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Advances in understanding subglacial meltwater drainage from past ice sheets

Published online by Cambridge University Press:  17 April 2023

Lauren M. Simkins*
Affiliation:
Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA
Sarah L. Greenwood
Affiliation:
Department of Geological Sciences, Stockholm University, 10691 Stockholm, Sweden
Monica C. M. Winsborrow
Affiliation:
Department of Geosciences, Centre for Arctic Gas Hydrate, Environment and Climate, UiT The Arctic University of Norway, 9037 Tromsø, Norway
Lilja R. Bjarnadóttir
Affiliation:
Geological Survey of Norway, 7491 Trondheim, Norway
Allison P. Lepp
Affiliation:
Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA
*
Author for correspondence: Lauren M. Simkins, E-mail: lsimkins@virginia.edu
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Abstract

Meltwater drainage beneath ice sheets is a fundamental consideration for understanding ice–bed conditions and bed-modulated ice flow, with potential impacts on terminus behavior and ice-shelf mass balance. While contemporary observations reveal the presence of basal water movement in the subglacial environment and inferred styles of drainage, the geological record of former ice sheets, including sediments and landforms on land and the seafloor, aids in understanding the spatiotemporal evolution of efficient and inefficient drainage systems and their impact on ice-sheet behavior. We highlight the past decade of advances in geological studies that focus on providing process-based information on subglacial hydrology of ice sheets, how these studies inform theory, numerical models and contemporary observations, and address the needs for future research.

Type
Letter
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
Copyright © The Author(s), 2023. Published by Cambridge University Press on behalf of The International Glaciological Society

1. Introduction

Liquid water beneath ice sheets influences ice-flow organization and velocity (Kyrke-Smith and others, Reference Kyrke-Smith, Katz and Fowler2014; Bell and others, Reference Bell, Banwell, Trusel and Kingslake2007; Larter and others, Reference Larter2019), subglacial sediment rheology and transport (Damsgaard and others, Reference Damsgaard, Goren and Suckale2020; Minchew and Meyer, Reference Minchew and Meyer2020), grounding-line behavior (Horgan and others, Reference Horgan2013; Fried and others, Reference Fried2015) and ice-shelf mass balance (Le Brocq and others, Reference Le Brocq2013; Alley and others, Reference Alley, Scambos, Siegfried and Fricker2016). Ice-sheet response, however, is contingent on subglacial water supply and drainage organization (Röthlisberger, Reference Röthlisberger1972; Walder, Reference Walder1986; Schoof, Reference Schoof2010). While some components of subglacial hydrological systems are relatively stable (i.e. fixed), such as large subglacial lakes beneath the East Antarctic ice sheet (Kapitsa and others, Reference Kapitsa, Ridley, de Robin, Siegert and Zotikov1996) and incised bedrock channels (Kirkham and others, Reference Kirkham2020), other reservoirs and drainage pathways are more transient and evolve through time and space with non-linear and spatially heterogeneous impacts on ice-sheet behavior (Schroeder and others, Reference Schroeder, Blankenship and Young2013; Andrews and others, Reference Andrews2014; Hoffman and others, Reference Hoffman2016; Siegfried and others, Reference Siegfried, Fricker, Carter and Tulaczyk2016; Rada and Schoof, Reference Rada and Schoof2018). Beyond the grounding line, contemporary sediment plumes emanating from marine-terminating outlet glaciers of the Greenland ice sheet observed via satellite imagery (Fried and others, Reference Fried2015; Schild and others, Reference Schild, Hawley and Morriss2016) and surficial expressions of channelization beneath Antarctic ice shelves (Le Brocq and others, Reference Le Brocq2013; Alley and others, Reference Alley, Scambos, Siegfried and Fricker2016) indicate active subglacial hydrological systems upstream.

