Behavior of Marine Sulfate in Variably Sulfidic Oceans

Linda C Kah, University of Tennessee, Department of Earth and Planetary Sciences, Knoxville, TN, United States, Miles A Henderson, University of Tennessee, Knoxville, TN, United States and Cara K Thompson, Santa Monica College, Santa Monica, CA, United States

Contact First Author: Linda C Kah; lckah@utk.edu

Previously Published Material: Related abstract investigating only the Ordovician material was presented at GSA 2014. All of this is currently in preparation for publication.

Abstract ID#: 34879

 

English Abstract:
Our understanding of the oxygenation history of ancient oceans commonly derives from reservoir modeling of marine S-isotopes. Traditional modeling of S-isotopes has emphasized a single-reservoir model, in which the composition of the marine sulfate reservoir depends only on the magnitude and isotopic composition of input fluxes and output fluxes. An inherent assumption in this model is that bacterially reduced HS− is either immediately extracted by reaction with available iron (e.g., within anoxic waters or sediment pore space) or immediately reoxidized, through a range of intermediate sulfur phases, to sulfate (e.g., within a well-oxygenated waters).

Much of the Proterozoic and early Paleozoic, however, preserves independent data that indicates persistent marine euxinia, wherein Fe2+ was at least locally insufficient to strip the water column of bacterially produced HS−. Such conditions demand modeling the marine sulfur cycle as a dual-reservoir system, wherein marine SO42– and HS− are treated as distinct, reactive reservoirs, with their own input and output fluxes that affect both the behavior of the individual reservoirs as well as the degree of linkage between the two reservoirs. In this case, the isotopic composition of marine sulfate is affected directly by the magnitude and isotopic composition of traditional input and output fluxes that act over long time scales, as well as a suite of transitory input and output fluxes, including bacterial sulfate reduction and a combination of chemical and biological sulfide oxidation.

Here we examine patterns of behavior in the marine sulfur cycle from Mesoproterozoic through Ordovician-aged strata (from China, Mauritania, Argentina, and Newfoundland), and interpret these patterns in terms of a dual-reservoir model. We show how systematic changes in sulfur isotope behavior related to the protracted oxygenation of the Earth’s biosphere and growth of the marine sulfate reservoir over nearly 1 billion years of Earth history.

Back to: The Neoproterozoic-Paleozoic Earth System: Records From an Earth in Transition