Widespread seawater intrusions beneath the grounded ice of Thwaites Glacier, West Antarctica

Warm water from the Southern Ocean has a dominant impact on the evolution of Antarctic glaciers and in turn on their contribution to sea level rise. Using a continuous time series of daily-repeat satellite synthetic-aperture radar interferometry data from the ICEYE constellation collected in March-June 2023, we document an ice grounding zone, or region of tidally-controlled migration of the transition boundary between grounded ice and ice afloat in the ocean, at the main trunk of Thwaites Glacier, West Antarctica, a strong contributor to sea level rise with an ice volume equivalent to a 0.6-m global sea level rise. The ice grounding zone is 6 km wide in the central part of Thwaites with shallow bed slopes, and 2 km wide along its flanks with steep basal slopes. We additionally detect irregular seawater intrusions, 5-10 cm in thickness, extending another 6 km upstream, at high tide, in a bed depression located beyond a bedrock ridge that impedes the glacier retreat. Seawater intrusions align well with regions predicted by the GlaDS subglacial water model to host a high-pressure distributed subglacial hydrology system in between lower-pressure subglacial channels. Pressurized seawater intrusions will induce vigorous melt of grounded ice over kilometers, making the glacier more vulnerable to ocean warming, and increasing the projections of ice mass loss. Kilometer-wide, widespread seawater intrusion beneath grounded ice may be the missing link between the rapid, past, and present changes in ice sheet mass and the slower changes replicated by ice sheet models. The dataset includes grounding line positions, all ICEYE radar interferograms and parameter files, files of tidal predictions and corrections for change in atmospheric pressure, and output products from the GlADS subglacial hydrology model.

Methods
Differential interferometry measures a differential change in ice displacement between 3 (or 4) epochs after eliminating the horizontal motion of ice through differencing of two pairs: we subtract two consecutive one-day pairs to eliminate the (steady) horizontal motion of the ice and leave the short term (vertical) motion of the ice, i.e. the vertical motion caused by changes in oceanic tide and atmospheric pressure, plus noise. If there is a data gap in data acquisition, we combine 2 x 1-day pairs acquired more than one day apart, hence 4 scenes. 

Predictions of SSH. We calculate changes in SHH using the CATS2008 tidal model corrected for IBE using ERA-5 atmospheric pressure fields.  

Subglacial hydrology is reproduced using the 2D finite-element Glacier Drainage System (GlaDS) model, which predicts the presence of long, high-pressured subglacial channels and an adjacent distributed drainage system. GlaDS calculates water discharge, flux, and pressure beneath Thwaites Glacier. The model allows coincident development of distributed and channelized systems. We test a range of conductivity of the distributed and channelised systems to converge to a stable solution. We use outputs that produce a) an upper limit for water pressure, beyond which the model does not converge, b) a lower limit when solutions have water with pressures far below the overburden pressure, and c) intermediate levels of pressure. We present an output (c) with a distributed system conductivity of 1 x 10-4 m3/2 kg-1/2 and a channel conductivity of 5 x 10-2 m3/2 kg-1/2. The model is applied with basal sliding velocity and water production (geothermal and frictional heat) from the ISSM IMSIP-6 Antarctic control run along with surface and bed topographies from BedMachine Antarctica version 1. The model is run on a mesh of 19,340 nodes with refining near the grounding line giving a minimum edge length of 280 m. The model is run for 20,000 days until near steady state.