Ice-sheet driven weathering input and water mass mixing in the Nordic Seas during the last 25,000 years
Introduction
One of the most important deep water masses ventilating large volumes of the global deep ocean is North Atlantic Deep Water (NADW). In the modern North Atlantic Ocean, dense waters formed in the Nordic Seas overflow the Iceland-Scotland Ridge (ISOW; Iceland-Scotland Overflow Water) and the Denmark Strait (DSOW; Denmark Strait Overflow Water) and form NADW upon mixing with ambient Labrador Sea Water (LSW) and local North Atlantic waters (e.g., Dickson et al., 1996, Hansen and Østerhus, 2000).
The overflow source waters forming in the Nordic Seas occupy large volumes north of the Greenland-Scotland Ridge (GSR) and can be categorized based on their formation characteristics (e.g., Aagaard et al., 1985, Hansen and Østerhus, 2000). The inflow of warm and saline Atlantic surface waters is subjected to substantial heat and buoyancy loss driving their subduction, in particular in the Norwegian Sea (Hansen and Østerhus, 2000; Fig. 1). The deeper layers are ventilated by open ocean convection involving lateral mixing of warm and saline Atlantic waters with cold and fresh polar waters in the Greenland and Iceland Seas resulting in Arctic Intermediate Water (AIW) (Aagaard et al., 1985, Hansen and Østerhus, 2000; Fig. 1). Likewise, Greenland Sea Deep Water (GSDW), is formed north of the GSR by local deep convection and lateral admixture of Arctic Ocean Deep Water (AODW) (Aagaard et al., 1985). From its formation area in the Greenland Sea, GSDW penetrates into the adjacent deep basins of the Norwegian Sea to form Norwegian Sea Deep Water (NSDW) as a result of further incorporation of AODW (Swift and Koltermann, 1988). The southward-directed components of these dense waters contribute to the overflow waters in different amounts, whereas their northward expansion ventilates the Arctic Ocean below the halocline (e.g., Aagaard et al., 1985, Hansen and Østerhus, 2000).
Reconstruction of past ocean circulation in the Nordic Seas has been based on conventional proxies such as benthic stable carbon and oxygen isotopes (e.g., Bauch et al., 2001; Sarnthein et al., 2003) or 14C-based ventilation ages (Thornalley et al., 2015, Ezat et al., 2017). However, these traditional proxies fail to provide more specific water provenance information. Independent water mass provenance information can be obtained via the radiogenic Nd isotope signature (expressed as = ((143Nd/144Ndsample)/(143Nd/144NdCHUR) − 1) × 10,000, where CHUR is the chondritic uniform reservoir; Jacobsen and Wasserburg, 1980) of seawater (cf. Frank, 2002). This is enabled by the oceanic residence time of Nd (∼500 yr) which is less than one ocean mixing time resulting in a heterogeneous distribution of Nd isotopes in the ocean (e.g., Frank, 2002, Tachikawa et al., 2003). Therefore, Nd isotopes extracted from authigenic seawater derived phases are a powerful tool to reconstruct past water mass mixing and ocean circulation (cf. Frank, 2002).
The seawater Nd and Pb isotope compositions are dictated by the geochemical characteristics of terrestrial input sources, which differ as a function of chemical differentiation of the Earth, radioactive decay and thus age (Frank, 2002). In the modern Arctic Ocean and the Nordic Seas, vigorous deep water exchange results in relatively homogeneous Nd isotope compositions of to −11 in deep waters north of the GSR (Lacan and Jeandel, 2004, Andersson et al., 2008, Porcelli et al., 2009, Laukert et al., 2017). However, the North Atlantic Current (NAC) entering the Norwegian Sea in the eastern part of the basin is unradiogenic with of ∼−13, whereas the overflow waters exiting the basin at depth show significantly more radiogenic of ∼−8 to −9 (Lacan and Jeandel, 2004; Fig. 1). This isotopic difference is the result of (i) water mass mixing, (ii) dissolved and particulate inputs from terrestrial sources, and (iii) interaction with marine deposits and bathymetric features within the Nordic Seas (von Blanckenburg and Nägler, 2001; Tachikawa et al., 2003, Lacan and Jeandel, 2004, Abbott et al., 2015, Du et al., 2018). For example, the more radiogenic values of the overflow waters relative to the precursor water masses are attributed to interaction of the deep waters with the basaltic margins of the GSR in the southern Norwegian Sea (Lacan and Jeandel, 2004, Morrison et al., 2019), which are characterized by signatures of up to +10 (e.g., Kempton et al., 2000, Fagel et al., 2014).
