An Ecological Perspective on Dolomite Formation in Great Salt Lake, Utah

Abstract

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. Protodolomite was abundant in a non-oolitic sediment core from the South Arm (SA) of GSL at < 17% salinity but was rare in an oolitic sediment core collected from the North Arm (NA) of GSL at > 26% salinity. 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 not detected in benthic photosynthetic mats from the SA. However, the mats comprised aragonite with halite and minimal calcite; benthic photosynthetic mats do not form in the NA. 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. 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. 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. Sediment grains from the SA core exhibit micrometer-sized euhedral protodolomite crystals that were not detected in the NA core. Collectively, these observations suggest that deposition of photosynthetic biomass drives the development of a sharp, anoxic lens of heterotrophic sulfate reduction. Sulfide, in turn, may promote dehydration of Mg 2+-water complexes and primary protodolomite nucleation and growth. 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.

Botryoidal and Spherulitic Aragonite in Carbonates Associated with Microbial Mats: Precipitation or Diagenetic Replacement Product?

“…Under SEM, the botryoidal and spherulitic aragonite consist of radially oriented euhedral fibrous crystals (20-300 μm long and 0.13-1 μm wide) with flattened terminations (Figure 4D and Figure 5F). The crystals are densely packed (Figure 5F) and often associated with biofilms, while the micritic nuclei consist of relict peloids (Figure 5D) or aggregations of spheroidal nanocrystals Duchi et al (1978) i Dunham et al (2020); those without indication are from this study.…”

“…Della Porta, 2015). The carbonates are mineralogically dominated by aragonite, but include also calcite and dolomite (Della Porta, 2015;Pace et al, 2016;Dunham et al, 2020).…”

“…In the case of Great Salt Lake, Pace et al (2016) proposed that the microbial bioherm formation is linked to microbial metabolisms via the following processes: 1) a Mg-Si phase forms on the EPS due to increased pH driven by cyanobacteria photosynthesis; 2) aragonite nucleates following cyanobacteria biofilm degradation by sulfate-reducing bacteria with intestine-like aragonite (as Pace et al, 2016 label the spherulitic aragonite) nucleating inside coccoid clusters, expanding in the extracellular organic matrix and embedding the Mg-Si phases; 3) dolomite partially replaces aragonite in neutral to acidic pore water by dissolutionreprecipitation reactions. Sulfate-reducing bacteria may play a fundamental role for the nucleation and growth of aragonite crystals (Pace et al, 2016) and dolomite formation (Dunham et al, 2020). Lindsay et al (2017) proposed that bioherm carbonate precipitation is instead primarily mediated by cyanobacteria photosynthesis.…”

Unexpected Abundance and Diversity of Phototrophs in Mats from Morphologically Variable Microbialites in Great Salt Lake, Utah

“…Both WW and WE environments are characterized as being subject to substantial wave action (i.e., a high-wave-energy environment) and are on a shallow ramp distant from the bedrock outcrop. Microbialites in these locations are up to ϳ1 m in diameter and are well cemented with carbonate (typically aragonite) mineral that has been suggested to be bioprecipitated (27,28). Microbialites tend to form in linear accumulations at WE sites and form distinct clusters (up to 5 m in diameter) at WW sites.…”

“…Conclusions. The thick, widely distributed benthic microbial mats that form in GSL bind and stabilize sediment grains (20) and have been suggested to also promote the precipitation of carbonate minerals, including aragonite, that can act as cement during mat lithification (27,28). Over extended periods of time, perhaps on the order of 2,500 to 20,000 years (24), numerous cycles of mat growth, sediment binding, and lithification lead to the formation of microbialites in GSL whose morphology largely reflects the average wave energy/direction and depositional setting of the local environment (20,24,25).…”