Sealevel changes control diagenetic dolomite formation in hemipelagic sediments of the Peru Margin Patrick Meister a,b, ⁎, Stefano M. Bernasconi b , Crisógono Vasconcelos b , Judith A. McKenzie b a Max-Planck-Institute for Marine Microbiology, Celsiusstrasse 1, 28359 Bremen, Germany b Geological Institute, ETH Zürich, 8092 Zürich, Switzerland ABSTRACT ARTICLE INFO Article history: Received 22 March 2007 Received in revised form 16 March 2008 Accepted 5 April 2008 Keywords: dolomite oxygen isotopes deep biosphere glacial/interglacial cycles paleo-temperatures Peru Margin 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 δ 18 O 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 δ 18 O 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 δ 18 O values in the porewater, which is supported by δ 18 O 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. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Dolomite layers and nodules are commonly observed in the geological record in organic carbon-rich deep-sea hemipelagic sedi- ments. They are found in the Miocene Monterey Fm. (Murata et al., 1969; Baker and Burns, 1985; Burns and Baker, 1987; Burns et al., 1988; Compton, 1988) and were first observed in recent ocean margin sediments during drilling in the California Borderland (DSDP Leg 63; Pisciotto and Mahoney, 1981), the Gulf of California (DSDP Leg 64; Kelts and McKenzie, 1982), and later on the Peru Margin (ODP Leg 112; Suess et al., 1988). These studies led to the formulation of the “organic dolomite model”, where the essential part of the inorganic carbon is delivered from organic matter degradation. Claypool and Kaplan (1974) concluded from the variable positive and negative δ 13 C values that dolomite layers form at different depths and in different diage- netic zones. In the sulphate reduction zone, carbonate with negative δ 13 C values is produced, and, in the methanogenic zone, carbonate with positive δ 13 C values is produced. The diagenetic zone, in which dolomite formation occurs, was considered to depend on the sedimentation rate, which limits the downward diffusion of SO 4 2- and, thus, the depth of organic matter degradation, which leads to supersaturation with respect to dolomite (Kelts and McKenzie, 1984). Marine Geology 252 (2008) 166–173 ⁎ Corresponding author. Max-Planck-Institute for Marine Microbiology, Celsius- strasse 1, 28359 Bremen, Germany. E-mail address: pmeister@mpi-bremen.de (P. Meister). 0025-3227/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2008.04.001 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo