Widespread iron-rich conditions in the mid-Proterozoic ocean

Abstract

The chemical composition of the ocean changed markedly with the oxidation of the Earth's surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean-atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth's middle age (1.8-1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe(2+)-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

“…In the modern ocean, anoxic seafloor is dominated by euxinia defined as having an excess of sulfide relative to Fe (i.e., above a molar ratio of 1Fe:2S, the stoichiometric proportions of pyrite). During the Proterozoic Eon, however, ferruginous anoxia is believed to have been more widespread Planavsky et al, 2011). Extrapolating from modern observations that reveal significant authigenic Re accumulation without Mo accumulation in anoxic sediments when dissolved H 2 S in pore waters is low, we infer that Re burial in sediments below anoxic waters does not depend on H 2 S availability in the water column.…”

“…The latter part of the GOE was marked by a protracted episode of elevated organic carbon burial (Lomagundi Event) between ca. 2.22 and 2.06 Ga, which suggests a long-lasting, but transient increase in atmosphere-ocean O 2 contents to levels that may not have occurred again until the Neoproterozoic Oxidation Event (NOE) (Karhu and Holland, 1996;Scott et al, 2008Scott et al, , 2014Kump et al, 2011;Planavsky et al, 2011Planavsky et al, , 2012Planavsky et al, , 2014Bekker and Holland, 2012;Partin et al, 2013).…”

“…Recent geochemical data (e.g., redox-sensitive trace metals and Fe speciation) suggest stratified ocean redox conditions for the middle portion of the Proterozoic (ca. 1.8-0.8 Ga; hereafter referred to as the mid-Proterozoic), wherein oxygenated surface waters were underlain by euxinic (anoxic and sulfidic) waters in highly productive marginal settings, and ferrous iron (Fe 2+ ) accumulated in suboxic to anoxic deep waters in offshore settings (ferruginous anoxia; Poulton et al, 2010;Planavsky et al, 2011;Reinhard et al, 2013a;Sperling et al, 2015). While Fe speciation data indicate ferruginous conditions at many times and localities in the mid-Proterozoic, the scarcity of preserved ancient seafloor, particularly of the deepest parts of the ocean, creates a challenge for constraining the marine redox landscape on a global scale.…”

Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

“…E nrichments in reactive iron are a consistent feature of marine sediments throughout most of the Proterozoic [1][2][3][4] , indicating persistent ocean anoxia well after Earth's first major rise in atmospheric oxygen (the Great Oxidation Event) at B2.32 Ga (ref. 5).…”

“…5). The emerging picture now suggests a predominance of ferruginous conditions in the deep ocean [1][2][3] , with euxinic conditions along productive continental shelves and in epicontinental seas, likely having a key role in reducing Fe(II) concentrations in comparison to before the Great Oxidation Event 6 (Fig. 1).…”