Abstract
Abstract. Despite advances regarding the microbial and organic-molecular impact on nucleation, the formation of dolomite in sedimentary environments is still incompletely understood. Since 1960, apparent dolomite formation has been reported from mud sediments of the shallow, oligohaline and alkaline Lake Neusiedl, Austria. To trace potential dolomite formation or diagenetic alteration processes in its deposits, lake water samples and sediment cores were analyzed with respect to sediment composition, hydrochemistry and bacterial community composition. Sediments comprise 20 cm of homogenous mud with 60 wt % carbonate, which overlies dark-laminated consolidated mud containing 50 wt % carbonate and plant debris. Hydrochemical measurements reveal a shift from oxic lake water with pH 9.0 to anoxic sediment pore water with pH 7.5. A decrease in SO42- with a concomitant increase in ΣH2S and NH4+ from 0 to 15 cm core depth indicates anaerobic heterotrophic decomposition, including sulfate reduction. The bacterial community composition reflects the zonation indicated by the pore water chemistry, with a distinct increase in fermentative taxa below 15 cm core depth. The water column is highly supersaturated with respect to (disordered) dolomite and calcite, whereas saturation indices of both minerals rapidly approach zero in the sediment. Notably, the relative proportions of different authigenic carbonate phases and their stoichiometric compositions remain constant with increasing core depth. Hence, evidence for Ca–Mg carbonate formation or ripening to dolomite is lacking within the sediment of Lake Neusiedl. As a consequence, precipitation of high-magnesium calcite (HMC) and protodolomite does not occur in association with anoxic sediment and sulfate-reducing conditions. Instead, analytical data for Lake Neusiedl suggest that authigenic HMC and protodolomite precipitate from the supersaturated, well-mixed aerobic water column. This observation supports an alternative concept to dolomite formation in anoxic sediments, comprising Ca–Mg carbonate precipitation in the water column under aerobic and alkaline conditions.
Highlights
Dolomite (CaMg(CO3)2) is the most abundant carbonate mineral in Earth’s sedimentary record
We further provide bacterial community analyses to address the potential role of microbes and their metabolisms in a carbonate mineral precipitation or alteration pathway
The detrital carbonates are distinguishable from authigenic carbonate phases by their bigger size and fractured shape
Summary
Dolomite (CaMg(CO3)2) is the most abundant carbonate mineral in Earth’s sedimentary record. It has rarely been observed forming in recent environments. Most occurrences of large dolomite deposits in the geological record are the result of pervasive dolomitization of precursor carbonates by fluids with high Mg : Ca ratios and temperatures during burial (e.g., Machel, 2004). The formation of dolomite near the sediment surface, so-called penecontemporaneous dolomite (Machel, 2004, and references therein), or even primary precipitation in shallow aquatic environments is often difficult to trace in the rock record and capture in modern environments. Dolomite does not form in sites where sufficient Ca, Mg and carbonate ions are provided, which is generally explained by the high kinetic barrier of dolomite nucleation and growth (e.g., Lippmann, 1973)
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