Ice-sheet driven weathering input and water mass mixing in the Nordic Seas during the last 25,000 years

https://doi.org/10.1016/j.epsl.2019.02.030Get rights and content

Highlights

  • First deep water Nd and Pb isotope records from the glacial–deglacial Nordic Seas.

  • Deglacial εNd changed by up to six units from unradiogenic LGM background.

  • Glacial–deglacial Nd and Pb isotope changes controlled by local weathering input.

  • Pb isotopes suggest synchronous postglacial weathering fluxes around the North Atlantic.

  • Holocene Nd isotope evolution dominated by re-invigorated water mass mixing.

Abstract

Neodymium (Nd) isotopes are a powerful proxy tool for reconstructing past changes in water mass mixing, but reliable application of this proxy requires constraints on past changes of source water compositions. A key region of the global deep water circulation system are the Nordic Seas, which provide dense waters fundamental for the formation of North Atlantic Deep Water (NADW). Yet, the Nd isotope evolution of past deep waters in the Nordic Seas is so far poorly constrained. Here we present the first reconstructions of seawater Nd isotope compositions extracted from marine sediments at two locations in the central and northern Nordic Seas covering the period from the last glacial to the present. Further insights into past changes in sediment provenance, weathering inputs and water mass mixing are provided by complementary seawater and detrital strontium (Sr) and lead (Pb) isotope compositions.

Our new data reveal that changes in source and magnitude of weathering inputs from the Scandinavian and Svalbard–Barents Ice Sheets (SIS and SBIS, respectively) controlled the Nd and Pb isotope composition of the Nordic Seas' deep waters during the last glacial period. During the Last Glacial Maximum (LGM), deep waters showed distinctly unradiogenic Nd and radiogenic Pb isotope signatures most likely driven by weathering inputs of the SBIS. In contrast, the deglaciation was characterized by enhanced SBIS ice dynamics and/or meltwater release delivering sediments from the distal Eurasian shelves to the Norwegian Sea. Pulses of volcanogenic sediment supply changed the deep water Nd isotope composition during Heinrich Stadials 1, 2 and the Bølling period. As such, the glacial–deglacial Nd and Pb isotope evolution was markedly different in the Nordic Seas, the North Atlantic and the Arctic Oceans, respectively. During the Holocene, Pb isotopes indicate synchronized weathering fluxes around the North Atlantic, whereas the influence of local weathering input on the Nd isotope evolution of the deep Nordic Seas ceased. Instead, the Holocene Nd isotope signal has been driven by the strength of the advection/convection of water masses in the study area. These new constraints on changes in the Nordic Seas provide important endmember information for Nd isotope based reconstructions of NADW export downstream in the Atlantic Ocean.

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 εNd = ((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 εNd10 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 εNd of ∼−13, whereas the overflow waters exiting the basin at depth show significantly more radiogenic εNd 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 εNd 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 εNd 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 εNd of northern Norwegian Sea core GIK23258 yielded relatively unradiogenic values of −12.2 ± 0.4 (2SD, n=5; 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 εNd 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 εNd 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

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