Dissolved neodymium isotopes in the Mediterranean Sea

Paolo Montagna (1, 2) , Christophe Colin (3) , Martin Frank (4) , Tjördis Störling (4, 5) , Toste Tanhua (4) , Micha J. A. Rijkenberg (6, 7) , Marco Taviani (8, 9) , Katrin Schroeder (10) , Jacopo Chiggiato (10) , Guohui Gao (11, 3) , Arnaud Dapoigny (12, 13) , Steven L. Goldstein (2, 14)

1 CNR-ISP - Institute of Polar Sciences [Venezia-Mestre]
2 LDEO - Lamont-Doherty Earth Observatory
3 GEOPS - Géosciences Paris Saclay
4 GEOMAR - Helmholtz Centre for Ocean Research [Kiel]
5 Department of Geology [Lund]
6 NIOZ - Royal Netherlands Institute for Sea Research
7 Universiteit Utrecht / Utrecht University [Utrecht]
8 WHOI - Biology Department
9 SZN - Stazione Zoologica Anton Dohrn
10 ISMAR-CNR - Istituto di Scienze Marine [Venezia]
11 UPCité - Université Paris Cité
12 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
13 GEOTRAC - Géochrononologie Traceurs Archéométrie
14 Columbia University [New York]

The neodymium isotopic composition (ε_Nd) of seawater is one of the most important geochemical tracers to investigate water mass provenance, which can also serve as a proxy to reconstruct past variations in ocean circulation. Nd isotopes have recently also been used to reconstruct past circulation changes in the Mediterranean Sea on different time scales. However, the modern seawater ε_Nd dataset for the Mediterranean Sea, which these reconstructions are based on, is limited and up to now only 160 isotopic measurements are available for the entire basin. The lack of present-day data also limits our understanding of the processes controlling the Nd cycle and Nd isotopic distribution in this semi-enclosed basin. Here we present new ε_Nd data from 24 depth profiles covering all Mediterranean sub-basins, which significantly increases the available dataset in the Mediterranean Sea. The main goal of our study is to better characterize the relationship between the dissolved Nd isotope distributions and major water masses in the Mediterranean Sea and to investigate the impact and relative importance of local non-conservative modifications, which include input of riverine particles and waters, aeolian-derived material and exchange with the sediments at continental margins. This comprehensive ε_Nd dataset reveals a clear ε_Nd - salinity correlation and a zonal and depth gradient with ε_Nd systematically increasing from the western to the eastern Mediterranean basin (average ε_Nd = -8.8 ± 0.8 and -6.7 ± 1 for the entire water column, respectively), reflecting the large-scale basin circulation. We have evaluated the conservative ε_Nd behaviour in the Mediterranean Sea and quantified the non-conservative components of the ε_Nd signatures by applying an Optimum Multiparameter (OMP) analysis and results from the Parametric Optimum Multiparameter (POMP) analysis of Jullion et al. (2017). The results of the present study combined with previously published Nd isotope values indicate that dissolved ε_Nd behaves overall conservatively in the open Mediterranean Sea and show that its water masses are clearly distinguishable by their Nd isotope signature. However, misfits between measured and OMP- and POMP-derived ε_Nd values exist in almost all sub-basins, especially in the eastern Levantine Basin and Alboran Sea at intermediate-deep depths, which can be explained by the influence of detrital lithogenic ε_Nd signatures through interaction with highly radiogenic Nile sourced volcanic fractions and unradiogenic sediments, respectively.

The radiogenic signature acquired in the eastern Levantine Basin is carried by the Levantine Intermediate Water and transferred conservatively to the entire Mediterranean at intermediate depths. Our measured ε_Nd values and OMP- and POMP-derived results indicate that non-conservative contributions originating from sediment sources are then propagated by water mass circulation (with distinct preformed ε_Nd) along the Mediterranean Sea through advection and conservative mixing. Mediterranean ε_Nd effectively traces the mixing between the different water masses in this semi-enclosed basin and is a suitable water mass tracer.

Mots clés: Neodymium isotopes; Seawater; Mediterranean Sea

Mots clés: PANOPLY

Domaines: Planète et Univers [physics]

Paolo Montagna, Christophe Colin, Martin Frank, Tjördis Störling, Toste Tanhua, et al.. Dissolved neodymium isotopes in the Mediterranean Sea. Geochimica et Cosmochimica Acta, 2022, 322, pp.143-169. ⟨10.1016/j.gca.2022.01.005⟩. ⟨insu-03659873⟩