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Project Progress

We continued our investigation of microbial oxidation of sulfides in the Río Tinto area, southwest Spain, where long-term exploitation of pyrite (ferrous iron sulfide) has produced acid mine drainage and sulfate deposits. We have made chemical and isotopic analyses of the waters most recently. Current models for the oxidation pathways of pyrite sulfur to sulfate, and the microbial influences on those pathways are contradictory. We analyzed of Δ¹⁷O (difference from the expected ¹⁷O-¹⁸O relationship) and δ¹⁸O (a measure of ¹⁸O/¹⁶O) from various Río Tinto sources to (1) attempt to identify biomarkers in sulfate, (2) use oxygen as a source tracer to determine the oxidation pathway of pyrite sulfur to sulfate. Δ¹⁷O values (calculated as differences from the usual and expected δ ¹⁷O- δ ¹⁸O slope of 0.528).

Near the source of the Río Tinto there are quite different and clearly differentiable waters, either rich in ferric iron, Fe³⁺ (Fig.1) or ferrous iron, Fe²⁺ (Fig. 2). The differences in the iron water chemistry can be explained by microbial survival and growth strategies. This in turn leads to different oxidation pathways where (1) molecular oxygen (δ¹⁸OO₂= 23.0‰) is the primary oxidant (high Fe²/Fe³⁺), (2) ferric iron (Fe³⁺) is the primary oxidant (low Fe²⁺/Fe³⁺) and water oxygen (δ¹⁸OH₂O=0.0‰) is completely incorporated into sulfate.

We measured a variety of different river inputs with δ¹⁸O values ranging from (+4.2 to 5.9)‰ in waters with high Fe²/Fe³⁺ concentrations (42%-65%), and (-2.3 to 1.1)‰ in waters with relatively low Fe²/Fe³⁺ concentrations (3.0%-24%), and are reproducible to 0.2‰. Although these δ¹⁸O results do indicate differences in the oxidation pathway of pyrite sulfur to sulfate, they cannot be used to quantify differences in the source of oxygen. This is because there are multiple intermediate steps during oxidation for which fractionation factors are still properly determined, and uncertainty resulting from fractionation during δ¹⁸O measurement. The Δ¹⁷O analyses of the same waters range between (+0.07 to +0.192)‰ and are reproducible to (0.007)‰. There is a strong linear relationship between δ¹⁷O and δ¹⁸O, which defines a slope or potential process specific slope of 0.5177, and an R squared value of 0.9995. This relationship has never before been measured for naturally produced sulfate via pyrite oxidation, and varies from the generally accepted relationship for terrestrial materials of 0.528 (Fig 3). This result rendered our initial strategy of using ∆¹⁷O as an oxygen source tracer infeasible, but potentially allows for the new method of measuring slope deviations specific to microbial influences.

The novel method of measuring Δ¹⁷O to determine the deviation from the 0.528 line eliminates much of the uncertainty inherent to δ¹⁸O measurement. Since the Δ¹⁷O value depends only on the relationship between δ¹⁷O/δ¹⁸O, which remains constant during mass dependent fractionation, any fractionation during measurement does not affect the Δ¹⁷O value. This also means that any slope deviations caused by microbial activity will be maintained during subsequent mass dependent fractionation processes, which would change the δ¹⁸O value.