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1. Sedimentary Signatures of Persistent Subglacial Meltwater Drainage From Thwaites Glacier, Antarctica

   https://doi.org/10.3389/feart.2022.863200  

   Lepp, A. P.; Simkins, L. M.; Anderson, J. B.; Clark, R. W.; Wellner, J. S.; Hillenbrand, C-D.; Smith, J. A.; Lehrmann, A. A.; Totten, R.; Larter, R. D.; et al (May 2022, Frontiers in Earth Science)

   Subglacial meltwater drainage can enhance localized melting along grounding zones and beneath the ice shelves of marine-terminating glaciers. Efforts to constrain the evolution of subglacial hydrology and the resulting influence on ice stability in space and on decadal to millennial timescales are lacking. Here, we apply sedimentological, geochemical, and statistical methods to analyze sediment cores recovered offshore Thwaites Glacier, West Antarctica to reconstruct meltwater drainage activity through the pre-satellite era. We find evidence for a long-lived subglacial hydrologic system beneath Thwaites Glacier and indications that meltwater plumes are the primary mechanism of sedimentation seaward of the glacier today. Detailed core stratigraphy revealed through computed tomography scanning captures variability in drainage styles and suggests greater magnitudes of sediment-laden meltwater have been delivered to the ocean in recent centuries compared to the past several thousand years. Fundamental similarities between meltwater plume deposits offshore Thwaites Glacier and those described in association with other Antarctic glacial systems imply widespread and similar subglacial hydrologic processes that occur independently of subglacial geology. In the context of Holocene changes to the Thwaites Glacier margin, it is likely that subglacial drainage enhanced submarine melt along the grounding zone and amplified ice-shelf melt driven by oceanic processes, consistent with observations of other West Antarctic glaciers today. This study highlights the necessity of accounting for the influence of subglacial hydrology on grounding-zone and ice-shelf melt in projections of future behavior of the Thwaites Glacier ice margin and marine-based glaciers around the Antarctic continent. 

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2. Brief communication: Heterogenous thinning and subglacial lake activity on Thwaites Glacier, West Antarctica

   https://doi.org/10.5194/tc-14-4603-2020  

   Hoffman, Andrew O.; Christianson, Knut; Shapero, Daniel; Smith, Benjamin E.; Joughin, Ian (January 2020, The Cryosphere)

   null (Ed.)

   Abstract. A system of subglacial lakes drained on Thwaites Glacier from 2012–2014. To improve coverage for subsequent drainage events, we extended theelevation and ice-velocity time series on Thwaites Glacier through austral winter 2019. These new observations document a second drainage cycle in2017/18 and identified two new lake systems located in the western tributaries of Thwaites and Haynes glaciers. In situ and satellite velocityobservations show temporary < 3 % speed fluctuations associated with lake drainages. In agreement with previous studies, these observationssuggest that active subglacial hydrology has little influence on thinning and retreat of Thwaites Glacier on decadal to centennial timescales. 

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3. Tidewater-glacier response to supraglacial lake drainage

   https://doi.org/10.1038/s41467-022-33763-2  

   Stevens, Laura A.; Nettles, Meredith; Davis, James L.; Creyts, Timothy T.; Kingslake, Jonathan; Hewitt, Ian J.; Stubblefield, Aaron (October 2022, Nature Communications)

   Abstract The flow speed of the Greenland Ice Sheet changes dramatically in inland regions when surface meltwater drains to the bed. But ice-sheet discharge to the ocean is dominated by fast-flowing outlet glaciers, where the effect of increasing surface melt on annual discharge is unknown. Observations of a supraglacial lake drainage at Helheim Glacier, and a consequent velocity pulse propagating down-glacier, provide a natural experiment for assessing the impact of changes in injected meltwater, and allow us to interrogate the subglacial hydrological system. We find a highly efficient subglacial drainage system, such that summertime lake drainage has little net effect on ice discharge. Our results question the validity of common remote-sensing approaches for inferring subglacial conditions, knowledge of which is needed for improved projections of sea-level rise. 

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4. Ice thickness estimates of Lemon Creek Glacier, Alaska, from active-source seismic imaging

   https://doi.org/10.1017/jog.2021.32  

   Veitch, Stephen A.; Karplus, Marianne; Kaip, Galen; Gonzalez, Lucia F.; Amundson, Jason M.; Bartholomaus, Timothy C. (October 2021, Journal of Glaciology)

   Abstract Lemon Creek Glacier, a temperate valley glacier in the Juneau Icefield of Southeast Alaska, is the site of long running (>60 years) glaciological studies. However, the most recent published estimates of its thickness and subglacial topography come from two ~50 years old sources that are not in agreement and do not account for the effects of years of negative mass balance. We collected a 1-km long active-source seismic line on the upper section of the glacier parallel and near to the centerline of the glacier, roughly straddling the equilibrium-line altitude. We used these data to perform joint reflection-refraction velocity modeling and reflection imaging of the glacier bed. We find that this upper section of Lemon Creek Glacier is as much as 150 m (~65%) thicker than previously suggested with a large overdeepening in an area previously believed to have a uniform thickness. Our results lead us to reinterpret the impact of basal motion on ice flow and have a significant impact on expectations of subglacial hydrology. We suggest that further efforts to develop a whole-glacier model of subglacial topography are necessary to support studies that require accurate models of ice thickness and subglacial topography. 

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5. Meltwater drainage and iceberg calving observed in high-spatiotemporal resolution at Helheim Glacier, Greenland

   https://doi.org/10.1017/jog.2021.141  

   Melton, Sierra M.; Alley, Richard B.; Anandakrishnan, Sridhar; Parizek, Byron R.; Shahin, Michael G.; Stearns, Leigh A.; LeWinter, Adam L.; Finnegan, David C. (January 2021, Journal of Glaciology)

   Abstract Marine-terminating glaciers lose mass through melting and iceberg calving, and we find that meltwater drainage systems influence calving timing at Helheim Glacier, a tidewater glacier in East Greenland. Meltwater feeds a buoyant subglacial discharge plume at the terminus of Helheim Glacier, which rises along the glacial front and surfaces through the mélange. Here, we use high-resolution satellite and time-lapse imagery to observe the surface expression of this meltwater plume and how plume timing and location compare with that of calving and supraglacial meltwater pooling from 2011 to 2019. The plume consistently appeared at the central terminus even as the glacier advanced and retreated, fed by a well-established channelized drainage system with connections to supraglacial water. All full-thickness calving episodes, both tabular and non-tabular, were separated from the surfacing plume by either time or by space. We hypothesize that variability in subglacial hydrology and basal coupling drive this inverse relationship between subglacial discharge plumes and full-thickness calving. Surfacing plumes likely indicate a low-pressure subglacial drainage system and grounded terminus, while full-thickness calving occurrence reflects a terminus at or close to flotation. Our records of plume appearance and full-thickness calving therefore represent proxies for the grounding state of Helheim Glacier through time. 

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