2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 336-11
Presentation Time: 4:00 PM


FAIIA, Anthony M., Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, SZYNKIEWICZ, Anna, Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 73996 and ZHANG, Zhaohui, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210093, China, afaiia@utk.edu

Fayetteville Green Lake (FGL), NY is a well-studied meromictic lake. The 54m depth of this glacially carved basin and groundwater influx from distinct levels have likely sustained the separation of the mixolimnion and monimolimnion since its formation. One of the most interesting features is the presence of an 18m deep mat made up of photosynthetic sulfur bacteria that accounts for most of the lake productivity. Both purple and green sulfur bacteria are present and result in a very distinct purple water horizon at times. These bacteria utilize H2S as an electron donor instead of H2O and produce elemental sulfur instead of O2. The production of H2S has been attributed to SO4 reduction taking place in the anoxic monimolimnion.

In June 2015, we sampled FGL to place more constraints on the sulfur and carbon cycles. Samples were taken every 3m and sampled for SO4, H2S, DIC, DOC, and CH4. The measured δ34S of SO4 and H2S is nearly identical to measurements taken by Deevey et al. 1963 and by Zerkle et al. 2010 suggesting a very stable sulfur cycle exists in the lake. δ34SSO4 increased with depth from 24.8‰ to 33.2‰. The δ34S of H2S also increased with depth from -29.8‰ at 22m to -22.9‰ at 52m and showed an apparent sulfur isotopic fractionation of 57‰ between SO4 and H2S, much larger than the 48‰ microbial fractionation expected from laboratory experiments. δ18OSO4 also increased with depth from 11.3‰ at 4m to 13.9‰ at 52m. To our knowledge the oxygen isotopic values of SO4 in FGL have not been reported before. There is a strong linear relationship between δ34SSO4, δ18OSO4, and depth suggesting an increasing importance of microbial sulfate reduction with depth. δ13C of DIC was likewise nearly identical to that reported in Deevey et al. 1963, suggesting a very stable carbon cycle in the monimolimnion of FGL. δ13CDIC was nearly constant at -9‰ throughout the mixolimnion and then decreased to value of -19.3‰ at 52m. DIC concentrations increased with depth as was previously observed by Deevey et al. 1963, Takahashi et al. 1968, and Brunskill 1969. We propose a simple model where sulfur is reduced in the sediment rather than in the water column followed by diffusion of H2S and HCO3 up to the chemocline. Photosynthetic sulfur bacteria then convert H2S and HCO3 to elemental sulfur, pyrite, and biomass which eventually settle back to the sediment to be recycled.