Abstract
Early diagenetic dolomite formation in methanogenic marine sediments is enigmatic because acidifi cation by CO2, a by-product of methanogenesis, should lead to carbonate dissolution and not precipitation. However, petrographic relationships indicate that dolomite breccia layers with δ13C values of ~+15‰, recovered from the lower slope of the Peru continental margin (Ocean Drilling Program Site 1230), formed deep in the methanogenic zone during tectonic activity of a decollement. Based on radiogenic Sr isotope ratios (87Sr/86Sr > 0.711) and positive δ18O values (+6‰), we present evidence that the dolomite breccias mainly formed from fl uids originating from deep sedimentary units within the accretionary prism, where they interacted with continental crust and/or siliciclastic rocks of continental affi nity. Due to silicate alteration and dehydration, such fl uids are likely alkaline and thus have the potential to neutralize the acidifi cation imposed by the high dissolved CO2 concentrations. This scenario provides a potential mechanism by which dolomite formation can be induced deep in a highly active methanogenic zone.
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