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Wu N.,University of Maryland University College | Farquhar J.,University of Maryland University College | Farquhar J.,Syddansk Universitetet | Strauss H.,University of Munster | And 2 more authors.
Geochimica et Cosmochimica Acta | Year: 2010

This study examines the sulfur isotope record of seawater sulfate proxies using δ34S and Δ33S to place constraints on the average global fractionation (Δ34Spy) associated with pyrite formation and burial and the exponent λ that relates variations of the 34S/32S to variations of the 33S/32S. The results presented here use an analysis of the sulfur isotope record from seawater sulfate proxies and sedimentary sulfide to extract this quantity as the arithmetic difference between δ34S of seawater sulfate and contemporaneous sulfide. It also uses an independent method that draws on inferences about the Δ33S evolution of seawater sulfate to evaluate this further. These two methods yield similar results suggesting that Δ34Spy and λ changed over the course of the Phanerozoic from slightly lower values of Δ34Spy (lower values of λ) in the early Phanerozoic (Cambrian-Permian) to higher values of Δ34Spy (higher values of λ) starting in the Triassic. This change of Δ34Spy and the exponent λ is interpreted to reflect a change in the proportion of sulfide that was reoxidized and processed by bacterial disproportionation on a global scale. The revised record of Δ34Spy also yields model pyrite burial curves making them more closely resemble model evolution curves for other element systems and global sea level curves. It is suggested that possible links to sea level may occur via changes in the area of submerged continental shelves which would provide additional loci for pyrite burial. The slightly different constraints used by the two approaches to calculate this fractionation may allow for additional information to be obtained about the sulfur cycle with future studies. For instance, the correspondence of these results suggests that the inferred variation of 34S/32S of pyrite is real, and that there is no significant missing sink of fractionated sulfur at the resolution of the present study (such as might be associated with organic sulfur). Burial of organic sulfur may, however, have been important at some times in the Phanerozoic and shorter timescale deviations between results provided by these methods may be observed with higher resolution sampling. If observed, this would suggest either that the record for pyrite (or less likely sulfate) is biased, or that another sink (possibly as organic sulfur) was important during these times in the Phanerozoic. © 2009 Elsevier Ltd. All rights reserved. Source


Farquhar J.,The Interdisciplinary Center | Farquhar J.,Syddansk Universitetet | Nanping W.U.,The Interdisciplinary Center | Canfield D.E.,Syddansk Universitetet | Oduro H.,The Interdisciplinary Center
Economic Geology | Year: 2010

Significant links exist between the sulfur cycle, sulfur geochemistry of sedimentary systems, and ore deposits over the course of Earth history. A picture emerges of an Archean and Paleoproterozoic stage of the sulfur cycle that has much lower levels of sulfate (<200μM), carries a signal of mass-independent sulfur, and preserves evidence for temporal and spatial heterogeneity that reflects lower amounts of sulfur cycling than today. A second stage of ocean chemistry in the Paleoproterozoic, with higher atmospheric oxygen and oceanic sulfate at low millimolar levels, follows this stage. The isotopic record in sedimentary rocks and in sulfide-bearing ore deposits suggests abundant pyrite burial and implies a missing 34S-depleted pool that may have been lost via deep ocean deposition and possibly subduction. Proterozoic ocean chemistry appears to be quite complex. The surface waters of the Proterozoic oceans are believed to have been oxygenated, but geologic evidence from ore deposits and sedimentary rocks supports coexistence of significant sulfidic and nonsulfidic, anoxic, intermediate water and deep-water pools in the Mesoproterozoic. This stage in ocean chemistry ends with the second major global oxidation event in the latest Neoproterozoic (-600 Ma). This event started the transition to more oxygenated intermediate and deep waters, and higher but variable oceanic sulfate concentrations. The event set the scene for the formation in the Phanerozoic of the first significant MVT deposits and possibly is reflected in changes in other sedimentary rock-hosted base metal sulfide deposits. © 2010 Society of Economic Geologists, Inc. Source

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