Drought induced pulses of SO42- from a Canadian shield wetland: use of δ34S and δ18O in SO42- to determine sources of sulfur

In small forested catchments in the Turkey Lakes Watershed (TLW), SO42- concentrations in streams draining wetlands can be elevated by up to a factor of 7 during post-drought discharge events compared to the annual average. Two neighbouring catchments, one with a series of cascading wetlands and one without any wetlands, were selected for comparison. Stable S and O isotope ratios were analyzed in samples of bulk precipitation, streams, and groundwaters to examine sources of SO42- in post-drought pulses. δ34S-SO42- in the streams and groundwaters show that SO42- is retained in the wetland via SO42- reduction and stored in the upper peat profile. Nitrate is elevated in soil and groundwaters at TLW due to high rates of nitrification in forest soils and the presence of NO3- can be used to identify piezometers unaffected by SO42- reduction. δ18O-SO42- shows that higher concentrations of SO42- in deeper groundwater are likely due to oxidation of organic S and not a geologic source of reduced S. Following drought, the low δ34S-SO42- in streams is consistent with wetland retention by SO42- reduction and much lower than SO42- released by weathering in deep glacial till and bedrock. High SO42- groundwaters and geologic sources do not contribute to the SO42- pulses in streams. Isotopic patterns over 6 a were similar. Pulses of SO42- in the wetland catchments following drought are a result of the oxidation of S previously reduced and stored in the wetland.