Abstract
This paper presents predictions of palaeo-subglacial lakes and their drainage pathways beneath the North American Ice Sheet during the last glaciation. We utilise data on the current topography and seafloor bathymetry, and elevation models of the ice- and ground-surface topography from data-calibrated glaciological modelling to calculate the hydraulic potential surface at the ice-sheets bed. Given that specific ice-surface elevations are only known from modelled outputs, and thus contain significant uncertainty, we utilise many such outputs to examine where on the bed that subglacial lakes are likely to have occurred. Our analysis demonstrates the potential for subglacial lake genesis, particularly beneath the former Cordilleran Ice Sheet; along the suture zone between the Laurentide and Cordilleran ice sheets; in Hudson Bay; in the Great Lake basins and deep trenches of the Canadian Archipelago. During the Last Glacial Maximum we suggest that at least 1000km3 of meltwater could have been stored subglacially. As the ice-sheet and the bed evolved subglacial lakes repeatedly formed and emptied, particularly in Hudson Bay and the suture zone between the Laurentide and Cordilleran ice sheets where lakes were characteristically broad and shallow (<10m deep). In contrast, the Cordilleran Ice Sheet was characterised by deep (up to ∼90m) and persistent lake genesis. Significantly, similar distributions and modes of predicted subglacial lakes are obtained irrespective of the model or model run, which suggests the results are robust. Subglacial meltwater drainage varied between stable networks, typically associated with strong topographic controls, and convoluted networks that underwent considerable dynamism, including repeated meltwater network capture. These lake likelihood predictions could usefully form targets for detailed field and remote investigations and we hypothesise and explore the potential that numerous deposits and spillways previously interpreted as arising from ice-marginal lakes may have emanated from their subglacial cousins.
Highlights
The generation, storage and evacuation of meltwater plays a fundamental role in modulating the behaviour of ice masses (e.g. Hubbard et al, 1995; Joughin et al, 2008; Stearns et al, 2008; Bartholomew et al, 2010)
There are a number of limitations to predicting subglacial lakes and meltwater drainage networks using the method outlined above: (i) the simplistic treatment of basal conditions; (ii) the digital elevation models (DEMs) includes some post-glacial filling of the true subglacial bed; (iii) the coarse resolution of the models; (iv) none of the models include dynamic coupling between the ice and subglacial meltwater, so ice-surface flattening above subglacial lakes is not accounted for; and (v) in reality some lakes do not form in hydraulic minima, such as those created by high geothermal heat fluxes or behind frozen margins
The subglacial hydrological modelling presented in this paper demonstrates the potential for subglacial lake formation beneath the North American Ice Sheet
Summary
The generation, storage and evacuation of meltwater plays a fundamental role in modulating the behaviour of ice masses (e.g. Hubbard et al, 1995; Joughin et al, 2008; Stearns et al, 2008; Bartholomew et al, 2010). Meltwater channels, tunnel valleys and eskers are widely observed across the formerly glaciated bed of North America, forming intricate networks detailing the composite history of meltwater drainage and ice-sheet behaviour Despite the wealth of data, ease of access (both remotely and in the field) and spatial coverage, disentangling the glacial meltwater history of North America is complex With this in mind, we use hydraulic calculations derived using an ensemble of palaeo-ice and bed topographies from thermomechanical ice-sheet models to simulate subglacial meltwater routeways and predict where meltwater may have ponded at the ice–bed interface. Meltwater drainage networks are simulated at discrete time-slices throughout the evolution of the last NAIS, which allows the spatial and temporal correlations between channels, subglacial lakes, and ice-streams to be investigated. Subglacial lake predictions provide a useful guide for detailed future field investigations and for elucidating their frequency, stability, and distribution through the ice-sheets evolution
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