Changing surface waters and deep-water formation in the eastern Nordic Seas
In the eastern Nordic Seas, by comparison, planktic δ18O values of 4.6–4.8 ‰ did not show any substantial HS-1 meltwater signal over the LGM and most of HS-1, from 18.4–16.3 cal. ka, except for a minor δ18O low ~17.6–17.2 cal. ka (by Δδ18Ο = 0.3 ‰; Fig. 2b). Figs. 2b and 3, however, display an almost sudden 2-‰ drop in the planktic δ18O record, that clearly document – after the first incursion of meltwaters at PS2644 in the west – a second major ice and meltwater outbreak during the last 1000 yr of HS-1a starting at 16.3-16.0 cal. ka (as with the onset of HS-1b, precisely coeval with a second major HS-1 cooling depicted in Greenland ice cores GRIP2 and NGRIP). The meltwater signal started from the Barents shelf, spread south down to Faeroe (Sarnthein et al., 2001). The ice breakout obviously induced a turbulent mixing down to intermediate waters at Site GIK23074 at 1157 m w. d. (Fig. 3), and lasted until ~14.7 cal. ka. This age estimate on top of HS-1a fits precisely a δ18O depletion found in Greenland records NGRIP and GISP2 (e.g., Grootes and Stuiver, 1997).
Over time segments I and II, eastern surface waters showed fairly low MRA of ~500 to 800 and 1200 yr (Figs. 2b and 3 that closely resemble MRA values recorded for the very early LGM by Simon et al. (2023). However, they differed strongly from the high MRA of 1900-2200 yr obtained from Site PS2644 in the west (Fig. 2a). Starting at 18.4 cal. ka, however, eastern MRA depict a fast rise to 1730 yr and 2000 yr, rapidly reaching a close match with the MRA found at western Site PS2644 over time segment III and early segment IV (Fig. 2b). Accordingly, the differential sense of eastern and western surface water circulation of time segments I and II ended and was briefly replaced by a close match during time segments III and IV, when ’old’ surface waters with an Arctic origin similar to those of the EGC had started also to cover the eastern Nordic Seas. Per analogy, we infer that this advection dominated until ~16 cal. ka.
During time segment I, extremely low bottom water ventilationages at Site GIK 23074 attest lively intermediate-water convection somewhere close to the site in the eastern Nordic Sea at least down to ~1200 m w.d. (Figs. 3 and 6; in harmony with Meland et al., 2008; Thornalley et al., 2015). In contrast, the regime at Site PS2644 was stratified. Occasionally, extremely young intermediate waters of Site GIK23074 found their way from the east up to the west, at Site PS2644 forming DSO mode 1 during time segment II (Fig. 4), where local deep-water convection continued in the eastern Nordic Seas, though somewhat attenuated (Fig. 3).
After 18.4 cal. ka, with the start of time segment III, the circulation geometry of the eastern Nordic Seas was strongly modified (Fig. 6; modified after Thornalley et al., 2015). Theinflow of NAC stopped . It was r eplaced by surface waters with high and very high MRA at Site GIK23074 / MD95-2311 (1157 m), that closely equate the MRA at Site PS2644 reflecting the EGC. Hence, the waters were probably Arctic-sourced. Their arrival was precisely coeval with the start of meltwater incursion through the Denmark Strait (Figs. 3, 4). Local intermediate and deep-water convection (sensu Siedler et al., 2001) then was replaced by stable stratification and dominantly seasonal, probably fairly modest volumes of brine-enhanced shelf waters leading to modest deep-water formation (Bauch and Bauch 2001; Waelbroeck et al., 2011; Thornalley et al., 2015). This shift is reflected by a local abrupt rise in bottom water ventilation ages from <1000 yr up to 2100-2200 yr, ages that slightly exceed the paired, likewise strongly raised MRA of surface waters reaching 1900-2100 yr (Fig. 3). In turn, the great iceberg and meltwater outbreak from the Barents Sea after 16.3 cal. ka obviously resulted in local stirring and an immediate overturning and renewed local ventilation of intermediate waters in the eastern Nordic Seas (Fig. 3).
Different from a model of Sessford et al. (2019), the brine water-induced intermediate waters of the eastern Nordic Seas during time segments III and IV apparently did never spread up to the westernmost Nordic Seas to replace DSO waters at Site PS2644. Our benthic ventilation ages and other proxy data suggest that the circulation geometry of the west was different from today being dominated by the North Iceland Jet (sensu Våge et al., 2011) that entrained DSO mode-3 waters from the east along the northern slope of Iceland and ultimately, from upper North Atlantic Intermediate Waters to the south and southeast of Iceland.