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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 µg/g and δ98Mo between +1.64 and +2.13‰. A linear decrease in pore water Mo concentrations to the depth where hydrogen sulfide accumulates, along with sedimentary authigenic δ98Mo values (δ98Mo 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 µg/g and δ98Mo auth signatures between –1.39 to +2.07‰. Pore water Mo concentrations are generally higher than ambient bottom water concentrations and the light δ98Mo signatures of the pore waters between +0.50 and +0.80‰ and δ98Mo auth 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 µg/g and δ98Mo auth values as high as +2.20‰. The relatively high Mo concentrations and seawater-like δ98Mo 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 δ98Mo 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 de-emphasize 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. [ABSTRACT FROM AUTHOR]

Enrichments of highly reactive iron (Fe) (sum of Fe (oxyhydr)oxide, carbonate and sulfide minerals) in marine sediments and sedimentary rocks are commonly interpreted as an indication of anoxic conditions in the bottom water at the time of deposition. The model system for this proxy rationale is the semi-restricted Black Sea, where sediments underneath the anoxic and sulfidic (i.e., euxinic) deep-water are enriched in reactive Fe, which was mobilized from the surrounding shelf areas. To test whether such a shelf-to-basin Fe shuttle can operate in semi-restricted basins without euxinic deep water, we investigated sedimentary Fe speciation and Fe isotope compositions in sediments of the Guaymas Basin, Gulf of California. Sediments on the slope underneath the eastern equatorial Pacific oxygen minimum zone and sediments within the oxic deep basin are both enriched in reactive Fe, with reactive Fe making up 45 ± 11% of the total Fe pool. The following mechanisms may contribute to these Fe enrichments: (1) Release of dissolved Fe from anoxic shelf and slope sediments followed by lateral transport of dissolved and/or particulate Fe in the water column; (2) preferential transport of fine-grained, terrigenous particles with a high reactive Fe content into the basin; (3) microbially mediated conversion of non-reactive silicate minerals to reactive Fe minerals during transport; (4) hydrothermal venting and lateral Fe transport within the deep water. The first process can explain reactive Fe enrichments in slope sediments, whereas all processes may contribute to sedimentary Fe enrichments in the deeper basin. The δ56Fe value of sediments increases from shelf to slope and decreases from the slope into the basin. This lateral pattern of δ56Fe, as well as the pattern of Fe enrichment, is similar to that observed in other marine systems with a Fe shuttle. However, the size of the Fe enrichment, and the range in δ56Fe (−0.06 to +0.16‰) is smaller. This difference is due to higher terrigenous sedimentation rates in the Guaymas Basin and, therefore, more intense dilution of shuttle-derived reactive Fe. We argue that, depending on the extent of bathymetric restriction and terrigenous background sedimentation, reactive Fe enrichments can form under a broad range of redox conditions and in diverse sedimentary environments. The concepts applied in this study can be used to identify those circumstances in the paleo-record. [ABSTRACT FROM AUTHOR]