HOW COMPARATIVE CARBONATE SEDIMENTOLOGY HELPED TO SOLVE THE “DOLOMITE PROBLEM”

The uneven distribution of dolomite in geologic time along with its scarcity in modern sediments has long been an enigma in sedimentology, often referred to as the “Dolomite Problem”. The discovery of modern dolomite-forming environments, such as the coastal sabkhas of Abu Dhabi, U.A.E. and the hypersaline Lagoa Vermelha, Brazil, offered sedimentologists study areas in which comparative carbonate sedimentology, ala Ginsburg (1974), could be applied. In particular, the peculiar physico-chemical characteristics of these special environments provided information to define variable paleoenvironments of ancient dolomite formation. From these studies, numerous models evolved which have been extensively used to interpret the geologic record of dolomite occurrence throughout Earth's history. The dolomite enigma, however, continued to challenge carbonate sedimentologists because the physico-chemical parameters defined from the natural environments could not be successfully translated into laboratory experiments in which dolomite precipitated under Earth surface conditions.

The discovery of specific microbes that mediate the precipitation of dolomite in natural environments and laboratory cultures opens a new research direction which integrates microbiology into the comparative carbonate sedimentology approach. The addition of a microbial factor allows for the routine precipitation of dolomite under variable conditions in order to simulate paleoenvironments and calibrate geochemical tracers locked into dolomite crystals. For example, we have experimentally determined the low-temperature oxygen-isotope fractionation factor for dolomite, which can now be applied to ancient dolomite to better interpret the formation environment. Further geomicrobiological studies, which will enhance our understanding of the origin of massive dolomite deposits, are in progress and demonstrate the continued applicability of using modern environments as analogues for ancient systems.