BEHAVIOR OF MARINE SULFUR IN THE ORDOVICIAN

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 (as in an anoxic water column or within sediment pore space) or immediately reoxidized, through a range of intermediate sulfur phases, to sulfate (as in a well-ventilated water column).

Unfortunately, utility of a single-reservoir model is limited at times when independent Fe speciation data suggests the presence of persistent marine euxinia, which indicates that ferrous iron was at least locally insufficient to strip the water column of bacterially produced hydrogen sulfide. Such a scenario demands modeling the marine sulfur cycle as a dual-reservoir system, wherein marine sulfate and marine hydrogen sulfide 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 utilize a two reservoir model to examine the behavior of marine sulfur in the Ordovician, and present evidence for a regional-scale (and potentially global-scale) oxygenation event in the late Middle Ordovician (Dariwillian) that may have had a profound influence in nutrient distribution in the Ordovician ocean.