Oxygen isotopes in marine sulfate and the sulfur cycle over the past 140 million years

A long-standing controversy in the geochemical community is whether the oxygen isotope composition (delta18O) of the ocean has changed significantly over Earth history. Resolving this conflict is essential for understanding the ocean's paleochemistry and temperature evolution. This thesis proposed to address whether the ocean was isotopically depleted by around 7‰ in the Proterozoic and has been increasing to today's value of 0‰ through the Phanerozoic by measuring oxygen isotopes in marine sulfate (delta 18OSO4) in well preserved marine barium sulfate (barite). A new procedure for the extraction of marine barite from pelagic sediments for the measurement of its delta18O was developed. The delta 18OSO4 of marine barite showed unexpected variability over the past 60 million years, including excursions of approximately 6‰. This suggested a reevaluation of the marine sulfur cycle, which highlighted the importance of sulfate reduction and sulfide reoxidation in organic-rich sediments. Although the residence time of marine sulfate for sulfur isotopes is over 10 million years, because of the rapid cycling of sulfur in organic rich sediments, the residence time for oxygen isotopes is around 1 million years. Coupled conservation equations were used to model temporal variability in the delta18OSO4 while simultaneously tracking sulfate concentrations. Because the delta18OSO4 does not correlate with the delta34S of sulfate, the primary driver of temporal variations in the delta18OSO4 must be changes in the sulfide reoxidation pathway in organic rich sediments. This suggests that the redox variability of the oceans over the Cenozoic may be significantly greater than previously thought. The delta18O SO4 may be a powerful tool to use at times in Earth's history when we know that the redox state of the oceans has changed dramatically, such as Cretaceous Ocean Anoxic Events (OAE). Initial results of delta18 OSO4 measurements around one of the largest OAEs suggest there were significant changes in the redox state of the oceans prior to the onset of whole-ocean anoxia. Unexpected variations in delta18O SO4 profiles in porewaters of organic-rich sediments highlight the complexity of the pathways of sulfate reduction and sulfide reoxidation, but suggest that the delta18OSO4 may help resolve redox coupling and pathways of organic matter remineralization.