Geochemical and microbial controls on dolomite formation in anoxic environments

The Middle Trassic Grenzbitumenzone is a 16 m thick sequence of interbedded, finely laminated organic matter-rich dolomites and black shales. Organic carbon contents in the dolomites reach 10 wt%, whereas black shales have organic carbon contents of up to 40 wt%. Geochemical calculations based on trace metal concentrations in the black shales together with paleontological and sedimentological data indicate that the sediments of the GBZ were deposited at extremely low sedimentation rates (2 to 5 m/my) in a silled shallow marine basin (30-100 m deep) under permanently anoxic conditions. Sedimentary structures indicate that the dolomites are the product of periodic turbiditic transport of calcium carbonate mud into the basin diluting a more or less constant organic matter-siliciclastic background sedimentation. Carbon isotope compositions of the dolomites range from -1.4 to -5.6%o (PDB) indicating that dolomite was formed in the sulfate reduction zone of organic matter diagenesis. No organic or inorganic geochemical evidence for methanogenetic activity is found in the sediments, suggesting that dolomite may have formed before sulfate was completely depleted from the pore waters. Sedimentary structures and the small variations in carbon isotopic compositions suggest that dolomite is mostly of replacement origin. This study shows that the combination of slow sedimentation rates and high supply of organic matter are the main factors that favored extensive early diagenetic dolomitization in the GBZ. The slow sedimentation rate allowed enough time for magnesium and sulfate to diffuse into the pore waters. The high alkalinity produced by organic matter decomposition through sulfate reducing bacteria, combined with the availability of magnesium, led to high dolomite supersaturation in the pore-waters and to the replacement of the abundant precursor calcium carbonate. Organic geochemical data indicate that the organic matter is immature and primarily of marine origin with a high bacterial contribution and can be classified as type II. A high contribution of bacterial lipids to the kerogen is indicated by high hopane concentrations. A model is presented in which sedimentation and organic matter accumulation and preservation is dominated by two major factors: the periodic deposition of carbonate mud turbidites and the presence of a permanently stratified water column in which cyanobacteria and chemoautotrophic bacteria formed a bacterial plate at the anoxic-oxic interface.