LINKING MICROBIAL DOLOMITE FORMATION AND GLOBAL BIOGEOCHEMICAL CYCLES IN THE PHANEROZOIC

The uneven distribution of dolomite in geologic time along with its scarcity in modern sediments has long been an enigma, referred to as the “Dolomite Problem”. In recent years, considerable evidence has been compiled that points to major variations in seawater chemistry during the Phanerozoic. Various authors have proposed that these variations may hold the key to understanding dolomite formation in the geologic past. In particular, comparison of dolomite abundance with the secular variations in seawater chemistry over the past 550 m.y. indicates that dolomite is relatively much more abundant during periods when calcite prevailed over aragonite precipitation, in so-called “calcite seas”, and KCl rather than MgSO4 evaporites predominated. Indeed, dolomite appears to have been more abundant during periods when the Mg:Ca ratio of seawater was considerably lower than the modern value. This correlation is, however, intuitively inconsistence with the thermodynamic factors thought to promote dolomite precipitation and further indicates that dolomite formation may be an important factor controlling the Mg:Ca ratio of seawater. Testing this hypothesis has proven difficult because the exact conditions facilitating dolomite precipitation under Earth surface conditions have remained controversial due to the inability to conduct controlled laboratory experiments.

Recently, with the recognition that modern dolomite does precipitate in specific microbial-mediated natural environments, culture experiments have been designed and carried out at low temperatures to replicate the biogeochemical conditions under which dolomite precipitation and diagenesis occurs. Additionally, the experimental results can be correlated with in situ biogeochemical and molecular measurements of the natural systems. Results show that important environmental factors include hypersalinity and the presence of specific bacterial species, which alter the microenvironment maintaining high alkalinity. Under specific anoxic conditions, consortium of Eubacteria and Archaea are prevalent. With the knowledge of the biogeochemical parameters favoring the microbial mediation of dolomite precipitation, it is now possible to evaluate the conditions favoring dolomite formation that may have prevailed in the Phanerozoic oceans.