Wednesday August 31, 2019
In this the second of three articles on dolomite in saline settings, we look at dolomite as a possible byproduct of microbial metabolism. Dolomite is a complex mineral and, due to an inherent requirement of lattice order, is not found in significant volumes in many Holocene marine-margin carbonate depositional settings. The figure illustrates some of the better-documented regions of Holocene protodolomite occurrence. Climatically, these sites are diverse, ranging from humid to semi-arid to arid deserts, geographically from deep offshore marine to coastal and hydrologically from eogenetic marine to shallow meteoric capillary and phreatic to hypersaline continental seepage. A prevailing association across most of the sites is that most can be tied to zones of active microbial metabolism, especially with saline anoxic settings exemplified the presence of a flourishing community of sulphate-reducing bacteria. The article discusses these occurrences and the concept of microbial facilitation as well as crystal ageing.
Wednesday July 31, 2019
This series of three articles looks at dolomite in saline settings, ranging from the modern lacustrine to ancient evaporitic platforms. The three articles offer partial updates to an old Warren (2000) paper. In the first article, we look at dolomite in saline marine-margin settings, focusing mostly on the nature of dolomite mineralogy and distribution in the Salt Creek lakes, Coorong region, South Australia. In the next article, we focus on the bacterial association with dolomites in some brackish to hypersaline saline settings in Brazilian carbonate lagoons and Abu Dhabi sabkhas and compare these Holocene mineralogies with the Coorong, while in the third, we will have a look if these Holocene occurrences relate to brine reflux and other ancient dolomitization models and occurrences.
More than 30 years ago, Lynton Land (1985) said; "No single model of dolomitization unequivocally accounts for all aspects of any massively dolomitized ancient limestone. All models have significant flaws, and our understanding of the dolomitization process and its relation to other diagenetic processes (silicification, stylolitization, organic maturation, etc.) is imperfect."
In many ways, this statement remains true, and as we will discuss in part 3, there is also a scaling dichotomy when we compare the thicknesses and lateral extents of Quaternary dolomite hydrologies with ancient platform dolomites. We shall discuss the hydrological contrasts tied to scaling difference in the third article of this series.