Abstract
Abstract. Increasing melt of ice sheets at their floating or vertical interfaces with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the “salt wedge” observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over flat or reverse-sloping impermeable beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through permeable till. Simulations using the Ice-sheet and Sea-level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10 %–50 % higher. Kilometers of intrusion-induced basal melt can cause projected ice sheet volume loss to more than double. These results suggest that further observational, experimental and numerical investigations are needed to determine the conditions under which seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.
Highlights
Where ice sheets come into contact with seawater, ice may be lost through melting and dissolution
In a stateof-the-art ice sheet model, we show that the transient rate of ice sheet mass loss depends sensitively on the extent of warm seawater intrusion upstream of marine ice sheet grounding lines
Future work can focus on answering three questions: (1) can we definitively confirm that seawater intrusion and melt are happening under grounded ice at some glaciers? (2) What are the properties of the subglacial hydrologic system under grounded ice near the ice–ocean interface? (3) Which processes play a role in the fluid dynamics of the freshwater– seawater interface?
Summary
Where ice sheets come into contact with seawater, ice may be lost through melting and dissolution. Previous studies have hypothesized that seawater intrusion may only be possible when transient tidal variations in the hydrostatic pressure of seawater at the grounding line either overcome this hydraulic barrier (Walker et al, 2013) or cause it to migrate upstream from the grounding line (Sayag and Worster, 2013). In such models, the compositional difference of seawater and subglacial discharge is not considered, leaving the horizontal variation in overburden pressure as the primary control on the subglacial intrusion of seawater. The interface between freshwater discharge and seawater can have a vertical structure in which dense seawater intrudes horizontally under the lighter freshwater forming a “salt wedge”, as has been observed in estuaries (Geyer and Ralston, 2011), enclosed wastewater outfalls (Adams et al, 1994) and coastal karst channels (Dermissis, 1993)
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