Abstract
Mahoney Lake is a permanently anoxic and sulfidic (euxinic) lake that has a dense plate of purple sulfur bacteria positioned at mid-water depth (~7 m) where free sulfide intercepts the photic zone. We analyzed the isotopic composition of sulfate (δ34SSO4 and δ18OSO4), sulfide (δ34SH2S), and the water (δ18OH2O) to track the potentially coupled processes of dissimilatory sulfate reduction and phototrophic sulfide oxidation within an aquatic environment with extremely high sulfide concentrations (>30 mM). Large isotopic offsets observed between sulfate and sulfide within the monimolimnion (δ34SSO4-H2S = 51‰) and within pore waters along the oxic margin (δ34SSO4-H2S > 50‰) are consistent with sulfate reduction in both the sediments and the anoxic water column. Given the high sulfide concentrations of the lake, sulfur disproportionation is likely inoperable or limited to a very narrow zone in the chemocline, and therefore the large instantaneous fractionations are best explained by the microbial process of sulfate reduction. Pyrite extracted from the sediments reflects the isotopic composition of water column sulfide, suggesting that pyrite buried in the euxinic depocenter of the lake formed in the water column. The offset between sulfate and dissolved sulfide decreases at the chemocline (δ34SSO4-H2S = 37‰), a trend possibly explained by elevated sulfate reduction rates and inconsistent with appreciable disproportionation within this interval. Water column sulfate exhibits a linear response in δ18OSO4-δ34SSO4 and the slope of this relationship suggests relatively high sulfate reduction rates that appear to respond to seasonal changes in the productivity of purple sulfur bacteria. Although photosynthetic activity within the microbial plate influences the δ18OSO4-δ34SSO4 relationship, the biosignature for photosynthetic sulfur bacteria is restricted to the oxic/anoxic transition zone and is apparently minor relative to the more prevalent process of sulfate reduction operative throughout the light-deprived deeper anoxic water column and sediment pore waters.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 1-23 |
| Number of pages | 23 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 189 |
| DOIs | |
| State | Published - Sep 15 2016 |
Bibliographical note
Funding Information:We thank BC Ministry of Environment Area Supervisors R. Gunoff and M. Weston for access to Mahoney Lake and their generous field support. S. Bates, A. Chappaz, A. Dekas, G. Druschel, D. Fike, B. Gill, J. Glass, M. McKibben, N. Planavsky, N. Riedinger, and A. Vossmeyer assisted in the field or laboratory. Hydrolabs were provided by M. Anderson UCR, and LacCore (National Lacustrine Core Facility, Department of Geology and Geophysics, University of Minnesota-Twin Cities). The manuscript benefited from helpful discussions with K. Hall, T. Northcote, C. Alpers, K. Mandernack, and J. Overmann. We also thank D. Johnston and two anonymous reviewers for their thoughtful and constructive reviews. Funding for our research was provided by NASA Exobiology and the NASA Astrobiology Institute (TL), the American Chemical Society Petroleum Research Fund (48736-ND2 to TL and WPG), and an Agouron Institute Geobiology Fellowship (WPG).
Publisher Copyright:
© 2016 Elsevier Ltd.
Copyright:
Copyright 2017 Elsevier B.V., All rights reserved.
Keywords
- Anoxygenic photosynthesis
- Mahoney Lake
- Photic Zone Euxinia
- Sulfate reduction rates
- Sulfur and oxygen isotopes
Continental Scientific Drilling Facility tags
- BC