Arctic river blockage and the formation of glacial deep ocean salinity anomalies

Substantial snow accumulation over northern continents during glacial periods contributed to the growth of the Laurentide and Eurasian ice sheets. As a result sea level dropped by ~120-130 m, which led to an increase in global mean ocean salinity by about 1 permil. Pore water chlorinity data from deep ocean sediment cores interestingly show even higher values regionally. Despite this superficial understanding of glacial ocean salinity shifts, the three-dimensional patterns of paleosalinity changes are still not well understood. Here, we argue that northern hemisphere ice-sheets effectively blocked pan-Arctic river discharge into the Arctic Ocean for millennia. In the absence of ice-sheet calving and melting, this process was responsible for the gradual accumulation of the 1 permil global mean salinity anomaly during glacial periods. To better understand the underlying physical mechanisms, we use the Community Earth System Model and mimic the freshwater withholding of the ice-sheets as an idealized negative freshwater perturbation. Applying this forcing scenario, we find that the river blockage due to the Laurentide and Eurasian ice-sheets removes the polar halocline, strengthens the Atlantic Meridional Overturning Circulation and contributes to the global increase of salinity at a rate of 0.1 permil/1000 years. Moreover, the process creates a characteristic pattern of deep ocean salinity anomalies, which is distinct from the vertical salinity redistribution due to sea-ice/brine formation in the Southern Ocean. Eventually, for glacial conditions both, the Arctic and Southern Ocean-generated salinity patterns combine.