Abstract
Abstract. An extensive compilation of published neodymium (Nd) concentrations and isotopic compositions (Nd IC) was realized in order to establish a new database and a map (using a high-resolution geological map of the area) of the distribution of these parameters for all the Mediterranean margins. Data were extracted from different kinds of samples: river solid discharge deposited on the shelf, sedimentary material collected on the margin or geological material outcropping above or close to a margin. Additional analyses of surface sediments were done in order to improve this data set in key areas (e.g. Sicilian strait). The Mediterranean margin Nd isotopic signatures vary from non-radiogenic values around the Gulf of Lion, (εNd values ∼ −11) to radiogenic values around the Aegean and the Levantine sub-basins up to +6. Using a high-resolution regional oceanic model (1/12° of horizontal-resolution), εNd distribution was simulated for the first time in the Mediterranean Sea. The high resolution of the model provides a unique opportunity to represent a realistic thermohaline circulation in the basin and thus apprehend the processes governing the Nd isotope distribution in the marine environment. Results are consistent with the preceding conclusions on boundary exchange (BE) as an important process in the Nd oceanic cycle. Nevertheless this approach simulates a too-radiogenic value in the Mediterranean Sea; this bias will likely be corrected once the dust and river inputs will be included in the model. This work highlights that a significant interannual variability of εNd distribution in seawater could occur. In particular, important hydrological events such as the Eastern Mediterranean Transient (EMT), associated with deep water formed in the Aegean sub-basin, could induce a shift in εNd at deep/intermediate depths that could be noticeable in the eastern part of the basin. This underlines that the temporal and geographical variations of εNd could represent an interesting insight of Nd as tracer of the Mediterranean Sea circulation, in particular in the context of palaeo-oceanographic applications.
Highlights
The general trend is that the margin Nd isotopic signatures vary from non-radiogenic values in the western Mediterranean Sea (WMed), to radiogenic values when reaching the Aegean and Egyptians coasts, the most radiogenic fields being located around the eastern border of the Levantine sub-basins, and in the volcanic region of the south of Italy (Fig. 2b)
The Algerian, Tunisian, French and Spanish coasts display relatively homogeneous values between −11.5 and −10. Such east–west gradient of Nd isotopic signature is observed in the seawater data, where poorly radiogenic waters from the Atlantic are progressively shifted toward more radiogenic values in the Levantine basin (Tachikawa et al, 2004)
The high-resolution simulation presented here provides a too-radiogenic signature of Nd isotopic signature in the Mediterranean Sea; this approach confirms the primordial role of the boundary exchange (BE) as the major source of Nd in the marine environment, similar to what has been previously demonstrated for the global ocean (Arsouze et al, 2007) and the Atlantic basin (Arsouze et al, 2010)
Summary
M. Ayache et al.: High-resolution neodymium characterization thermohaline circulation cell (Malanotte-Rizzoli and Robinson, 1988), strait transports and dynamics or cross-shore exchanges. It is a region receiving the highest aerosol loads owing to air masses carrying numerous and various aerosol types (Lelieveld et al, 2002; Nabat et al, 2014), where oligotrophy occurs and with a characteristic dynamic of the deep chlorophyll maximum (Mignot et al, 2014). The radiogenic isotope 143Nd is produced by the radioactive α-decay of 147Sm. At the continent surface, the Nd isotopic composition (usually expressed as εNd 1 of a given material) is a function of the Sm / Nd ratio characterizing this material, which is primarily a function of its age and lithology. The εNd of the continents presents a heterogeneous distribution (Goldstein and Hemming, 2003; Jeandel et al, 2007)
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