Major advances in observing contemporary ice-sheet hydrology, such as radar specularity (Schroeder and others, Reference Schroeder, Blankenship and Young2013) and repeat satellite measurements (Fricker and others, Reference Fricker, Siegfried, Carter and Scambos2016), reveal spatiotemporal evolution of basal water transmission on sub-decadal scales. Yet, the limited nature of long-term (decadal to millennial) observations impedes holistic perspectives on the modes and magnitudes of water drainage beneath ice sheets and their consequences for ice-sheet behavior. In formerly glaciated landscapes and continental margins, relict subglacial water drainage is recorded by meltwater landforms and sedimentological successions (Fig. 1; Kehew and others, Reference Kehew, Piotrowski and Jørgensen2012; Lee and others, Reference Lee, Wakefield, Phillips and Hughes2015; Greenwood and others, Reference Greenwood, Clason, Helanow and Margold2016; Esteves and others, Reference Esteves, Bjarnadóttir, Winsborrow, Shackleton and Andreassen2017). Channels (broadly defined) incised into bedrock and sediments and positive-relief esker ridges record drainage styles and organization (Storrar and others, Reference Storrar, Stokes and Evans2014; Zoet and others, Reference Zoet, Muto, Rawling and Attig2019; Lewington and others, Reference Lewington, Livingstone, Clark, Sole and Storrar2020) and, in some cases, associated ice-margin retreat behavior (Livingstone and others, Reference Livingstone2020; Simkins and others, Reference Simkins2021) and creation of tidal embayments in grounding lines (Horgan and others, Reference Horgan2013). Distinct meltwater plume deposits and hydrologically sorted sediments reveal relative magnitudes and frequency of water drainage into the ocean, precursory, synchronous or resulting glacial environment changes, and geochemical signatures of sediment and water provenance (Witus and others, Reference Witus2014; O'Regan and others, Reference O'Regan2021; Lepp and others, Reference Lepp2022). Such empirical observations based on the geological record have long guided and continue to inform, and challenge, glaciological theory (e.g. Walder and Hallet, Reference Walder and Hallet1979; Boulton and others, Reference Boulton, Hagdorn, Maillot and Zatsepin2009; Hewitt, Reference Hewitt2011).

Fig. 1. (a) Murtoo pathway within glacially streamlined terrain in central Finland (Mäkinen and others, Reference Mäkinen, Kajuutti, Palmu, Ojala and Ahokangas2017; Ojala and others, Reference Ojala2019). Data: LiDAR-based DEM from the National Land Survey of Finland. (b) Meltwater channels and eskers drape and incise drumlins in the Bothnian Sea (Greenwood and others, Reference Greenwood, Clason, Nyberg, Jakobsson and Holmlund2017). Data: MBES-based DEM from the Swedish Maritime Administration. (c) Meltwater channel incised retreat moraines (red dots) on Thor Iversenbanken in the Central Barents Sea (Esteves and others, Reference Esteves, Bjarnadóttir, Winsborrow, Shackleton and Andreassen2017). Data: MAREANO MBES-based bathymetry from the Norwegian Mapping Authority. (d) Meltwater corridor in which channels cross-cut grounding zone wedges (red dots) in the Ross Sea, Antarctica (Simkins and others, Reference Simkins2021). Data: MBES-based DEM from cruise NBP15-02, available through the United States Antarctic Program Data Center. In (a)–(d), red dashed lines outline the encompassing areas of meltwater landforms. (e) CT scan and photograph of the upper 250 cm of sediment core NBP19-02 KC-08, collected in the Amundsen Sea, records meltwater plume events that emanated from the Thwaites Glacier grounding line (Lepp and others, Reference Lepp2022).

