Stable oxygen isotopes in sulfate: Implications for Phanerozoic pO2

Abstract

Atmospheric pO2 and pCO2 reflect the balance of physical and biological processes on Earth that regulate life and climate. On million year timescales, the oxygen and carbon cycles are linked to the global sulfur cycle, primarily through oxidation and reduction of sulfur minerals that consumes and produces O2. A record of these oxidation and reduction processes is preserved in the stable isotope composition of sulfate. As O2 is consumed, it may be directly incorporated into the sulfate anion (SO42-) when conditions allow. The stable isotope composition of atmospheric O2 (in particular the triple isotope composition, D'17O) is directly proportional to the ratio pO2/pCO2 in the stratosphere. After weathered sulfate enters the ocean reservoir, it consistently precipitates, forming mineral phases that are stable over millions to billions of years. For the first time, we reconcile the triple oxygen isotope proxy in marine sulfate to the modern and more recent past (last 542 million years). This work begins in chapter 2 with a model for the D'17O proxy based on measurements of modern seawater sulfate. It is understood that the atmospheric effect is not resolvable in the modern system. Chapter 3 presents the marine sulfate record based on marine barite minerals covering the last ~130 million years. Similarly, our findings suggest that the global sulfur cycle is dominated by alternate earth surface processes (i.e. microbial sulfur cycling) and does not apparently record information about atmospheric O2. While we do not extract atmospheric information from this analysis, we do learn that the fundamental sulfur cycle processes have remained strikingly consistent across this time. To interpret the mineral records preserved deeper in Earth's past and bridge the Proterozoic to Phanerozoic, we calibrate the isotope composition of a marine sulfate evaporite basin relative to seawater sulfate in chapter 4. We determine the nature of sulfate isotope fractionation during the life history of marine sulfate that is restricted and precipites from an evaporite basin. Then, a compilation of marine sulfate evaporite mineral basins covering the last 1050 million years is presented in chapter 5. We highlight a transition in marine sulfate D'17O from marine sulfate that preserved some atmospheric O2 anomaly to marine sulfate that does not conserve a resolvable effect. To account for the scale of change that we observe, we constrain the preservation of O2 in sulfate to be ~1% of the total sulfate signal. These findings call into question previous estimates of Proterozoic pO2 and primary production (GPP). At a minimum, these estimates require revision to roughly 10X lower pO2 values and/or 10X lower GPP. Most conservatively, we suggest that the marine sulfate D'17O records be interpreted more generally, in terms of directionality rather than inferring fine-scale atmospheric information.