Benthic Microbial Communities and Dolomite Formation in Marine and Lacustrine Environments — A New Dolomite Model
Published:January 01, 2000
David T. Wright, 2000. "Benthic Microbial Communities and Dolomite Formation in Marine and Lacustrine Environments — A New Dolomite Model", Marine Authigenesis: From Global to Microbial, Craig R. Glenn, Liliane Prévôt-Lucas, Jacques Lucas
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Conventional hydrologically-driven models of dolomite formation, though popular, often lack empirical support, and encounter fundamental chemical problems related to kinetic impediments in saline solutions: these include the high hydration energy of the magnesium ion, the extremely low activity of the carbonate ion, and the presence of even very low concentrations of sulfate. Although an organogenic dolomite model exists, it has been mainly limited in application to modern, organic-rich, deep marine sediments. However, growing evidence from modern and ancient sediments points to a greatly enhanced and fundamental role for benthic microbial communities in dolomite formation, linked to anoxic organic diagenesis driven by sulfate reduction during which all kinetic inhibitors to dolomite formation are overcome. Magnesium concentrated in cyanobacterial sheaths may be liberated during degradation into sulfate-free solutions of high ionic strength and carbonate ion activity, to become available for dolomite formation. Another possible source of magnesium in modified saline solutions is from dissociation of complexed ions. These processes, which provide the basis for a new organogenic model for dolomite formation, have operated throughout the geologic record on a range of scales, from relatively minor modern occurrences to the thick, extensive platformal dolostones of the Proterozoic which developed in association with microbially-dominated shallow marine environments.
Analyses of lake waters and sediments from ephemeral Coorong lakes of South Australia reveal seasonally high carbonate alkalinities and magnesium concentrations in association with intense bacterial sulfate reduction and cyanobacterial degradation, indicating a genetic link between microbial mediation of ambient waters and dolomite formation. The ecosystem of the Coorong distal ephemeral lakes during late stages of evaporation provides a small scale analogue for biogeochemical processes operating in microbially-dominated shallow marine environments typical of the Proterozoic, where sulfate reduction would have been a major, shelf-wide phenomenon operating just beneath the sediment surface, continually driving biochemical modification of interstitial waters and sustaining high carbonate alkalinity in an underlying sulfate-free medium.
Thin sections of late Archaean platformal Conophyton stromatolites from South Africa reveal progressive disintegration of constituent cyanobacterial sheaths accompanied by the appearance and increasing crystal size of replacive, authigenic dolomite in a calcite matrix, indicating that magnesium was derived from in situ degradation of cyanobacterial sheaths. Total loss of sheath material resulted in a coarse dolospar fabric preserving no evidence of its microbial origin. Petrographic analysis of silicified microbial shelf sediments, including cyanobacterial mat, ooids, peloids and intraclasts, from the Cambrian Eilean Dubh Formation of northwestern Scotland reveals a common diagenetic trend in which sequential anoxic biofabric degradation resulted in the appearance and progressive increase in abundance of dolomite, culminating in a dolospar fabric. Structureless dolostones may thus evolve through a sequence of ephemeral fabrics by authigenic precipitation or replacement linked to microbial degradation, but evidence for their origin depends on the preservation of successive stages in their development from organic-rich sediments to massive dolomites. The predominance of stromatolites and other microbialitic sediments in thick dolostone platformal successions throughout much of the Precambrian and early Paleozoic, indicates that organic diagenesis has played a major role in dolomite formation from the microbial to the global scale.
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Marine Authigenesis: From Global to Microbial
This volume is a collection of 33 state-of-the-art papers focusing on various aspects of authigenic and diagenetic marine minerals and related global elemental cycling. The commingling of the various studies of authigenic minerals in this volume, including the most recent advances in knowledge concerning the occurrence and origins of phosphorites, glauconites, dolomites, siderites, manganese-iron associations, barites, ironstones, and other marine chemical sediments/sedimentary rocks of early authigenic/diagenetic origin, is partly the result of the increasing awareness that there are many overlaps, even direct co-associations, between different authigenic minerals, both in time, space, and genesis. Taken together, this compilation represents a holistic approach towards marine authigenesis that considers the integrated whole more than the simple sum of its parts.