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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
3. (May), Pages 378-390
Microbial Mediation of Modern Dolomite Precipitation and Diagenesis Under
Anoxic Conditions (Lagoa Vermelha, Rio de Janeiro, Brazil)
Crisogono Vasconcelos (1,2), Judith A. McKenzie (1)
ABSTRACT
Dolomite, despite its thermodynamic stability and abundance in the ancient
rock record, is rarely found forming in Holocene environments. This enigma
is frequently called the Dolomite Problem. The recent discovery of modern
dolomite formation in Lagoa Vermelha, a shallow-water isolated coastal
lagoon east of Rio de Janeiro, Brazil, provides a new environment to investigate
the factors promoting dolomite precipitation under earth surface conditions.
Lagoa Vermelha serves as a natural laboratory in which the dolomite formation
process was studied using an integrated hydrologic, geochemical, and sedimentological
approach. The results of this study indicate that Ca-dolomite precipitation
occurs under anoxic hypersaline conditions within a black sludge layer
directly overlying the water/se iment interface. With deposition, the dolomite
undergoes an "ageing" process, whereby increased ordering of the crystal
structure occurs. Both the initial precipitation and subsequent early diagenesis
are strongly mediated by microbial activity. In fact, using sulfate-reducing
bacteria cultured from Lagoa Vermelha samples, a highly ordered dolomite
has been produced in the laboratory at low temperatures. These experimental
results combined with the study of the natural environment mandate that
a microbial factor be added to the list of factors capable of causing dolomite
precipitation. Considering the Lagoa Vermelha system, we propose a new
actualistic model for dolomite formation, which we call the microbial dolomite
model.
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