Shallow water anoxia in the Mesoproterozoic ocean: Evidence from the Bashkir Meganticlinorium, Southern Urals

The apparent lag between the first permanent rise of atmospheric oxygen to appreciable levels and oxygenation of the deep ocean has focused efforts in deciphering the evolution of seawater chemistry across the Proterozoic Eon (2.5-0.542 Ga). It is generally accepted that from ∼1.85 Ga oxic shallow marine waters were widespread while the deep ocean remained dominantly ferruginous (Fe(II)-rich), with episodic euxinia confined to productive continental margins and intracontinental basins. The geochemical record that informs this picture, however, is currently sparse, and further studies are required to adequately evaluate temporal and spatial variability in ocean redox conditions across this vast expanse of time. Here, we report Fe-S-C systematics, alongside major (Al, Mn) and trace metal (Mo, U) data for ∼1.6-0.8 Ga marine sediments from the Bashkir Meganticlinorium (BMA), Southern Urals, Russia. Our Fe speciation data reveal a water column dominated by ferruginous conditions, no evidence for euxinia, and oxygenated waters confined to only the shallowest, near-coastal settings. Trace metal data support these findings, with Mo and U enrichment factors in anoxic sediments implying the operation of a particulate Fe (oxyhydr)oxide shuttle under ferruginous conditions as the main mechanism for generating moderate authigenic Mo enrichments. Sulfur isotope systematics imply that predominantly low dissolved sulfate concentrations prevailed throughout the deposition of the BMA succession, which, in combination with a low organic carbon loading, promoted the development of ferruginous, rather than euxinic, water column conditions. The restriction of oxic conditions to the shallowest, near-coastal depositional settings within the Bashkir basin contrasts to other redox studies from this period. Such discrepancies highlight major uncertainty in our understanding of the temporal and spatial evolution of water column redox chemistry across the Mesoproterozoic Era, and signify the need for further detailed work to constrain the chemical evolution of the oceans during this crucial time period.