Due to its higher particle reactivity, the residence time of Pb in the ocean is only 20–200 yr (Henderson and Maier-Reimer, 2002). Thus the Pb isotope signature of seawater is more sensitive to partial dissolution of input material rather than to advective transport. Conversely, seawater Nd isotope signatures are less prone to alteration from partial dissolution and therefore more likely to trace water mass mixing. Accordingly, the combination of Nd and Pb isotopes from authigenic seawater-derived phases can provide unique insight into the relative influence of past water mass mixing and weathering inputs (e.g., von Blanckenburg and Nägler, 2001, Frank, 2002, Haley et al., 2008a, Haley et al., 2008b; Crocket et al., 2013, Crocker et al., 2016). It has been shown that the growth and decay of Northern Hemisphere ice sheets induced pronounced changes of particulate and dissolved terrigenous input fluxes which influenced the Nd and Pb isotope composition of ambient water masses (e.g., von Blanckenburg and Nägler, 2001, Frank, 2002, Kurzweil et al., 2010, Crocket et al., 2013, Roberts and Piotrowski, 2015, Howe et al., 2016, Blaser et al., 2019). Despite their important role for NADW formation, the Nd and Pb isotope evolution of dense waters in the Nordic Seas is still largely unconstrained during major climatic transitions.
Here we present the first seawater Pb and Nd isotope records from the central and northern Norwegian Sea covering the last glacial–interglacial transition. By combining radiogenic isotope and conventional proxy records, we demonstrate that inputs from the surrounding ice sheets significantly altered the radiogenic isotope signatures of advected deep waters in the Nordic Seas and discuss their export into the adjacent ocean basins.
Section snippets
Sample locations and age models
Kasten core GIK23258-2 was recovered from the Barents Sea slope in the northern Norwegian Sea (75.00°N and 13.97°E, 1768 mbsl; Fig. 1) and the data are spliced with neighboring trigger box core GIK23258-3 (Sarnthein et al., 2003). The spliced record is referred to as GIK23258 (Supplementary Tables S1a, b). The published age model of Sarnthein et al. (2003) was extended by two additional radiocarbon dates at 600 cm (13,210 ± 60 14C yr BP) and 800 cm (18,780 ± 100 14C yr BP) measured at the
Results
Our new radiogenic isotope data from the Norwegian Sea show distinct changes during the past ∼25 ka (Fig. 2). The leach fraction of northern Norwegian Sea core GIK23258 yielded relatively unradiogenic values of −12.2 ± 0.4 (2SD, ; Table S2 and Fig. 2) for the LGM (18–24 ka BP), followed by large fluctuations between −8.1 ± 0.3 and −11.8 ± 0.3 during the deglaciation. The most radiogenic values were recorded during Heinrich Stadials HS1 and HS2, and the Bølling warming. In contrast,
Significance of the radiogenic isotope signal of the leached phase
We observed covariation of HH-AcOH-leached authigenic, seawater-derived Pb and Nd isotope records alongside changes in sediment geochemistry in GIK23258 until the early Holocene (Fig. 2c). Such covariation can result from concurrent natural changes of the detrital and authigenic signals or partial dissolution of detrital phases during chemical extraction (Gutjahr et al., 2007). The latter occurs in particular when the carbonate fraction is removed and does not sufficiently buffer the leach
Conclusions
We show that authigenic precipitates extracted from deep Norwegian Sea sediments recorded the Nd isotope signature of ambient bottom waters varying by ∼6 units during the last deglaciation. Provenance analyses of the detrital fraction suggest that sediment supply to the Norwegian Sea changed between local and more distal sources on the Eurasian shelves in response to ice-sheet dynamics.
