Evaluating the Relationship Between the Carbon and Sulfur Cycles in the Later Cambrian Ocean: An Example from the Port au Port Group, Western Newfoundland, Canada

We present a high-resolution δ34S (sulfate and pyrite) and δ13Ccarbonate record from the Middle–Upper Cambrian Port au Port Group, a mixed carbonate-siliciclastic succession exposed in western Newfoundland, Canada. The results illustrate systematic δ34Ssulfate shifts of > 15‰ over relatively short stratigraphic intervals (10 m, likely 15‰ δ34Ssulfate shifts recognized in Port au Port strata. Such large shifts can be generated only if fluctuating oceanic redox is invoked; marine anoxia forces reduced C/S burial and elevated Δ34S, driving larger δ34S changes per mole of organic carbon buried. The conclusion that later Cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late Early Cambrian extinction.Organismic and Evolutionary Biolog

δ44/40\delta^{44/40}Ca-δ88/86\delta^{88/86}Sr multi-proxy constrains primary origin of Marinoan cap carbonates

The Neoproterozoic Earth experienced at least two global-scale glaciations termed Snowball Earth events. 'Cap carbonates' were widely deposited after the events, but controversy surrounds their origin. Here, we apply the novel $\delta^{44/40}$Ca-$\delta^{88/86}$Sr multi-proxy to two Marinoan (ca. 635 Ma) cap carbonate sequences from Namibia and show that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone $\delta^{44/40}$Ca and $\delta^{88/86}$Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates correlate with elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show the greatest Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects in the rock record provides a basis for developing the $\delta^{44/40}$Ca-$\delta^{88/86}$Sr multi-proxy as an indicator of saturation state and $p$CO$_2$

Risk of lung cancer mortality in nuclear workers from internal exposure to alpha particle-emitting radionuclides

BACKGROUND: Carcinogenic risks of internal exposures to alpha-emitters (except radon) are poorly understood. Since exposure to alpha particles-particularly through inhalation-occurs in a range of settings, understanding consequent risks is a public health priority. We aimed to quantify dose-response relationships between lung dose from alpha-emitters and lung cancer in nuclear workers. METHODS: We conducted a case-control study, nested within Belgian, French, and UK cohorts of uranium and plutonium workers. Cases were workers who died from lung cancer; one to three controls were matched to each. Lung doses from alpha-emitters were assessed using bioassay data. We estimated excess odds ratio (OR) of lung cancer per gray (Gy) of lung dose. RESULTS: The study comprised 553 cases and 1,333 controls. Median positive total alpha lung dose was 2.42 mGy (mean: 8.13 mGy; maximum: 316 mGy); for plutonium the median was 1.27 mGy and for uranium 2.17 mGy. Excess OR/Gy (90% confidence interval)-adjusted for external radiation, socioeconomic status, and smoking-was 11 (2.6, 24) for total alpha dose, 50 (17, 106) for plutonium, and 5.3 (-1.9, 18) for uranium. CONCLUSIONS: We found strong evidence for associations between low doses from alpha-emitters and lung cancer risk. The excess OR/Gy was greater for plutonium than uranium, though confidence intervals overlap. Risk estimates were similar to those estimated previously in plutonium workers, and in uranium miners exposed to radon and its progeny. Expressed as risk/equivalent dose in sieverts (Sv), our estimates are somewhat larger than but consistent with those for atomic bomb survivors.See video abstract at, http://links.lww.com/EDE/B232

Neoproterozoic sulfur isotopes, the evolution of microbial sulfur species, and the burial efficiency of sulfide as sedimentary pyrite

