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 trace metals (micro-nutrients) from the coast to the oceans. Yet, the mechanisms through which trace metals are delivered to seawater across the land-ocean continuum 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 carried out in the oligotrophic eastern Mediterranean Sea.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. Varying anthropogenic inputs resulted in inter-cruise temporal variations in Zn, Pb, and Pb isotopes. In contrast, low temporal variability was registered for PO4, SiOH4, Cu, and Co. 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 compositions, light REE depletion (shale-normalized Nd/Yb<0.19) and a negative 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 removal, evident 300-500 km away from the shore. This study highlights the dynamic role of continental margins in modulating terrestrial and anthropogenic elemental inputs to the oceans.

Keywords: Trace metals, Pb isotopes, GEOTRACES, Mediterranean Sea