Evaluating the S-isotope fractionation associated with Phanerozoic pyrite burial

This study examines the sulfur isotope record of seawater sulfate proxies using delta S-34 and Delta S-33 to place constraints oil the average global fractionation (Delta S-34(py)) associated with pyrite formation and burial and the exponent lambda that relates variations of the S-34/S-32 to variations of the S-33/S-32. 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 seawater delta S-34 of seawater sulfate and contemporaneous sulfide. It also uses an independent method that draws on inferences about the Delta S-33 evolution of seawater sulfate to evaluate this further. These two methods yield similar results Suggesting that Delta S-34(py) and lambda changed over the course of the Phanerozoic from slightly lower values of Delta S-34(py) (lower Values of lambda) in the early Phanerozoic (Cambrian-Permian) to higher values of Delta S-34(py) (higher values of lambda) starting in the Triassic. This change of Delta S-34 and the exponent lambda 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 Delta S-34(py) 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 S-34/S-32 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 for less likely sulfate) is biased, or that another sink (possibly as organic sulfur) was important during these times in the Phanerozoic. (C) 2009 Elsevier Ltd. All rights reserved.