Experimental determination of Fe isotope fractionations in the diagenetic iron sulphide system

Initial published work suggested that Fe isotope fractionations recorded in sediments were a product of biological activity. Experiments and measurements of natural samples now indicate that Fe isotope fractionation can be the product of both biological and inorganic processes. Sedimentary iron sulphides provide unique information about the evolution of early life which developed under anoxic conditions. It is in these sedimentary Fe-S species and in particular in Archean and Proterozoic pyrites that the largest Fe isotope variations (up to a range of ~5‰ for δ56/54Fe) have been measured. Most research has focussed on potential processes responsible for the formation of a 56Fe depleted Fe(II) pool from which iron sulphides would precipitate without additional fractionation, recording the light Fe isotope composition of the pool. Much less attention has been given to the possibility that the iron sulphide forming mechanisms themselves could produce significant fractionations. The Fe-S system constitutes a diverse group of stable and metastable phases, the ultimate Fe sequestrating phase being pyrite. The aim of this study was to examine experimentally where Fe isotope fractionations occur during the abiotic formation of iron sulphides in order to assess whether or not the measured Fe isotope signatures in natural pyrite could be explained by chemical mechanisms only. Both analytical and experimental protocols were developed in order to determine the partition of Fe isotopes for each step towards diagenetic pyrite formation. 56/54Fe and 57/54Fe ratios were measured on an IsoProbe-P Micromass MC-ICP-MS, and all experiments were performed under oxygen-free N2 atmosphere. Supporting previously published data, the results indicate that the precipitation of the nanoparticulate iron(II) monosulphide mackinawite (FeSm) kinetically fractionates lighter isotopes with initial fractionations of Δ56FeFe(II)aq-FeS = 1.17 ± 0.16 ‰ at 25°C and Δ56FeFe(II)aq-FeS = 0.98 ± 0.16 ‰ at 2°C. The rate of isotopic exchange between Fe(II)aq and FeSm decreases as FeSm nanoparticles grow. Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which case Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation– growth mechanism. Because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. The equilibrium Fe isotope fractionation between Fe2+ aq and FeSm was determined using the three isotope method and is Δ56FeFe(II)-FeS = -0.33 ± 0.12 ‰ at 25°C and Δ56FeFe(II)-FeS = -0.52 ± 0.16 ‰ at 2°C. This suggests that at equilibrium, FeSm incorporates heavier isotopes with respect to Fe2+ aq, and the isotopic composition of most naturally occurring FeSm does not represent equilibrium. During pyrite formation, pyrite incorporates kinetically lighter isotopes with a fractionation Δ56FeFeS-pyrite ~ 2.2 ‰. Because pyrite is sparingly soluble in sedimentary environments, isotope exchange is prevented and pyrite does not equilibrate with its Fe(II) source. Combined fractionation factors between Fe2+ aq, mackinawite (FeSm) and pyrite permit the generation of pyrite with Fe isotope signatures that encapsulate the full range of sedimentary δ56Fepyrite recorded in both Archean and modern sediments. Archean Fe isotope excursions reflect various degrees of pyritisation, extent of Fe(II)aq utilisation, and variations in source composition rather than microbial dissimilatory Fe(III) reduction only. Our results show that sedimentary pyrite is not a passive recorder of the Fe isotope composition of the reactive Fe(II) reservoir forming pyrite. It is the formation process itself that influences pyrite Fe isotope signatures with consequent implications for the interpretation of sedimentary pyrite Fe isotope compositions throughout geological time

Simple performance evaluation of pulsed spontaneous parametric down-conversion sources for quantum communications

Fast and complete characterization of pulsed spontaneous parametric down conversion (SPDC) sources is important for applications in quantum information processing and communications. We propose a simple method to perform this task, which only requires measuring the counts on the two output channels and the coincidences between them, as well as modeling the filter used to reduce the source bandwidth. The proposed method is experimentally tested and used for a complete evaluation of SPDC sources (pair emission probability, total losses, and fidelity) of different bandwidths. This method can find applications in the setting up of SPDC sources and in the continuous verification of the quality of quantum communication links

Oxygen minimum zones in the early Cambrian ocean

The relationship between the evolution of early animal communities and oceanic oxygen levels remains unclear. In particular, uncertainty persists in reconstructions of redox conditions during the pivotal early Cambrian (541-510 million years ago, Ma), where conflicting datasets from deeper marine settings suggest either ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeoredox proxies with a record of organic-walled fossils from exceptionally well-defined successions of the early Cambrian Baltic Basin, we provide evidence for the early establishment of modern-type oxygen minimum zones (OMZs). Both inner-and outer-shelf environments were pervasively oxygenated, whereas mid-depth settings were characterised by spatially oscillating anoxia. As such, conflicting redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean transition.This work was funded by NERC (NE/K005251/1). SWP acknowledges support from a Royal Society Wolfson Research Merit Award