2. Recent scientific advances from paleo-ice sheets

Over the past decade, advances in geophysical methods, growing data accessibility and sedimentological studies from deglaciated terrains have pushed the boundaries of subglacial hydrology understanding and challenged concepts of the spatiotemporal evolution of water drainage beneath ice sheets. Near complete coverage of light detection and ranging (LiDAR) and satellite photogrammetry elevation data across terrestrial landscapes formerly glaciated by the European and North American ice sheets, give unprecedented views of paleo-subglacial meltwater landforms that range in relief from 10−1 to 102 m and lengths of 101–105 m, allowing holistic ice-sheet scale assessment of controls on drainage. While nowhere near as complete, increasing coverage and quality of bathymetry data from deglaciated continental shelves provide perspectives on water flow beneath marine-based ice sheets and implications for ice-sheet behavior at fine scales previously unseen. These offshore advances via multibeam echo sounding (MBES) surveys are facilitated by national hydrographic programs such as MAREANO (e.g. Esteves and others, Reference Esteves, Bjarnadóttir, Winsborrow, Shackleton and Andreassen2017), marine geological repositories such as the Marine Geoscience Data System and researcher-led surveying and compilations (e.g. Greenwood and others, Reference Greenwood, Simkins, Winsborrow and Bjarnadóttir2021). Additionally, 3-D seismic survey grids, albeit sparse but increasing in spatial coverage due to industry-academic relations, are unique datasets to assess temporal evolution of drainage pathways and internal architecture of meltwater landforms (e.g. Kirkham and others, Reference Kirkham2021, Reference Kirkham2022). These advances in terrain data acquisition and availability have not only permitted ice-sheet scale documentation of small-scale (meter to sub-meter) and intricate meltwater landforms, demonstrating their variability as well as near ubiquity, but have also uncovered both new types of landforms and little recognized meltwater landform assemblages, stimulating new hypotheses for meltwater landform genesis and understanding of the coupling to ice-flow and ice-margin behavior (Storrar and others, Reference Storrar, Stokes and Evans2014; Ojala and others, Reference Ojala2019; Kirkham and others, Reference Kirkham2020).

Challenging the traditional binary categorization of subglacial drainage through either ‘channelized’ or ‘distributed’ pathways (Röthlisberger, Reference Röthlisberger1972; Kamb, Reference Kamb1987), meltwater corridors found in the Northern Hemisphere (e.g. Peterson and others, Reference Peterson, Johnson, Dahlgren, Påsse and Alexanderson2018; Lewington and others, Reference Lewington, Livingstone, Clark, Sole and Storrar2020), and on the Antarctic seafloor (Simkins and others, Reference Simkins2021) represent broad subglacial drainage pathways of meltwater landform assemblages that span 101–102 km in length. Corridor-like drainage systems have also been identified beneath the contemporary Greenland ice sheet (Hoffman and others, Reference Hoffman2016; Davison and others, Reference Davison, Sole, Livingstone, Cowton and Nienow2019) with complex configurations evolving depending on hydraulic gradients, meltwater input to the subglacial environment and sediment deformation (Davison and others, Reference Davison, Sole, Livingstone, Cowton and Nienow2019). These corridors indicate co-existence of drainage styles, varying genetic erosional and depositional processes and waxing and waning of drainage magnitudes in time and space. Additionally, newly observed landforms in Scandinavia, termed murtoos, potentially bridge the long-standing gap in recognizing geomorphic evidence for distributed subglacial drainage (Mäkinen and others, Reference Mäkinen, Kajuutti, Palmu, Ojala and Ahokangas2017; Ojala and others, Reference Ojala2019). These low-relief triangular subglacial landforms oriented with their apex in the ice-flow direction (Fig. 1a) often occur with other meltwater landforms such as channels and eskers, and sediments which have undergone hydraulic sorting, ductile deformation and liquefaction (Becher and Johnson, Reference Becher and Johnson2021). Collectively, murtoo presence suggests efficient transitional drainage between channelized and distributed under high-pressure conditions, potentially in response to transient linked cavity-type drainage systems (Ojala and others, Reference Ojala, Mäkinen, Kajuutti, Ahokangas and Palmu2022) similar to those beneath the Greenland ice sheet (Hoffman and others, Reference Hoffman2016). Both corridors and murtoos point to variable modes of drainage that co-exist or evolve in time and space including ‘efficient’ and ‘inefficient’ components, thus questioning the validity of assuming or parameterizing singular modes of subglacial water drainage.

