Direct observations of melting, freezing, and ocean circulation in an ice shelf basal crevasse

Abstract

Ocean conditions near the grounding zones of Antarctica’s ice shelves play a key role in controlling the outflow and mass balance of the ice sheet. However, ocean observations in these regions are largely absent. Here, we present a detailed spatial survey collected with an underwater vehicle in a basal crevasse located in the ocean cavity at the Ross Ice Shelf grounding zone. The observations depict fine-scale variability in ocean forcing that drives asymmetric melting along the lower crevasse sidewalls and freezing in the upper reaches of the crevasse. Freshwater release from melting at depth and salt rejection from freezing above drives an overturning circulation. This vertical circulation pattern overlays a dominant throughflow jet, which funnels water parallel to the coastline, orthogonal to the direction of tidal currents. Importantly, these data reveal that basal crevasses influence ocean circulation and mixing at ice shelf grounding zones to an extent previously unknown.

Swirls and scoops: Ice base melt revealed by multibeam imagery of an Antarctic ice shelf

“…The older fractures show considerable basal melt and widening compared with younger ones. Elevated water velocities within fractures have been observed below Thwaites Ice Shelf ( 25 ) and Kamb Ice Stream ( 30 , 31 ), which can explain the enhanced melting. However, it is also possible that the relatively warm and buoyant meltwater mixtures formed in the cavity ( Fig.…”

“…Basal channels, stretching many kilometers (10,26,27), have been linked to enhanced basal melt (28). Elevated melt rates and decimeter-scale scallops have been observed on the steep sides of basal fractures (25,(29)(30)(31). Analysis of these processes has been limited by a lack of data from the ice base, particularly high-resolution surveys and complementary datasets covering sufficiently large areas to understand the extent and representativeness of the features.…”

High Basal Melt Rates and High Strain Rates Lead to More Fractured Ice

“…Recent studies continue to emphasize the intricate interplay between basal melt and crevasses (Alley et al, 2021;Washam et al, 2023). Similarly, Larter (2022) recently suggested that roughness may serve as a health indicator for ice shelves.…”

“…Similarly, where strain rates are high, the ice will fracture, causing crevasses (and potentially rifts) to form. Melt and fracture may also work in combination, where high melt rates increase the stress (Vaughan et al, 2012), resulting in crevasses, and/or basal melt erodes existing crevasses (Washam et al, 2023). The combined effect of these processes could increase roughness and potentially decrease ice stability.…”

Seawater Intrusion in the Observed Grounding Zone of Petermann Glacier Causes Extensive Retreat

“…We find the maximum seawater intrusion distance dependent on C d , which is the measurable drag coefficient of water flowing past ice. Estimates of C d for the ice‐ocean interface are typically between 0.001 and 0.01 (Johannessen, 1970; Kottmeier & Engelbart, 1992; Lu et al., 2011; McPhee, 1989; Shirasawa, 1986), with C d values calculated directly from observations under ice shelves falling within this range (Washam et al., 2023). Using the end member values of C d , we find seawater intrusion to range from 1.5 to 15 km.…”