Dynamics of dissolved trace metals, rare earth elements and Pb isotopes across the eastern margins of the Mediterranean Sea

Abstract

Continental margins support marine primary productivity by transferring nutrients and micro-nutrients (trace metals) from the coast to the oceans. Yet, the mechanisms governing the delivery of trace metals across the land-sea continuum, and how they vary temporally, are still poorly constrained. Here, we report high spatial resolution depth profiles of dissolved trace metals (Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb), rare earth elements (REEs), nutrients (PO₄, TON, and SiOH₄) and Pb isotopes from two transect cruises in the oligotrophic eastern Mediterranean Sea.

Varying anthropogenic inputs resulted in inter-cruise variations in Zn and Pb concentrations and Pb isotopes. In contrast, low temporal variability was registered for PO₄, SiOH₄, Cu, and Co. The isotopic composition of Pb in the eastern Mediterranean Sea (²⁰⁶Pb/²⁰⁷Pb = 1.161–1.173 and ²⁰⁸Pb/²⁰⁶Pb = 2.085–2.101) is controlled by advected Atlantic surface water and anthropogenic inputs delivered via continental runoff (terrestrial) or atmospheric shuttles. The deep-water inventory of Pb is partially controlled by historical anthropogenic sources. An enrichment in Zn and Cd (81 and 17 %, respectively) and a 50 % depletion in Pb relative to open-waters was observed in Intermediate Levantine Waters, in tandem with terrestrial Pb isotopic signatures, light REE depletion (shale-normalized Nd/Yb < 0.22) and a strong Ce anomaly (Ce/Ce* < 0.20). These are driven by intermediate nepheloid layers from the margins, which act as both a source and a sink for trace metals through release and scavenging, evident 300–500 km away from the shore. This study highlights the dynamic role of continental margins in modulating terrestrial and anthropogenic inputs to the oceans.