Influence of particulate versus diffusive molybdenum supply mechanisms on the molybdenum isotope composition of continental margin sediments

The sedimentary concentration and stable isotope composition of molybdenum (Mo) is widely used as a proxy for paleo redox conditions in the marine environment. However, the behavior of Mo during early diagenesis is still not fully understood, which complicates the application of the Mo proxy in ancient continental margin environments. Here, we present Mo concentrations and isotope compositions of sediment and pore water samples from the Guaymas Basin in the Gulf of California. Our sample set covers a broad range of depositional environments, including sediments from within the eastern equatorial Pacific oxygen minimum zone (OMZ), from a semi-restricted oxic graben, and from near a hydrothermal vent-field. By investigating Mo cycling in these different settings, we provide new insights into different modes of Mo fixation and the associated isotope fractionation. Sediments from the OMZ have authigenic Mo concentrations (Mo-auth) between 3.3 and 17.2 mg/g and delta Mo-98 between +1.64 and +2.13 parts per thousand. A linear decrease in pore water Mo concentrations to the depth where hydrogen sulfide accumulates, along with sedimentary authigenic delta Mo-98 values (delta Mo-98(auth)) close to seawater, indicate diffusion of Mo from the bottom water into the sediment with little isotope fractionation during quantitative Mo removal. Sediments from the site with oxic bottom water within the basin reveal Mo-auth concentrations ranging from 1.2 to 14.7 mg/g and delta Mo-98(oauth) signatures between 1.39 to +2.07%. Pore water Mo concentrations are generally higher than ambient bottom water concentrations and the light delta Mo-98 signatures of the pore waters between +0.50 and +0.80% and delta Mo-98(oauth) signatures of the sediments indicate continuous Mo exchange between the pore water and Mn and Fe oxides during early diagenesis. Sediment samples from the vent field mainly consist of black smoker debris and are characterized by Mo-auth concentrations ranging from 8.6 to 33.2 mg/g and delta Mo-98(auth) values as high as +2.20%. The relatively high Mo concentrations and seawater-like delta Mo-98 can be explained by near-quantitative Mo scavenging from hydrothermal solutions with little isotope fractionation at high temperatures. Comparison of our new data for the OMZ sediments in the Gulf of California with previously published data for sediments from the Peruvian OMZ highlights that Mo isotope compositions in this kind of setting strongly depend on how Mo is delivered to the sediment. If diffusive Mo delivery from the bottom water into the sediment significantly contributes to Mo accumulation in the solid phase, as is the case in the Guaymas Basin, sedimentary Mo-auth concentrations are relatively low but the isotope values are close to the delta Mo-98 signal of seawater. If Mo is exclusively delivered by particles, like on the Peruvian shelf, much higher sedimentary Mo-auth concentrations can be attained. In the latter case, Mo-auth isotope values will be lighter because the sediments preserve the isotopic offset that was generated during adsorption or uptake of Mo by particles. Our findings deemphasize the role of dissolved Mo speciation in pore waters but highlight the importance of the mode of Mo delivery for the Mo concentration and isotope composition preserved in the paleo-record. (C) 2020 Elsevier Ltd. All rights reserved.