GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 97-11
Presentation Time: 9:00 AM-6:30 PM


HAGEN, Cedric J., CREVELING, Jessica R. and MIX, Alan C., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Bldg, Corvallis, OR 97331

Large amplitude negative excursions in the stable carbon isotopic composition of Neoproterozoic marine strata (δ13Ccarb < -5‰) defy explanation by traditional steady-state carbon cycle dynamics. Locally, the carbonate strata hosting a subset of these excursions––the Bitter Springs, Islay, and Shuram––also include intercalated sulfate evaporite deposits. Here we develop a simplified steady-state carbon cycle model to explore the hypothesis that the dissolution of evaporite supplied sulfate to the oxidant-limited Neoproterozoic ocean, spurred microbial anaerobic oxidation of methane (AOM) coupled to sulfate reduction, and induced a negative δ13C excursion. The model tracks nine fluxes of carbon—photosynthesis, remineralization, volcanic/metamorphic degassing, carbonate burial, organic carbon burial, carbonate weathering, silicate weathering, organic carbon weathering, and carbon-fixation from AOM—between marine dissolved inorganic and organic carbon reservoirs. The model predicts that dissolution of a Messinian-sized evaporite deposit can produce a δ13Ccarb excursion comparable to the Shuram (~ -12‰) whose nadir is reached in ~1.1 Myr. A smaller magnitude model excursion requires less evaporite dissolution. A more protracted model excursion duration and time to nadir requires a slower evaporite dissolution rate, and vice versa. Although AOM primarily impacts δ13CDIC, an attenuated excursion in δ13Corg also results from the ensuing ‘bloom’ of sulfate reducing bacteria and the contemporaneous photosynthetic exchange. Thus, we speculate that large amplitude Neoproterozoic δ13Ccarb excursions with attenuated δ13Corg excursions could relate to sulfate evaporite dissolution, although global mixing of these isotopic reservoirs allows for the expression of these excursions in some strata without related evaporite deposits.