Upward thinning propagation as a surge precursor of marine-terminating glaciers in Svalbard

Land-terminating glacier surges begin at the upstream region, followed by the flow acceleration propagating downward. However, marine-terminating glacier surges in Svalbard may be triggered by frontal thinning and retreat, indicating different driving factors of surge initiation from land-terminating glaciers. Here, we present detailed observations of glacier surface elevations for two marine-terminating glaciers, Wahlenbergbreen and Negribreen, and characterize the evolution from frontal thinning to glacier-wide surges. Wahlenbergbreen and Negribreen entered an active surge phase in 2016 and 2017, respectively, with a surface elevation drop of ~60 m at their thinning centers. Interestingly, our analysis using the ArcticDEM strip data reveals that the intensive frontal thinning took place three years before the active surge phase. The centers of frontal thinning then gradually shifted upstream at a rate of 2-3 km/yr during the following three years until the glacier-wide surges occurred. Based on these observations, we propose a physical framework for surge initiation due to ocean-induced thinning. This thinning signal can kinematically propagate inland, increase the surface slope and driving stress until a certain threshold is achieved, and finally accelerate the entire glacier with an inefficient subglacial drainage system. This proposed mechanism can contribute to surge initiation with other driving factors, such as excessive meltwater supply to the bed. A region-wide survey for this surge precursor (inland thinning propagation at the glacier front) is now being planned to answer whether all marine-terminating glacier surges in Svalbard have a terminus origin and whether these surge events are fundamentally different from land-terminating glacier surges driven by thickened upstream bulges.