LINKING WATER COLUMN AND SEDIMENTARY SULFUR ISOTOPE PATTERNS IN A EUXINIC LAKE

Mahoney Lake is a shallow (14 m) meromictic lake located in British Columbia, Canada likely formed 9 Kyr ago following glacial retreat. The alkaline water chemistry supports unusually high concentrations of dissolved sulfate in excess of 400 mM and dissolved sulfide levels above 35 mM. The anoxic and sulfidic (euxinic) water column represents extreme reducing conditions beyond traditional end member environments such as the Black Sea. The oxic-anoxic interface supports a dense microbial plate populated by a single species of purple sulfur bacteria (Amoebobacter purpureus) that thrive where the dissolve sulfide gradient intercepts the photic zone. The elevated sulfide concentration coupled with intense microbial activity make this lake an ideal study site to track sulfur redox transformations, particularly the sulfur and oxygen isotopic composition of dissolved sulfate (either interstitial or water column) and carbonate-associated sulfate (CAS). We present isotope data from the water column and sedimentary sulfur species (CAS, AVS, pyrite, total organic sulfur) that may fingerprint paleoecological and biogeochemical signals associated with anoxygenic photosynthesis.

That downcore sulfate isotope profiles (δ³⁴S_SO4 = 22.1‰; δ¹⁸O_SO4 = 17.3‰) within oxic and euxinic sediments remain unchanged with depth reflects the vast reservoir of sulfate (400 mM) available for sulfate reduction. Respective isotopic enrichments in sulfate sulfur and oxygen, by as much as 4 and 3‰, are observed across the chemocline. Porewater sulfate isotopes are near identical to the maximum isotopic compositions attained above the sediment water interface (δ³⁴S_SO4 = 28.4‰ and δ¹⁸O_SO4 = 20.7‰). The δ³⁴S/δ¹⁸O water column sulfate ratio of 1.6 suggests sulfide-reoxidation and formation of secondary sulfate at the chemocline. Paired pyrite extracts (CRS) also reflect the isotopic composition of water column sulfides suggesting that the sulfur buried (as CAS or pyrite) in the euxinic depocenter of the lake record redox transformations driven by microbial activity within the plate situated at the chemocline.