<patrick.blaser@unil.ch>
ABSTRACT (246 w.)
Today, the sub-surface Denmark Strait Overflow (DSO) and the Iceland Scotland Overflow form the starting points of Atlantic Meridional Overturning Circulation and compensate for the poleward flowing Norwegian and Irminger branches of the North Atlantic surface current that drive the ’Nordic Heat Pump’. During peak glacial and early deglacial times, ice sheets on Iceland and Greenland, and ice-induced isostatic and eustatic sea-level changes reduced the Denmark Strait aperture and DSO. Nonetheless, extremely high benthic stable carbon and oxygen isotope values together with very high ventilation ages of bottom waters reflect a north-south density gradient of intermediate-waters and persistent flow of partially Arctic-sourced waters through both the Denmark Strait and the Faeroe Channel, similar to today. The first arrival of Heinrich­­­ -1 meltwaters northwest of Iceland, arriving from the southwest around 18.4 cal ka, accompanied a tipping point in DSO circulation, documented by reduced ventilation and ventilation ages, a 3°C warming, and increased radiogenic Nd isotope signatures in sediments at luff-side Site PS2644. These records suggest a sudden subsurface incursion of Atlantic intermediate waters across basaltic sediments from S.E. of Iceland. Deep-water convection off Norway then was replaced by weak brine water formation, coeval with a breakdown of the ’Nordic Heat Pump’ evidenced by a temperature drop on Greenland. After 16.2 cal. ka, a major meltwater outbreak from the Barents ice shelf led to modified Heinrich-1-style circulation until ~15.1 cal ka. Vice-versa, the DSO intensified during interstadial and Holocene times, then causing sediment hiatuses at site PS2644.
KEY WORDS:
Nordic Seas, Deep-water formation, Denmark Strait Overflow, Age control of Heinrich stadial 1, Nordic Heat Pump, Atlantic Meridional Overturning Circulation
KEY POINTS:
The Overflow of Nordic Sea’s deep waters forms the onset of Atlantic Meridional Overturning Circulation being enhanced at interstadial times
Sediments off N. Iceland show a joint onset of deglacial meltwater dilution and 3-degree warming of the Denmark Strait Overflow 18400 yr ago
Deglacial changes in circulation caused Heinrich Stadial 1, a temporary end of heat advection to European high latitudes 18400–15000 yr ago
PLAIN LANGUAGE SUMMARY (200 w.)
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 the peak last glacial and early deglacial from two high-resolution marine sediment cores with centennial-scale age resolution based on the technique of radiocarbon plateau tuning (22-15 cal. ka). Sediment data suggest that the salinity of surface water, advected from the North Atlantic, started to drop about 18 400 years ago. This drop accompanied a 3°C rise in temperature and a drop in ventilation and radiocarbon ventilation age of Denmark Strait Overflow waters feeding the AMOC. Also, it paralleled a massive rise in the radiocarbon reservoir age of surface waters up to 2000 yr and an abrupt breakdown of Nordic Seas’ deep-water convection off Norway. Accordingly, Atlantic waters were replaced by less saline polar waters, marking a breakdown of the Nordic Heat Pump and start of ’Heinrich Stadial 1’ as reflected by a coeval cooling documented on top of the Greenland ice sheet, lasting until ~15.1 cal. ka.
INTRODUCTION
The Denmark Strait Overflow (DSO; Fig. 1) with its exceptionally high density plays a key role for ~50% of the Atlantic meridional overturning circulation (AMOC) and thereby, for global deep-ocean circulation. The DSO is closely associated with the ”Nordic Heat Pump”, and any changes therefore have major implications for northern hemispheric climate, moreover, it is crucial for deep-ocean ventilation and radiocarbon uptake. Today, the flux of DSO is long-term fairly stable with >3 Sv entering from the southern Greenland Sea and Arctic Ocean (Brakstad et al., 2023). Further 5 Sv are entrained into DSO during its overflow in the Irminger Sea to the south of the Denmark Strait, where a 2.5 km deep descend acts in stabilizing the DSO (data of Biastoch et al., 2003 and 2021; Kösters, 2004, Kösters et al., 2005; Kaese & Oschlies, 2000, Kaese et al. 2003 and refs. therein)
East of Faroe the DSO is paralleled by the Iceland-Scotland Overflow Water (ISO), also enfolding ~1.4–2.4 Sv, being supplied to the N.E. North Atlantic and subsequently, after having passed the Charlie Gibbs Fracture Zone, into the AMOC in the western Atlantic (Hansen et al., 2000; Sessford et al., 2019; Biastoch et al., 2021).
