On the seasonal freshwater stratification in the proximity of fast‐flowing tidewater outlet glaciers in a sub‐Arctic sill fjord

TLDR

The Greenland Ice Sheet discharges large volumes of freshwater into adjacent fjords, many of which are directly connected to tidewater outlet glaciers and are characterized by dense glacial and sea ice cover. The study provides the first seasonal hydrographic observations of a sub‑Arctic fjord within 4–50 km of a fast‑flowing tidewater outlet glacier. The authors measured runoff, subglacial freshwater, glacial, and sea ice melt above 50–90 m and used a thermodynamic mixing model on late‑summer temperature and salinity profiles to quantify freshwater content. During summer a layered temperature structure below the low‑salinity surface layer was observed, while winter showed stepwise halo‑ and thermoclines; the warm intermediate water mass (>1 °C) served as a significant subsurface heat source, and thermodynamic mixing analysis revealed 5–11 % freshwater below the summer surface, with glacial melt contributing 1–2 % and subglacial freshwater 3–10 %, whereas winter halocline freshwater was ~1 % with negligible subglacial input.

Abstract

Abstract The Greenland Ice Sheet releases large amounts of freshwater into the fjords around Greenland and many fjords are in direct contact with the ice sheet through tidewater outlet glaciers. Here we present the first seasonal hydrographic observations from the inner part of a sub‐Arctic fjord, relatively close to and within 4–50 km of a fast‐flowing tidewater outlet glacier. This region is characterized by a dense glacial and sea ice cover. Freshwater from runoff, subglacial freshwater (SgFW) discharge, glacial, and sea ice melt are observed above 50–90 m depth. During summer, SgFW and subsurface glacial melt mixed with ambient water are observed as a layered structure in the temperature profiles below the low‐saline summer surface layer (<7 m). During winter, the upper water column is characterized by stepwise halo‐ and thermoclines formed by mixing between deeper layers and the surface layer influenced by ice melt. The warm ( T > 1°C) intermediate water mass is a significant subsurface heat source for ice melt. We analyze the temperature and salinity profiles observed in late summer with a thermodynamic mixing model and determine the total freshwater content in the layer below the summer surface layer to be between 5% and 11%. The total freshwater contribution in this layer from melted glacial ice was estimated to be 1–2%, while the corresponding SgFW was estimated to be 3–10%. The winter measurements in the subsurface halocline layer showed a total freshwater content of about 1% and no significant contribution from SgFW.