The role of bacterial sulfate reduction during dolomite precipitation: Implications from Upper Jurassic platform carbonates

Abstract

The early diagenetic formation of dolomite in modern aquatic environments is limited mostly to evaporitic and marine-anoxic, organic-rich sediments dominated by bacterial sulfate reduction (BSR). In such environments, bacterial activity lowers the energy barriers for the nucleation and growth of dolomite and thus promotes the formation of non-stoichiometric, highly disordered and metastable (proto)dolomite. Although the boundary conditions for the formation of modern (proto)dolomites are considered to be generally understood, the role of BSR during limestone dolomitization in ancient marine environments remains questionable.Herein, we present a study about the physicochemical conditions and processes, which led to the formation of partly dolomitized limestone and dolostone in the presence of BSR on a stable carbonate platform during the Upper Jurassic at Oker (Northern German Basin). The dolomite textures, the spatial trace elemental patterns of the dolomite and of the surrounding limestone and the results of δ18O and δ13C isotope analyses reveal that the Oker dolomite has been formed by the early diagenetic replacement of magnesian calcite precursors at temperatures between 26°C and 37°C. We interpret the mineralizing fluids responsible for dolomitization as pristine-marine to slightly evaporitic and reducing seawater being modified during shallow seepage reflux and/or evaporitic tidal pumping. The elevated δ34SCAS values (+17.9 to +19.7‰, V-CDT) of the Oker dolomite, compared to ambient Upper Jurassic seawater, indicate that BSR facilitated dolomite formation. For the first time, we show that a linear anti-correlation exists between decreasing carbonate-associated sulfate (CAS) contents in dolomite and increasing ordering ratio of the dolomite lattice structure, with the degree of cation order in dolomite to be given by: degree of cation order(Dol): =−0.018·CAS(Dol)+68.3 (R2=0.98).This correlation implies that the CAS content of sedimentary dolomite can be used as a measure for dolomite maturity. The relationships between the ambient (paleo)environmental controls, the resultant dolomitization pathways and subsequently the structure and the composition of the precipitating dolomite are presented and discussed in relation to the stability of modern and ancient (proto)dolomites throughout burial diagenesis.