Tracing the stepwise oxygenation of the Proterozoic ocean

Abstract

Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon (2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep-ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox-sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen. Subsequent expansion of sulphidic conditions after about 1,800 Myr ago (refs 8, 9) maintained a mid-Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic (sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms. By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.

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

“…A poorly oxygenated atmosphere-ocean system in the Archean is indicated by several lines of evidence in the sedimentary record (Farquhar et al, 2000;Bekker et al, 2010;Sverjensky and Lee, 2010;Lyons et al, 2014), such as abundant banded iron formations (BIF), common occurrence of redox-sensitive detrital minerals, and preservation of sulfur mass-independent fractionation (S-MIF). In addition, the low concentrations of some redox-sensitive elements (e.g., Mo, U) in sedimentary archives suggest low seawater concentrations of these elements because of their limited oxidative mobilization from the Archean continental crust (Scott et al, 2008;Partin et al, 2013). The Great Oxidation Event (GOE) is marked by a permanent increase of atmospheric O 2 content to >0.001% present atmospheric level (PAL), starting between 2.45 and 2.32 Ga (Pavlov and Kasting, 2002;Bekker et al, 2004;Gumsley et al, 2017).…”

“…The Great Oxidation Event (GOE) is marked by a permanent increase of atmospheric O 2 content to >0.001% present atmospheric level (PAL), starting between 2.45 and 2.32 Ga (Pavlov and Kasting, 2002;Bekker et al, 2004;Gumsley et al, 2017). This transition was accompanied by the appearance of new mineral species containing redox-sensitive elements in their highest oxidation states, reduction in BIF deposition, disappearance of S-MIF, and an increase in seawater Mo, U, and sulfate concentrations Schrö der et al, 2008;Scott et al, 2008Scott et al, , 2014Sverjensky and Lee, 2010;Hazen et al, 2011;Planavsky et al, 2012;Reuschel et al, 2012;Partin et al, 2013;Reinhard et al, 2013a). The latter part of the GOE was marked by a protracted episode of elevated organic carbon burial (Lomagundi Event) between ca.…”

“…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).…”

“…Once an environment becomes authigenically active for a particular redox-sensitive trace metal, the degree of authigenic enrichment of that metal in sediments will be broadly proportional to its dissolved seawater inventory Scott et al, 2008;Reinhard et al, 2013a). On a global scale, that inventory will be controlled mainly by the collective state of marine redox conditions-once pervasive oxidative continental weathering and the associated riverine flux of dissolved anionic metal complexes are established (Scott et al, 2008;Partin et al, 2013;Reinhard et al, 2013a).…”

“…Once an environment becomes authigenically active for a particular redox-sensitive trace metal, the degree of authigenic enrichment of that metal in sediments will be broadly proportional to its dissolved seawater inventory Scott et al, 2008;Reinhard et al, 2013a). On a global scale, that inventory will be controlled mainly by the collective state of marine redox conditions-once pervasive oxidative continental weathering and the associated riverine flux of dissolved anionic metal complexes are established (Scott et al, 2008;Partin et al, 2013;Reinhard et al, 2013a). In an ancient ocean that was more anoxic than today, a globally higher rate of metal burial in seafloor sediments should occur, leading to a decrease in seawater metal concentrations and thus lower metal enrichments in coeval ORM (Scott et al, 2008;Sahoo et al, 2012;Partin et al, 2013;Reinhard et al, 2013a).…”