Important characteristics of modern DSO waters are low bottom water temperatures near -1°C, high bottom water salinities of 34.9 psu, and densities (σ) of 27.8-28.05 kg/m3 for Theta=2°C at the bottle neck of the Denmark Strait (Whitehead et al., 1974; Macrander and Käse, 2007; Haine, 2021). A slight but persistent north-south density gradient is indicated for the Last Glacial Maximum (LGM, 19–23 thousand calibrated yr before present; cal. ka) by δ18O records of single specimens of epibenthic foraminifera (Millo et al., 2006) and confirmed by the distribution pattern of εNd isotopes, a tracer of dominantly continental or basaltic sediments overflown by bottom waters (Larkin et al., 2022; Blaser et al., 2020).
To monitor short-term variations in the DSO over peak glacial-to-early deglacial climate change (22 - 15 cal. ka; here ’ka’ is used for ’kilo yr ago’ in the sense of BP = Before Present = 1950 yr AD) we now employ, partly refine, and supplement published centennial-scale proxy records obtained from sediment Core PS2644, a site that marks the southern margin of the Blosseville Basin, that is the northern funnel to the Denmark Strait north of westernmost Iceland (Fig. 1; Voelker, 1999; Millo et al., 2006). We compare the effect and origin of long-term climate trends and intermittent short, centennial-to-millennial-scale episodes of major cold spells with episodes of fast climate warming, regimes in part lasting until today. The very last major Greenland stadial we redefined as Heinrich Stadial 1a and b (HS-1a and b), in contrast to a redefinition of stadials proposed by Andrews and Voelker (2018). On the basis of advanced age control (details are given below) our definition of HS-1 differs strongly from that previously given by Hodell et al. (2017) based on sediments from the southern subpolar North Atlantic.
Moreover, we compare our records with proxy data and circulation models of stadial and interstadial analogue variations over Marine Isotope Stage 3 (MIS3) near to the northern entrance of the Denmark Strait as proposed by Hagen and Hald (2002), Sadatzki et al. (2020), and Sessford et al. (2018, 2019). The latter authors report on distinct changes in MIS3 bottom water temperature, which we now try to reproduce for late MIS2. Sessford et al. (2019) proposed pathways of brine water-induced intermediate waters that finally might have been funneled into the DSO, assuming a water column homogenous in the Nordic Seas down to 1500 m.
We test this model by means of a number of multiproxy records from Site PS2644, compared to pertinent records obtained from outside on the basis of centennial-scale age resolution (Sarnthein et al., 2020): (i) We employ records from two neighbor sites at the western margin of the Vøring Plateau in the eastern Nordic Seas (Fig. 1; GIK23074 and MD95-211), where brine water formation is widely accepted to have occurred during HS-1 (Meland et al., 2008; Waelbroeck et al., 2011). (ii) We compare coeval paleoceanographic records obtained south of Iceland in the northern North Atlantic (Thornalley et al., 2011; Millo et al., 2006; Sarnthein et al., 1994).
In this way we focus on minor and major changes in the origin of early deglacial DSO waters and related changes in the flow geometry of the eastern Nordic Seas and of North Atlantic intermediate- and deep-water circulation. Changes in flow geometry were possibly linked to crucial tipping points in the composition of DSO waters, items to be constrained in this study. The origin of these changes appears highly important for a better understanding of past and future trends of the ”Nordic Heat Pump” and European climate change (sensu Jansen et al., 2020; Ditlevsen and Ditlevsen, 2023).
