Episodic glacier calving drives vertical mixing in cold Greenland fjords

Marine-terminating glaciers in shallow, cold Greenland fjords are not in direct contact with warm and deeper Atlantic Water. In these fjords, stratification near glacier termini is primarily driven by freshwater inputs. Surface runoff enhances near-surface stratification, while subglacial discharge drives localized mixing at depth. Meanwhile, the role of calving events as drivers of fjord-scale mixing remains poorly constrained. Here, we combine hydrographic measurements and oxygen isotope data with remote sensing and three-dimensional computational fluid dynamics simulations to resolve the immediate fjord response to a major calving event. We show that a large, buoyancy-driven calving event triggers rapid vertical mixing that redistributes water masses throughout the upper and intermediate water column, entraining Polar Water into the surface layer and inducing episodic contributions from deeper fjord waters. This mixing results in a salinity increase in the upper ∼ 25 m, counteracting the expected summer freshening from glacier meltwater. Oxygen isotope (δ18O) profiles and simulations further indicate episodic inputs from deeper water masses. Together, these results demonstrate that calving events can exert significant, and previously poorly constrained, controls on mixing in cold, shallow glacier–ocean systems.