Assessing Pyrite-Derived Sulfate in the Mississippi River with Four Years of Sulfur and Triple-Oxygen Isotope Data.

Riverine dissolved sulfate (SO42-) sulfur and oxygen isotope variations reflect their controls such as SO42- reduction and reoxidation, and source mixing. However, unconstrained temporal variability of riverine SO42- isotope compositions due to short sampling durations may lead to mischaracterization of SO42- sources, particularly for the pyrite-derived sulfate load. We measured the sulfur and triple-oxygen isotopes (δ34S, δ18O, and Δ'17O) of Mississippi River SO42- with biweekly sampling between 2009 and 2013 to test isotopic variability and constrain sources. Sulfate δ34S and δ18O ranged from -6.3‰ to -0.2‰ and -3.6‰ to +8.8‰, respectively. Our sampling period captured the most severe flooding and drought in the Mississippi River basin since 1927 and 1956, respectively, and a first year of sampling that was unrepresentative of long-term average SO42-. The δ34SSO4 data indicate pyrite-derived SO42- sources are 74 ± 10% of the Mississippi River sulfate budget. Furthermore, pyrite oxidation is implicated as the dominant process supplying SO42- to the Mississippi River, whereas the Δ'17OSO4 data shows 18 ± 9% of oxygen in this sulfate is sourced from air O2.