OCEANOGRAPHIC WATCHDOG POSITIONS of SITES PS2644 AND GIK23074
Bathymetry and modern patterns of surface and bottom water circulation in the Denmark Strait (Fig. 1; today: ~630 m w.d.) are detailed by Kösters et al. (2004). Macrander et al. (2007) present details of the modern DSO structure being ruled by geostrophic forcing. Sediment Core PS2644 lies within the lower portion of the modern plume funneled into the DSO (Käse et al., 2003). Prior to entering the Denmark Strait, Norwegian Sea Deep Water (NSDW) today is entrained from both the Greenland Basin and Arctic Ocean (Brakstad etal., 2023), a scenario to be traced back over last glacial and early deglacial times 22–15 cal. ka.
Sediments of Core PS2644 are well suited to monitor past glacial-to-early deglacial variations in the flow of surface and deep-water masses since the site lies close to the Polar Front, that is the mean position of the border of perennial sea ice, the boundary between the East Iceland Current (EIC) and the nearshore North Iceland Irminger Current (NIIC) (Fig. 1; Voelker, 1999), while not lying too close to the Denmark Strait where extreme winnowing due to strong bottom currents largely prevents any sediment deposition. To achieve an undisturbed and largely continuous sediment record of past changes in the geometry of ocean water masses, Site PS2644 at 777 m water depth on top of a narrow hemipelagic sediment ridge was chosen after careful selection by means of a high-resolution PARASOUND parametric echosounder system (Hubberten et al., 1995). As detailed in Voelker (1999) the site shows a sediment drift free from lateral near-bottom sediment input like sediment slumps and/or turbidity currents (Fig. 2a; Fig. S1).
Core GIK23074 was retrieved from the western margin of the Vøring Plateau right below the North Atlantic Current (NAC), but far away from the Norwegian margin (Fig. 1; Voelker, 1999). The site is marked by exceptionally high, yet undisturbed hemipelagic sedimentation rates (25-60 cm /ky; Fig. 2b), thus providing a unique high-resolution (17-50 yr per cm sediment depth) record of ocean history in the eastern Nordic Seas at 1157 m water depth.
AGE CONTROL and PALEOCEANOGRAPHIC PROXIES
Precise age control of marine sediment records PS2644 and GIK23074 is essential for constraining the objectives of this study. Age control has been based on various conventional, correlation-based age tie points by Voelker (1999) and Voelker et al. (1998 and 2000). Finally, however, we based our age model on the technique of14C plateau tuning (Figs. 2a and b) (Sarnthein et al., 2020 and 2023). The different approaches are listed below:
• Distinct features in the planktic δ18O records were correlated to apparently analogous age-calibrated δ18O oscillations in ice core records GISP2 and NGRIP, where ages are based on incremental time scales (Grootes and Stuiver, 1997; Svensson et al., 2008) (Fig. S1; Voelker, 1999).
• Likewise, distinct variations in % Neogloboquadrina pachydermasin (Nps; in ”old” teminology) served as tracer of short-term SST changes that were used as stratigraphic markers by comparison to temperature variations dated in Greenland ice core records.
• The Vedde volcanic ash layer was used as tracer dated by land plants at ~10.31±50 yr 14C ka (= 12.1 cal. ka) on the basis of ambient plant macrofossils (age revised by Birks et al., 2017; Bronk Ramsey et al., 2020).
• Initial age control was established by means of a high-resolution suite of planktic 14C ages in Core PS2644, using the general assignment of a hypothetical Marine Reservoir Age (MRA) of 400 yr (Voelker, 1999; Voelker et al., 1998). This assumption, however, has now been subject to major revision (Sarnthein et al., 2015 and 2020) as shown below.