Surface charge and growth of sulphate and carbonate green rust in aqueous media

We report the first determination of the point of zero charge of sulphated and carbonated green rust particles. Green rust has been recognised as a prevalent mineral in environments such as hydromorphic soils, groundwaters and anoxic Fe(II)-rich water bodies, and the evolution of its net surface charge with pH has direct implications for the uptake of contaminants, metals and nutrients in such settings. We find that the surface of both sulphated and carbonated green rust is positively charged at pH 8.3. Thus, alkaline settings will promote enhanced adsorption of metallic cations. However, the behaviour of ionic species surrounding green rust is more complicated than that predicted by simple pH-dependent adsorption, as our experiments suggest that green rust likely grows via dissolution-reprecipitation during Ostwald-ripening. This implies that adsorbed species are potentially subject to repetitive steps of release into solution, re-adsorption and co-precipitation during particle growth. The growth rate of green rust particles is highest within the first 50. min of aging, and appears to decrease towards an asymptote after 200. min, suggesting that particle growth controls on the uptake of dissolved species will be most important during the early steps of green rust growth. Our findings thus contribute to a better understanding of the controls that green rust may exert on dissolved ions in a variety of anoxic environments

Triple iron isotope constraints on the role of ocean iron sinks in early atmospheric oxygenation

International audienceThe role that iron played in the oxygenation of Earth’s surface is equivocal. Iron could have consumed molecular oxygen when Fe3+-oxyhydroxides formed in the oceans, or it could have promoted atmospheric oxidation by means of pyrite burial. Through high-precision iron isotopic measurements of Archean-Paleoproterozoic sediments and laboratory grown pyrites, we show that the triple iron isotopic composition of Neoarchean-Paleoproterozoic pyrites requires both extensive marine iron oxidation and sulfide-limited pyritization. Using an isotopic fractionation model informed by these data, we constrain the relative sizes of sedimentary Fe3+-oxyhydroxide and pyrite sinks for Neoarchean marine iron. We show that pyrite burial could have resulted in molecular oxygen export exceeding local Fe2+ oxidation sinks, thereby contributing to early episodes of transient oxygenation of Archean surface environments

Fe isotope exchange between Fe(II)(aq) and nanoparticulate mackinawite (FeSm) during nanoparticle growth

We detail the results of an experimental study on the kinetics of Fe isotope exchange between aqueous Fe(II)aq and nanoparticulate mackinawite (FeSm) at 25 °C and 2 °C over a one month period. The rate of isotopic exchange decreases synchronously with the growth of FeSm nanoparticles. 100% isotopic exchange between bulk FeSm and the solution is never reached and the extent of isotope exchange asymptotes to a maximum of ~ 75%. We demonstrate that particle growth driven by Ostwald ripening would produce much faster isotopic exchange than observed and would be limited by the extent of dissolution–recrystallisation. We show that Fe isotope exchange kinetics are consistent with i) FeSm nanoparticles that have a core–shell structure, in which Fe isotope mobility is restricted to exchange between the surface shell and the solution and ii) a nanoparticle growth via an aggregation–growth mechanism. We argue that because of the structure of FeSm nanoparticles, the approach to isotopic equilibrium is kinetically restricted at low temperatures. FeSm is a reactive component in diagenetic pyrite forming systems since FeSm dissolves and reacts to form pyrite. Isotopic mobility and potential equilibration between FeSm and Fe(II)aq thus have direct implications for the ultimate Fe isotope signature recorded in sedimentary pyrite

A procedural development for the analysis of ^56/54Fe and ^57/54Fe isotope ratios with new generation IsoProbe MC-ICP-MS

We have developed a procedure for iron isotope analysis using a hexapole collision cell MC-ICP-MS which is capable of Fe isotope ratio analysis using two different extraction modes. Matrix effects were minimised and the signal-to-background ratio was maximised using high-concentration samples (~ 5μg Fe) and introducing 1.8 mL/min<sup>-1</sup> Ar and 2 mL/min H<sub>2</sub> into the collision cell to decrease polyatomic interferences. The use of large intensity on the faraday cups considerably decreases the internal error of the ratios and ultimately, improves the external precision of a run. Standard bracketing correction for mass bias was possible when using hard extraction. Mass bias in soft extraction mode seems to show temporal instability that makes the standard bracketing inappropriate. The hexapole rf amplitude was decreased to 50 % to further decrease polyatomic interferences and promote the transmission of iron range masses. We routinely measure Fe isotopes with a precision of ± 0.05 ‰ and ± 0.12 ‰ (2σ) for δ<sup>56</sup>Fe and δ<sup>57</sup>Fe respectively

