Molybdenum Stable Isotope Fractionation: - and the Interpretation of Molybdenum Burial Pathways from Sulfidic Waters to Sediments

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Molybdenum (Mo) is a redox-sensitive element which can track the level of free oxygen in Earth 's surface environments through the geological record. The build-up of oxygen in the Earth’s atmosphere and oceans are intimately linked to the evolution of the biosphere, since the production of O2 is purely biogenic and high oxygen levels are required for the respiration of animals, including ourselves. The concentration and isotopic composition of Mo provides information about the paleo-environment, if we understand the processes that fractionate Mo isotopes in sedimentary environments.

This thesis includes an investigation of the Mo burial pathway into modern sulfidic sediments of Lake Cadagno, Switzerland, combined with two applications to the ancient rock record: The ~750 Ma Chuar Group, Grand Canyon USA and ~500 Ma Swedish Alum shale. The use of molybdenum isotope compositions to trace ocean oxygenation is founded on the observation that sea water carries an isotope composition that depends on whether Mo is predominantly sequestered into sulfidic sediments or oxic (non-sulfidic) sediments. It has previously been assumed that marine euxinic sediments (underlying sulfidic waters) have a molybdenum isotope composition identical to sea water, due to quantitative scavenging at high hydrogen sulfide concentrations. Thus, euxinic sediments will carry the isotope composition of contemporaneous oceans and ancient euxinic sediments will provide a signal of the oxygenation of ancient oceans. The study in Lake Cadagno tests this assumption by investigating the isotopic consequences of the Mo burial pathway into sulfidic sediments. The results show that Mo removal is kinetically slow and isotopes do fractionate even in waters with high sulfide concentrations. However, the expressed isotope fractionation is indeed small and does not negate the use of molybdenum isotopes as a paleo-ocean redox indicator. Furthermore, the study offers a detailed description of the burial pathway from its riverine sources to its final uptake in the sediment. Mo enters the oxic surface waters as dissolved molybdate which is converted to particle reactive thiomolybdates in the sulfidic bottom waters that accumulate onto particles and settle to the sediments. The nature of these particles is mostly organic matter which degrades during early diagenesis and releases Mo into interstitial waters. At this point, Mo is probably bound to humic substances and is not reduced to insoluble molybdenite (Mo4+), even 4 though this reduced species is the thermodynamically stable form. The emerging picture of the Mo burial pathway is consistent with the final products that we find in the rock record. The second manuscript investigates Mo concentrations and isotopes in euxinic sediments of the ~ 750 Ma Chuar Group, USA. The data shows very low Mo concentration and negligible isotope fractionation to occur with very little variation throughout the geological succession. This provide evidence that sulfidic water masses were widespread ~200 million years before the explosion of animal life forms, consistent with the current model for Proterozoic ocean chemistry. A third manuscript is an investigation of ~500 Ma Alum shale succession deposited after the rise and diversification of animal life forms. One would expect to see a more modern level of ocean oxygenation. However, the data supports largely oxygen-deficient conditions also in the Cambrian with anoxic and non-sulfidic sediments playing a role. There is an interesting co-variation between carbon, sulfur and molybdenum isotopes in the sections, where a globally significant carbon isotope excursion, the Streptoean Positive Carbon Isotope Excursion (SPICE), marks the last in a series of excursions that is interpreted to reflect extinction events. The molybdenum isotope profile shows that the event is characterized by a shift from anoxic into highly sulfidic ocean water. Possibly, these oceanic redox excursions would be associated with extinction event of aerobic life forms in shallow basins and increased the environmental stress and, thus, the selection pressure on the biosphere.

The evolution of the environment, in which the biosphere unfolds, can be explored through the molybdenum isotope composition in sulfidic sediments. The burial pathway from waters to sediments can change the isotope composition slightly and thus complicates quantitative inferences of ocean oxygenation. Improved understanding of the major burial pathways of Mo is expected to improve our understanding of the Earth’s surface environments and, in particular, the accumulation of atmospheric oxygen through Earth history.
Original languageEnglish
Number of pages205
Publication statusPublished - 25 Feb 2009
Externally publishedYes

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