Leibniz Institute of Marine Sciences, Germany

Early diagenetic dolomite formation in methanogenic marine sediments is enigmatic because acidification by CO (sub 2) , a by-product of methanogenesis, should lead to carbonate dissolution and not precipitation. However, petrographic relationships indicate that dolomite breccia layers with delta (super 13) C values of approximately +15 per mil, 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 ( (super 87) Sr/ (super 86) Sr>0.711) and positive delta (super 18) O values (+6 per mil), we present evidence that the dolomite breccias mainly formed from fluids originating from deep sedimentary units within the accretionary prism, where they interacted with continental crust and/or siliciclastic rocks of continental affinity. Due to silicate alteration and dehydration, such fluids are likely alkaline and thus have the potential to neutralize the acidification imposed by the high dissolved CO (sub 2) concentrations. This scenario provides a potential mechanism by which dolomite formation can be induced deep in a highly active methanogenic zone.