MOLYBDENUM ISOTOPE SYSTEMATICS IN NATURAL AND EXPERIMENTAL SETTINGS
Scheiderich, Kathleen Dwyer
Walker, Richard J
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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 δ<super>98</super>Mo of Mo dissolved in the water and authigenic Mo in the sediments. Increased variability of δ<super>98</super>Mo 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 δ<super>98</super>Mo, 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 δ<super>98</super>Mo 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.