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During Ocean Drilling Program (ODP) Leg 201, different diagenetic minerals were recovered in cored deep-sea sediments. Dolomite was recovered as fragments of discrete layers or lenses at different depths in the sediments of the Peru Margin and as cemented breccia layers at the Peru Trench. Other diagenetic minerals, such as Carich rhodochrosite, microcrystalline quartz and calcite cement, occur in the deep-sea sediments of the Eastern Equatorial Pacifi c. This study investigates the formation of these minerals in a stratigraphic, geochemical and microbiological context. Different geological, geochemical and microbiological methods were applied to correlate these processes with microbial activity in the “deep biosphere”. On the Peru Margin, dolomite occurs in the sedimentary sequences as hard-lithifi ed layers surrounded by limited amounts of friable dolomite. This occurrence is directly related to the organic carbon content in the diatomaceous sediments. The Sr-isotopic composition of the dolomite layers found in sediments on the shelf and upper slope is similar to the composition of Quaternary seawater indicating that they formed near the sediment surface (in the uppermost 30 m below seafl oor) and not at greater depths where a hypersaline brine is present. δ13C values measured in these layers are not in equilibrium with the modern porewater and vary considerably with depth, ranging from –15‰ to +6‰ (PDB). We propose that precipitation of distinct dolomite layers occurs associated with the methane sulphate interface (MSI). At this geochemical boundary, bacterial activity is high, alkalinity reaches peak values and sulphate, which was proposed as an inhibitor for dolomite formation, is removed. The high variability of the δ13C values probably refl ects variability in the rates of methanogenic and methane oxidizing microbial activity at this boundary, which causes an upward or downward migration of the MSI. A numeric diffusion model for the sulphate concentration confi rms that such non-steady state conditions are realistic. The location of the discrete dolomite layers in the sedimentary section is probably related to variations in abundance and composition of organic matter in the shelf sediment due to changes in the oceanographic conditions, such as glacial/interglacial variations. The infl uence of glacial-interglacial variations is also refl ected in lower paleo-porewater temperatures calculated from the δ18O values of the dolomites. Brecciated dolomite layers were found within the accreted sediment of the Peru Trench. Petrographic relationships indicate that these layers were formed at depth, along the thrust faults. Although sulphate is not measurable below 10 m beneath the sediment surface today, and δ13C-values indicate strong methanogenic microbial activity, high amounts of pyrite were found associated with the dolomite. The pyrite is probably the result of I

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Linking the formation of dolomite and other early diagenetic minerals with microbial activity in deep-sea sediments: results from ODP Leg 201, Peru Margin and Eastern Equatorial Pacific