Quadruple sulfur isotope analysis for transition of the S cycle in the terminal of the Proterozoic: Estimates of 34epsilon and 33lambda values from the Proterozoic to the Phanerozoic

Abstract

Sulfur isotopes have been used to understand the sulfur cycle throughout the Earth's history. The apparent isotopic fractionation between sulfides and sulfates is believed to have increased in response to global oxygenation due in part to changes in the fractionation factors for microbial sulfate reduction. However, accurate estimates of the fractionation factors are difficult due to the lack of sulfate in the Precambrian period, and there are few examples of fractionation factors for 33S and 36S using Proterozoic sedimentary rocks. In this study, we established an analytical method to estimate fractionation factors for microbial sulfate reduction (34epsilon), mass-dependent exponents (33lambda and 36lambda), and initial d34S for sulfate (d34Si) using only pyrite isotopic ratios without sulfate analysis. This method was validated by using isotopic ratios of modern sedimentary sulfides. The estimated d34Si are consistent with measured sulfate values. Re-evaluation of multiple sulfur isotope records of the Proterozoic using this method suggests that the fractionation factors in the Proterozoic are significantly smaller than that in the Phanerozoic and increased from the Ediacaran.