Simulating meteoric and mixing zone carbonate diagenesis with a two-dimensional reactive transport model

Abstract

Meteoric and mixing-zone diagenesis can dramatically alter the geochemical signatures of shallow marine carbonates. Most preserved pre-Cretaceous carbonates were deposited in shallow marine environments and thus may have been susceptible to meteoric and mixing-zone diagenesis. However, a quantitative understanding of how the geochemical composition of carbonates changes during diagenesis still requires further development. Here, we present a new two-dimensional (2D) reactive transport model coupled with a 2D coastal hydrology model to simulate carbonate diagenesis and provide insights into its impact on the isotopic and elemental compositions of carbonates in the geological record. Using this model, we have simulated the stratigraphic trends and relationship between isotopic records (for example, δ13C and δ18O values) observed in modern (Recent-Miocene) sections where the impact of meteoric diagenesis has been clearly characterized. Our model can also reproduce anomalous Neoproterozoic carbonate geochemical profiles where the effects of meteoric diagenesis have been debated. Further, our model indicates that linear carbonate C-O isotope co-variations can either be generated in the mixing zone between freshwater and marine pore waters, or in the freshwater phreatic zone with a downward decrease in recrystallization (with no net carbonate dissolution or precipitation) rate. In addition, numerous processes were observed to decouple δ18Ocarb values from other isotopic and elemental signatures during carbonate diagenesis, indicating that a lack of linear correlation between δ18Ocarb values and other geochemical variables does not necessarily suggest limited meteoric alteration. Sensitivity analyses show that the steady-state timescale is controlled by compositional differences between fluid endmembers, the calcite-water element distribution coefficient, the recrystallization rate, porosity, and the groundwater discharge rate. Given that reactive transport models have proven to be powerful theoretical tools in many disciplines of Earth sciences, our hope is that this model will promote a more quantitative understanding of meteoric and mixing zone diagenesis of marine carbonates.