Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas
Abstract 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 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%0 lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2 2.5%0) even under oxic bottom water conditions. However, the sensitivity of bulk carbonate d 98 Mo 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 mM H 2 S aq which is achieved only in organic and sulfide rich carbonate sediments.