International audienceSignificant variability in ?34Spyrite values in Neoproterozoic sedimentary rocks has been attributed to the evolution of nonphotosynthetic sulfide-oxidizing bacteria and the advent of sulfur disproportionation reactions in response to Earth's evolving redox chemistry. We analyzed trace sulfate in carbonates from South Australia and Namibia and reconstructed the sulfur isotope evolution of seawater sulfate. Comparison of our ?34Ssulfate record with published ?34Spyrite data from the same or equivalent successions indicates that ?34Ssulfate - ?34Spyrite (?34S) rose gradually through the second half of the Neoproterozoic and fluctuated coincident with episodes of glaciation, but did not exceed 46? before ca. 580 Ma. Large variability in ?34Spyrite in the Neoproterozoic can be explained as a consequence of low sulfate concentrations and rapidly fluctuating ?34Ssulfate in seawater rather than the onset of sulfur disproportionation reactions mediated by nonphotosynthetic sulfide-oxidizing bacteria

Sulfur isotope evidence for low and fluctuating sulfate levels in the Late Devonian ocean and the potential link with the mass extinction event

High amplitude positive carbon isotope excursions in the Late Devonian, the punctata and Kellwasser events, reflect major perturbations in the global carbon cycle that have been attributed to increased continental weathering and subsequent ocean eutrophication. Despite the comparable carbon isotope anomalies, however, a major extinction has been reported only for the Kellwasser Events, while the punctata Event is marked by low extinction intensity. This study presents multiple sulfur isotope records of carbonate-associated sulfate (CAS) and pyrite from Late Devonian sections in the Great Basin, USA, in order to document changes in the coupled (or decoupled) geochemical cycles of carbon and sulfur during the punctata and Upper Kellwasser events. A positive sulfur isotope shift in both CAS and pyrite accompanies the onset of the punctata Event, but to a larger extent in the latter. As a result, the sulfur isotope offset between CAS and pyrite (Δ^(34)S_(CAS-py)) dropped to less than 10‰. In the middle of the punctata Event, a sharp negative δ^(34)S_(CAS) excursion and negative Δ^(34)S_(CAS-py) values coincide with the Alamo impact. Unlike the rapid δ^(34)S_(py) and δ^(34)S_(CAS) oscillations associated with the punctata Event, the Upper Kellwasser was a period of relative stability, except for a brief δ^(34)S_(CAS) drop before the event. Paired sulfur isotope data, aided by a simple box model, suggest that the geochemical cycle of sulfur may have been partly responsible for the contrasting biological responses that define these events. High stratigraphic δ^(34)S_(py) and δ^(34)S_(CAS) variability, coupled with strong reservoir effect, demonstrates a relatively small oceanic sulfate pool existed during the punctata Event. Further, the Alamo impact likely triggered the rapid oxidation of microbially-produced sulfide within this event. The expansion of sulfidic bottom water thus may have been impeded during the punctata Event. In contrast, the lack of a positive shift in δ^(34)S_(CAS) and sizable Δ^(34)S_(CAS-py) values (>15‰) throughout the Upper Kellwasser Event imply higher relative sulfate levels. A larger seawater sulfate reservoir may have promoted the development of sulfidic bottom waters in the eutrophic epicontinental seas, increasing biological stress and potentially contributing to the mass extinction

Sulfur cycling in the aftermath of a 635-Ma snowball glaciation: Evidence for a syn-glacial sulfidic deep ocean

International audienceWe have analyzed ?34S (sulfate and pyrite), ?18Ocarbonate and ?13Ccarbonate, and major and trace elemental concentrations, including extractable Fe and Mn phases, in four sections of the Maieberg Formation, the cap-carbonate sequence to the Marinoan glaciation in northern Namibia. ?34Ssulfate profiles and other geochemical characteristics in the basal, transgressive cap dolostone (Keilberg Member) are nearly identical in all sections and indicate deposition from a water mass with very low sulfate concentrations. In the overlying interval that consists of rhythmites deposited during the transgressive high-stand, large geochemical disparities occur between sections that we interpret to have been deposited in open-ocean versus restricted settings. In the former, a large negative shift in ?34Ssulfate of ? 20? above the cap dolostone accompanies a change in mineralogy from dolomite to limestone, a sharp decrease in ?18O and ?13C, and a positive spike in Fe and Mn concentrations. In the latter, dolomite persists above the cap dolostone, ?34Ssulfate increases abruptly, ?18O and ?13C are invariant, and a spike in Mn and Fe concentrations is present, but subtle. These contrasting geochemical signatures in coeval sections can be explained by strong lateral chemical gradients that developed as cold, euxinic deep water and a cap of warm, oxic, brackish water that flooded the continental shelf during the post-glacial transgression. The geochemical differences between open-ocean and restricted sections diminishes upsection, presumably recording the gradual mixing and homogenization of the waters along the Otavi platform

