Sulfur and Oxygen Isotope Compositions of Seawater Sulfate as Recorded in Marine Barite

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Kastner 9628479 Global changes in climate and atmospheric chemistry are intimately related to the sulfur and carbon sedimentary cycles. Evidence for large scale transfers of S and C between the sedimentary reservoirs is provided by the isotope records of oceanic sulfate and carbon. Coupling between the S and C reservoirs maintains a steady-state atmospheric oxygen reservoir. Thus, an increase in the rate of sulfide oxidation must be compensated by an increase in the rate of organic C burial, resulting in an overall inverse relationship between marine 34Ssulfate and 34Scarbonate, as observed. A high resolution marine carbonate 13C age curve exists. The evidence that S and O isotope compositions of seawater sulfate have fluctuated significantly through the Phanerozoic, is mostly based on the comprehensive isotope data obtained from marine evaporite sulfates; but the geological record of marine evaporites is episodic, with gaps of millions of years. Evaporites are also susceptible to diagenesis, their age control could be problematic, and the purely marine origin of some is questionable; it is therefore important to identify another marine phase that reliably records and maintains seawater sulfate S and O isotope compositions. Marine barite is a readily available ubiquitous minor phase in deep-sea sediments, it is continuously deposited, its associated sediment ages can be determined, it is not susceptible to diagenetic alteration in oxic sediments, and has been shown to record reliably seawater Sr isotope composition; it seems to be a very suitable mineral for this study. Thus, the objectives of this research are: (I) to confirm that marine barite is a reliable recorder of seawater sulfate S and O isotope values, (II) to construct detailed, continuous, well dated seawater sulfate S and O isotope records over the past 65 m.y., (III) to construct high resolution S and O isotope records during three special time intervals (1) the Messinian event, (2) the Cretaceous-Tertiary boundary , and (3) the distinct minimum in 34S near the Albian-Aptian boundary, using well dated deep-sea sediments. The refined S and O isotope curves of seawater sulfate will be related to the oceanic 13C record of carbonates and interpreted in terms of the processes controlling the dynamic balance between the oceanic S and C input and output fluxes. The role of increased weathering fluxes, especially from the Himalayan uplift over the past -40 m.y., will also be evaluated by correlating data from this research with the existing 12C/12C and 87Sr/86Sr seawater data for this time interval. The emphasis of the work will be on barite that will be separated from sediments of Holocene to 65Ma age, at about 1 to 2 million year intervals. Barite samples from the Holocene to 38 Ma have previously been analyzed by us for their Sr-isotope composition. For the three "special" geologic time intervals higher resolution sampling will be carried out at an interval of about 0.5 m.y. The habit of the barite crystals that will be studied by SEM, and Sr- isotope composition of the separated barites will be tests for possible diagenesis. Continuous records of seawater sulfate S and O isotope values, by analyses of marine barite, and their correlation with existing high resolution oceanic C and Sr isotope age curves, should help to unravel the climate-tectonic coupling during these three important geological events.