Differential DSO modes and Nordic Sea stratification – Implications for short-term glacial-to-deglacial climate change in the northern hemisphere
After ~18.4 cal. ka (note: before 1950 AD) (Figs. 2a, 4), intermediate waters ultimately derived from upper intermediate and/or subsurface waters in the North Atlantic started to dominate DSO mode 3 at Site PS2644 north of Iceland below a layer of highly aged, less saline, and southward flowing Arctic surface waters. This switch in bottom water circulation in the Icelandic Sea was precisely coeval (i) with a very first incursion of meltwaters through the Denmark Strait and (ii) with a pertinent switch in the geometry of surface and intermediate-water circulation in the Norwegian Sea, that is, coeval within the error range of the 14C plateau tuning technique (Figs. 4 and 5) (Sarnthein et al., 2020).
The overall switch from an anti-estuarine to a weakened anti-estuarine flow regime was almost instantaneous, taking from 18.7-18.39 cal. ka. It presented a tipping point in circulation geometry of the complete Nordic Seas, and in turn, of AMOC (e.g., major abrupt rise of MRA near to the Azores Islands; Balmer and Sarnthein, 2018). Accordingly, the switch triggered a prompt breakdown of the poleward advection of warm Atlantic surface waters, that previously had driven the ”Nordic Heat Pump” up to northern Norway and Svalbard over peak glacial time segments I and II per analogy to today, though with somewhat lower SST (based on % Nps and Artificial Neural Network = ANN records of Weinelt et al., 2003). We are confronted with a tipping point in North Atlantic circulation geometry and climate, that marked already the onset of the HS-1b cold spell at a very early tie point in deglacial time. Below, the age and potential time span covered by the oceanographic shift itself are compared with the climate history constrained by temperature records on Greenland in ice cores, independently dated with an incremental time scale based on annual-layer counts.
­Based on the NGRIP δ18O record (GICC05; Wolf et al., 2010), the temperatures in North Greenland reflect an overall trend of gradual deglacial warming from 22 to ~17 cal. ka (Figs. 4 and 5). The trend, however, was interrupted by a significant first deglacial breakdown in the advection of warm atmospheric moisture, an event here named ’HS-1b’ that started between 18,500 and 18,380 cal. yr ago (before 2k). The event was less distinct in ice core GISP2, here depicted between 18,630 and 18,430 cal. yr b2k (Grootes and Stuiver, 1997). This age range matches within the range of a century the great switch in seawater stratification of Nordic Sea circulation independently measured at two different marine sediment records at the base of time segment III, when dated by means of 14C plateau tuning. Per se , the age match may form a proof for the robustness of 14C plateau-based age estimates. Also, the event is reflected directly by a short atmospheric14C plateau named ’4a’ lasting from ~18.6 to ~18.4 cal. kyr before 1950 AD (Figs. 2a and b; Sarnthein et al., 2023). Based on GICC05 ages, the fundamental switch in seawater stratification and the resulting drop of the Nordic heat pump may have taken no longer than 80–100 yr.
Unfortunately, it is widely impossible to compare this narrow age range with any of the numerous IRD and SST records published for the onset of HS-1 from the North Atlantic ”Heinrich-1 IRD Belt”. Except for some sporadic and wide-spaced single age tie points (Hodell et al., 2017, and refs. therein), local MRA levels and even more so, the precise timing of their short-term variations are largely unknown due to a lack of sediment records with absolute datings such as high-resolution14C plateau tuning. By comparison to pertinent changes recovered in the Nordic Seas over HS-1 (Figs. 2a and b) local MRAs may actually have shown short-term variations between 200 and 2000 yr. Also, the age range of meltwater advection and ice-rafted debris input was subject to major regional variations, while winds and currents were driving icebergs over vast sea regions. Off southwestern Portugal, for example, sediment record SHAK06-5K the ages of which were calibrated by14C plateau tuning (Ausin et al., 2021) show a δ18O-based local meltwater incursion and an IRD input not starting prior to 17.8/17.9 cal. ka, that is 500-600 yr after the start of HS-1b, as defined in our Nordic Seas cores. – During the Alpine Late Glacial, the HS-1 cold spell is reflected by the well-dated ”Gschnitz Stadial” that showed temperatures and aridity closely similar to those estimated for the LGM (Ivy-Ochs et al., 2023).
CONCLUSIONS
• On the basis of the 14C PT technique we define a suite of four peak glacial to early-deglacial time segments from 22-15 cal. ka, that show differential MRAs and bottom water ventilation ages at two core sites in the western and eastern Nordic Seas.
• On the basis of sediment-based quantitative proxy-data we distinguish three glacial-to-deglacial modes of Denmark Strait Overflow (DSO), each of them revealing differences in source region, formation mode, and/or in flow intensity.
