Is seawater intrusion accelerating glacier loss?

The Antarctic Ice Sheet has been a major contributor to sea level rise over the past several decades. Ice sheet melt has accelerated due to increased contact of warm, salty ocean waters with the continental shelf, ice cavities and glaciers. This is caused by the combined effect of rapid climate warming from human-induced greenhouse gas emissions and a cooling of the Antarctic stratosphere from the human-induced depletion of ozone.

Physical processes taking place at the transition boundary between grounded ice and ice afloat in the ocean – the ice grounding zone - are critical to glacier loss. This study used a unique time series of high-resolution satellite observations and subglacial hydrologic modeling to define an ice grounding zone on the Thwaites Glacier in West Antarctica that expanded and contracted with the tides. In addition, the study helped to explain the role of seawater intrusions on glacial melt and discussed the resultant magnitudes of ice mass loss and sea level rise (Figure 1).

Figure 1: Ocean grounding zone vs. ice grounding zone of an ice sheet/ice shelf system. Circumpolar deep water (CDW) filling the ice cavity is colour coded by temperature from blue (cold) to red (warm). The ocean grounding zone is always flooded with CDW, while the ice grounding zone alternates between flooded and unflooded phases driven by changes in oceanic tide and atmospheric pressure. Seawater intrusion propagates beyond the ice grounding zone at irregular intervals. Under grounded ice, the glacier bed is overlaid by a thin sheet (10 cm) of pressurized subglacial water, which facilitates intrusion and hydraulic jacking of the ice due to seawater. The panel on the Top shows differential, interferometric fringes associated with ice flexing in the ocean and ice grounding zones at low (Top) and high sea surface height (Below). Bull’s eye deformation fringes farther upstream reflect ice subsidence vs. uplift around a bed depression at low vs. high sea surface height.

The study found the ice grounding zone to stretch 6 km inland in the central part of Thwaites Glacier with shallow bed slopes and 2 km inland along its flanks with steep basal slopes. A portion of the ice grounding zone abuts a bedrock ridge (Mouginot Ridge) that provides a temporary barrier against glacier retreat as its bed elevation rises inland. Further inland is a second ridge (South Ridge) after which the bed elevation drops significantly, accelerating any glacier retreat which may not stop until the entire basin drains to sea.

The study revealed a slow change in the grounding line position, except at the top of the tidal cycle where the differential sea surface height signal was large and the limit of tidal impacts well defined. Along the main trunk of Thwaites, the analysis detected a vertical motion beyond the ice grounding zone attributed to seawater intrusion that occurred on an irregular basis with extreme events. In most cases, this seawater intrusion “bull’s eye” moved in phase with the change in sea surface height, consistent with seawater rushing beneath grounded ice and lifting it up. At the upper part of the tidal cycle, the study detected seawater intrusions up to 12 km inland. In other cases, the seawater intrusions were out of phase with sea surface height, indicating that seawater was trapped in the cavity or being flushed out. Seawater intrusion was also identified in the northern flank of South Ridge, but was disconnected from that in the main trunk (Figure 2).

Figure 2: Grounding line positions on the main trunk of Thwaites Glacier in March–June 2023. (A) Red lines delimit regions moving in phase with changes in SSH or the ice grounding zone. Orange lines delineate irregular seawater intrusion moving in phase with SSH and green lines delineate seawater extrusion out of phase with SSH. (B–L) Example differential interferogram (DInSAR) with Ascending Right (AscR), Ascending Left (AscL) and Descending Right (DscR) looking geometry and Descending (Dsc) right looking geometry at different UTC time.

Subglacial Antarctic hydrology is a system at near-steady state in contrast to seasonal systems observed in Greenland and in mountain glacier which instead develop during summer melt and shut down during the winter freeze.  Because of the lack of seasonality in Antarctica, the subglacial system is highly pressurized, which facilitates seawater intrusion and hydraulic jacking at high sea surface heights. GlaDS predicted an average water thickness of 8 ± 1 cm in the main trunk of Thwaites with little spatial variability, and a set of subglacial channels aligned with bed troughs with discharges up to 40 m³/s. The bull’s eye of seawater intrusions near the ice grounding zone aligned between two GlaDs channels over an area of high-water pressure while subglacial channels were predicted in narrow zones of lower water pressure, coincident with bed troughs (Figure 3).

Figure 3: Ice Grounding Zone vs. GlaDS subglacial hydrology. Counts of grounding line position color coded from black (low, 1) to purple and yellow (high, 80) overlaid on (A) water pressure, P, as fraction of overburden pressure; (B) water discharge, D, from 0 to 40 m³/s, overlaid on bed topography from 0 to 1,100 m; and (C) hydraulic potential, ϕ, from 0.4 to 4.0 MPa; along with location of “Ridge Mouginot” and “Ridge South” ridges, and "Bull’s eye.”