During the LGM and Younger Dryas cold intervals, ice driven inputs of local sediments altered seawater
Acknowledgments
We thank Jo Clegg, Vicky Rennie and Jutta Heinze for technical help, and Lisa Bretschneider is acknowledged for supplementary mass spectrometric analyses. We also thank Derek Vance for editorial handling as well as Nathalie Fagel and two anonymous reviewers for their helpful comments improving this manuscript. This work was funded by NERC grant NE/F007132/1 to AMP with additional support from St John's College Cambridge, DFG-Schwerpunktprogramm SPP 1266 INTERDYNAMIK and the Mainz Academy of
References (59)
- et al.
The impact of sedimentary coatings on the diagenetic Nd flux
Earth Planet. Sci. Lett.
(2016) - et al.
Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic–Atlantic gateways
Geochim. Cosmochim. Acta
(2008) - et al.
A multiproxy reconstruction of the evolution of deep and surface waters in the subarctic Nordic seas over the last 30,000 yr
Quat. Sci. Rev.
(2001) - et al.
Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea sediment by chemical leaching
Chem. Geol.
(2016) - et al.
The resilience and sensitivity of Northeast Atlantic deep water to overprinting by detrital fuxes over the past 30,000 years
Geochim. Cosmochim. Acta
(2019) - et al.
A chemical technique for the separation of ferro-manganese minerals, carbonate minerals and adsorbed trace elements from pelagic sediments
Chem. Geol.
(1967) - et al.
Neodymium isotopic composition of deep-sea corals from the NE Atlantic: implications for past hydrological changes during the Holocene
Quat. Sci. Rev.
(2010) - et al.
Geochemical response of the mid-depth Northeast Atlantic Ocean to freshwater input during Heinrich events 1 to 4
Quat. Sci. Rev.
(2016) - et al.
A Pb isotope tracer of ocean-ice sheet interaction: the record from the NE Atlantic during the Last Glacial/Interglacial cycle
Quat. Sci. Rev.
(2013) - et al.
Experimental evidence for a mineral-controlled release of radiogenic Nd, Hf and Pb isotopes from granitic rocks during progressive chemical weathering
Chem. Geol.
(2019)
Long-term coordinated changes in the convective activity of the North Atlantic
Prog. Oceanogr.
Grain size separation and sediment mixing in Arctic Ocean sediments: evidence from the strontium isotope systematic
Chem. Geol.
Lead isotope systematics of granitoid weathering
Geochim. Cosmochim. Acta
Late Quaternary evolution of sediment provenances in the Central Arctic Ocean: mineral assemblage, trace element composition and Nd and Pb isotope fingerprints of detrital fraction from the Northern Mendeleev Ridge
Quat. Sci. Rev.
Provenance of Late Quaternary ice-proximal sediments in the North Atlantic: Nd, Sr and Pb isotopic evidence
Earth Planet. Sci. Lett.
Reliable extraction of a deepwater trace metal isotope signal from Fe–Mn oxyhydroxide coatings of marine sediments
Chem. Geol.
North Atlantic–Nordic Seas exchanges
Prog. Oceanogr.
Systematic changes in lead isotopic composition with soil age in glacial granitic terrains
Geochim. Cosmochim. Acta
Advection and removal of 210Pb and stable Pb isotopes in the oceans: a general circulation model study
Geochim. Cosmochim. Acta
Sm–Nd isotopic evolution of chondrites
Earth Planet. Sci. Lett.
Glacial freshwater discharge events recorded by authigenic neodymium isotopes in sediments from the Mendeleev Ridge, western Arctic Ocean
Earth Planet. Sci. Lett.
The Iceland plume in space and time: a Sr–Nd–Pb–Hf study of the North Atlantic rifted margin
Earth Planet. Sci. Lett.
