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.
Temperature and salinity effects on 18O fractionation for rapidly precipitated carbonates:Laboratory experiments with alkaline lake water —Perspective
