On the retreat of lagoon-terminating glaciers: sometimes fresh, sometimes salty

Journey Berry , Erin C Pettit, Åsa K Rennermalm, Nicole Abib, Aurora Roth, Deborah Kurtz

Coastal lagoon-terminating glaciers flow into water bodies of variable salinity and temperature, shaped by mixing meltwater, ocean water, and ice. They remain understudied yet increasingly important as coastal glacier margins retreat globally. Lagoon salinity influences circulation through interactions with buoyant subglacial discharge. Water temperature, modulated by ocean connectivity and latent heat processes, controls thermal forcing on the ice. Thus, lagoon circulation, freshwater energetics, and retreat behavior tightly intertwine with lagoon volume and its ocean connection. Bear Glacier, fed by the Harding Icefield in Southcentral Alaska, transitioned to a glacier-lagoon as recently as 2014. Glacial lake outburst floods, tides, and storms regulate the ocean connection, contributing to the glacier’s retreat. We document this evolution using historical maps, satellite imagery, local knowledge, and lagoon salinity and temperature measurements. Variable salinity reflects changes in channel flow alongside the morainal spit separating lagoon from ocean, where periodic erosion from outburst floods allows the channel to connect the two water bodies closer. Recent retreat beyond the glacier’s narrowing, combined with upstream overdeepening, saltier water, an eroding spit, and regional warming, suggests continued inland glacier retreat. This lagoon-mediated retreat style is a likely analog for future retreat of other coastal glaciers in Alaska, Patagonia, and Greenland.