EVERY OCEAN HAS AN OXIC LINING: EVIDENCE FOR SHALLOW WATER ANOXIA IN THE MESOPROTEROZOIC

The Mesoproterozoic Era (1.6 – 1.0 Ga) has long been considered a period of relative environmental stasis. Current understanding of ocean redox from this Era involves a ferruginous deep ocean overlain by an oxic shallow-marine layer, with episodic euxinia occurring along productive continental margins. This record is, however, based on a limited number of studies, and further evidence is required to gain a more detailed perspective on temporal and spatial variability in ocean chemistry across this vast expanse of time. Here we present Fe speciation, total organic carbon (TOC), major element (Mn and Al), and trace (Mo, V, U) metal data from the ~1.6 – 0.8 Ga Bashkirian Anticline (BA) in the South Urals of Russia, to test accepted models of Proterozoic redox.

Fe speciation results reveal a dominantly ferruginous water column, with oxic conditions confined to very shallow depositional settings. Trace metal data support these findings, but also identify a persistent Mo enrichment. Limited organic matter in the sediments and low sulphide availability in the water column suggest the input of highly reactive Fe to the shallow shelf overwhelmed the flux of dissolved sulphate, thus shifting the oceanic system to ferruginous conditions. Consequently, a particulate Fe shuttle is proposed as the main mechanism for the authigenic drawdown of Mo, and is supported by the relationship between Mo and U enrichment factors.

In contrast to previous work, our data indicate the predominance of ferruginous conditions where anoxia is present, with no indication of a nearshore euxinic wedge. It also suggests that the shallow marine environment was not completely oxygenated at this time, highlighting redox variability on a local and temporal scale. Sediments from the BA thus illustrate how ineffective it is to couple palaeo-redox studies from this period into one model of ocean chemistry.