THE BIOGEOCHEMICAL CONTROLS OF d¹⁸OSO4 VALUES FROM SULFIDE OXIDATION

In order to better understand the reaction pathways of sulfide oxidation and the biogeochemical controls of d18OSO4 values in acid mine drainage (AMD) and other natural environments, we conducted a series of metal sulfide oxidation experiments in the laboratory. Previous studies have shown that d18OSO4 values vary as a result of changes in the incorporation into sulfate of O2 versus water derived oxygen, which in turn depends on reaction pathways and conditions. Our experiments were conducted biotically and abiotically, under aerobic and anaerobic conditions, with varying d18OH2O values. Parent sulfur minerals used in these experiments included pyrite (FeS2) and sphalerite (ZnS). Elemental sulfur, which can be a side-product of acid-leaching of monosulfide minerals, was also used for aerobic bacterial experiments. Biotic experiments were conducted at pH ~ 2.2-2.7 in the presence of the sulfide oxidizing bacterium, Acidithiobacillus ferrooxidans. For biotic and abiotic anaerobic experiments with pyrite, and aerobic bacterial experiments with elemental sulfur, plots of d18OSO4 versus d18OH2O are linear with slopes of ~1.0. The results for all of these experiments indicate H2O as the sole source of oxygen for sulfate. Enrichment factors (D(SO4-H2O) ) of 8.9 ‰, 4.8 ‰, and 3.6 ‰ were estimated for the aerobic So, anaerobic biotic pyrite, and anaerobic abiotic pyrite experiments, respectively. Enrichment factors from anaerobic biotic and abiotic sphalerite experiments were 8.5 ‰ and 7.9‰, respectively. The results of anaerobic oxidation of pyrite and sphalerite both indicate small differences in the oxygen isotopic signature between biotic and abiotic experiments. However, d18OSO4 from anaerobic sphalerite and the aerobic So experiments were each enriched in 18O by ~5 ‰ compared to anaerobic pyrite experiments, which may be interpreted to result from the incorporation of atmospheric oxygen into sulfate. However, our results indicate that other important factors influence the d18OSO4 values and must be considered before accurate (paleo)environmental assessment of sulfide/sulfur oxidation can be made based on measured d18OSO4 values.