ABSTRACT

Presumed barriers to early dolomitization in normal seawater-derived pore fluids at earth-surface temperatures appear to be overcome within some anoxic, organic-rich sediments as a result of bacterial sulfate reduction and methanogenesis. These processes may promote early dolomitization, particularly during methanogenesis and late stages of sulfate reduction, by concurrently raising and sustaining high pH and high total alkalinity and CO₃^2- concentrations in pore fluids, and by simultaneously either decreasing Mg and Ca hydration or by promoting crystal surface reactions with less hydrated Mg-Ca neutral ion pairs. Volumetrically significant quantities of dolomite are associated with sulfate reduction and/or methanogenesis within peritidal, shallow-marine, and deep-sea deposits. Average concentrations of organogenic dolomite are as much as 70% in some Holocene peritidal deposits, and 28% in Mesozoic to Quaternary deep-sea sediments.

Organogenic dolomites are mainly cements, and contain relatively low concentrations of Sr and Mn. Sulfate-reduction dolomites generally are Fe-deficient because of concurrent pyrite precipitation, whereas methanogenetic dolomites may be somewhat more ferroan as Fe substitutes for depleting Mg. Sources of Mg and Ca for dolomite are diffusion from overlying seawater and/or dissolution of precursor carbonate sediments. Dolomites are characterized by a wide range in ¹³C values wherein those of sulfate reduction versus methanogenetic origin typically are ¹³C-depleted and ¹³C-enriched, respectively. The extent of ¹³C depletion or enrichment, however, depends on the extent of organodiagenetic reactions and amount of ¹³C contributed by seawater diffusion, and commonly results in overlap of ¹³C_dolomite values. The range of ¹⁸O_dolomite values is somewhat more restricted, and generally reflects differences in pore-fluid temperature and salinity. Some of the unresolved issues in organogenic dolomitization are: the relative efficiencies of sulfate reduction versus methanogenesis in promoting dolomitization, depths of dolomite formation inferred on the basis of ¹⁸O_dolomite values and probable sources of Mg and Ca, and the mode of dolomitization with progressive burial into methanogenetic zones.