Variable DSO modes linked to short-term changes in surface water composition
On centennial timescales, the immense shift from peak glacial DSO modes 1 and 2 to deglacial mode 3 was directly paired with the distinct onset of a δ18O depletion of (sub-) surface waters at PS2644. It started right at the base of deglacial time segment III, 18.4 cal. ka, subsequent to a long-term constant LGM δ18O maximum near ~4.5 ‰ that had prevailed since ~22.0 cal. ka (Fig. 4). The start of a gradual decrease in planktic δ18O by up to 1.5 ‰ displays the onset of both warmer and meltwater-diluted surface waters advected through the Denmark Strait, a precursor of the North Iceland branch of the Irminger Current. After 17.2 cal. ka, the shift resulted in an SST rise by 2°–4°C during time segment IV (based on census counts of planktic foraminifera of Voelker, 1999; Pflaumann in Sarnthein et al., 2001; Hagen and Hald, 2002). The rise was paired with a meltwater-induced salinity reduction that necessarily led to continued sea ice cover (You et al., 2023; Sadatzki et al., 2020). Thus, any local CO2 and 14C exchange between atmosphere and surface waters off East Greenland was continuously suppressed until ~15.1 cal. ka (Fig. 2a).