• The proxy records suggest that waters of DSO modes 1 and 2 were largely fed by a mix of Arctic intermediate waters and deep-water convection in the eastern Nordic Seas, either reflecting a weaker (mode 2) or stronger and even erosive (mode 1) anti-estuarine flow geometry at the northern entrance to the Denmark Strait, similar to today.
• Starting at 18.4 cal. ka, the proxy records of time-segments III and IV show a meltwater-diluted Irminger Current that entered the Denmark Strait from southwest. It induced an immediate switch to DSO mode 3, then dominated - as we hypothesize - by subsurface and upper intermediate waters advected from the northern North Atlantic southeast of Iceland, hence suggesting a strongly weakened anti-estuarine flow geometry.
• Near 18.4 cal. ka, the climate tipping point between DSO modes 2 to 3 is marking the onset of Heinrich stadial 1b. This switch in ocean flow geometry caused a temporary end of the Nordic Heat Pump. The switches took hardly more than a century, a time span independently confirmed by incremental age estimates obtained from the Greenland temperature records of GISP2 and NGRIP.
• The fundamental switch in Nordic Seas flow geometry ~18.6–18.4 cal. ka is also reflected by a special atmospheric 14C plateau, by 14C plateau 4a.
• After 16.3 cal. ka, a subsequent meltwater outbreak from the Barents shelf (Weinelt et al., 1991) caused a major prolongation of the HS-1 flow geometry in the Nordic Seas and further cooling of HS-1, then named Heinrich stadial 1a until Dansgaard-Oeschger Event 1, independently documented in Greenland ice core records (~16.1 – 14.7 cal. kyr b2k).
IMPLICATIONS
The onset of cold spell HS-1b forms a scenario enticing to speculate about potential trends found today and in the near future (e.g., in view of models of Ditlevsen and Ditlevsen, 2023; Jansen et al., 2020; Caesar et al., 2018) on the basis of following boundary conditions: (i) Within few decades the onset of the cold spell near 18.4 cal. ka was found coeval at two records from Greenland ice cores and two marine sediment records that showed a fundamental switch from anti-estuarine to modified sources of anti-estuarine flow geometry in the western and a switch to weak anti-estuarine in the eastern Nordic Seas and accordingly, also in AMOC. (ii) The tipping point of seawater stratification was met as soon as first meltwaters had arrived through the Denmark Strait. (iii) The switch to estuarine and/or weak anti-estuarine flow geometry in the Nordic Seas and consequentially, the breakdown of Nordic Heat Pump took probably less than 100 yr.
In turn, however, we don’t have any evidence for the actual source of these meltwaters somewhere in the Irminger and Labrador Seas. Even less, we can quantify the past meltwater fluxes needed to trigger the almost abrupt onset of HS-1b by comparison to meltwater fluxes from Western Greenland and Baffin Island, that today are admixed to the North Atlantic Drift and finally may serve as potential origin of a future tipping point in AMOC geometry. Thus, the analog of the start of HS-1 is left with open questions.
Data availability
All primary radiocarbon data and cal. ages assigned are stored at PANGAEA.de® under ”still to be processed”.
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ACKNOWLEDGMENTS
We thank Pieter M. Grootes, Kiel, and Antje Voelker, Lisboa, for substantial critical comments on our manuscript and for references to recent publications. In particular we thank A. Biastoch, Kiel, for his advice in potential questions of physical oceanography. We gratefully acknowledge the help of Ralf Tiedemann, AWI Bremerhaven, with the acquisition of special additional sediment samples from Core PS2644, John Southon, Irvine CA, and Gesine Mollenhauer, AWI Bremerhaven, for analyzing 14C ages of benthic foraminifera samples of Core GIK23074, and Michael Bollen for help with the analysis of Nd and Pb isotope data at the University of Lausanne. Sebastian Beil, Kiel, and Hugo Ortner, Innsbruck, kindly helped us with software problems. P.B. acknowledges funding from the European Union’s Horizon 2020 research and innovation program (grant agreements No 101065424, project OxyQuant).
FIGURE CAPTIONS
Figure 1. Location of twin sites PS2644 and GS15-198-36GC in the Icelandic Sea (67°52’N, 21°46’W, 777 m w.d.) and twin sites GIK23074 and MD95-2311 in the Norwegian Sea (66°40’N, 4°54’E, 1157 m w.d.). Yellow arrows mark warm surface water currents entering the Nordic Seas (NAC: North Atlantic Current, NIIC: North Iceland Irminger Current), green arrow shows the EGC. Arrows on top of broken lines depict intermediate-water currents such as the Denmark Strait Overflow (DSO), the North Iceland Jet (NIJ), and the Iceland Scotland Overflow Water (ISO). Bathymetry based on Ocean Data View (Schlitzer, 2002), highlighted by faint 250-m isoline.