A long-standing challenge in glacial geomorphology has been how to interpret the temporal significance of meltwater landforms: the time required, and the stability of discharge required, for both landform and whole drainage pathway formation. Meltwater landform relations to other subglacial and ice-marginal landforms provide insights in this regard (e.g. Greenwood and others, Reference Greenwood, Clason, Nyberg, Jakobsson and Holmlund2017; Simkins and others, Reference Simkins2017; Ojala and others, Reference Ojala2019; Livingstone and others, Reference Livingstone2020). For example, drumlins and mega-scale glacial lineations incised by channels and draped by eskers in the Bothnian Sea indicate a geomorphic switch from active bedform shaping to channelized water drainage overprinting stable bedforms, shortly before deglaciation (Fig. 1b; Greenwood and others, Reference Greenwood, Clason, Nyberg, Jakobsson and Holmlund2017). Here, interlinking channels and eskers of comparable sizes within a coherent drainage path highlight the transitory dominance of erosion and deposition in the subglacial environment. Episodic esker segment (‘bead’) deposition has long been inferred from the terrestrial landform-sediment record (De Geer, Reference De Geer1897; Banerjee and McDonald, Reference Banerjee, McDonald, Jopling and McDonald1975; Mäkinen, Reference Mäkinen2003). Livingstone and others (Reference Livingstone2020) demonstrate a tight relationship between esker beads and De Geer moraines in central Nunavut and infer time-transgressive landform building by drainage pathways to the ice margin. Similarly, meltwater channel incision through retreat moraines in the Barents Sea (Fig. 1c; Esteves and others, Reference Esteves, Bjarnadóttir, Winsborrow, Shackleton and Andreassen2017) and variable incision of or draping by retreat moraines in the western Ross Sea (Simkins and others, Reference Simkins2017) indicate the relative persistence of channelized drainage during active ice-margin retreat. Embayments can form where meltwater channels drain at grounding lines (Horgan and others, Reference Horgan2013; Simkins and others, Reference Simkins2017) likely through grounding-line sediment non-deposition, with potential to enhance tidal action at and upstream of the grounding line as ocean water flushes in and out of the channel path as observed at the contemporary Whillans Ice Stream (Horgan and others, Reference Horgan2013). A corridor of over 80 meltwater channels on the Antarctic continental shelf (Fig. 1d; Simkins and others, Reference Simkins2021) had prolonged impacts on grounding-line behavior as larger magnitude grounding-line retreat events and grounding zone wedge deposition occurred while the channels within the corridor were active, compared to smaller retreat events and moraine deposition when the channels were inactive. While the mechanism for this relationship remains unknown, it possibly results from hydrological controls on sediment rheology and mobility that influence building of ice-marginal landforms that may or may not reduce effective water depths enough to counterbalance grounding-line buoyancy-driven retreat. Such observations of meltwater–grounding-line landform associations and the potential to document these over large tracts of paleo-ice-sheet beds offer new possibilities for constraining the time component of the meltwater landform record, as well as quantifying sediment loads and, for example, seasonal deposition of individual esker beads.

Complementary to geomorphological studies, sediment records from deglaciated continental shelves and proglacial lake basins elucidate the temporal persistence of subglacial and grounding-line water discharge and associated changes in ice-sheet configuration and behavior (e.g. Rüther and others, Reference Rüther2012; Lee and others, Reference Lee, Wakefield, Phillips and Hughes2015; Avery and others, Reference Avery2021; O'Regan and others, Reference O'Regan2021; Lepp and others, Reference Lepp2022). Meltwater plume deposits offshore of Thwaites Glacier, Antarctica and Ryder Glacier, Greenland are a common feature associated with (or precursor to) glacier retreat and ice-shelf break up events, indicated by the millimeter-scale stratigraphy resolved by computed tomography (CT) scans and by grain-scale sedimentology (Fig. 1e; O'Regan and others, Reference O'Regan2021; Lepp and others, Reference Lepp2022). Downcore stratigraphy and trace elemental ratios in cores that sample meltwater plume deposits reveal differences in relative magnitudes and frequencies of subglacial drainage into the ocean offshore of western and eastern Thwaites Glacier, and suggest greater magnitudes of sediment-laden water were delivered to the ocean in recent centuries compared to the past several thousand years (Lepp and others, Reference Lepp2022). In the Baltic Sea basin, where proglacial varved sediments have long been used to document the pattern and pace of Fennoscandian ice-margin retreat, Avery and others (Reference Avery2021) find multi-decadal cycles of enhanced meltwater discharge through a 725 year varve series, ~15 000 years ago. In these paleo cases, and particularly where the former ice-sheet bed is now exposed, there is great potential for examining links between the temporal information archived in the distal sedimentological record of meltwater events and longevity of discharge, and the high-resolution geomorphology of the hydrological system responsible.

