MOLYBDENUM ISOTOPE SYSTEMATICS IN NATURAL AND EXPERIMENTAL SETTINGS

Molybdenum isotopes have a broad potential applicability for paleoenvironmental analysis, particularly with respect to questions of eutrophication history, development of anoxia, and sedimentation under conditions of varying oxygenation. Using a double-spike method, the Mo isotope proxy was applied to sediments and water samples from the Chesapeake Bay, where the severity of seasonal anoxic episodes has been increasing over the last century. It was discovered that isotopic fractionation is occurring in the estuary, as indicated by the large differences between the δ98Mo of Mo dissolved in the water and authigenic Mo in the sediments. Increased variability of δ98Mo values and increased authigenic Mo deposition were likely related to the onset of coastal anoxic episodes in the Bay. Sediment samples from the Eastern Mediterranean were also analyzed for δ98Mo, along with redox-sensitive element concentrations (Re, Mo, V, Ba, and Fe). Over the past 5 million years, climatic shifts have driven cyclic oceanographic changes in the Mediterranean, specifically basin-wide anoxic episodes, which are visible in the sedimentary sequence as layers that are highly enriched in redox-sensitive elements and organic matter (sapropels). I investigated whether δ98Mo values, in conjunction with other proxies, could be used to infer the degree to which the deep basin was affected by anoxic conditions, and how this may have changed between individual anoxic episodes. There were clear temporal differences in the apparent severity of anoxia in the Mediterranean, as reflected by the proxies in the sapropels. The amount of Mo in Mediterranean seawater did not change during sapropel deposition, and therefore, the basin likely remained open to circulation. I collaborated in a project to determine whether Mo isotopes could be fractionated at high temperature and pressure in an experimental system, designed to mimic natural hydrothermal-type porphyry systems. It was found that Mo isotopes are fractionated between a melt and vapor phase under the experimental conditions, and in a manner consistent with equilibrium exchange processes. Molybdenum entering the melt phase undergoes a coordination change to higher coordination number, thus preferentially enriching the vapor phase in the heavier Mo isotopes

Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

Redox conditions and the mechanisms of redox development are a critical aspect of Eastern Mediterranean sapropels, whose formation in oxygen-depleted waters is closely related to water column stratification at times of global sea level rise and insolation maxima. Sapropels in the Nile Fan formed at relatively shallow water depths under the influence of the monsoon-driven freshwater output from the River Nile. This work evaluates the redox evolution of Holocene sapropel S1 in VALPAMED cruise core MD9509, recovered at 880 mbsl in the NE Nile Fan, using a combination of geochemical element proxies, Fe speciation, Fe and Mo isotopes studies. The productivity and redox proxies (Ba/Al, Mo/Al, U/Al, V/Al, Sb/Al) show well-defined enrichments in the sapropel, but with a marked minimum at ca 8.2 ka indicative of reventilation corresponding to a well known global cooling event. Peak productivity and reducing signals occur close to the initiation of sapropel formation. The proxy signals in sapropel 9509 are stronger and of longer duration than those of a second sapropel S1, recovered at the same depth, but 380 km to the north (MD9501), supporting the notion (suggested in previous studies) of more reduced conditions in the Nile Fan. The MoEF vs. UEF enrichment factor variations in core 9509 infer a transition from open marine suboxic conditions in the enclosing non-sapropel sediments to anoxic non-sulphidic water column conditions in the sapropel. Correspondingly, the highly reactive Fe pool (FeHR) measured in Fe speciation studies is dominated by Fe(oxyhydr) oxide minerals in the background sediments, whereas pyrite (Fepy) becomes the dominant component of the FeHR pool in the sapropel. Maximum Fepy values in the sapropel coincide with peak productivity and reducing conditions, implying a clear link between trace element uptake, diagenetic bacterial sulphate reduction in anoxic porewater and Fe mobilization in the sapropel. Iron isotope compositions (δ56Fe) in the sapropel do not show any departure from primary (marine and detrital) source sediment values, and the absence of an Fe/Al vs. δ56Fe trend strongly argues against an Fe shuttle. Molybdenum isotopes, however, show marked non-conservative fractionation patterns. Background sediment δ98/95Mo values (0.2 to 0.7‰) are compatible with fractionation upon absorptive uptake by Fe (oxyhydr)oxides and pyrite. In contrast, minimum δ98/95Mo values exhibited at peak sapropel (reducing and pyrite producing) conditions are most closely modeled by Mo isotope fractionation during kinetically controlled conversion of aqueous molybdate to thiomolybdate species. The conservative Fe isotope behavior/Mo isotope fractionation minima in the sapropel may be a characteristic of organic-rich sediment diagenesis below an anoxic non-sulphidic water body, without the operation of a benthic Fe shuttle