While common in older rocks, dolomite and protodolomite are rare in modern marine sediments despite waters being supersaturated with respect to these minerals, which should favor their formation ( Fairbridge, 1957). It has been demonstrated that VHMC formation always precedes the formation of ordered dolomite in high-temperature laboratory synthesis experiments ( Gregg et al., 2015). Prior studies have distinguished high- and very-high-magnesium calcite from protodolomite (> 36% MgCO 3, weak cation ordering) and dolomite (> 36% MgCO 3 and strong cation ordering) ( Fang and Xu, 2019). The crystal structure of dolomite differs from that of calcite in that the ordering of calcium (Ca 2+) and magnesium (Mg 2+) ions into alternating layers results in reduced crystal symmetry ( Gregg et al., 2015). The scarcity of dolomite in the NA sediment core may result from constraints imposed by a combination of extreme hypersalinity and depositional environment on phototrophs and sulfate reducers, their activities, and the thermodynamics of protodolomite formation.ĭolomite is common in ancient sedimentary rocks of marine origin and is generally thought to have formed through diagenetic processes involving high temperature and pressure or through post-depositional replacement of calcite or aragonite (CaCO 3) ( Rosenberg and Holland, 1964 Rosenberg et al., 1967 Hardie, 1987). Sulfide, in turn, may promote dehydration of Mg 2+-water complexes and primary protodolomite nucleation and growth.
Collectively, these observations suggest that deposition of photosynthetic biomass drives the development of a sharp, anoxic lens of heterotrophic sulfate reduction. Sediment grains from the SA core exhibit micrometer-sized euhedral protodolomite crystals that were not detected in the NA core. Differences in the quality of organic matter between the SA and NA cores, as indicated by carbon to nitrogen ratios, indicate fresh deposition of photosynthetic biomass at the SA sediment core site but not in the NA sediment core site. Transcripts affiliated with a dominant halophilic, heterotrophic sulfate-reducing bacterium were detected in the uppermost sections of the SA core and their abundance was positively correlated with rates of acetate oxidation/assimilation and concentrations of sulfide. To begin to identify potential controls on the formation of protodolomite in the SA and NA of GSL, the composition and abundance of 16S rRNA gene transcripts, carbon cycling activities, and porewater geochemistry of the sediment cores were characterized. However, the mats comprised aragonite with halite and minimal calcite benthic photosynthetic mats do not form in the NA. Protodolomite was not detected in benthic photosynthetic mats from the SA. Protodolomite was also abundant in a non-oolitic NA sediment hand sample yet was absent in a nearby oolitic sediment hand sample in locations that likely receive allochthonous nutrients. Protodolomite was abundant in a non-oolitic sediment core from the South Arm (SA) of GSL at 26% salinity. Protodolomite was detected in sediments from Great Salt Lake (GSL), Utah, United States, that have no history of elevated temperature or pressure, conditions that are thought to promote dolomitization of sedimentary carbonates. 6Utah Geological Survey, Salt Lake City, UT, United States.5Great Salt Lake Institute, Department of Biology, Westminster College, Salt Lake City, UT, United States.4Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, United States.
3Department of Earth Sciences, Montana State University, Bozeman, MT, United States.2Department of Geoscience, University of Wisconsin-Madison, Madison, WI, United States.1Department of Microbiology and Immunology, Montana State University, Bozeman, MT, United States.Baxter 5, David Lageson 3, David Mogk 3, Andrew Rupke 6, Huifang Xu 2 and Eric S. Lindsay 1, Christopher Steuer 3, Nicholas Fox 3, Madelyne Willis 4, Alatna Walsh 3, Daniel R.
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