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.
How much dissolved oxygen was present in the mid-Proterozoic oceans between 1.8 and 1.0 billion years ago is debated vigorously. One model argues for oxygenation of the oceans soon after the initial ...rise of atmospheric oxygen approximately 2.3 billion years ago. Recent evidence for H(2)S in some mid-Proterozoic marine basins suggests, however, that the deep ocean remained anoxic until much later. New molybdenum isotope data from modern and ancient sediments indicate expanded anoxia during the mid-Proterozoic compared to the present-day ocean. Consequently, oxygenation of the deep oceans may have lagged that of the atmosphere by over a billion years.
The ocean‐atmosphere system is typically envisioned to have gone through a unidirectional oxygenation with significant oxygen increases in the earliest (ca. 635 Ma), middle (ca. 580 Ma), or late (ca. ...560 Ma) Ediacaran Period. However, temporally discontinuous geochemical data and the patchy metazoan fossil record have been inadequate to chart the details of Ediacaran ocean oxygenation, raising fundamental debates about the timing of ocean oxygenation, its purported unidirectional rise, and its causal relationship, if any, with the evolution of early animal life. To better understand the Ediacaran ocean redox evolution, we have conducted a multi‐proxy paleoredox study of a relatively continuous, deep‐water section in South China that was paleogeographically connected with the open ocean. Iron speciation and pyrite morphology indicate locally euxinic (anoxic and sulfidic) environments throughout the Ediacaran in this section. In the same rocks, redox sensitive element enrichments and sulfur isotope data provide evidence for multiple oceanic oxygenation events (OOEs) in a predominantly anoxic global Ediacaran–early Cambrian ocean. This dynamic redox landscape contrasts with a recent view of a redox‐static Ediacaran ocean without significant change in oxygen content. The duration of the Ediacaran OOEs may be comparable to those of the oceanic anoxic events (OAEs) in otherwise well‐oxygenated Phanerozoic oceans. Anoxic events caused mass extinctions followed by fast recovery in biologically diversified Phanerozoic oceans. In contrast, oxygenation events in otherwise ecologically monotonous anoxic Ediacaran–early Cambrian oceans may have stimulated biotic innovations followed by prolonged evolutionary stasis.
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•Electrodeposition of cobalt is significantly suppressed by cyclohexane dioxime.•Adsorbed intermediate chelate species were proposed to be involved in the suppression.•Diffusion of ...chelates in electrolyte is in equilibrium with the consumption of the adsorbate.•The presence of a pair of adjacent oxime groups is necessary for a strong and fast suppression.•Bulky molecular structure and strong coordination with Co2+ are beneficial for suppression.
Electrodeposition of cobalt was studied in presence of two additives, cyclohexane oxime (CHO) and cyclohexane dioxime (CHD), which differ by one oxime group. Cyclic voltammetry (CV), potentiostatic deposition with pulsed rotation rates, and galvanostatic deposition with injections of additives were carried out to understand the formation and breakdown of intermediate species that suppress cobalt electrodeposition. While strong and fast suppression was observed for CHD on cobalt deposition, the suppression effect of CHO was much less pronounced. A negative differential resistance in CV was observed for CHD due to a breakdown of suppression, which was related to the reduction of absorbed Co-CHD chelate species. A critical rotation rate was defined for CHD as the minimum rotation required to enable a fully suppressed deposition state. An inversely proportional relation between additive concentration and the square root of critical rotation rate demonstrated an equilibrium between the diffusion of the intermediate in solution and the consumption of intermediate by incorporation into deposit. While a pair of adjacent oxime groups was necessary for a strong fast suppression effect, bulkier molecular structures and stronger chelating resulted in stronger suppression.
Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to −12‰). The ...nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13Ccarb and δ13Corg) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34S of carbonate‐associated sulfate (CAS) and increased 87Sr/86Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13C presented in this study.
Major hydrocarbon accumulations occur in traps associated with salt domes. Whereas some of these hydrocarbons remain to be extracted for economic use, significant amounts have degraded in the ...subsurface, yielding mineral precipitates as byproducts. Salt domes of the Gulf of Mexico Basin typically exhibit extensive deposits of carbonate that form as cap rock atop salt structures. Despite previous efforts to model cap rock formation, the details of subsurface reactions (including the role of microorganisms) remain largely unknown. Here we show that cap rock mineral precipitation occurred via closed-system sulfate reduction, as indicated by new sulfur isotope data.
C-depleted carbonate carbon isotope compositions and low clumped isotope-derived carbonate formation temperatures indicate that microbial, sulfate-dependent, anaerobic oxidation of methane (AOM) contributed to carbonate formation. These findings suggest that AOM serves as an unrecognized methane sink that reduces methane emissions in salt dome settings perhaps associated with an extensive, deep subsurface biosphere.
Here, we present results from sediments collected in the Argentine Basin, a non‐steady state depositional marine system characterized by abundant oxidized iron within methane‐rich layers due to ...sediment reworking followed by rapid deposition. Our comprehensive inorganic data set shows that iron reduction in these sulfate and sulfide‐depleted sediments is best explained by a microbially mediated process—implicating anaerobic oxidation of methane coupled to iron reduction (Fe‐AOM) as the most likely major mechanism. Although important in many modern marine environments, iron‐driven AOM may not consume similar amounts of methane compared with sulfate‐dependent AOM. Nevertheless, it may have broad impact on the deep biosphere and dominate both iron and methane cycling in sulfate‐lean marine settings. Fe‐AOM might have been particularly relevant in the Archean ocean, >2.5 billion years ago, known for its production and accumulation of iron oxides (in iron formations) in a biosphere likely replete with methane but low in sulfate. Methane at that time was a critical greenhouse gas capable of sustaining a habitable climate under relatively low solar luminosity, and relationships to iron cycling may have impacted if not dominated methane loss from the biosphere.
The concept of the Great Oxidation Event (GOE), during which atmospheric oxygen rose precipitously and perhaps to near-modern levels around 2.4–2.1billionyears ago (Ga), has become entrenched in our ...views on secular atmospheric evolution. Multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the Archean–Proterozoic boundary. However, it remains unclear precisely when this transition occurred, due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures. Here, we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium (U) concentrations in iron formations (IF). Just as IF are important archives of the redox state of seawater, concentrations of redox-sensitive U in IF are faithful proxies for oxidative continental weathering and associated delivery of dissolved U to seawater. Our dataset suggests that there was an increase in U redox cycling and transport at ca. 2.47Ga, just before the permanent loss of mass-independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the GOE. Further, there is significant temporal variability in the IF U record that we propose reflects dynamic Precambrian redox conditions. We provide additional support for earlier suggestions that the GOE was a protracted event marked by vacillating oxygen levels.
•U in iron formations (IF) tracks oxidative U cycling.•There is an increase in the U content of IF at 2.47–2.43Ga.•We compare the IF record of seawater U with other proxies to examine GOE timing.•The GOE was a protracted process initiated near the Archean–Proterozoic boundary.
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