Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post–Snowball Earth hothouse

Obscuring Complexity and Performing Progress: Unpacking SDG Indicator 6.5.1 and the Implementation of IWRM

At a rhetorical level, the SDGs provide a unified global agenda, and their targets and indicators are believed to drive action for social and environmental transformation. However, what if the SDGs (and their specific goals and indicators) are more of a problem than a solution? What if they create the illusion of action through a depoliticised and technical approach that fails to address fundamental dilemmas of politics and power? What if this illusion continues to reproduce poverty, inequality, and environmental degradation? This paper addresses these questions through a focus on SDG 6.5.1 – the implementation of integrated water resources management (IWRM), measured on a 0-100 scale through a composite indicator. The paper presents an empirical analysis of SDG 6.5.1 reporting in Colombia, Ethiopia, India, Malaysia, and the UK, drawing on research from the Water Security and Sustainable Development Hub. An evidence review and series of expert interviews are used to interrogate the local politics of IWRM measurement, specifically three dilemmas of global composite indicator construction: (1) reductive quantification of normative and contested processes; (2) weak analysis of actually existing institutional capability, politics, and power; and (3) distracting performativity dynamics in reporting. The paper concludes that SDG 6.5.1 is an example of a 'fantasy artefact', and that in all countries in this study, IWRM institutions are failing to address fundamental and 'wicked' problems in water resources management. We find little evidence that these numbers, or the survey that gives rise to them, drive meaningful reflection on the aims or outcomes of IWRM. Instead, they tend to hide the actually-existing political and institutional dynamics that sit behind the complexity of the global water crisis

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

Re-Os geochronology of the Neoproterozoic-Cambrian Dalradian Supergroup of Scotland and Ireland: Implications for Neoproterozoic stratigraphy, glaciations and Re-Os systematics

New Re–Os geochronology for the Ballachulish Slate Formation of the Dalradian Supergroup, Scotland yields a depositional age of 659.6 ± 9.6 Ma. This age represents the first successful application of the Re–Os system to rocks that have extremely low Re and Os abundances (<1 ppb and <50 ppt, respectively). The Re–Os age represents a maximum age for the glaciogenic Port Askaig Formation and refutes previous chemostratigraphic and lithostratigraphic studies which correlated the Port Askaig Formation with a series of middle Cryogenian (ca. 715 Ma) glacials. Additionally, the Re–Os age strongly suggests that the Port Askaig Formation may be correlative with the 650 Ma end-Sturtian glaciations of Australia. As a consequence, the correlation of the Ballachulish Limestone Formation with the ca. 800 Ma Bitter Springs anomaly is not tenable. Initial Os isotope data from the Ballachulish Slate Formation coupled with data from Australia reveals a radiogenic 187Os/188Os isotope composition (0.8–1.0) for seawater during the Neoproterozoic, which is similar to that of modern seawater (1.06). We also report a young, highly imprecise Re–Os age (310 ± 110 Ma) for the Early Cambrian Leny Limestone Formation which is constrained biostratigraphically by a polymerid and miomerid trilobite fauna. We suggest, based on the mineralogy of the Leny Limestone, (kaolinite, muscovite and a serpentine group mineral, berthierine), that the Re–Os systematics have been disturbed by post-depositional fluid flow associated with Palaeozoic igneous intrusions. However, it is evident from the Ballachulish Slate Formation results that anhydrous metamorphism does not disturb the Re–Os geochronometer

Anoxia in the terrestrial environment during the Late Mesoproterozoic

A significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the Precambrian. In contrast, the oxygenation history of terrestrial aquatic environments has received little attention, despite the significance of such settings for early eukaryote evolution. To address this, we provide here a geochemical and isotopic assessment of sediments from the late Mesoproterozoic Nonesuch Formation of central North America. We utilize rhenium-osmium (Re-Os) geochronology to yield a depositional age of 1078 ± 24 Ma, while Os isotope compositions support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich (ferruginous) water column. Thus, similar to the marine realm, anoxia persisted in terrestrial aquatic environments in the Middle to Late Proterozoic, but sulfidic water column conditions were not ubiquitous. Our data suggest that oxygenation of the terrestrial realm was not pervasive at this time and may not have preceded oxygenation of the marine environment, signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution

Re–Os geochronology of a Mesoproterozoic sedimentary succession, Taoudeni basin, Mauritania: Implications for basin-wide correlations and Re–Os organic-rich sediments systematics

