near the Wilkes Subglacial Basin outflow, and enables comparisons to ice sheet model‐inferred thermal states. The signs of intermixed or near thawed conditions raises the possibility that changes in basal thermal state could impact the stability of Adélie‐George V Land, adding to the region's potentially vulnerable topographic configuration and sensitivity to ocean forcing driven grounding line retreat. 

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3. Greenland Subglacial Discharge as a Driver of Hotspots of Increasing Coastal Chlorophyll Since the Early 2000s

   https://doi.org/10.1029/2022GL102689  

   Oliver, Hilde; Slater, Donald; Carroll, Dustin; Wood, Michael; Morlighem, Mathieu; Hopwood, Mark J. (May 2023, Geophysical Research Letters)

   Abstract Subglacial discharge emerging from the base of Greenland's marine‐terminating glaciers drives upwelling of nutrient‐rich bottom waters to the euphotic zone, which can fuel nitrate‐limited phytoplankton growth. Here, we use buoyant plume theory to quantify this subglacial discharge‐driven nutrient supply on a pan‐Greenland scale. The modeled nitrate fluxes were concentrated in a few critical systems, with half of the total modeled nitrate flux anomaly occurring at just 14% of marine‐terminating glaciers. Increasing subglacial discharge fluxes results in elevated nitrate fluxes, with the largest flux occurring at Jakobshavn Isbræ in Disko Bay, where subglacial discharge is largest. Subglacial discharge and nitrate flux anomaly also account for significant temporal variability in summer satellite chlorophyll a (Chl) within 50 km of Greenland's coast, particularly in some regions in central west and northwest Greenland. 

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4. Evaluation of Iceberg Calving Models Against Observations From Greenland Outlet Glaciers

   https://doi.org/10.1029/2019JF005444  

   Amaral, Tristan; Bartholomaus, Timothy_C; Enderlin, Ellyn_M (June 2020, Journal of Geophysical Research: Earth Surface)

   Abstract Frontal ablation processes at marine‐terminating glaciers are challenging to observe and difficult to represent in numerical ice flow models, yet play critical roles in modulating ice sheet mass balance. Current ice sheet models typically rely on simple iceberg calving models to prescribe either terminus positions or iceberg calving rates, but the relative accuracies and uncertainties of these calving models remain largely unconstrained at the ice sheet scale. Here, we evaluate six published iceberg calving models against spatially and temporally diverse observations from 50 marine‐terminating outlet glaciers in Greenland. We seek the single model that best reproduces observed conditions across all glaciers, at all observation times, and with low sensitivity to calibration uncertainty. Five of six calving models can produce unbiased estimates of calving position or calving rate at the ice sheet scale. However, time series analysis reveals that, when using a single, optimized model parameter, rate‐predicting calving models frequently yield calving rate errors in excess of 10 m d⁻¹. In comparison, terminus position‐predicting calving models more accurately track observed changes in terminus position (remaining within ~1 km of the observed grounded terminus position). Overall, our results indicate that the crevasse depth calving model provides the best balance of high accuracy and low sensitivity to imperfect parameter calibration. While the crevasse depth model appears unlikely to capture the true controls on crevasse penetration, numerically, it reproduces observed terminus dynamics with high fidelity and should be considered a leading candidate for use in models of the Greenland Ice Sheet. 

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5. The dynamics of a subglacial salt wedge

   https://doi.org/10.1017/jfm.2020.308  

   Wilson, Earle A.; Wells, Andrew J.; Hewitt, Ian J.; Cenedese, Claudia (July 2020, Journal of Fluid Mechanics)

   Marine-terminating glaciers, such as those along the coastline of Greenland, often release meltwater into the ocean in the form of subglacial discharge plumes. Though these plumes can dramatically alter the mass loss along the front of a glacier, the conditions surrounding their genesis remain poorly constrained. In particular, little is known about the geometry of subglacial outlets and the extent to which seawater may intrude into them. Here, the latter is addressed by exploring the dynamics of an arrested salt wedge – a steady-state, two-layer flow system where salty water partially intrudes a channel carrying fresh water. Building on existing theory, we formulate a model that predicts the length of a non-entraining salt wedge as a function of the Froude number, the slope of the channel and coefficients for interfacial and wall drag. In conjunction, a series of laboratory experiments were conducted to observe a salt wedge within a rectangular channel. For experiments conducted with laminar flow (Reynolds number $Re<800$ ), good agreement with theoretical predictions are obtained when the drag coefficients are modelled as being inversely proportional to $Re$ . However, for fully turbulent flows on geophysical scales, these drag coefficients are expected to asymptote toward finite values. Adopting reasonable drag coefficient estimates for this flow regime, our theoretical model suggests that typical subglacial channels may permit seawater intrusions of the order of several kilometres. While crude, these results indicate that the ocean has a strong tendency to penetrate subglacial channels and potentially undercut the face of marine-terminating glaciers. 

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