Meltwater-induced salinity drop in Greenland Sea induced changes in AMOC and the onset of Heinrich-1 stadial 18 400 years ago – Potential analog to modern trends

Differences in salt content of North Atlantic surface waters drive variations in Nordic Seas' overturning circulation. These form a switchboard for changes in the oceanic heat transport to North European high latitudes, the 'Nordic Heat Pump', and for Atlantic Meridional Overturning Circulation (AMOC). We deduced changes in the Nordic Seas' overturning circulation during peak last glacial and early deglacial times (22-15 cal. ka) from two marine sediment cores with centennial-scale age resolution synchronized via radiocarbon (¹⁴C) plateau tuning. Sediment data suggest that the salinity of surface waters, advected through the Denmark Strait from the northwesternmost Atlantic, started to drop significantly near 18.4 cal. ka. This accompanied precisely an abrupt rise in bottom water temperature by up to 3.5°C and a drop in both ventilation and ¹⁴C ventilation ages of Denmark Strait overflow waters feeding the AMOC. Moreover, it paralleled a change in (detrital) Pb and Nd radiogenic isotopes suggesting that overflow waters then started to have their dominant source in the North Iceland Jet of upper North Atlantic Intermediate Water that overflows the shallow basaltic Iceland-Scotland Ridge east of Iceland. Off Norway, the salinity reduction north of Iceland went along with a fast rise in the ¹⁴C reservoir age of surface waters from ~600–1200 years up to ~2000 years and an abrupt breakdown of Nordic Seas' convection of young deep waters. Accordingly, warm Atlantic waters were replaced by slightly cooler Arctic polar waters aged like those of the East Greenland Current, inducing a breakdown of the 'Nordic Heat Pump' and start of 'Heinrich Stadial 1' as reflected by a precisely coeval cooling documented on top of the Greenland ice sheet, lasting until ~15 cal. ka. The outlined circulation changes starting near 18.4 cal. ka remind us of potential implications of the meltwater flow from West Greenland strongly enhanced today.