Authigenic Pb isotopes from the Laurentian Fan: changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation
Earth Planet. Sci. Lett.
Ocean circulation and freshwater pathways in the Arctic Mediterranean based on a combined Nd isotope, REE and oxygen isotope section across Fram Strait
Geochim. Cosmochim. Acta
Geochemical signatures of sediments documenting Arctic sea-ice and water mass export through Fram Strait since the Last Glacial Maximum
Quat. Sci. Rev.
Deglaciation of the Eurasian ice sheet complex
Quat. Sci. Rev.
Reconstructing deglacial North and South Atlantic deep water sourcing using foraminiferal Nd isotopes
Earth Planet. Sci. Lett.
The distribution of neodymium isotopes in Arctic Ocean basins
Geochim. Cosmochim. Acta
Rare earth element association with foraminifera
Geochim. Cosmochim. Acta
Cited by (9)
Neodymium isotopes as a paleo-water mass tracer: A model-data reassessment
2022, Quaternary Science ReviewsCitation Excerpt :Hence, it still remains to be determined how substantial the impact of these non-conservative effects truly were. Moreover, here we are not able to simulate a range of observations, such as a highly unradiogenic glacial deep Labrador Sea (Blaser et al., 2020) and Nordic Seas (Struve et al., 2019), or highly radiogenic signatures in the Northeast (Roberts and Piotrowski, 2015) and South Atlantic (Huang et al., 2020; Skinner et al., 2013), which either hint to missing processes or lacking resolution and general shortcomings of the model. Here, we extended the implementation of Nd isotopes in the Bern3D model by revising a number of critical parameterizations, which provides us with an improved description of the marine Nd cycle.
Active North Atlantic deepwater formation during Heinrich Stadial 1
2021, Quaternary Science ReviewsCitation Excerpt :The latter was previously reported also for HS11 in the Nordic Seas (Bauch, 2013). Though potentially subject to changes in the erosion patterns (Struve et al., 2019), εNd and sortable silt records from the Irminger Sea (Crocker et al., 2016 and citations therein) further support the export of deepwater from the Irminger Sea during HS1. Thus, a mechanism needs to be found that can explain northward NAC transport and ISOW formation during HS1.
Evolution of the Global Overturning Circulation since the Last Glacial Maximum based on marine authigenic neodymium isotopes
2020, Quaternary Science ReviewsCitation Excerpt :It may be countered that the influence of volcanic detrital sediments via the Nordic Sea pathway could have also been reduced due to limited export of glacial NSOW (Thornalley et al., 2015). However, glacial NSOW was less radiogenic than today (Struve et al., 2019), and thus cannot have been a significant contributor to the preformed εNd of NSW in the LGM (Fig. 9A). Regarding the reduction of unradiogenic sources, the contribution of Canadian Shield Nd sources to the NSW preformed εNd was likely reduced in the LGM, because of reduced LSW formation (Hillaire-Marcel et al., 2001), a diminished DWBC pathway (Fagel et al., 1999) along which deep-sea boundary exchange with low-εNd detrital sediments happens (Lacan and Jeandel, 2005b) (Fig. 2E, F), and a removal of shelf area in the Baffin-Labrador Basin for sedimentary flux (Arsouze et al., 2008).
Labrador Sea bottom water provenance and REE exchange during the past 35,000 years
2020, Earth and Planetary Science LettersCitation Excerpt :Today, NWABW is mainly sourced from dense water produced in the Nordic Seas that overflows the Greenland-Scotland Ridge, thereby entraining significant amounts of LSW. A recently published ϵNd record from the deep Norwegian Sea indicates that deep water in the Nordic Seas was considerably less radiogenic during MIS 2 and 3 than today (Fig. 8 and Struve et al., 2019). In fact, the Nd isotope signatures were similar to those at Site U1305 at the entrance to the LS, and thus the Nordic Seas are a possible source region for glacial NWABW, provided the actual overflow waters had similar isotope signatures to deep waters (as is the case today).
Active Nordic Seas deep-water formation during the last glacial maximum
2022, Nature Geoscience