Ocean redox structure across the Late Neoproterozoic Oxygenation Event: A nitrogen isotope perspective

International audienceThe end of the Neoproterozoic Era (1000 to 541 Ma) is widely believed to have seen the transition from a dominantly anoxic to an oxygenated deep ocean. This purported redox transition appears to be closely linked temporally with metazoan radiation and extraordinary perturbations to the global carbon cycle. However, the geochemical record of this transition is not straightforward, and individual data sets have been variably interpreted to indicate full oxygenation by the early Ediacaran Period (635 to 541 Ma) and deep ocean anoxia persevering as late as the early Cambrian. Because any change in marine redox structure would have profoundly impacted nitrogen nutrient cycling in the global ocean, the N isotope signature of sedimentary rocks (δ15Nsed) should reflect the Neoproterozoic deep-ocean redox transition. We present new N isotope data from Amazonia, northwest Canada, northeast Svalbard, and South China that span the Cryogenian glaciations (∼750 to 580 Ma). These and previously published data reveal a Nisotope distribution that closely resembles modern marine sediments, with a mode in δ15N close to +4 and range from −4 to +11. No apparent change is seen between the Cryogenian and Ediacarian. Data from earlier Proterozoic samples show a similar distribution, but shifted slightly towards more negative δ15N values and with a wider range. The most parsimonious explanation for the similarity of these Nisotopedistribution is that as in the modern ocean, nitrate (and hence O2) was stable in most of the middle–late Neoproterozoic ocean, and possibly much of Proterozoic Eon. However, nitrate would likely have been depleted in partially restricted basins and oxygen minimum zones (OMZs), which may have been more widespread than in the modern ocean

Spontaneous restoration of functional β-cell mass in obese SM/J mice

Maintenance of functional β-cell mass is critical to preventing diabetes, but the physiological mechanisms that cause β-cell populations to thrive or fail in the context of obesity are unknown. High fat-fed SM/J mice spontaneously transition from hyperglycemic-obese to normoglycemic-obese with age, providing a unique opportunity to study β-cell adaptation. Here, we characterize insulin homeostasis, islet morphology, and β-cell function during SM/J\u27s diabetic remission. As they resolve hyperglycemia, obese SM/J mice dramatically increase circulating and pancreatic insulin levels while improving insulin sensitivity. Immunostaining of pancreatic sections reveals that obese SM/J mice selectively increase β-cell mass but not α-cell mass. Obese SM/J mice do not show elevated β-cell mitotic index, but rather elevated α-cell mitotic index. Functional assessment of isolated islets reveals that obese SM/J mice increase glucose-stimulated insulin secretion, decrease basal insulin secretion, and increase islet insulin content. These results establish that β-cell mass expansion and improved β-cell function underlie the resolution of hyperglycemia, indicating that obese SM/J mice are a valuable tool for exploring how functional β-cell mass can be recovered in the context of obesity

The geochemistry of zinc and copper stable isotopes in marine hydrothermal brine pools: Perspectives from metalliferous sediments of the Atlantis II Deep, Red Sea

Highlights • Zinc and copper isotope characterization of metalliferous sediments formed by brine pool deposition in the Atlantis II Deep. • Enrichments of light zinc and copper isotopes in sediments when compared to the hydrothermal inputs to the basin. • Isotopic data are consistent with metal sulfide precipitation and lesser adsorption of metals onto Si-Fe-OOH particles. • Increase of δ66Zn with increasing distance from hydrothermal metal release due to uptake of light isotopes by metal sulfides. • Zinc isotope patterns due to metal sulfide precipitation could be of interest in mineral exploration. Abstract This study provides a zinc and copper stable isotope characterization (δ66Zn, δ68Zn, and δ65Cu) of metalliferous seafloor sediments from the Atlantis II Deep, a hydrothermally influenced brine basin in the Red Sea. Samples collected from box cores that capture the entire stratigraphy in the Deep have δ66Zn and δ65Cu values of −0.31 to 0.34 ‰ (0.02 ‰ median) and − 1.81 to 1.02 ‰ (−0.34 ‰ median) relative to the JMC-Lyon and NIST SRM 976 standards, respectively. These results suggest enrichments of light stable isotopes in sediments compared to the hydrothermal inputs to the basin, which likely overlap the mantle-like isotopic signatures of basalts beneath the Deep. Such shifts to lower δ66Zn and δ65Cu values are consistent with widespread metal sulfide deposition from the brines because sulfide anions preferentially consume the light stable isotopes of zinc and copper. However, this interpretation contrasts with observations in the open ocean, where the fractionation of zinc and copper stable isotopes is strongly influenced by biological utilization and organic matter. Previous studies proposed that metal deposition in the Atlantis II Deep is also driven by adsorption onto iron oxides/hydroxides and their weakly crystalline (Si-)Fe-OOH precursor phases within the brines. However, because this process should accumulate heavy zinc and copper stable isotopes, its influence on isotopic fractionation is likely limited. Controls by metal sulfide precipitation are also indicated by spatial covariations between δ66Zn and concentrations of zinc and copper, that is, δ66Zn values increase whereas metal contents decrease with distance away from hydrothermal venting. Comparable trends are lacking for copper isotopes, perhaps because of additional influence by redox processes or, compared to zinc, a much stronger influence by adsorption onto (Si-)Fe-OOH phases, particularly in areas distal to hydrothermal venting where reduced sulfur could be scarce. Collectively, our results from the Atlantis II Deep indicate that zinc and copper stable isotopes could provide information about base and precious metals deposits from similar paleoenvironments. Firstly, zinc and copper stable isotopes shed light on metal sourcing and accumulation processes. Secondly, mineral precipitation in hydrothermally influenced brine pools produces zinc stable isotope patterns that, at least theoretically, could be of interest in mineral exploration at sub-basin and deposit scales

