Neoproterozoic oxygenation event

The Neoproterozoic Oxygenation Event (NOE), also called the "Second Great Oxidation Event", was a geologic time interval between around 850 and 540 million years ago during the Neoproterozoic era, during which the oxygen concentration in Earth's atmosphere and oceans rose significantly. Taking place after the end to the Boring Billion, a euxinic period of extremely low atmospheric oxygen spanning from the Statherian period of the Paleoproterozoic era to the Tonian period of the Neoproterozoic era, the atmospheric oxygen concentration rose from <0.1% PAL throughout most of the early/mid-Proterozoic to >13% PAL during the early Neoproterozoic from 900 to 815 Ma, and possibly up to ~50% PAL during the Cryogenian Sturtian–Marinoan interglacial. The NOE was the second major increase in atmospheric and oceanic oxygen concentration on Earth, after the Great Oxidation Event (GOE) of the Neoarchean-Paleoproterozoic boundary. Unlike the GOE, it is unclear whether the NOE was a synchronous, global event or a series of asynchronous, regional oxygenation intervals with unrelated causes.

Evidence for oxygenation

Carbon isotopes Beginning around 850 Mya to around 720 Mya, a time interval roughly corresponding to the Late Tonian, between the end of the Boring Billion and the onset of the Cryogenian "Snowball Earth", marine deposits record a very significant positive carbon isotope excursion. These elevated δ13C values are believed to be linked to an evolutionary radiation of eukaryotic plankton and enhanced organic burial, which in turn indicate a spike in oxygen production during this interval. Further positive carbon isotope excursions occurred during the Cryogenian. Although several negative carbon isotope excursions, associated with warming events, are known from the Late Tonian all the way up to the Proterozoic-Phanerozoic boundary, the carbon isotope record nonetheless maintains a noticeable positive trend throughout the Neoproterozoic.

Nitrogen isotopes δ15N data from 750 to 580 million year-old marine sediments hailing from four different Neoproterozoic basins show similar nitrogen isotope ratios to modern oceans, with a mode of +4% and a range from -4% to +11%. No significant change is observed across the Cryogenian-Ediacaran boundary, implying that oxygen was already ubiquitous in the global ocean as early as 750 Mya, during the Tonian period.

Sulfur isotopes Seawater sulfate δ34S values, which saw a gradual increase over most of the Neoproterozoic punctuated by major drops during glaciations, show a significant positive excursion during the Ediacaran, with a corresponding decrease in pyritic δ34S. High fractionation rates between sulfate and sulfide indicate an increase in the availability of sulfate in the water column, which in turn is indicative of increased reaction of pyrite with oxygen. In addition, genetic evidence points to the occurrence of a radiation of non-photosynthetic sulfide-reducing bacteria during the Neoproterozoic. Through bacterial sulfur disproportionation, such bacteria further deplete marine sulfide of heavier sulfur isotopes. Because such bacteria require significant amounts of oxygen to survive, an oxygenation event during the Neoproterozoic raising oxygen concentrations to over 5-18% of modern levels is believed to have been a necessary prerequisite for the diversification of these microorganisms.

Strontium isotopes δ13C can reliably indicate changes in net primary productivity and oxygenation if the rates of weathering into the oceans and carbon dioxide outgassing remain constant or increase, since a decrease in either of these could cause a positive δ13C excursion through continued preferential biological consumption of carbon-12 by existing communities while the supply of available carbon decreased, without indicating an increase in primary productivity and oxygen production. The ratio of strontium-87 to strontium-86 is used as a determinant of the relative contribution of continental weathering to the ocean's nutrient supply; an increase in this ratio, as observed throughout the Neoproterozoic and into the Cambrian until reaching a peak at the end of the Cambrian, suggests a rise in continental weathering and bolsters evidence from carbon isotope ratios for high oxygenation in this interval of time.

Chromium isotopes Surface oxidation of Cr(III) to Cr(VI) causes isotopic fractionation of chromium; Cr(VI), typically present in the environment as either chromate or dichromate, has elevated values of δ53Cr, or the ratio of chromium-53 to chromium-52, whereas bacterial reduction of Cr(VI) to Cr(III) is associated with negative chromium isotope excursions. Following the riverine transport of oxidised chromium into the ocean, the reaction reducing Cr(VI) back into Cr(III) and subsequently oxidising ferrous iron into ferric iron is highly efficient at sequestering Cr(VI), as is the precipitation of Cr(III) with ferric oxyhydroxide, meaning that chemically precipitated chromium isotope ratios in sediments abundant in ferric iron accurately reflect seawater chromium isotope ratios at the time of deposition. Because efficient oxidation of Cr(III) to Cr(VI) is only possible in the presence of the catalyst manganese dioxide, which is only stable and abundant at high oxygen fugacities, a positive excursion of δ53Cr indicates an increase in atmospheric oxygen concentrations. Banded iron formations (BIFs) deposited during the Neoproterozoic consistently display highly positive δ53Cr values, from 0.9% to 4.9%, demonstrating the era's oxygenation of the atmosphere. Oxidative chromium cycling began approximately 0.8 Ga, indicating that oxygen level rise began well before the Cryogenian glaciations. Chromium isotopes also show that during the Cryogenian interglacial interval, between the Sturtian and Marinoan glaciations, oxygenation of the ocean and atmosphere was slow and subdued; this interval marked a lull in the NOE.

Molybdenum isotopes δ98Mo values were slightly higher during the Late Ediacaran than in the Cryogenian or the Early and Middle Ediacaran. This isotopic proxy indicates the level of oxygenation of the Late Ediacaran ocean was comparable to that of Mesozoic oceanic anoxic events.

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