Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas

- Mo concentrations in shallow carbonate sediment cores exceed concentrations found in primary carbonate precipitates.
- The degree of Mo enrichment appears to be controlled by pore water sulfide concentrations during syndepositional diagenesis.
- Mo isotopes in carbonate sediments appear primarily controlled by pore water sulfide concentrations.
- Sediments deposited with low pore water sulfide have Mo isotope values lighter than seawater.
- Sediments deposited with high pore water sulfide have Mo isotope values approaching that of seawater.

Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide-a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record.Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain < 0.2 ppm Mo, we find that carbonate sediments deposited under fully oxic shallow bottom water can experience extensive syndepositional authigenic Mo enrichment (1 to > 10 ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~ 1-1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2-2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ98Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50-100 μM H2Saq, which is achieved only in organic- and sulfide-rich carbonate sediments.