SULFUR CYCLING AND PYRITE FORMATION IN MODERN EUXINIC LAKES

Examining modern euxinic systems can provide a better understanding for sulfur (S) cycling and pyrite formation during the Precambrian. Mahoney Lake, Canada and Green Lake, NY contain phototrophic S-oxidizing bacteria and sulfate reducers that trigger an active redox chemistry of S, producing significant levels of sulfide within the water column. Reduced S in the sedimentary record is mainly in the form of pyrite. Thus Fe speciation analysis will be conducted in order to identify the reactive Fe pools available. The high content of organic matter, especially in Mahoney Lake (~2.5 wt.%), suggests that sulfide is potentially incorporated to organic S compounds as well.

Water column samples were collected from both lakes for bulk and isotope analysis (δ³⁴S; V-CDT) of sulfate and sulfide. High levels of sulfate (>400mM) and sulfide (>30mM) were observed in the euxinic waters of Mahoney Lake. The isotope offset between sulfate and sulfide of ~52‰ (δ³⁴S_SO4=28‰, δ³⁴S_H2S=-24‰), suggests that dissimilatory sulfate reduction is the dominant process that fractionates S isotopes in Mahoney Lake. In order to examine the sulfide pathway during diagenesis and pyrite formation sediment cores were subsampled at 10 cm intervals for S chemistry and 5 cm for reactive Fe analysis. Total organic S (TOS), acid-volatile S (AVS), and pyrite were extracted from each sample. Pyrite-S ranges from 0.1 to 0.6 wt.%, while AVS exhibits lower concentrations (up to 0.2 wt.%) in Mahoney Lake. Isotope analysis of pyrite-S revealed light δ³⁴S values (-15 to -33‰) similar to the sulfide in the water column, suggesting that pyrite was formed syngenetically. Prelimenary results in Green Lake sediments showed that AVS has levels up to 0.05 wt.% while pyrite-S is up to 0.8 wt.%.

Ongoing work includes bulk and isotope analysis of Green Lake water samples, S isotope analysis in the remaining sediment extractants and Fe speciation. Iron speciation can provide information for the formation of S intermediate species as Fe can also be an oxidant to reduced S. Although pyrite levels seem significant in these small lakes, the Fe redelivery mechanisms remain to be characterized. To address this issue, we will couple Fe distribution patterns in the sedimentary record with phosphorus (P), that might give insights into how P fluxes into the lake systems can control Fe availability.