SULFATE EVAPORITE DISSOLUTION, AOM, AND THE NEOPROTEROZOIC CARBON CYCLE

Large amplitude negative excursions in the stable carbon isotopic composition of Neoproterozoic marine strata (δ¹³C_carb < -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 δ¹³C_carb 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 δ¹³C_DIC, an attenuated excursion in δ¹³C_org also results from the ensuing ‘bloom’ of sulfate reducing bacteria and the contemporaneous photosynthetic exchange. Thus, we speculate that large amplitude Neoproterozoic δ¹³C_carb excursions with attenuated δ¹³C_org 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.