Sulfur isotopic evolution of Phanerozoic and Ediacaran seawater sulfate
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Dynamics of &sigma34S and &sigma33S of oceanic sulfate and sedimentary pyrite over geologic time has been used to understand and extract information about the marine sulfur cycle. My Ph.D project uses this approach to study the evolution of the marine sulfur cycle and focuses on: 1)Providing temporal &sigma34S and &sigma33S records for Phanerozoic and Ediacaran seawater sulfate based on the analysis of carbonate associated sulfate 2) Reinterpreting previously documented variations of sulfur isotope fractionation using new measurements of &sigma33S of oceanic sulfate. The sulfur isotope fractionation between sulfate and pyrite appears to have varied widely, between 25 / to 40 / over the course of Phanerozoic. For the earlier part of Phanerozoic, the values of sulfur isotope fractionation are approximately 35 /. The fractionation then decreases to 25 / during the Carboniferous Period. Following this, the sulfur isotope fractionations progressively increase, reaching approximately 40 / during the Cenozoic Period. 3) Evaluating the connection between sulfate concentration, sulfide re-oxidation and sulfate exchange between water column and marine sediment systems using steady state and also non steady state models. The model results and the data presented here suggest the sulfur cycle in the Ediacaran Oman basin evolved from one that was similar to the global sulfur cycle to a sulfur cycle that was disconnected or partially was disconnected from the open ocean sulfur cycle. It also implies that the sulfate levels drop at this transition due to weakening of vertical bioturbation or weakening of other physical processes that involve in mixing of sulfate in pore water and overlying sulfate in marine settings. The significance of these three directions is the new information that they provide about the evolution of the sulfur cycle. It is relevant to understanding the environmental changes and their connections to sulfur ecosystem evolution for the geological time interval extending from present until latest Precambrian.