Zusammenfassung
The first experimentally determined temperature dependent oxygen-18 fractionation factor between dolomite and water at low temperatures [Vasconcelos, C., McKenzie, J.A., Warthmann, R., Bernasconi, S., 2005. Calibration of the δ18O paleo-thermometer with dolomite formed in microbial cultures and natural environments. Geology 33, 317–320] allows now the precise calculation of temperatures during early diagenetic dolomite precipitation. We use δ18O values of early diagenetic dolomite beds sampled during ODP Legs 112 and 201 on the Peru continental margin (Sites 1227, 1228 and 1229) [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032] to calculate paleo-porewater temperatures at the time of dolomite precipitation. We assumed unaltered seawater δ18O values in the porewater, which is supported by δ18O values of the modern porewater presented in this study. The dolomite layers in the Pleistocene part of the sedimentary columns showed oxygen isotope temperatures up to 5 °C lower than today. Since Sites 1228 and 1229 are located at 150 and 250 m below sealevel, respectively, their paleo-porewater temperatures would be influenced by considerably colder surface water during glacial sealevel lowstands. Thus, Pleistocene dolomite layers in the Peru Continental margin probably formed during glacial times. This finding is consistent with a model for dolomite precipitation in the Peru Margin recently discussed by Meister et al. [Meister, P., McKenzie, J.A., Vasconcelos, C., Bernasconi, S., Frank, M., Gutjahr, M., Schrag, D.P., 2007. Dolomite formation in the dynamic deep biosphere, results from the Peru Margin, OPD Leg 201. Sedimentology 54, 1007–1032], where dolomite forms episodically at the sulphate methane interface. It was shown that the sulphate methane interface migrates upwards and downwards within the sedimentary column, but dolomite layers may only form when the sulphate–methane interface stays at a fixed depth for a sufficient amount of time. We hypothesize that the sulphate–methane interface persists within TOC-rich interglacial sediments, while this zone is buried by TOC-poor sedimentation during glacial times. Thus, the presented oxygen isotope data provide additional information on the timing of early diagenetic dolomite formation and a possible link between episodicity in dolomite formation and sealevel variations. A similar link between early diagenesis and oceanography may also explain spacing of dolomite layers in a Milankovitch type pattern observed in the geological record, such as in the Miocene Monterey Formation.
