Authigenic phase formation and microbial activity control Zr, Hf, and rare earth element distributions in deep-sea brine sediments
Résumé
Sediments collected from hypersaline and anoxic
deep-sea basins in the eastern Mediterranean (Thetis, Kryos,
Medee, and Tyro) were characterised in terms of their miner-
alogical composition, the distributions of rare earth elements
(REE), Zr, and Hf and their content of microbial DNA. We
identified two major mineralogical fractions: one fraction of
detritic origin was composed of quartz, gypsum, and low-
Mg calcite bioclasts (with 0
<
Mg
<
0
.
07 %) and another
fraction of authigenic origin constituted of halite, dolomite,
high-Mg calcite (with a Mg content of up to 22 %) and rare
bischofite and showed a textural evidence of microbial as-
semblages.
We found that in the Medee and Tyro sediments, the shale-
normalised REE pattern of these sediments is strongly en-
riched in middle REE (MREE), whereas in the Thetis and
Tyro basins, a positive Gd anomaly in the residue was ob-
tained after the removal of the water-soluble fraction. In all
investigated basins, Y / Ho ratio clustered around chondritic
values, whereas Zr / Hf ratio ranged from slightly subchon-
dritic to superchondritic values. Subchondritic Y / Ho and
Zr / Hf values were mainly found in the high-Mg carbonate
having a microbial origin. The observed preferential removal
of Zr with respect to Hf without significant partitioning of
Y with respect to Ho indicates that the Zr / Hf ratio and Y–
Ho fractionations are influenced by the microbial activity in
the sediments. We propose that the concurrent Y-Ho and Zr–
Hf fractionations are a suitable tracer of microbial activity in
marine sediments