Décomposition en valeurs singulières randomisée et positionnement multidimensionel à base de tâches

The multidimensional scaling (MDS) is an important and robust algorithm for representing individual cases of a dataset out of their respective dissimilarities. However, heuristics, possibly trading-off with robustness, are often preferred in practice due to the potentially prohibitive memory and computational costs of the MDS. The recent introduction of random projection techniques within the MDS allowed it to be become competitive on larger testcases. The goal of this manuscript is to propose a high-performance distributed-memory MDS based on random projection for processing data sets of even larger size (up to one million items). We propose a task-based design of the whole algorithm and we implement it within an efficient software stack including state-of-the-art numerical solvers, runtime systems and communication layers. The outcome is the ability to efficiently apply robust MDS to large datasets on modern supercomputers. We assess the resulting algorithm and software stack to the point cloud visualization for analyzing distances between sequencesin metabarcoding.Le positionnement multidimensionnel (MDS) est un algorithme important et robuste pour représenter les cas individuels d’un ensemble de données en fonction de leurs dissimilarités respectives. Cependant, les heuristiques, qui peuvent être un compromis avec la robustesse, sont souvent préférées en pratique en raison de sa consommation mémoire et de ses coûts potentiellement prohibitifs. L’introduction récente de techniques de projection aléatoire dans le MDS lui a permis de devenir compétitif sur des cas test plus importants. L’objectif de ce manuscrit est de proposer un MDS haute performance basé sur la projection aléatoire pour le traitement d’ensembles de données de taille encore plus grande (jusqu’à un million d’éléments). Nous proposons une conception de l’algorithme et nous l’implémentons dans une pile logicielle efficace, comprenant des solveurs numériques de pointe ainsi des systèmes d’exécution et des couches de communication optimisés. L’aboutissement de ce travail résultat est la capacité d’appliquer efficacement le MDS robuste à de grands ensembles de données sur des super-ordinateurs modernes. Nous évaluons l’algorithme etla pile logicielle résultants à la visualisation de nuages de points pour l’analyse des distances entre séquences de metabarcoding

A palaeoecological model for the late Mesoproterozoic – early Neoproterozoic Atar/El Mreïti Group, Taoudeni Basin, Mauritania, northwestern Africa

Reconstructing the spatial distribution of early eukaryotes in palaeoenvironments through Proterozoic sedimentary basins provides important information about their palaeocology and taphonomic conditions. Here, we combine the geological context and a reconstruction of palaeoenvironmental redox conditions (using iron speciation) with quantitative analysis of microfossil assemblages (eukaryotes and incertae sedis), to provide the first palaeoecological model for the Atar/El Mreïti Group of the Taoudeni Basin. Our model suggests that in the late Mesoproterozoic – early Neoproterozoic, the availability of both molecular oxygen and nutrients controlled eukaryotic diversity, higher in oxic shallow marginal marine environments, while coccoidal colonies and benthic microbial mats dominated respectively in anoxic iron-rich and euxinic waters during marine highstands or away from shore where eukaryotes are lower or absent

A global transition to ferruginous conditions in the early Neoproterozoic oceans

Eukaryotic life expanded during the Proterozoic eon1, 2.5 to 0.542 billion years ago, against a background of fluctuating ocean chemistry2, 3, 4. After about 1.8 billion years ago, the global ocean is thought to have been characterized by oxygenated surface waters, with anoxic and sulphidic waters in middle depths along productive continental margins and anoxic and iron-containing (ferruginous) deeper waters5, 6, 7. The spatial extent of sulphidic waters probably varied through time5, 6, but this surface-to-deep redox structure is suggested to have persisted until the first Neoproterozoic glaciation about 717 million years ago8, 9, 10, 11. Here we report an analysis of ocean redox conditions throughout the Proterozoic using new and existing iron speciation and sulphur isotope data from multiple cores and outcrops. We find a global transition from sulphidic to ferruginous mid-depth waters in the earliest Neoproterozoic, coincident with the amalgamation of the supercontinent Rodinia at low latitudes. We suggest that ferruginous conditions were initiated by an increase in the oceanic influx of highly reactive iron relative to sulphate, driven by a change in weathering regime and the uptake of sulphate by extensive continental evaporites on Rodinia. We propose that this transition essentially detoxified ocean margin settings, allowing for expanded opportunities for eukaryote diversification following a prolonged evolutionary stasis before one billion years ago