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

P Censi,F Saiano,M Raso,P Zuddas,A Nicosia,S Mazzola

doi:10.5194/bg-11-1125-2014

Abstract

Abstract. Sediments collected from hypersaline and anoxic deep-sea basins in the eastern Mediterranean (Thetis, Kryos, Medee, and Tyro) were characterised in terms of their mineralogical 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 assemblages. We found that in the Medee and Tyro sediments, the shale-normalised REE pattern of these sediments is strongly enriched in middle REE (MREE), whereas in the Thetis and Tyro basins, a positive Gd anomaly in the residue was obtained 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 subchondritic 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.

Highlights

The growing interest in rare earth elements (REE – lanthanides and yttrium, but not including scandium) Zr and Hf distributions in marine sediments started with the increased exploitation of these elements in several industrial practices (Andrianov et al, 2011; Du and Graedel, 2013; Moriwaki et al, 2013; Hein et al, 2013, and references therein)
We found that in the Medee and Tyro sediments, the shalenormalised REE pattern of these sediments is strongly enriched in middle REE (MREE), whereas in the Thetis and Tyro basins, a positive Gd anomaly in the residue was obtained after the removal of the water-soluble fraction
Tectonic settling of the seafloor in the eastern Mediterranean led to deep-sea basins filled with reducing hypersaline brines, extreme environments where the occurrence of authigenic minerals (Mg-rich carbonates and halides) and microbial colonies in the sedimentary assemblages influenced the geochemical distribution of REE, Zr, and Hf

Summary

Introduction

The growing interest in rare earth elements (REE – lanthanides and yttrium, but not including scandium) Zr and Hf distributions in marine sediments started with the increased exploitation of these elements in several industrial practices (Andrianov et al, 2011; Du and Graedel, 2013; Moriwaki et al, 2013; Hein et al, 2013, and references therein). Several studies carried out during the last 25 yr have identified the processes responsible for REE adsorption onto the surface of Fe–Mn crusts in marine environments (Bau and Koschinsky, 2009, and reported references), but few have focused on processes occurring during the crystallisation of authigenic minerals (Bach et al, 2003; Moller and Dulski, 1983; Himmler et al, 2010; Azmy et al, 2011; Ehya, 2012). There is a paucity of knowledge regarding processes affecting REE, Zr, and Hf distributions during carbonate crystallisation and their soluble salts in natural systems, and only limited number of studies have been carried out under laboratory conditions (Rimstidt et al, 1998; Pokrovsky et al, 1999). The identification of processes allowing the accumulation of REE, Zr, and Hf in natural sedimentary assemblages requires a detailed knowledge of the mineralogical composition of the assemblage as well as the identification of microbial activity. Censi et al (2013) recently demonstrated the capability of microbial assemblages to influence distributions of REE, Zr, and Published by Copernicus Publications on behalf of the European Geosciences Union

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