Abstract
Abstract. Meltwater and sediment-laden plumes at tidewater glaciers, resulting from the localized subglacial discharge of surface melt, influence submarine melting of the glacier and the delivery of nutrients to the fjord's surface waters. It is usually assumed that increased subglacial discharge will promote the surfacing of these plumes. Here, at a western Greenland tidewater glacier, we investigate the counterintuitive observation of a non-surfacing plume in July 2012 (a year of record surface melting) compared to the surfacing of the plume in July 2013 (an average melt year). We combine oceanographic observations, subglacial discharge estimates and an idealized plume model to explain the observed plumes' behavior and evaluate the relative impact of fjord stratification and subglacial discharge on plume dynamics. We find that increased fjord stratification prevented the plume from surfacing in 2012, show that the fjord was more stratified in 2012 due to increased freshwater content and speculate that this arose from an accumulation of ice sheet surface meltwater in the fjord in this record melt year. By developing theoretical scalings, we show that fjord stratification in general exerts a dominant control on plume vertical extent (and thus surface expression), so that studies using plume surface expression as a means of diagnosing variability in glacial processes should account for possible changes in stratification. We introduce the idea that, despite projections of increased surface melting over Greenland, the appearance of plumes at the fjord surface could in the future become less common if the increased freshwater acts to stratify fjords around the Greenland ice sheet. We discuss the implications of our findings for nutrient fluxes, trapping of atmospheric CO2 and the properties of water exported from Greenland's fjords.
Highlights
Over the last 2 decades, the rate of mass loss from the Greenland ice sheet (GrIS) has quadrupled (Rignot et al, 2011; Shepherd et al, 2012)
This study began with the counterintuitive observation of a surfacing subglacial discharge plume in Saqqarleq Fjord in late July 2013 but a subsurface trapped plume during late July 2012
By combining oceanographic observations together with support from a plume model we have shown that the difference between the 2 years can be explained by the increased freshwater content of the fjord in 2012 relative to 2013, resulting in stronger fjord stratification and a suppression of the vertical extent of the plume
Summary
Over the last 2 decades, the rate of mass loss from the Greenland ice sheet (GrIS) has quadrupled (Rignot et al, 2011; Shepherd et al, 2012). Buoyant plumes typically occur in localized areas along the glacier front, at times visible on the fjord surface as patches of turbid water (e.g., How et al, 2019; Mankoff et al, 2016) Since they are driven primarily by subglacial discharge deriving from ice sheet surface melting, their appearance is limited mainly to summer (e.g., Motyka et al, 2013; Schild et al, 2016), and, due to the sediments they carry, they control sedimentation rates and distribution in the vicinity of the glacier front (Mugford and Dowdeswell, 2011). As they rise up the calving front, plumes entrain large volumes of ambient fjord waters, increasing their initial volume by more than an order
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