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

<jats:p>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. Here we present a seven-year (2019–2025) Sentinel-1 C-band SAR backscatter time series for all five sites using the Extra Wide swath mode in HH+HV dual polarisation, providing the first systematic C-band characterisation of seasonal backscatter contrasts and interannual trends across the Cook et al. (2018) priority sites. Radiometrically calibrated gamma0 statistics were extracted from 180 valid scene-site observations using site-specific geographic bounding boxes 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 comparison sites (p &lt; 0.001), consistent with a year-round anomalous firn dielectric state such as brine infiltration. Sulzberger Ice Shelf shows a non-significant seasonal contrast of +0.24 dB (p = 0.112) and a suppressed HH/HV ratio (6.84 dB, p &lt; 0.001), consistent with moderate brine-related suppression of seasonal firn variability. Shackleton and Abbot exhibit positive seasonal contrasts of +2.73 and +1.46 dB, consistent with the absence of persistent brine, though East Antarctic site statistics may include seasonal sea ice contributions. 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 Sentinel-1A radiometric stability (0.1 dB; Schmidt et al., 2020), indicating progressive changes in firn or surface properties. These findings provide the first C-band satellite evidence for brine-related backscatter anomalies at Nickerson and Sulzberger and reveal systematic backscatter decline across multiple Antarctic ice shelves over the study period.</jats:p>