Suppressed basal melting in the eastern Thwaites Glacier grounding zone

Abstract

Thwaites Glacier is one of the fastest-changing ice–ocean systems in Antarctica1–3. Much of the ice sheet within the catchment of Thwaites Glacier is grounded below sea level on bedrock that deepens inland4, making it susceptible to rapid and irreversible ice loss that could raise the global sea level by more than half a metre2,3,5. The rate and extent of ice loss, and whether it proceeds irreversibly, are set by the ocean conditions and basal melting within the grounding-zone region where Thwaites Glacier first goes afloat3,6, both of which are largely unknown. Here we show—using observations from a hot-water-drilled access hole—that the grounding zone of Thwaites Eastern Ice Shelf (TEIS) is characterized by a warm and highly stable water column with temperatures substantially higher than the in situ freezing point. Despite these warm conditions, low current speeds and strong density stratification in the ice–ocean boundary layer actively restrict the vertical mixing of heat towards the ice base7,8, resulting in strongly suppressed basal melting. Our results demonstrate that the canonical model of ice-shelf basal melting used to generate sea-level projections cannot reproduce observed melt rates beneath this critically important glacier, and that rapid and possibly unstable grounding-line retreat may be associated with relatively modest basal melt rates.

Heterogeneous melting near the Thwaites Glacier grounding line

“…Ocean currents weaken within 5 m of the ice from a background speed near 3 cm s −1 (ref. 28 ) to near zero close to the interface (Fig. 4a ).…”

“…Although no SGW source is observed directly, bathymetry near the GL along T1 suggests a recent subglacial channel (Fig. 2c ), and SGW outflow measured downstream varies over time 28 . Estimates of SGW concentration calculated from T – S and DO– S properties indicate maximum values of 7 ml l −1 and 24 ml l −1 , respectively.…”

“…As part of the National Science Foundation (NSF)-Natural Environment Research Council (NERC) International Thwaites Glacier Collaboration (ITGC), a comprehensive field campaign was carried out over two austral summers, with an ice-shelf-drilling campaign in 2019–2020 to access the ocean cavity 28 and sediments below the TEIS to observe the changing system directly. We conducted detailed in situ hydrographic measurements in an area of the TEIS referred to as the ‘butterfly’ 12 .…”

“…The grey lines denote the bottom of the terrace. Vertical profiles of ocean-current speed ( U ), thermal driving ( Θ − Θ f ), absolute salinity ( S A ) and dissolved oxygen (DO) binned with distance from the ice base show that, although the water is warm close to the interface, the current velocity slows in the boundary layer, suggesting breaking from friction at the interface 28 . b , c , As in a for the furthest crevasse from the GL, observed along both T1 ( b ) and T2 ( c ).…”

“…5 ). We compared these to results from three autonomous phase-sensitive radio echo sounders (ApRES) and the oceanographic mooring at the borehole 28 (Fig. 5 and Methods ).…”