RESERVOIR SIZE, RESIDENCE TIME, AND THE RATE OF ISOTOPIC CHANGE: EXAMINING THE BEHAVIOR OF MESOPROTEROZOIC MARINE SULFATE

The large reservoir size and long residence time of marine sulfate in the modern ocean significantly affects observed rates of isotopic change. Over long time scales, S-isotope variation is governed by steady-state behavior and controlled by global redox cycles, via tectonically induced changes in sedimentation and coupling of C-S redox cycles. Over short time scales, S-isotope variation is attributed to non-steady state behavior driven by local effects such as basin restriction, evaporite deposition, or locally high rates of sulfide burial.

In the Mesoproterozoic, data from marine evaporites and carbonate-associated sulfate (CAS) reveal a global pattern of short term S-isotope variation, suggesting non-steady state conditions. Here, stratigraphically and geochronologically constrained CAS data from the 1.2 Ga Society Cliffs Fm. and the 1.3 Ga Dismal Lakes Grp., Canada, are used to reconstruct behavior of the marine sulfate reservoir. Because non-steady state conditions are highly dependent upon reservoir size as well as the isotopic composition of input/output fluxes, modeling the Mesoproterozoic marine sulfate reservoir as a non-steady state system permits an unprecedented glimpse into the evolution of the Precambrian ocean-atmosphere system.

Modeling suggests that the Mesoproterozoic marine sulfate reservoir was <1/3 that of the modern. Although C-S redox linkages were important, isotopic perturbation of the global sulfate reservoir was decoupled from C-isotopic change, driven instead by the interaction of weathering input (seafloor spreading rates), extent of S-reduction (reservoir effects), and pyrite burial (related to extent of euxinic bottom water). Ultimately, increased ocean-atmospheric oxygenation in the Neoproterozoic, driven by changes in marine C-cycling, resulted in an increase in the size of the marine sulfate reservoir, evolution of the bacterial sulfate community and an increase in DS, and increased coupling of the C-S isotopic systems.