Seawater 87Sr/86Sr ratios along continental margins: Patterns and processes in open and restricted shelf domains

Type Article
Date 2020-12
Language English
Author(s) El Meknassi Sofia1, Dera Guillaume1, de Rafélis Marc1, Brahmi Chloé3, Lartaud Franck2, Hodel Florent1, Jeandel Catherine4, Menjot Ludovic1, Mounic Stéphanie1, Henry Manuel1, Besson Philippe1, Chavagnac Valérie1
Affiliation(s) 1 : Géosciences Environnement Toulouse (GET), Université Paul Sabatier Toulouse 3, CNRS UMR 5563, IRD, Toulouse, France
2 : Sorbonne Université, CNRS UMR 8222, Laboratoire d'Ecogéochimie des Environnements Benthiques (LECOB), Observatoire Océanologique de Banyuls, Banyuls-sur-Mer, France
3 : Université de la Polynésie Française, UMR 241 Ecosystèmes Insulaires Océaniens (EIO), Faa'a, Tahiti, French Polynesia
4 : Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS), Université Paul Sabatier Toulouse 3, CNRS UMR 5566, IRD, Toulouse, France
Source Chemical Geology (0009-2541) (Elsevier BV), 2020-12 , Vol. 558 , P. 119874 (15p.)
DOI 10.1016/j.chemgeo.2020.119874
WOS© Times Cited 5
Keyword(s) Strontium isotopes, Seawater, Shells, Coastal environment, Groundwater, Open shelf
Abstract

To better constrain the Sr isotope budget in marginal domains without any fluvial inputs, we analyzed the chemical composition and 87Sr/86Sr ratio of waters and shells from four locations: two coastal lagoons, one hemipelagic platform and one open marine shelf. Our results highlight homogeneous 87Sr/86Sr ratios typical of oligotrophic oceanic waters (OOW) (i.e., 0.709172 ± 0.000023) in the Pacific Tatakoto atoll and along a Mediterranean shore to offshore transect (~25 km off Banyuls-sur-Mer, BSM). This attests that oceanic inputs from oligotrophic areas remain the main Sr source in open shelf areas compared with submarine groundwater discharges (SGD) or particulate dissolution influences. In BSM, only foreshore data are more radiogenic, possibly due to rainwater mixing, local groundwater springs or more efficient particle dissolution in the intertidal zone. In restricted areas, we report variable 87Sr/86Sr ratios between the Salses-Leucate (France) and Oualidia (Morocco) lagoons. The first one has homogeneous 87Sr/86Sr ratio typical of OOW except close to groundwater discharges. In Oualidia, 87Sr/86Sr ratios decrease by 1.2 × 10−3 from OOW values close to the Atlantic inlet to progressively less radiogenic ones upstream within the interior of the lagoon. These differences depend on several factors including the leaky, restricted or choked morphology of lagoons modulating the oceanic Sr inputs, but also SGD fluxes whose 87Sr/86Sr ratios and Sr concentrations are highly variable according to the nature of rocks leached in karstic aquifer. In Oualidia, the low 87Sr/86Sr ratios correspond to high Sr concentrations (up to 150 μmol·l−1) issued from the dissolution of Mesozoic evaporites, leading to SGD fluxes accounting for 60% of the local Sr budget. Through data compilation, we show that similar 87Sr/86Sr gradients and processes prevail at the whole Mediterranean scale. Finally, we postulate that high coastal water retention times can also account for anomalous coastal 87Sr/86Sr ratios and that the combination of water mass restriction, SGD, bioadsorption and early diagenetic processes could decrease seawater Sr concentrations in some marginal areas.

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El Meknassi Sofia, Dera Guillaume, de Rafélis Marc, Brahmi Chloé, Lartaud Franck, Hodel Florent, Jeandel Catherine, Menjot Ludovic, Mounic Stéphanie, Henry Manuel, Besson Philippe, Chavagnac Valérie (2020). Seawater 87Sr/86Sr ratios along continental margins: Patterns and processes in open and restricted shelf domains. Chemical Geology, 558, 119874 (15p.). Publisher's official version : https://doi.org/10.1016/j.chemgeo.2020.119874 , Open Access version : https://archimer.ifremer.fr/doc/00648/76047/