Authigenic phase formation and microbial activity control Zr, Hf, and rare earth element distributions in deep-sea brine sediments

[摘要] Sediments collected from hypersaline and anoxic deep-sea basins in theeastern Mediterranean (Thetis, Kryos, Medee, and Tyro) were characterised interms of their mineralogical composition, the distributions of rare earthelements (REE), Zr, and Hf and their content of microbial DNA. We identifiedtwo major mineralogical fractions: one fraction of detritic origin wascomposed of quartz, gypsum, and low-Mg calcite bioclasts (with 0 < Mg < 0.07%) and another fraction of authigenic origin constitutedof halite, dolomite, high-Mg calcite (with a Mg content of up to 22%) andrare bischofite and showed a textural evidence of microbial assemblages.

We found that in the Medee and Tyro sediments, the shale-normalised REEpattern of these sediments is strongly enriched in middle REE (MREE),whereas in the Thetis and Tyro basins, a positive Gd anomaly in the residuewas obtained after the removal of the water-soluble fraction. In allinvestigated basins, Y / Ho ratio clustered around chondritic values, whereasZr / Hf ratio ranged from slightly subchondritic to superchondritic values.Subchondritic Y / Ho and Zr / Hf values were mainly found in the high-Mgcarbonate having a microbial origin. The observed preferential removal of Zrwith respect to Hf without significant partitioning of Y with respect to Hoindicates that the Zr / Hf ratio and Y–Ho fractionations are influenced by themicrobial activity in the sediments. We propose that the concurrent Y-Ho andZr–Hf fractionations are a suitable tracer of microbial activity in marinesediments.

[学科分类] 地球化学与岩石