Intermediate and Vacillating Redox in Ancient Marine Settings and Their Biological Implications

The presence of marine oxygen is an essential precursor for faunal habitability, driving distribution in coastal systems today and linked to evolutionary patterns throughout the geologic past. In oxygen-deficient marine systems, the reducing capacity can be defined by the zonation of redox-sensitive elements (e.g. O, I, N, Mn, Fe, S, C), with their availability in the rock record in some cases linked to the water column redox state during deposition and subsequent burial processes. In this dissertation, I reconstruct paleoredox in environments defined by water column oxygen deficiency at biologically critical levels, specifically extreme low oxygen to mildly sulfidic, a redox window most representative of reducing systems today but largely underexplored in the geologic past. First, I present the initial surface ocean-specific Precambrian redox record, the location and period hosting the emergence of both oxygenic photosynthesis and multicellular life, evaluated through iodine-to-calcium-magnesium ratios in mostly shallow carbonate and supplemented through study of iodine retention during carbonate diagenesis. Oxygen became widespread in shallow marginal marine settings starting in the Paleoproterozoic, likely at levels necessary to support simple animals. The ensuing Proterozoic record was dynamic, with oxygen fluctuating at and below thresholds similar to waters within and directly overlying modern oxygen minimum zones and largely constrained to shallow settings, a condition particularly exacerbated during the middle Proterozoic. This is a new view of the Precambrian surface ocean and such variability and spatially restricted marine oxygen at critically low levels is hypothesized to explain the delayed emergence of animals until the Neoproterozoic. A second study defines natural variations in water column redox in the Baltic Sea over the Holocene, the world’s largest modern anthropogenically forced and spatially and temporally variable low-oxygen basin. An iron-manganese-molybdenum geochemical approach reveals distinct phases of large-scale anoxia during the Holocene, with the magnitude and frequency of anoxia increasing with increasing distance from the sill, but never beyond mild sulfide accumulation. This work specifically indicates that spatial and temporal variability in Baltic redox appears to have driven both long-term oxygen depletion and restrained the maximum reducing potential over the Holocene