Tuesday, June 19, 2012 7:28:12 PM
A research team examined globally distributed marine bacteria that use sulphur compounds instead of oxygen to generate energy (i.e. sulphate respiration). They discovered that primary dolomite crystals are formed under conditions that are currently found in marine sediments. The dolomite precipitates exclusively within a mucus matrix, secreted by the bacteria to form biofilms. Different chemical conditions prevail within the biofilm compared to the surrounding water. In particular, the alteration of the magnesium to calcium ratio plays an important role. (The dolomitization principle is to take some limestone, which consists of calcite (CaCO3) and add some Magnesium (Mg) to get dolomite CaMg(CO3)2)). The magnesium to calcium ratio changes allow for the formation of dolomite crystals. The team was able to show that the ratio of different isotopes of calcium between the ambient water, the biofilm and dolomite crystals is different. This ratio is an important tool to reconstruct past environmental conditions. The fact that bacteria are involved in this process allows more precise interpretations of climate signals that are stored in sediments.
A strong possibility is that massive primary dolomite can form particularly during times when large quantities of organic matter in the seabed are degraded by sulphate-respiring bacteria. Such conditions exist when the sea water above the seafloor is free of oxygen. In Earth's history, several such oxygen-free periods have occurred, partly consistent with time periods of intensified dolomite deposition.
Over 90% of (sedimentary) dolomite is made up of the mineral dolomite. Researchers became aware of large deposits of primary dolomite as old as 600 million years. Dolomite was first scientifically described in 1791 as a rock by Dolomieu from exposures in what are now known as the Dolomite Alps of northern Italy. Experts say the process of recent primary dolomite formation is restricted to extreme ecosystems, including bacterial mats in highly saline lakes and lagoons. As these systems are very limited in space, there is an explanation gap for geologists for the widespread presence of fossil dolomite.
See also my post on Microbial Dolomite in Abu Dhabi [http://my.opera.com/nielsol/blog/2010/10/08/microbial-dolomite-in-abu-dhabi]