Acid-volatile sulfide and acid-extractable iron sediment profiles do not track changes in lake trophic status and atmospheric sulfur deposition

Résumé

Sulfide formation in anaerobic lake sediments depends on supply rates of organic carbon and sulfate. Improvements to wastewater treatment plant effluent quality (e.g., lower total phosphorus (P)) and sulfur emissions can affect sulfide formation rates, which, in turn, can affect metal chemistry including internal iron loading. To determine if these improvements corresponded with sulfide and iron-related signals in sediments over time, acid-volatile sulfide (AVS) and acid-extractable iron accumulation rates were measured in sediment cores of two lakes with anaerobic hypolimnia, the formerly eutrophic, now mesotrophic central basin of Lake Erie, one of the Laurentian Great Lakes, and a small meso-eutrophic moraine lake in southern Ontario, Lake St. George. AVS accumulation rates declined gradually in both lakes after 1980 by 95% and 57%, respectively. Acid-extractable iron accumulation rate profiles were similar to AVS in both lakes, but acid-extractable iron rates were several orders of magnitude larger than AVS rates, hence most of the iron was not chemically associated with AVS. In contrast to the gradual declines in AVS, total P loading to Lake Erie did not decline much after 1980, total P concentrations in Lake St. George remained relatively constant between 1980 and 2014, and sulfate concentration decreases were too small in both lakes to account for the large AVS declines after 1980. Hence, productivity and sulfur emission signals appear to have been overridden by diagenetic processes, which produced similar profiles. Therefore, AVS and acid-extractable iron do not appear to be useful as paleo-indicators of trophic status and sulfate deposition.