Impact of surface melt and brine infiltration on fracture toughness of ice shelves

Abstract. Ice shelves are heterogeneous composites of firn, meteoric ice, refrozen melt and brine-saturated ice. The properties and distribution of these elements control ice shelf response to stress and susceptibility to fracturing. Here, we quantify how surface-melt and brine infiltration modify the Mode I fracture toughness (K_Ic) of meteoric ice on the Brunt Ice Shelf (BIS), Antarctica. During the 2023/24 austral summer, we recovered a 37 m core sequence from meteoric infill ice near Halley VI, where radar mapping shows continuous brine horizons at ∼ 37 m depth and line scans indicate that the upper 37 m contain ∼ 7 % refrozen melt. We combined density, salinity, temperature and grain size measurements with semi-circular three-point bending tests on samples representing (i) meteoric ice, (ii) melt-modified meteoric ice, and (iii) brine-infiltrated meteoric ice. Our results show melt-modified samples are consistently tougher than melt-free meteoric ice, with K_Ic increases up to ∼ 40 %. This is despite their larger grain size, indicating densification dominates over grain-size effects. In contrast, brine-saturated meteoric ice exhibits markedly lower K_Ic, by 14 %–34 % relative to density-matched, brine-free meteoric ice, consistent with chemical weakening and lower freezing temperatures. Our results demonstrate that as K_Ic varies strongly with density, salinity and depth, a spatially and temporally constant toughness value is unlikely to reproduce calving behaviour accurately. Implementing spatially and vertically variable K_Ic values, and understanding how ice shelf structure and composition evolves over time, is essential to improve predictions of rift propagation and calving.