Drought induced pulses of SO42- from a Canadian shield wetland: use of delta S-34 and delta O-18 in SO42- to determine sources of sulfur

摘要

Following summer drought periods, pulses of elevated SO42- concentrations are frequently observed in streams draining forested catchments that contain wetlands. Delays in the recovery of freshwater streams and lakes in eastern Canada from historically high levels of acidic precipitation have been partially ascribed to these periodic pulses of SO42-. Climate in eastern Canada has also changed within the last 25 a, with a documented increase in summer dryness and duration of droughts.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. delta(34)S-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. delta(18)O-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 delta(34)S-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 S02- 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. (c) 2004 Elsevier Ltd. All rights reserved.