Zn isotope evidence for immediate resumption of primary productivity after snowball Earth

The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during postglacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fl uctuations in primary productivity. Here we report the fi rst Zn isotopic data, together with carbon and oxygen isotopic profi les from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly 66Zn-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove δ66Zn (66Zn/64Zn, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in δ66Zn records a vigorous increase in primary production and export from a nutrient-laden surface ocean

Zn isotope evidence for immediate resumption of primary productivity after snowball Earth

The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during post-glacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity. Here we report the first Zn isotopic data, together with carbon and oxygen isotopic profiles from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly ^(66)Zn-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove δ^(66)Zn (^(66)Zn/^(64)Zn, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in δ^(66)Zn records a vigorous increase in primary production and export from a nutrient-laden surface ocean

The early Neoproterozoic Chandindu Formation of the Fifteenmile Group in the Ogilvie Mountains

Annual Report Yukon Exploration and Geology 2013Studies of biogeochemical and evolutionary change in the Neoproterozoic require a detailed understanding of stratigraphic successions and their intrabasinal correlation to integrate those records into regional and global frameworks. The early Neoproterozoic Fifteenmile Group in the Ogilvie Mountains has previously been shown to archive important information on the evolution of the biosphere, including ocean redox and early evolution of eukaryotes. Here, we formally define the Chandindu Formation, a 150 to 420-m-thick siltstone-dominated mixed carbonate-siliciclastic succession of the lower Fifteenmile Group in the Coal Creek and Hart River inliers. We present ten sections of the Chandindu Formation and propose a type section and formalization to promote the development of a consistent stratigraphic framework for Proterozoic successions in northwest Canada. The Chandindu Formation begins with muddy tidal flat facies, which are succeeded by shale-siltstonesandstone coarsening-upward cycles deposited in a predominantly subtidal environment. However, carbonate occurrences throughout the entire unit suggest localized carbonate buildups, likely nucleated on fault-bound paleohighs where siliciclastic background sedimentation was low. These paleohighs originated from rift-inherited complex basin topography and syn-depositional faulting during deposition of the upper Chandindu Formation

Studies of the contribution of alternating electromagnetic fields toward material fatigue in 100Cr6

Purpose - The purpose of this paper is to show how controlled exposure of electromagnetic fields toward bearing steel vulnerates the microstructure. The ability of Barkhausen Noise signal processing is used for detecting phenomena Such as dislocation and subgrain formation processes as the beginning of later failures. Design/methodology/approach - A Barkhausen noise signal measurement equipnient is used for detecting subsurface distress of 100Cr6 as a function of the applied electromagnetic and mechanical stress. Barkhausen noise signal is mathematically processed by use of fractal dimension analysis. Findings - The paper cleary reveals significant impact of electromagnetic field in junction with mechanical loading. Electromagnetic impact depends on the magnitude of the field. Research limitations/implications - Research limitations are given by the fact that in real field applications, e.g. wind power plants, bearings are exposed by multiple influences and the methodology is not applicable to those conditions. Practical implications - The methodology can be applied to real field applications in condition monitoring systems. Up to now, no reasonable on-line measurement is in use determining sub surface fatigue phenomena. The paper hence, reveals the possibility to raise condition monitoring into a new perspective. Originality/value - The use of Barkhausen noise signal processing, as presented here, is original with respect to real field applications, such as wind power plants with a high demand in condition monitoring, especially off-shore plants