Abstract
Intensive study of the Greenland Ice Sheet's (GrIS) subglacial drainage has been motivated by its importance for ice dynamics and for nutrient/sediment export to coastal ecosystems. This has revealed consistent seasonal development of efficient subglacial drainage in the lower ablation area. While some hydrological models show qualitative agreement with field data, conflicting evidence (both field- and model-based) maintains uncertainty in the extent and rate of efficient drainage development under thick (∼1 km) ice. Here, we present the first simultaneous time series of directly-observed subglacial drainage evolution, supraglacial hydrology and ice dynamics over 11 weeks in a large GrIS catchment. We demonstrate development of a fast/efficient subglacial drainage system extending from the margin to beneath ice >900 m thick, which then persisted with little response to highly variable moulin inputs including extreme melt events and extended periods (2 weeks) of low melt input. This efficient system evolved within ∼3 weeks at a moulin initiated when a fracture intersected a supraglacial river (rather than hydrofracture and lake drainage). Ice flow response to surface melt inputs at this site follows a pattern commonly observed in the lower GrIS ablation area, and by assuming a strong relationship between ice dynamics and subglacial hydrology, we infer that efficient subglacial drainage evolution is widespread under 900 m-thick ice in west Greenland. This time series of tracer transit characteristics through a developing and then persistent efficient drainage system provides a unique data set with which to validate and constrain existing numerical drainage system models, extending their capability for simulating drainage system evolution under current and future conditions.
Highlights
The Greenland Ice Sheet’s (GrIS) subglacial drainage system has been intensively studied over the last two decades, both to assess ice sheet response to increasing melt (e.g. Zwally et al, 2002; Sundal et al, 2011), and more recently to quantify nutrient and sediment export from the ice sheet to coastal/fjord environments (e.g. Sejr et al, 2014; Hawkings et al, 2015; Hopwood et al, 2020)
In Greenland, the inland propagation of efficient drainage is linked to the establishment of hydrological connections between the surface and subglacial drainage pathways under increasingly thick ice, enabled by fracturing and subsequent moulin development, sometimes associated with supraglacial lake drainage events (Nienow et al, 2017; Hoffman et al, 2018)
We find clear evidence for the development of efficient subglacial drainage extending from the margin to beneath ice at least ∼900 m thick, in a region of the ablation zone where the extent of seasonal drainage evolution has remained uncertain in previous field studies and numerical simulations
Summary
The Greenland Ice Sheet’s (GrIS) subglacial drainage system has been intensively studied over the last two decades, both to assess ice sheet response to increasing melt (e.g. Zwally et al, 2002; Sundal et al, 2011), and more recently to quantify nutrient and sediment export from the ice sheet to coastal/fjord environments (e.g. Sejr et al, 2014; Hawkings et al, 2015; Hopwood et al, 2020). The Greenland Ice Sheet’s (GrIS) subglacial drainage system has been intensively studied over the last two decades, both to assess ice sheet response to increasing melt Some aspects of the GrIS subglacial drainage system are understood in considerable detail, with field and remotesensing data revealing a persistent, widespread seasonal pattern in the ablation area, characterised by inland-propagation of evolution from slow/inefficient to fast/efficient drainage (Nienow et al, 2017). In Greenland, the inland propagation of efficient drainage is linked to the establishment of hydrological connections between the surface and subglacial drainage pathways under increasingly thick ice, enabled by fracturing and subsequent moulin development, sometimes associated with supraglacial lake drainage events (Nienow et al, 2017; Hoffman et al, 2018). Efficient summer drainage reverts to slow, inefficient drainage during winter except very near the ice margin, as ice creep gradually closes subglacial conduits once melt inputs have ceased
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