Formation of dolomite layers was observed in organic carbon–rich upwelling-related sediments of the shelf and upper slope of the Peru margin. Dolomite forms early as a primary precipitate in uncompacted sediment and is not replacing the precursor carbonate. These findings, together with the results of an isotopic study (Meister et al., unpubl. data; see discussion above), are consistent with a model for dolomite formation associated with a biogeochemical interface, such as the methane/sulfate boundary. Dolomite breccias were precipitated on the lower slope of the Peru margin contemporary with the brittle deformation along faults in the accretionary prism. These dolomites were formed in association with hydrothermal fluid flow. The presence of great amounts of pyrite indicates the in situ reduction of sulfur species and thus, dolomite formation in the fault zones may also be related to microbial activity.
Even though no crystal morphologies typical for microbially mediated carbonates were found, the dolomite formation is probably related to zones of increased microbial activity in support of the microbial dolomite model. This activity provides high alkalinity and removes sulfate from the pore water, which would be consistent with the sulfate inhibition model. Thus, for the Peru margin environment, dolomite formation may be promoted by both microbial mediation and sulfate depletion. Further geochemical and isotopic studies are required in order to determine the major control on deep-sea dolomite formation.