| dc.description.abstract | The role that microorganisms play in low-temperature dolomite formation is still unresolved. Given the scarcity of dolomite in modern low-temperature depositional settings, controls on its formation remain unclear. Textural evidence from modern and ancient geomicrobiological dolomite-forming environments, combined with laboratory experiments that produce dolomite phases associated with microbial surfaces and metabolism, support a microbial model for dolomite formation. However, more research is needed to substantiate this model. This study here aims to clarify the role of organic matter abundance and solution chemistry in a putative microbial dolomite: the Neoproterozoic dolomite of the Dengying Formation, Sichuan Basin, China. Researchers debate the origin of dolomite with different data supporting a microbial model conflicting with late-state hydrothermal dolomite. This study tested the microbial model for dolomite formation in the Dengying using controlled laboratory experiments constructed to emulate Ediacaran seawater chemistry (ESW) with abundant microbial mats. The constructed Ediacaran solution chemistry accounted for alkalinity, pH, temperature, and dissolved sulfate. Experimental variables included differing Mg:Ca (1:1) ratios, the presence of dissolved silica in solution (ESW no silica), and presence/absence of synthetic organic matter (in the form of carboxyl microspheres). Solution pH, alkalinity and dolomite saturation index decreased through the incubation period in all solutions. Further, aragonite and Mg-calcite (with mol % of Mg ranging from 15-25% MgCO3) precipitates were the main mineral suite of the ESW solution, while calcite and small yields of aragonite predominated in the 1:1 solution. Further, microscopy analysis shows an association between magnesium ions and dissolved silica, the presence of spheroidal precipitates associated with the microspheres and possible Mg-carbonate precipitation in the carboxylated surfaces. Despite trends in the literature that suggest both silica and organic carbon play crucial roles in the availability of Mg for the dolomite reaction, our results did not document dolomite nucleation and precipitation from any of the studied solutions. Our summarized results suggest parallelisms with likely initial stages of carbonate precipitation in the Ediacaran prior to early or late stage dolomitization processes. While results from this study do not substantiate a microbial model of dolomite formation nor do they abrogate this possibility, our data underscore the complex interactions between solution chemistry and surface nucleation and their interaction to produce dolomite at low temperature. | |
