Ocean Drilling Program, United States

Dolomite sampled during Leg 201 occurs as hard lithified layers surrounded by small amounts of friable dolomite, and its presence is related to organic carbon-rich diatomaceous sediments. Petrographic relationships indicate a shallow depth of dolomite precipitation on the Peru margin. Dolomitic breccia layers found on the lower slope, however, were precipitated contemporaneous with brecciation, which probably took place at greater depth under the influence of tectonic deformation processes in the accretionary prism. In most cases, the formation of coexisting framboidal pyrite postdates the growth of dolomite rhombs. No crystal morphologies indicating a direct influence of bacteria in the precipitation process were observed. Occurrence and morphology of the carbonate layers, however, suggest focused precipitation within the sedimentary sequence. We hypothesize that microbial hot spots discovered during Leg 201 at geochemical boundaries such as the methane/sulfate boundary may be sites of strongly enhanced microbial mediation of dolomite precipitation.