THE TRIPLE OXYGEN ISOTOPE COMPOSITION OF MARINE SULFATE: A METRIC FOR ATMOSPHERIC OXYGEN

The geological record offers only incomplete snapshots of the chemistry of paleo-environments. This is especially true when the goal is reconstructing atmospheric chemistry and O₂. Here we present a new data set for an emerging proxy: the ¹⁷O contents of seawater sulfate. There is a rich history of analyzing the stable sulfur and oxygen isotope composition of both seawater and evaporite minerals preserved in the sedimentological record. For instance, the most common example of this approach is the use of sulfur isotopes in sulfate and sulfide (both aqueous and in mineral form) to track everything from rates of microbial processes through to the presence/absence of certain metabolic processes in a given environment. The use of oxygen isotope ratios in sulfate has developed in a similar fashion, providing complementary information to that of sulfur isotopes. Through our current work, we will extend the application of oxygen isotopes to include the trace stable oxygen isotope, ¹⁷O. These data are facilitated by a new laser F₂ fluorination technique running at Harvard, and accompanied by the calibration of a suite of common sulfate standards. At first blush, ¹⁶O - ¹⁷O - ¹⁸O systematics should carry mass-dependent microbial fractionations with process-specific mass laws that are resolvable at the level of our analytical precision. In compliment, riverine sulfate (the sulfate input to the ocean) is an oxidative weathering product and is posited to carry a memory effect of tropospheric O₂. Interestingly, the ¹⁷O/¹⁶O of that O₂ carries a mass-independent signal reflecting the balance between stratospheric reactions and Earth surface biospheric fluxes. Thus, once the biospheric component is calibrated, what remains is a relatively clean picture of the atmospheric contribution. Through this presentation we will present a new Phanerozoic and Ediacaran record of the ¹⁷O of sulfate. These data and associated models will then be related to independent measures of atmospheric O₂ changes over the same time interval.