Bryan Killingsworth, Huiming Bao , Issaku Kohl

Riverine dissolved sulfate (SO42−) sulfur and oxygen isotope variations reflect their controls such as SO42− reduction and re-oxidation, 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 ∆’17O) of Mississippi River SO42− with biweekly sampling between 2009-2013 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 ∆’17OSO4 data shows 18 ±9% of oxygen in this sulfate is sourced from air O2.