A stable isotope study of microbial dolomite formation in the Coorong Region, South Australia

Abstract

The Coorong Region of South Australia is a classic location for the study of modern dolomite formation. A number of ephemeral lakes here contain dolomite, whilst other lakes in close proximity, contain aragonite with minor magnesite. Our previous field and experimental work has revealed that sulphate-reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area. We here use a combination of oxygen, carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments. We report isotopic data from four dolomite and three aragonite containing lakes. δ 34S data for dissolved lake- and porewater sulphates reveal 34S enrichment of residual sulphate in all seven lakes, consistent with bulk closed system microbial fractionation by sulphate-reducing bacteria. Calculations using temperature-dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes. A comparison of δ 18O data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt-like saturated mud horizon coincident with the zone of sulphate reduction. Surprisingly, δ 13C data from the carbonates do not show the characteristic wide range of values often associated with ‘organogenic’ carbonates, but instead show that these bacterially-mediated precipitates incorporate carbon primarily from the inorganic, lakewater reservoir, partially diluted by an organic component. A ‘Coorong type’ microbial model of dolomite formation may be particularly applicable to, for example, Precambrian sedimentary dolomites where benthic microbial communities, which are today restricted to extreme environments, dominated the shallow-marine ecosystem prior to the emergence of Metazoa.