Tracking hydrological processes during the formation of Marinoan cap dolostone; insight from reordered clumped isotopes

Widespread dolomite precipitates, known as cap dolostone, were deposited in the aftermath of the Neoproterozoic Marinoan glaciation, and are thought to reflect dramatic perturbations of seawater chemistry in the aftermath of the glaciation. However, controversy still remains on the nature and the hydrodynamic process of fluids that produced these dolomites. In this study, we present clumped isotope values (Delta (sub 47) ) of Neoproterozoic Marinoan cap carbonates from five sections in Namibia and South China and examine the spatial variation of the post-glacial hydrodynamics across the two carbonate platforms. The Delta (sub 47) -temperatures show the mineral-specific variation in dolostone and limestone, a feature that can be attributed to the solid-state reordering driven by short-term hydrothermal events. Based on the results of theoretical calculations and laboratory experiments, we present a framework that enables the utilization of the reordered clumped isotopes to trace the dolomitization process. Under such a framework, we find the reordering patterns determined by clumped isotopes are indistinguishable from the inner to outer platforms as well as between the dolostone and limestone, which suggests the mixture of meltwater and seawater as the source of dolomitizing fluids. Our findings demonstrate the clumped isotope can be used to trace hydrological processes for deep-time successions, and offer insight into the enigmatic origin of cap carbonates

Ca isotope stratigraphy across the Cenomanian–Turonian OAE 2: Links between volcanism, seawater geochemistry, and the carbonate fractionation factor

The Ca isotope composition of marine carbonate rocks offers potential to reconstruct drivers of environmental change in the geologic past. This study reports new, high-precision Ca isotope records (View the MathML source; 2σSD=±0.04‰) for three sections spanning a major perturbation to the Cretaceous ocean-climate system known as Ocean Anoxic Event 2 (OAE 2): central Colorado, USA (Portland #1 core), southeastern France (Pont d'Issole), and Hokkaido, Japan (Oyubari, Yezo Group). In addition, we generated new data for selected samples from Eastbourne, England (English Chalk), where a previous Ca isotope study was completed using different methodology (Blättler et al., 2011). Strata of the Yezo Group contain little carbonate (∼1 wt.% on average) and accordingly did not yield a clear View the MathML source signal. The Portland core and the Pont d'Issole section display comparable View the MathML source values, which increase by ∼0.10–0.15‰ at the onset of OAE 2 and then decrease to near-initial values across the event. The Eastbourne View the MathML source values are higher than previously reported. They are also higher than the View the MathML source values for the Portland core and the Pont d'Issole section but define a similar pattern. According to a numerical model of the marine Ca cycle, elevated hydrothermal inputs have little impact on seawater View the MathML source values. Elevated riverine (chemical weathering) inputs produce a transient negative isotope excursion, which significantly differs from the positive isotope excursions observed in the Portland, Pont d'Issole, and Eastbourne records. A decrease in the magnitude of the carbonate fractionation factor provides the best explanation for a positive shift in View the MathML source values, especially given the rapid nature of the excursion. Because a decrease in the fractionation factor corresponds to an increase in the Ca/CO3 ratio of seawater, we tentatively attribute the positive Ca isotope excursion to transient ocean acidification, i.e., a reduction in the concentration of View the MathML source during CO2 uptake. Recent studies utilizing a variety of isotope proxies, e.g., Nd, Os, and Pb, implicate eruption of the Caribbean Large Igneous Province as a likely source of increased CO2. Moreover, integration of C, Ca, and Os isotope data reveals new information about the timing of events during the onset of OAE 2