• On top of a simple conventional stratigraphic alignment of planktic14C ages, we further refined the age control by defining local planktic 14C age plateaus and plateau boundaries that were tuned as cal. age tie points to pertinent age-calibrated structures in the atmospheric 14C record of Lake Suigetsu (Table 1; Sarnthein et al., 2023). In turn, Suigetsu age control is based on U/Th-based model ages of Bronk Ramsey et al., 2020 (details of age derivation in Sarnthein et al., 2020 and 2023) (Figs. 2a and b). We are aware that our approach is in conflict with allegations of Bard and Heaton (2021) claiming that the approach is flawed. However, we trust in 14C plateau tuning for various crucial reasons: (i) Different from Bard and Heaton (op. cit.) we have clearly shown that past centennial to millennial-scale fluctuations of the atmospheric 14C record have been authentic (Sarnthein et al. (2023). (ii) Our technique of plateau tuning solely relies on the tuning of a whole suite of 14C plateaus in the atmosphere and a sediment core each (Figs. 2a+b), thus can clearly distinguish and/or exclude potential fake plateaus in a sediment core, such as given for Core GIK23074 for the top portion of HS-1 (Fig. 2b). (iii) Our text (Figs. 4 and 5) is demonstrating two prominent cases of abrupt deglacial climate change during Heinrich stadial 1 at 18.4 and 16.2 cal. ka, where the 14C plateau-based age estimates precisely match pertinent estimates based on the incremental age scale of the North GRIP ice core with less than 100 years deviation, results that are far from incidental and would not be revealed by any other stratigraphic method. (iv) On the basis of numerous details, we have refuted one-by-one the allegations of Bard and Heaton (op.cit.), as published in the discussion section of their article (Grootes and Sarnthein, 2021; Sarnthein and Grootes, 2021), though ignored by Bard and Heaton.
Calendar-age uncertainties of the atmospheric 14C plateau boundaries employed for our tuning approach hardly exceed ~50 to ~100 yr each. Local MRA of planktic foraminifers and surface waters were derived from the age difference between the average 14C age estimated for paired, that is, coeval atmospheric and marine plateaus based on14C plateau tuning (Sarnthein et al., 2020, and references therein).
• In addition, a single U/Th-based cal. age was obtained from a solitary coral in Core MD95-2011. This age was compared with the14C ages of paired planktic and benthic foraminifera specimens (Fig. 3), thereby largely confirming the paired age estimates and MRA derived on the basis of 14C plateau tuning of the planktic 14C record (unpubl. comm. of Dreger, 2000; thorium/uranium data of Lomitschka and Mangini [1999]. The ratio of thorium to uranium in each sample, which yield the calendar age of the coral, were measured by thermal ionisation mass spectrometry (TIMS) at the Heidelberger Akademie der Wissenschaften (Heidelberg, Germany) according to the method outlined by Bollhöfer et al. (1996) and Neff et al. (1999).
• Sedimentation rates with multi-centennial time resolution were derived from the age interpolation of sediment sections using the cal. age of planktic 14C plateau boundaries (Fig. 2a and b). The estimates were widely supported by means of ages independent of14C plateau tuning such as the linear age interpolation between conventional age tie points to pertinent ages of ice core record GISP2 (Voelker, 1999; Fig. S1).
• To a large extent, Site PS2644 lacks modern and Holocene reference values for most proxy records employed in this study. Except for a few cm thick 14C-dated sediment layer that forms the actual core top, a strong DSO flow has hindered any Holocene sediment deposition and/or led to sediment erosion prevalent over most of the last ~15 cal. ka (Voelker, 1999; Kuijpers et al., 2003).
• The derivation and constraints of various paleoceanographic proxy values employed in this study are given in Supplementary Text no. 1.
PS2644: FOUR STRATIGRAPHIC TIME SEGMENTS ~22–15 cal. ka
The PS2644 sediment section representing LGM and HS-1 times starts and ends with major stratigraphic gaps prior to ~21.9 cal. ka and subsequent to ~15.1 cal. ka (Fig. 2a). The gaps occur near the end of Greenland Interstadial (GI) 2 and close to the end of HS1, shortly prior to the Bølling-Allerød (B/A) interstadial that is lost by erosion or non-deposition (Fig. S1; Voelker, 1999, and Voelker et al., 2000). This sediment gap is documented by X-ray radiography as distinct unconformity at 54-53 cm composite core depth, right below the Vedde Ash layer, 12.6 cal. ka (sensu Voelker, 1999, and Voelker and Haflidason, 2015). The stratigraphic gaps probably result from enhanced sediment winnowing due to the constriction of an enhanced inflow to the Denmark Strait near to its northern entrance.
