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

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• 作者：Myriam Lambelet^a ; ^{m.lambelet@imperial.ac.uk" class="auth_mail" title="E-mail the corresponding author} ; Tina van de Flierdt^a ; Kirsty Crocket^b ; Mark Rehkä ; mper^a ; Katharina Kreissig^a ; Barry Coles^a ; Micha J.A. Rijkenberg^c ; Loes J.A. Gerringa^c ; Hein J.W. de Baar^c ; Reiner Steinfeldt^d
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 March 2016
• 年：2016
• 卷：177
• 期：Complete
• 页码：1-29
• 全文大小：4391 K

文摘

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), ε_Nd = −14.2 ± 0.3; Labrador Sea Water (LSW), ε_Nd = −13.7 ± 0.9; Northeast Atlantic Deep Water (NEADW), ε_Nd = −12.5 ± 0.6; Northwest Atlantic Bottom Water (NWABW), ε_Nd = −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 ε_Nd values of −13.2 ± 1.0 (2sd) and lower-NADW exhibits values of ε_Nd = −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 (ε_Nd = −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.