Neodymium isotopic composition and concentration in the western North Atlantic Ocean: Results from the GEOTRACES GA02 section

Myriam Lambelet,Tina Van De Flierdt,Kirsty Crocket,Mark Rehkämper,Katharina Kreissig,Barry Coles,Micha J.A Rijkenberg,Loes J.A Gerringa,Hein J.W De Baar,Reiner Steinfeldt

doi:10.1016/j.gca.2015.12.019

Abstract

The neodymium (Nd) isotopic composition of seawater is commonly used as a proxy to study past changes in the thermohaline circulation. The modern database for such reconstructions is however poor and the understanding of the underlying processes is incomplete. Here we present new observational data for Nd isotopes and concentrations from twelve seawater depth profiles, which follow the flow path of North Atlantic Deep Water (NADW) from its formation region in the North Atlantic to the northern equatorial Atlantic. Samples were collected during two cruises constituting the northern part of the Dutch GEOTRACES transect GA02 in 2010. The results show that the different water masses in the subpolar North Atlantic Ocean, which ultimately constitute NADW, have the following Nd isotope characteristics: Upper Labrador Sea Water (ULSW), εNd=−14.2±0.3; Labrador Sea Water (LSW), εNd=−13.7±0.9; Northeast Atlantic Deep Water (NEADW), εNd=−12.5±0.6; Northwest Atlantic Bottom Water (NWABW), εNd=−11.8±1.4. In the subtropics, where these source water masses have mixed to form NADW, which is exported to the global ocean, upper-NADW is characterised by εNd values of −13.2±1.0 (2sd) and lower-NADW exhibits values of εNd=−12.4±0.4 (2sd). While both signatures overlap within error, the signature for lower-NADW is significantly more radiogenic than the traditionally used value for NADW (εNd=−13.5) due to the dominance of source waters from the Nordic Seas (NWABW and NEADW). Comparison between the concentration profiles and the corresponding Nd isotope profiles with other water mass properties such as salinity, silicate concentrations, neutral densities and chlorofluorocarbon (CFC) concentration provides novel insights into the geochemical cycle of Nd and reveals that different processes are necessary to account for the observed Nd characteristics in the subpolar and subtropical gyres and throughout the vertical water column. While our data set provides additional insights into the contribution of boundary exchange in areas of sediment resuspension, the results for open ocean seawater demonstrate, at an unprecedented level, the suitability of Nd isotopes to trace modern water masses in the strongly advecting western Atlantic Ocean.

Highlights

Ocean circulation has an important role in modulating global heat transport and the carbon cycle, two key players in understanding climate change and in the past (Broecker, 1991; Ganachaud and Wunsch, 2000; Rahmstorf, 2002; Alley et al, 2003; Marshall and Speer, 2012)
We report the first results on dissolved Nd isotopic compositions and concentrations from GEOTRACES section GA02 along the flow path of North Atlantic Deep Water (NADW) in the western Atlantic Ocean from Iceland in the north to the equator in the south (Rijkenberg et al, 2014)
The track for the Dutch GA02 section cruise followed the deepest part of the Northwest Atlantic Ocean and the flow path of NADW (Figs. 1 and 2), starting from the Irminger Sea down to the south of the Sargasso Sea (Table 2) and further into the Southwest Atlantic to the Falkland Islands (third leg – see http://www.geotraces.org/, Rijkenberg et al (2014) and Middag et al (2015) for details on the entire cruise track; not part of this study)

Summary

Introduction

Ocean circulation has an important role in modulating global heat transport and the carbon cycle, two key players in understanding climate change and in the past (Broecker, 1991; Ganachaud and Wunsch, 2000; Rahmstorf, 2002; Alley et al, 2003; Marshall and Speer, 2012). Elderfield and Greaves, 1982; Goldstein et al, 1984), changes in open ocean Nd isotopic compositions have been mainly attributed to water mass mixing (e.g. Piepgras et al, 1979; Albarede and Goldstein, 1992). This assumption has been substantiated by observed co-variations of Nd isotopes in deep waters in the western Atlantic Ocean with salinity (von Blanckenburg, 1999) and silicate concentrations (Goldstein and Hemming, 2003), but has been challenged by more recent observations. Following surfaces of equal density (isopycnals), this nonconservative behaviour of Nd in the vicinity of continental margins can leave a fingerprint in the open ocean, away from the area of actual seawater–sediment interaction (Lacan and Jeandel, 2001; Rickli et al, 2014)

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Conclusion