This peak-glacial-to-early-deglacial sediment record of PS2644 for MIS2 is partitioned into four stratigraphic time segments I through IV that lasted from 21.8–19.8, 19.8–18.4, 18.4–17.2, and 17.2–~15.1 cal. ka, based on epibenthic ventilation ages, changes in the stable C and O isotope composition of planktic and epibenthic foraminifera (Nps and mainly Cibicidoides lobatulus ), and less distinct, planktic MRA. The partitioning is also reflected by the εNd and Pb isotope records, moreover, by distinct changes in sedimentation rate (Fig. 4). It is important to note that the time segments apply to the suite of structures in the proxy records of both the cores PS2644 near Iceland and GIK23074 from the eastern Nordic Seas.
Time segment (I), 21.8–19.8 cal. ka, in PS22644 covers the top portion of the Last Glacial Maximum (LGM; as defined by Mix et al., 2001), while time segment (II) 19.8–18.4 cal. kaalready reflects the end of the LGM, as suggested by a first major deglacial rise in eustatic sea level starting at 19.4 cal. ka (based on purely atmospheric 14C ages of Hanebuth et al., 2009). During segments I and II, sea ice-covered subsurface waters of the East Greenland Current (EGC) at PS2644 (Sadatzki et al., 2020) are marked by minimum temperature and peak salinity values as reflected by fairly persistent planktic δ18O values of 4.5 ‰ and minimum SST of 3.7°C based on census counts of planktic foraminifera species (Pflaumann et al., 2003; Millo et al., 2006). Time segment I shows a maximum MRA near 2200 yr, that slightly dropped to ~1900 yr after 19.8 cal. ka (Fig. 4).
During time segment I, bottom waters at PS2644 were marked by a bipartite population of epibenthic δ13C values. One of them presents the highest δ13C values and, despite all processes of ocean mixing, the highest deep-water ventilation recorded in the global ocean (Millo et al., 2006; Duplessy et al., 2002), clearly predominant over 21.8–20.3 cal. ka, when bottom water ventilation ages reached up to 2500 14C yr.
Radiogenic isotopes in the detrital sediment fraction indicate a mixture of Arctic and European sources of the sediment, with a reduced contribution from nearby Iceland (Fig. 4 and 5; Struve et al., 2019; Larkin et al., 2021; Blaser et al., 2020). We hypothesize that this could have been caused by the complete glaciation of Iceland and/or a change in ocean currents shielding the site from Icelandic input. Interestingly, the authigenic sediment fraction exhibited significantly more radiogenic signatures in both Nd and Pb. A similar effect has been observed in front of the Barents Shelf (Struve et al. 2019) and in the eastern subpolar North Atlantic (ODP980; Crocket et al. 2013) and attributed to the glacial erosion of terrestrial metal oxides in Northwest Europe and their supply to the ocean.
During time segment II, after 19.8 cal. ka, the antecedent high in bottom water ventilation age was short-term replaced by very low ages of 100-400 yr. North of Iceland they mark an early deglacial incursion of waters reflecting a direct contact with the atmosphere nearby, closely resembling ages that already marked the end of GI 2, and more important, LGM bottom water ages found at Site GIK23074 in the eastern Nordic Seas. Moreover, we find a distinct rise in radiogenic εNd values of the detritus up to -5, which traces an increased contribution by Icelandic basalts passed by DSO waters prior to reaching Site PS2644 (Fig. 4), or increased supply through an early deglaciation of parts of Iceland (sensu Crocket et al., 2012).
