Microbial cycling, oxidative weathering, and the triple oxygen isotope consequences for marine sulfate

Abstract:

Microorganisms are responsible for most geochemical sulfur cycling in the ocean. On both modern and geological time scales, stable isotope ratios often serve as a mechanism to track conspicuous or coupled microbial processes, which in turn inform burial fluxes. 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. We look to calibrate these biogeochemical effects through the integrated picture captured in marine pore water sulfate profiles, where the ¹⁸O/¹⁶O is known to evolve. 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. Through this presentation, we look to calibrate the controls on the balance between biospheric and atmospheric contributions to the marine sulfate reservoir. This is enabled by a series of isotope mass-balance models and with the ultimate goal of developing the geological triple oxygen isotope records of sulfate as a new environmental proxy for paleo-environmental reconstructions.