Imaging suspended sediment plumes in Greenland’s fjords using a multi-frequency multibeam echo-sounder and an acoustic Doppler current profiler

Sediment delivery to Greenland’s glacial fjords is expected to increase significantly in response to accelerated atmospheric and oceanic warming. As glaciers retreat on to land, melt-water will enter the fjords at the water surface rather than rising as buoyant plumes from the base of the calving front, thus reducing mixing with the nutrient-rich waters below. Increased surface sediment concentrations will prevent light penetration which, together with decreased nutrient availability, will cause a reduction in primary production and ultimately effect rates of seafloor carbon burial. Glacial fjords are a major global carbon sink, but it remains unclear how sediment delivery and transport processes in glacial fjords will change as deglaciation progresses. We present water column data acquired with an Acoustic Doppler Current Profiler (ADCP) and a multi-frequency Sonic 2026 multibeam echo-sounder deployed on a vessel in a Greenland fjord with a land-retreated glacier and a fjord with a recently-retreated glacier. The results demonstrate the capability of the multibeam echo-sounder to image suspended sediment plumes in the water column, which we compare with backscatter acquired with the ADCP. The water column imaging demonstrates how mixing processes between the freshwater plumes and tidally-driven oceanic saltwater causes sediment plumes to form near-bed concentrations of fluid mud that align with seafloor channels observed in the bathymetry data acquired with the Sonic 2026, providing new insights into sediment transport processes in fjords at different stages of deglaciation.