Authigenic phases and biomass contents drive Zr, Hf and REE distributions in anoxic lake sediments

Abstract

Abstract. REE, Zr and Hf distributions in seafloor sediments collected from the hypersaline, anoxic Thetis, Kryos, Medee and Tyro deep-sea basins from the Eastern Mediterranean were determined in light of their mineralogical composition, and biomass contents. Mineralogical investigations demonstrate that all the studied sediments show a similar mineralogy. Detritic assemblages mainly consist of quartz, gypsum and calcite with Mg contents ranging from 0 to about 7%, often of a bioclastic nature. On the contrary, authigenic parageneses are formed by halite, bischofite, dolomite and calcite, with Mg contents up to 22%. Textural evidences of biological activity were also identified. In sediments from the Medee and Tyro basins, REE, Zr and Hf distributions were analysed in the fraction soluble in nitric acid, whereas in materials coming from the Thetis and Kryos basins, the water-soluble sediment fraction had been previously removed and REE, Zr and Hf distributions were investigated in the residue. This approach evidenced that shale-normalised REE patterns of the whole fraction soluble in nitric acid show strong intermediate REE (MREE) enrichments that give way to positive Gd anomalies once water-soluble minerals are removed. Y/Ho ratios are clustered around chondritic values justified by the occurrence of detritic minerals whereas Zr/Hf values span a~wider range from slightly subchondritic to superchondritic terms. Negative Gd anomalies, subchondritic Y/Ho and Zr/Hf values are found in Mg-carbonate rich samples suggesting that authigenic Mg-carbonates partition Ho and Hf with respect to Y and Zr during their crystallization from brines. Textural observations and biomass analyses highlighted effects of biological activities in sediments involving Zr and Hf enrichments and the highest Zr/Hf values according to the preferential Zr removal onto biological surfaces, without partitioning Y with respect to Ho. These first data suggest that Zr/Hf ratio and REE distributions can represent tracers of biological activity in sediments.