HR: 0800h
AN: V41F-1527    [Abstracts]
TI: Magnesium Isotopic Evidence for Widespread Microbial Dolomite Precipitation in the Geological Record.
AU: * Carder, E A
EM: ecar02@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: Galy, A
EM: albert00@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AU: McKenzie, J A
EM: sediment@erdw.ethz.ch
AF: Geological Institute, ETH-Zentrum, Zurich, 8092 Switzerland
AU: Vasconcelos, C
EM: crisogono.vasconcelos@erdw.ethz.ch
AF: Geological Institute, ETH-Zentrum, Zurich, 8092 Switzerland
AU: Elderfield, H
EM: he101@esc.cam.ac.uk
AF: Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ United Kingdom
AB: The enigma surrounding the `Dolomite Problem' is the relative abundance of dolomite in the geological record versus its very rare occurrence on the surface of the modern Earth despite a particularly favourable modern seawater chemistry. Recent studies of modern dolomite from hypersaline coastal lagoons in Brazil and Pleistocene dolomite from ODP cores collected during ODP Leg 201 on the Peru Margin suggest microbial mediation is an important factor [1]. Indeed, cultures of sulfate-reducing bacteria isolated from the lagoons mediate dolomite precipitation in the laboratory [2, 3]. In this study we report magnesium isotopic analyses of these modern microbial associated dolomites and ancient dolomites of a range of geological ages and environments. The application of stable magnesium isotopes to study dolomite formation and the nature of the processes involved represents a new frontier in isotope geochemistry. Highly accurate determination of the magnesium isotopic composition allows us to distinguish between kinetic and equilibrium isotope fractionation on the basis of the excess of 25Mg. A significant kinetic isotope fractionation is observed in laboratory cultures and surfical microbial mats from the Brazilian lagoons. Older dolomites (<3000 yrs.) taken from cores recovered from the lagoon are much closer to equilibrium. We interpret our data as evidencing an initial microbial mediated nucleation of dolomite that is a kinetic process and a subsequent inorganic addition of dolomite overprinting an equilibrium signature. This is in agreement with a previous major element and crystallographic study of the Brazilian dolomites [1]. The ancient dolomites analysed range in age from Neoproterozoic to Pleistocene and come from diverse geological environments including submarine diagenetic zones, platform carbonates and lagoonal environments. Magnesium isotopic analysis shows evidence of a varying component of kinetic fractionation, smaller than the kinetic end member as typified by the laboratory cultures. The ancient dolomites appear to evidence the same initial kinetic nucleation and subsequent equilibrium growth as the modern. In contrast, hydrothermal dolomite exhibits only equilibrium fractionation. Taken together, our results argue for a strong biological role in magnesium fixation into sedimentary dolomite in both the ancient and modern and suggest microbial processes are important in resolving the `Dolomite Problem'. References [1] Vasconcelos, C. and McKenzie, J.A., (1997), J. Sed. Res., 67, 378-390. [2] Vasconcelos, C., McKenzie, J.A., Bernasconi, S., Grujic, D. and Tien, A.J., (1995), Nature 377, 220-222. [3] Warthmann R., van Lith Y., Vasconcelos C., McKenzie J.A. and Karpoff A.M., (2000), Geology 28, 1091-1094.
DE: 1000 GEOCHEMISTRY
DE: 1030 Geochemical cycles (0330)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1051 Sedimentary geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005