Seven-year Sentinel-1 C-band SAR time series reveals brine infiltration signatures and progressive firn densification across Antarctic ice shelves

Seawater infiltration into Antarctic ice shelf firn has been predicted by firn densification models but observational confirmation from satellite remote sensing remains limited, particularly at C-band frequencies. Cook et al. (2018) identified five ice shelves — Shackleton, Abbot, Nickerson, Sulzberger, and Rennick — as the highest-priority candidates for brine infiltration based on firn air content, annual mean air temperatures, and ice flow speeds. Here we present a seven-year (2019–2025) Sentinel-1 C-band SAR backscatter time series for all five priority sites using the Extra Wide (EW) swath mode in HH+HV dual polarisation, providing the first systematic C-band characterisation of seasonal backscatter contrasts and interannual trends across the complete set of Cook et al. recommended sites. Radiometrically calibrated gamma0 backscatter statistics were extracted from 180 valid scene-site observations across six sampled months per year. Nickerson Ice Shelf exhibits anomalously low mean winter HH backscatter of −14.69 dB, a near-zero seasonal contrast of +0.54 dB, and a significantly suppressed HH/HV ratio of 5.73 dB relative to all other sites (p < 0.001), consistent with year-round liquid brine in firn suppressing volume scattering. Sulzberger Ice Shelf shows a statistically non-significant seasonal contrast of +0.24 dB (p = 0.112), also consistent with persistent brine infiltration. Shackleton and Abbot exhibit positive seasonal contrasts of +2.73 and +1.46 dB respectively, indicating dry firn behaviour. All four main sites show significant negative trends in winter HH backscatter over 2019–2025 (slopes: −0.138 to −0.281 dB yr-1; r2 = 0.394 to 0.869; p ≤ 0.003), substantially exceeding the documented radiometric stability of Sentinel-1A (0.1 dB; Schmidt et al., 2020), indicating progressive firn densification across multiple Antarctic ice shelves. These findings provide the first C-band satellite evidence of brine infiltration signatures at Nickerson and Sulzberger ice shelves and reveal progressive firn change across multiple Antarctic ice shelves over the study period.