The four major Isotopes of Dolomite (C, O, Mg, Ca): Closing the Calcium Isotope (d44/42Ca) Gap

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Dolomite is a rock forming mineral that consists of Ca2+, Mg2+, and CO32- and naturally occurs in a variety of sedimentological and diagenetic environments. Owing to its predominance in the deep time record, elemental and isotopic proxy data from various dolomites have been extensively studied. In contrast, data on the calcium isotope signatures of dolomites remain scarce. This is relevant as non-stoichiometric dolomites, the bulk of dolomites in Earth’s history, are often Ca-rich (molar ratio of Ca:Mg>1) and hence, Ca isotope (d44/40Ca or d44/42Ca) signatures clearly deserve attention. Previous data sets compiled by the proponents and coworkers document that dolomite Ca isotope signatures are variable and significant fractionation processes affect early diagenetic dolomite. Along similar lines, (replacive and precipitative) dolostone Ca isotope ratio data reflect solid-fluid exchange processes. Here we argue that due to high concentrations of several key elements, early marine diagenetic dolomites are a complex but promising target material for multi-isotope-proxy approaches. Accordingly, previous work of the applicants documented that the d44/42Ca, d26Mg, d13C, and d18O isotope signatures of a variety of early diagenetic dolomites/dolostones represent distinct diagenetic domains and time intervals. This project aims to go significantly beyond this point and follows a dual approach: (i) analysis of dolomite/dolostone samples from selected field areas and time intervals. The focus is placed on early diagenetic phases: marine evaporative, i.e. sabkha, reflux, non-marine evaporative, and palustrine environments. We intend to test the potential of these early diagenetic fabrics as archives of their primary and evolved marine pore-fluids considering kinetics and microbiogeological constraints. (ii) This requires calibration by means of dolomite formation experiments at well-known physicochemical conditions (Graz University of Technology). Amorphous calcium/magnesium carbonate at low temperature experiments (25-60°C), and aragonite at elevated temperatures (100-250°C) will be used as seed material. Both experimental setups are reasonable analogues to natural dolomite environments. The objectives of the proposal are to: (i) Establish a well-constrained characterization of Ca isotope signatures for early diagenetic dolomite types; (ii) Combine multi-proxy Ca, Mg, C, O (and stable Sr) isotope data in order to reduce noise and enhance signal in data sets by assessing covariance patterns; and (iii) Assess the formation and diagenetic alteration of different stoichiometric dolomite types and their isotopic and elemental patterns relative to syn-sedimentary non-stoichiometric phases. Overall, we aim to provide the community with the foundations to establish a well-constrained palaeo-seawater archive.