Zr/Hf ratio and REE behaviour: A coupled indication of lithogenic input in marginal basins and deep-sea brines

Abstract

The distribution of dissolved Zr, Hf and Rare Earth Elements (yttrium and lanthanides, hereafter referred to as REE) in the Eastern Mediterranean seawater column was measured in the Kryos basin to evaluate the lithogenic contribution from both Nile River and Sahara and Arabian desert dust. We found dissolved Zr/Hf ratios below the signature of crustal rocks and chondrites; a phenomenon likely driven by the dissolution of the Mn-rich coating of atmospheric dust particles delivered from the desert. In deeper waters, Zr/Hf ratios are clustered close to the signature of crustal rocks and chondrites according to the different Zr and Hf dissolved speciation. The Zr/Hf ratio observed in the deeper 3300 m seawater column corresponds to the value found in the Atlantic Ocean. The REE distribution displays a negative Ce anomaly and a positive La anomaly throughout the shallow waters above a 75 m depth. Zr, Hf, and REE concentrations measured in the anoxic deep-sea brines from Tyro, Medee, and Thetis basins indicate that the dissolution of evaporite within Messinian levels is responsible for the high salt content of these brines and influence Zr/Hf fractionation in the aqueous phase. Hafnium shows stronger affinity relative to Zr for surfaces of salts and Mn–Fe oxyhydroxides. Zr/Hf values and the observed shale-normalised REE features suggest that Zr, Hf and REE are leached from Mn and Fe-rich coatings of desert particles in shallow water layers. This process can be responsible for the observed distribution of studied elements in the studied Eastern Mediterranean oxic seawater. In deep-sea brines, Zr, Hf and REE are released from Messinian evaporites.