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.