Impact of freshwater runoff from the southwest Greenland Ice Sheet on fjord productivity since the late 19th century

Climate warming and the resulting acceleration of freshwater discharge from the Greenland Ice Sheet are impacting Arctic marine coastal ecosystems, with implications for their biological productivity. To accurately project the future of coastal ecosystems and place recent trends into perspective, palaeo-records are essential. Here, we show runoff estimates from the late 19th century to the present day for a large sub-Arctic fjord system (Nuup Kangerlua, southwest Greenland) influenced by both marine- and land-terminating glaciers. We followed a multiproxy approach to reconstruct spatial and temporal trends in primary production from four sediment core records, including diatom fluxes and assemblage composition changes and biogeochemical and sedimentological proxies (total organic carbon, nitrogen, C/N ratio, biogenic silica, δ¹³C, δ¹⁵N, and grain-size distribution). We show that an abrupt increase in freshwater runoff in the mid-1990s was reflected by a 3-fold increase in biogenic silica fluxes in the glacier-proximal area of the fjord. In addition to increased productivity, freshwater runoff modulates the diatom assemblages and drives the dynamics and magnitude of the diatom spring bloom. Our records indicate that marine productivity is higher today than it has been at any point since the late 19th century and suggest that increased mass loss of the Greenland Ice Sheet may continue promoting high productivity levels at sites proximal to marine-terminating glaciers. We highlight the importance of palaeo-records in offering a unique temporal perspective on ice-ocean-ecosystem responses to climate forcing beyond existing remote sensing or monitoring time series.