A recent body of work examining physical processes of and conditions for subglacial fluvial erosion, deposition and sediment mobility is an important step forward (Beaud and others, Reference Beaud, Flowers and Venditti2016, Reference Beaud, Flowers and Venditti2018; Damsgaard and others, Reference Damsgaard2017; Hewitt and Creyts, Reference Hewitt and Creyts2019; Kirkham and others, Reference Kirkham2022; Stevens and others, Reference Stevens2022; Vérité and others, Reference Vérité2022). These studies build towards an integrated or continuum view of meltwater organization, depending on water supply and sources, basal conditions and substrate properties. Importantly, they make advances towards knowledge of where, over what timescales, and with what meltwater discharge regimes, sediments are mobilized and landforms may form, opening up the vast landform record to much more effective and accurate use as a document of coupled meltwater – ice flow – ice margin behavior in paleo-ice sheets over seasonal-to-millennial timescales.

3. Looking forward

To push the field of subglacial hydrology forward using the landform and sediment records of deglaciated regions, we need: increased geophysical data coverage in regions proximal to contemporary ice-sheet margins; coupled remote-sensing and field-based observations in terrestrial landscapes; reporting of quantitative sedimentologic and morphometric data and community building to work across disciplinary bounds and study-area silos. Of promise in narrowing knowledge gaps are emerging themes of research on deeper groundwater interactions with the ice–bed interface (Gustafson and others, Reference Gustafson2022) and its implications for landform genesis (Boulton and others, Reference Boulton, Hagdorn, Maillot and Zatsepin2009; Hermanowski and Piotrowski, Reference Hermanowski and Piotrowski2019), understanding subglacial lake and ice-sheet surface connections to subglacial drainage systems (Greenwood and others, Reference Greenwood, Clason, Helanow and Margold2016; Simkins and others, Reference Simkins2017), assessment of the role of local (100–101 m relief) variability in bed conditions on drainage organization (Simkins and others, Reference Simkins2021) and the continued pursuit of constraining time for meltwater landform construction and evolution; each of these pursuits will benefit from more seamless integration of theory, numerical models and coupled geomorphological and chronological studies (Kirkham and others, Reference Kirkham2022; Stevens and others, Reference Stevens2022). Additionally, higher-resolution topographic data from the surface of Mars offer opportunities to dig deeper into the surficial expressions of meltwater landforms (e.g. Butcher and others, Reference Butcher2020), whereby comparison with Earth's meltwater landforms may offer new insights into key processes controlling their genesis. Here on Earth and on Mars, we need to be mindful of what we are not seeing, such as evidence for distributed, transient and hard-bed systems that might not leave their mark, in geomorphological and sedimentological records of subglacial hydrological systems. Additionally, when planning new research projects, those of us working on records of paleo-ice sheets and those studying contemporary ice sheets should draw on literature from the two respective fields to identify key gaps in understanding of subglacial hydrological forms and processes that will aid in assessing the future of the Greenland and Antarctic ice sheets.

Acknowledgments

The authors acknowledge their funding sources past, present and future that allow them to work toward better understanding subglacial processes; the traditional stewards of the global lands in which they work, some of whom directly interacted with extinct ice sheets and newly deglaciated landscapes following the Last Glacial Maximum; and those who collect and curate the data that support this work.

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Fig. 1. (a) Murtoo pathway within glacially streamlined terrain in central Finland (Mäkinen and others, 2017; Ojala and others, 2019). Data: LiDAR-based DEM from the National Land Survey of Finland. (b) Meltwater channels and eskers drape and incise drumlins in the Bothnian Sea (Greenwood and others, 2017). Data: MBES-based DEM from the Swedish Maritime Administration. (c) Meltwater channel incised retreat moraines (red dots) on Thor Iversenbanken in the Central Barents Sea (Esteves and others, 2017). Data: MAREANO MBES-based bathymetry from the Norwegian Mapping Authority. (d) Meltwater corridor in which channels cross-cut grounding zone wedges (red dots) in the Ross Sea, Antarctica (Simkins and others, 2021). Data: MBES-based DEM from cruise NBP15-02, available through the United States Antarctic Program Data Center. In (a)–(d), red dashed lines outline the encompassing areas of meltwater landforms. (e) CT scan and photograph of the upper 250 cm of sediment core NBP19-02 KC-08, collected in the Amundsen Sea, records meltwater plume events that emanated from the Thwaites Glacier grounding line (Lepp and others, 2022).