Glacial meltwater is the primary source of subsurface freshening off the Western Antarctic Peninsula

The Western Antarctic Peninsula (WAP) is one of the fastest warming regions on Earth, with increasing freshwater input from melting glaciers and ice shelves. Although surface-layer freshening is well documented, the extent to which glacial meltwater influences subsurface waters remains poorly constrained. Here, we investigated the vertical distribution and origin of freshwater anomalies using hydrographic, isotopic (δ ¹⁸ O, δ ² H), and major-ion data from three sites along the WAP: Cierva Cove, Petermann Island, and Paradise Bay. The data show consistent freshening below 50 m depth. Chloride dilution and isotope depletion define conservative mixing between local seawater and a strongly δ ¹⁸ O-depleted glacial meltwater endmember, with this signal extending to depths greater than 90 m in the more enclosed embayments. Estimated meltwater fractions at these depths are approximately 0.5 to 2%. Major-ion and halogen ratios also vary conservatively, supporting mixing between seawater and glacial meltwater rather than addition of a chemically distinct subsurface fluid. Despite limited vertical sampling, the deepest samples at each site remain consistent with the inferred surface-to-depth mixing relationships. These findings indicate that glacial meltwater can be stored well below the surface layer along parts of the WAP, likely through plume-driven neutral-buoyancy intrusions, lateral advection, and mixing. Recognising this subsurface meltwater reservoir is important for understanding local stratification and for improving representation of freshwater input in ocean models of the region.