The exceptionally well-preserved sedimentary rocks of the Taoudeni basin, NW Africa represent one of the world's most widespread (> 1 M km2) Proterozoic successions. Hitherto, the sedimentary rocks were considered to be Mid Tonian based on Rb–Sr illite and glauconite geochronology of the Atar Group. However, new Re–Os organic-rich sediment (ORS) geochronology from two drill cores indicates that the Proterozoic Atar Group is 200 Ma older (1107 ± 12 Ma, 1109 ± 22 Ma and 1105 ± 37 Ma). The Re–Os geochronology suggests that the Rb–Sr geochronology records the age of diagenetic events possibly associated with the Pan African collision. The new Re–Os geochronology data provide absolute age constraints for recent carbon isotope chemostratigraphy which suggests that the Atar Group is Mesoproterozoic and not Neoproterozoic. The new Re–Os ORS geochronology supports previous studies that suggest that rapid hydrocarbon generation (flash pyrolysis) from contact metamorphism of a dolerite sill does not significantly disturb the Re–Os ORS systematics. Modelled contact conditions suggest that the Re–Os ORS systematics remain undisturbed at 650 °C at the sill/shale contact and ≥ 280 °C 20 m from the sill/shale contact. Moreover, the Re–Os geochronology indicates that the West African craton has a depositional history that predates 1100 Ma and that ORS can be correlated on a basin-wide scale. In addition, the Re–Os depositional ages for the ORS of the Taoudeni basin are comparable to those of ORS from the São Francisco craton, suggesting that these cratons are correlatable. This postulate is further supported by identical Osi values for the Atar Group and the Vazante Group of the São Francisco craton

Monte Carlo sampling for error propagation in linear regression and applications in isochron geochronology

Geochronology is essential for understanding Earth’s history. The availability of precise and accurate isotopic data is increasing; hence it is crucial to develop transparent and accessible data reduction techniques and tools to transform raw mass spectrometry data into robust chronological data. Here we present a Monte Carlo sampling approach to fully propagate uncertainties from linear regressions for isochron dating. Our new approach makes no prior assumption about the causes of variability in the derived chronological results and propagates uncertainties from both experimental measurements (analytical uncertainties) and underlying assumptions (model uncertainties) into the final age determination. Using synthetic examples, we find that although the estimates of the slope and y-intercept (hence age and initial isotopic ratios) are comparable between the Monte Carlo method and the benchmark “Isoplot” algorithm, uncertainties from the later could be underestimated by up to 60%, which are likely due to an incomplete propagation of model uncertainties. An additional advantage of the new method is its ability to integrate with geological information to yield refined chronological constraints. The new method presented here is specifically designed to fully propagate errors in linear regressions especially in geochronological applications involves linear regressions such as Rb-Sr, Sm-Nd, Re-Os, Pt-Os, Lu-Hf, U-Pb (with discordant points), Pb-Pb and Ar-Ar

First evidence for radical anions in metathesis catalysis

The Grubbs’ catalyst, (PCy3)2RuCl2([double bond, length half m-dash]CHPh), generates persistent radical anions on treatment with π-acceptors such as p-benzoquinones and a remarkably wide range of dienes and even simple alkenes

Neoproterozoic Re–Os systematics of organic-rich rocks in the São Francisco Basin, Brazil and implications for hydrocarbon exploration

The São Francisco Basin contains a remarkable archive of Neoproterozoic strata and its hydrocarbon-bearing strata are receiving increasing attention as global oil and gas exploration targets progressively deeper and older rocks. New Re–Os geochronology for the Paracatu Slate Formation of the Canastra Group, Brazil, yields a depositional age of 1002 ± 45 Ma. This age represents the first successful application of the Re–Os system to rocks of this group and indicates excellent agreement with a previously published U–Pb detrital zircon age (Rodrigues et al., 2010). Together with TOC values of ca. 2 wt.% (despite greenschist metamorphism), it might be argued that the São Francisco Basin has had the potential for hydrocarbon generation since the Tonian (1000–850 Ma). In addition, we also report an imprecise Re–Os age (1304 ± 210 Ma) for the Serra do Garrote Formation, a further potential source rock of the Vazante Group. We suggest, based on petrological evidence, that the Re–Os systematics were disturbed by post-depositional fluid flow that was most likely associated with Vazante ore deposit mineralization. An attempt to determine a Re–Os date for the Sete Lagoas Formation, a putative post-Sturtian cap carbonate, is precluded owing to low Re abundances (≤100 ppt). Major environmental changes in the aftermath of the Jequitaí glaciation, particularly the development of palaeotopography such as subglacial tunnel valleys, may account for the apparent random distribution of TOC enrichment in these Cryogenian/Ediacaran post-glacial deposits. This scenario might thus have major implications for the hydrocarbon prospectivity of this post-glacial succession