CHARACTERIZING OXYGEN LEVELS IN THE PRECAMBRIAN OCEAN USING FE/AL AND MO/AL RATIOS IN SHALES

It has been proposed that the ocean remained anoxic until the Neoproterozoic when a global-scale event triggered the onset of oxygenation throughout the water column. Despite this assertion, oxygen levels in the Precambrian ocean remain elusive, particularly as related to local- versus global-scale patterns in marine oxygen availability. Given the lack of Precambrian bioturbating organisms and the failure of the other traditional approaches, laminated Precambrian black shales are less clearly products of anoxic deposition than their Phanerozoic equivalents.

Recent work in modern anoxic settings shows that Fe/Al ratios in fine-grained sediments are intrinsically linked to the presence or absence of hydrogen sulfide in the water column. In anoxic-sulfidic environments, the Fe/Al ratio is elevated relative to oxic sediments due to the scavenging of dissolved iron during water-column (syngenetic) pyrite formation. Delivery of this scavenged Fe is decoupled from the local siliciclastic influx and therefore can drive up the Fe/Al ratio in anoxic basins during periods of low terrigenous accumulation. Unfortunately, few data exist for coeval, shallow-shelf Proterozoic red shales of known oxic origin. These red shales provide the essential Fe/Al baseline for the continental influx, against which anoxic Fe enrichment and corresponding distributions of Proterozoic marine anoxia can be measured. It is our assertion that these red shales represent the oxic shallow waters of a Proterozoic ocean that may have otherwise been largely anoxic and sulfidic at depth. In this presentation, we will discuss the theory behind this method, results from modern and ancient Phanerozoic settings and our preliminary data from Proterozoic shales of the Belt and Bylot supergroups, northwestern United States and arctic Canada, respectively. We will also highlight analogous relationships for Mo/Al ratios, wherein Mo is effectively scavenged in the sulfidic water column so that, as for iron, the magnitude of enrichment relative to oxic crustal end members reflects the presence of sulfide in the water column in combination with the extent of dilution from the terrigenous influx.