Evidence for episodic oxygenation in a weakly redox-buffered deep mid-Proterozoic ocean

• Published in: Chemical geology 2018-04, Vol.483, p.581-594
• Main Authors: Planavsky, Noah J., Slack, John F., Cannon, William F., O'Connell, Brennan, Isson, Terry T., Asael, Dan, Jackson, John C., Hardisty, Dalton S., Lyons, Timothy W., Bekker, Andrey
• Format: Article
• Language: English
• Subjects: Animikie Basin; Ocean oxygenation; Ocean redox; Paleoceanography; Proterozoic
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2018.03.028

Over the last two decades, popular opinion about prevailing conditions in the mid-Proterozoic deep ocean has evolved from fully oxygenated to globally euxinic (sulfidic) to a more heterogeneous, stratified water column with localized pockets of euxinia existing in predominantly iron-rich (ferruginous) deep waters. The Animikie Basin in the Lake Superior region has been essential in shaping our view of marine redox evolution over this time period. In this study, we present a multi-proxy paleoredox investigation of previously unanalyzed strata of the late Paleoproterozoic Animikie Basin using drill cores through the ~1.85 Ga Stambaugh Formation (Paint River Group) in the Iron River-Crystal Falls district of the Upper Peninsula of Michigan, USA. Based on previous tectonic reconstructions and analysis of sedimentary regimes, the Iron River-Crystal Falls section captures strata from among the deepest-water facies of the Animikie Basin. In contrast to previous work on sedimentary rocks in this basin, we find evidence from iron speciation, trace metal, and Mo isotope data for episodes of at least local deep-water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions. While trace-metal enrichments and iron speciation data suggest predominantly anoxic conditions, the occurrence of Mn-rich intervals (up to 12.3 wt% MnO) containing abundant Mn-Fe carbonate, and a wide range of Mo isotope data with extremely negative values (δ98/95Mo = −1.0 to +1.1‰), record the shuttling of Mn-oxides from surface waters through oxic or suboxic waters to the sediment-water interface. We propose that such conditions are analogous to those of locally restricted modern and Holocene basins in the Baltic Sea, which receive episodic inflow of oxygenated water, producing similar geochemical signatures to those observed for the Animikie Basin. We argue that the mid-Proterozoic was characterized by a lack of a strong redox buffer (low sulfide, ferrous iron, and oxygen contents), and thus was vulnerable to dramatic, and at least local, redox shifts—including briefly oxygenated bottom waters. A refined view of the mid-Proterozoic ocean is emerging: one that was still predominantly anoxic, but marked by regional heterogeneities and short-term redox variability that may, in part, reflect a transitional state between prevailingly anoxic Archean and predominantly oxic Phanerozoic oceans.