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

(Table 3) Geochemistry ODP Hole 160-969D sediments

Sediment depth is given in mbsf. Ages ere linearly interpolated using sedimentation rates and the mid-point ages of sapropels (Emeis et al., 1996, doi:10.2973/odp.proc.ir.160.1996; 2000, doi:10.1016/S0031-0182(00)00059-6), which are based on the astronomical age model of Lourens et al. (1998, doi:10.2973/odp.proc.sr.160.017.1998)

(Table 2) Mo isotope ratios of ODP Hole 160-969D sediments

(Table 2) Mo isotope ratios of ODP Hole 160-969D sediment

(Table 4) Sulfur isotope ratios of pyrite of ODP Hole 160-969D sediments

(Table 4) Sulfur isotope ratios of pyrite of ODP Hole 160-969D sediment

Molybdenum and sulphur isotope ratios of early Pleistocene sapropels of ODP Hole 160-969D

Organic-rich sediments (sapropels) deposited in the Mediterranean are presumed to have formed during periods of increased productivity, and/or deep water oxygen depletion, possibly including the development of sulfidic conditions (euxinia). Geochemical redox proxies (Re, Mo, Mo isotopes, V, Fe/Al, and multiple S isotopes) in 8 sapropels from the Pleistocene confirm water column euxinic conditions of varying intensity during sapropel deposition. These same proxies indicate an oxic origin for hemipelagic sediments deposited between sapropel-forming episodes. In one intensively sampled sapropel, deposited between 1.450 and 1.458 Ma, changing concentrations of organic carbon, Ba, Re, Mo, V, and Fe/Al track one another closely, reflecting coupling between water column euxinia and biological productivity. Multiple S isotope data from this sapropel suggest that the redox interface where oxidative sulfur cycling occurred was present in the sediments during hemipelagic sedimentation, but moved into the water column during sapropel deposition. Molybdenum isotopes of these 8 sapropels encompass a range of values (d98Mo = +0.2 to +1.7), but are all 98Mo-depleted relative to seawater (d98Mo = +2.3 per mil), suggesting that quantitative removal of Mo did not occur. This finding contrasts with modern Black Sea sediments. In general, Re/Mo ratios in sapropels are greater than in modern seawater, implying that the water column was not sufficiently sulfidic during sapropel-forming episodes to induce complete removal of both these elements. Surprisingly, the heaviest d98Mo values are found within hemipelagic sediments. Very few of the hemipelagic samples preserve the negative d98Mo values commonly associated with modern oxic marine sediments. Many of the hemipelagic samples also contained higher concentrations of Re and Mo than are common in oxic sediments. These features may be attributable to diffusion from the sapropels of a 98Mo-enriched component into the hemipelagic sediments