Time segment III, 18.4–17.2 cal. ka, reflects a phase of transition, when planktic δ18O values at PS2644 show a marked gradual drop by more than 1.5 per mil (Fig. 4). The drop primarily reflects a drop in sea surface salinity induced by a lateral advection of meltwaters from southwest, from the Irminger Sea, and thus records the onset of HS-1. Also, foraminifera census counts reflect a slight SST rise up to 4°C (Sarnthein et al., 2001). Over this time, local MRA dropped to 1670-1780 yr, a value slightly lower than before, but still reflecting a reduced carbon exchange of sub-surface waters with the atmosphere, impeded by ongoing perennial sea ice cover. At the eastern Site GIK23074 MRAs at the onset of time segment III showed an impressive sudden rise from 1175 yr up to 1900 yr, a level equating that found at the western Site PS2644 (Figs. 2b and 3).
Like surface waters, bottom waters document a major change near 18.4 cal. ka: The maxima of benthic δ18O values measured on single epibenthic specimens reveal a rapid, centennial-scale18O depletion by 0.8 ‰, equivalent to a rise in bottom water temperature of up to 3.4°C (Fig. 4). At this time, the data population with extremely high epibenthic δ13C values has disappeared in favor of medium high values, coeval with a renewed short-term reduction of bottom water ventilation ages down to 750 yr near 17.5 cal. ka. Once more, εNd signatures in both the detrital and authigenic fractions have increased, indicating an elevated supply of Icelandic volcanogenic material overflown by DSO waters prior to reaching Site PS2644 (Fig. 4) or delivered to the site directly by surface currents such as the North Iceland Irminger Current (Fig. 1). The contemporaneous authigenic206Pb/204Pb -based ratios show a pronounced short-lasting maximum (Fig. 5), which could result from an increased delivery of Pb glacially eroded from Northwest Europe 18.4-17.0 cal. ka.
Time segment IV, 17.2–15.1 cal. ka (i.e., until the onset of a major hiatus), presents the full maturation of the HS-1 sub-ice meltwater regime as reflected by a persistent minimum in planktic δ18O. Near the very onset, this interval was marked by a short-term SST peak of 7.5°C (census counts of planktic foraminifera species; Fig. 15c of Voelker, 1999). As in antecedent periods, MRA of 1900 years continued. By contrast, paired bottom water ages were as low as 1100 to 1550 14C years, significantly lower than during the LGM. As in time segment III, epibenthic δ13C values of bottom water ventilation were modestly high (0.6-1.4 ‰).
The onset of time segment IV shows a remarkable sudden rise in bulk sedimentation rates at PS2644, almost by a factor of three, with sediments marked by a high in the concentration of ice-rafted hematite-stained quartz grains originating from (North-) East Greenland (Voelker, 1999). At the same time, detrital206Pb/204Pb ratios decreased while εNd became more radiogenic (Fig. 4), which would agree with an increased delivery of Tertiary basalt sediment. On the expense of the ”European” source (Fig. 5) the basalt signal then may have come from the nearby Iceland-Scotland Ridge east of Iceland, overflown by the North Iceland Jet, when heading west for Site PS2644. Also, Fig. 5 may suggest an input of basalt debris from major basalt sources in East Greenland south of 70°N. Conversely, however, we regard this east-west sediment transport as unlikely, since it would need to cross the (sea-ice covered) frontal systems of the East Greenland Current flowing toward southwest (Fig. 1).
Altogether, MRAs and bottom water ventilation ages at Site GIK23074 (Fig. 3) reveal a suite of changes in differential stratification of the eastern Nordic Seas contemporary with that in the west, at PS2644 (Fig. 1). In addition, however, time segment IV has been dissected near 16.15 cal. ka by a subsequent major drop in both MRA and bottom water ventilation age (Fig. 2b and 3). The drops are tied to the great late-deglacial meltwater outbreak from the Barents Shelf as documented by a short-term extreme low in planktic δ18O values extending south to the Faeroe Isles (Weinelt et al, 1991; Voelker, 1999).
DISCUSSION – LINKAGES BETWEEN CHANGING SOURCES OF DSO WATERS AND SHORT-TERM CHANGES IN SEA SURFACE SALINITY AND CLIMATE