C³Corpus Christi Carbonates
Research

Experimental Proxy Calibration

Using high-temperature, replacement dolomitization experiments to test and correct the geochemical proxies that we use to reconstruct the paleoredox conditions of ancient seawater.

Overview

Dolomite comprises a significant proportion of ancient carbonate rocks, yet it rarely forms in modern sedimentary systems under near-surface, low-temperature conditions. This apparent contradiction, known as “the dolomite problem,” is a major obstacle for geochemistry — many proxies that are used to reconstruct the geochemical composition of ancient seawater are measured in rocks that have undergone some degree of diagenetic alteration, and it is often unclear whether the original seawater signal has survived.

To get an answer under carefully controlled conditions, we react CaCO3 with a Mg-rich fluid of known composition at elevated temperature and pressure, watching it convert to CaMg(CO3)2 over weeks to months. Given that the starting compositions are fixed, any trace element or isotope measured in the product can be directly attributed to how it partitioned during the reaction, rather than inferred from a geological sample with an unknown diagenetic history. The current focus of these experiments is uranium: δ238U in carbonate rocks is one of the best tools for reconstructing the oxygenation history of Earth's oceans, since the 238U/235U ratio shifts predictably with seafloor anoxia. Nevertheless, whether dolomitization preserves or resets that 238U/235U signal remains unconstrained.

Our results show that U(VI) is highly rock-buffered during dolomitization. In other words, uranium concentrations in ancient dolostones are largely controlled by how much uranium was available in the precursor limestone. That distinction matters beyond uranium — if one geochemical proxy is rock-buffered while others (e.g., strontium and iodine) are fluid-buffered, a single correction factor cannot be applied across the geologic record. Each proxy must be calibrated independently before it can be trusted to reconstruct the geochemical composition of ancient seawater.

Key Figures

Chart showing the abundance of dolomite versus limestone in carbonate rocks from 3.2 billion years ago to the present, with dolomite dominant through most of Earth's history but nearly absent in Cenozoic strata.
Dolomite (tan) forms a significant proportion of the carbonate rocks throughout Earth history, yet it is essentially absent from Cenozoic strata and rarely form under modern seawater-like conditions. Fig. 1C, McCormick et al. (2026), Geochimica et Cosmochimica Acta, 420, 228–245.
Cross-section schematic of a sealed hydrothermal reaction vessel showing the Teflon liner containing CaCO3 reactant submerged in solution.
Schematic diagram of the 23 mL Teflon-lined, stainless steel reaction vessels that are used for high temperature, replacement dolomitization experiments. Here we react a CaCO3 powder with a Mg-rich fluid of known composition to evaluate the partitioning behaviour and isotopic fractionation of trace elements during dolomitization. Fig. SM1, McCormick et al. (2026), Supplementary Material, Geochimica et Cosmochimica Acta, 420, 228–245.
Reaction progress curve showing mineral abundance transitioning from calcium carbonate to dolomite over one to one thousand hours, passing through induction, replacement, and recrystallization stages.
Conceptual model for the experimental replacement of CaCO3 by CaMg(CO3)2, showing the relative mineral abundances vs. reaction time. Fig. 2, McCormick et al. (2026), Geochimica et Cosmochimica Acta, 420, 228–245.
Schematic diagram of a thin solution film at the dolomitization reaction interface, showing uranium atoms moving between the dissolving parent carbonate, the solution film, and the bulk diagenetic fluid before being incorporated into the growing dolomite.
Schematic representation of a diagenetic reaction front, showing dolomitization (via dissolution-precipitation) within a ‘thin-solution film’. Uranium added by dissolution of the parent carbonate (fdiss) is exchanged with the surrounding fluid (fdiff) before being incorporated into the growing dolomite by precipitation (fppt). Fig. 8A–B, McCormick et al. (2026), Geochimica et Cosmochimica Acta, 420, 228–245.

Selected Publications

McCormick, C.A., Hardisty, D., Pederson, C., Dietrich, W.L., Bondzie-Selby, I., Hashim, M., Lau, K.V. (2026). Experimental constraints on the partitioning behavior of uranium (VI) in dolomite. Geochimica et Cosmochimica Acta, 420, 228–245.

View Paper →

Bondzie-Selby, I., Swart, P.K., McCormick, C.A., Lu, C., Lau, K.V., Hardisty, D., Dietrich, W.L., Pederson, C.L. Time-Resolved Dual Carbonate Clumped Isotope (Δ47 and Δ48) and Non Linear Mixing Investigation of Dolomite. Poster Presentation at: Goldschmidt Conference, Montreal, Canada.

Bondzie-Selby, I., McCormick, C.A., Swart, P.K., Lau, K.V., Hardisty, D., Dietrich, W.L., Pederson, C. (2025). Dual clumped isotope (Δ47 and Δ48) calibration between dolomite and temperature. Poster Presentation at: 38th IAS Meeting of Sedimentology, Huelva, Spain.

Dietrich, W.L., McCormick, C.A., Hashim, M.S., Lau, K.V., Pederson, C., Bondzie-Selby, I., Hardisty, D.S. (2024). Constraints on dolomite as an archive of seawater sulfate from dolomitization experiments. Poster Presentation at: Goldschmidt Conference, Chicago, USA.