The Dziani Dzaha Lake: A long-awaited modern analogue for superheavy pyrites.

Sedimentary records of superheavy pyrites in Phanerozoic and Proterozoic successions (i.e., extremely positive δ34 Spyrite values together with higher δ34 Spyrite than coeval δ34 SCAS ) are mostly interpreted as resulting either from secondary postdepositional processes or from multiple redox reactions between sulfate and sulfide in stratified sulfate-poor environments. We report here the first observation of strongly positive δ34 S values for both dissolved sulfate and sulfide (average δ34 Sdiss.sulfate value of 34.6‰ and δ34 Sdiss.sulfide values of 36.7‰) compared to the present-day seawater δ34 Sdiss .sulfate (~21‰), with a negative apparent fractionation between sulfate and sulfide (∆34 Sdiss.sulfate-diss.sulfide ~ -2.1 ± 1.4‰), in the sulfate-poor (<3 mm) modern thalassohaline lacustrine system Dziani Dzaha (Mayotte, Indian Ocean). Overall, surface sediments faithfully record the water column isotopic signatures including a mainly negative ∆34 Ssed.sulfate-sed.sulfide (-4.98 ± 4.5‰), corresponding to the definition of superheavy pyrite documented in the rock record. We propose that in the Dziani Dzaha this superheavy pyrite signature results from a two-stage evolution of the sulfur biogeochemical cycle. In a first stage, the sulfur cycle would have been dominated by sulfate from initially sulfate-rich marine waters. Overtime, Raleigh distillation by microbial sulfate reduction coupled with sulfide burial in the sediment would have progressively enriched in 34 S the water column residual sulfate. In a second still active stage, quantitative sulfate reduction not only occurs below the halocline during stratified periods but also in the whole water column during fully anoxic episodes. Sulfates are then regenerated by partial oxidation of sulfides as the oxic-anoxic interface moves downward. These results demonstrate that the atypical superheavy pyrite isotope signature does not necessarily require postdepositional or secondary oxidative processes and can result from primary processes in restricted sulfate-poor and highly productive environments analogous to the Dziani Dzaha.

Early oxidation of the martian crust triggered by impacts.

Despite the abundant geomorphological evidence for surface liquid water on Mars during the Noachian epoch (>3.7 billion years ago), attaining a warm climate to sustain liquid water on Mars at the period of the faint young Sun is a long-standing question. Here, we show that melts of ancient mafic clasts from a martian regolith meteorite, NWA 7533, experienced substantial Fe-Ti oxide fractionation. This implies early, impact-induced, oxidation events that increased by five to six orders of magnitude the oxygen fugacity of impact melts from remelting of the crust. Oxygen isotopic compositions of sequentially crystallized phases from the clasts show that progressive oxidation was due to interaction with an 17O-rich water reservoir. Such an early oxidation of the crust by impacts in the presence of water may have supplied greenhouse gas H2 that caused an increase in surface temperature in a CO2-thick atmosphere.

The growth of lithospheric diamonds.

Natural diamonds contain mineral and fluid inclusions that record diamond growth conditions. Replicating the growth of inclusion-bearing diamonds in a laboratory is therefore a novel diagnostic tool to constrain the conditions of diamond formation in Earth's lithosphere. By determining the carbon isotopic fractionation during diamond growth in fluids or melts, our laboratory experiments revealed that lithospheric monocrystalline and fibrous and coated diamonds grow similarly from redox reactions at isotopic equilibrium in water and carbonate-rich fluids or melts, and not from native carbon. These new results explain why most of the lithospheric diamonds are characterized by a common carbon isotopic fingerprint, inherited from their common parent fluids and not from the mantle assemblage.

Globally asynchronous sulphur isotope signals require re-definition of the Great Oxidation Event.

The Great Oxidation Event (GOE) has been defined as the time interval when sufficient atmospheric oxygen accumulated to prevent the generation and preservation of mass-independent fractionation of sulphur isotopes (MIF-S) in sedimentary rocks. Existing correlations suggest that the GOE was rapid and globally synchronous. Here we apply sulphur isotope analysis of diagenetic sulphides combined with U-Pb and Re-Os geochronology to document the sulphur cycle evolution in Western Australia spanning the GOE. Our data indicate that, from ~2.45 Gyr to beyond 2.31 Gyr, MIF-S was preserved in sulphides punctuated by several episodes of MIF-S disappearance. These results establish the MIF-S record as asynchronous between South Africa, North America and Australia, argue for regional-scale modulation of MIF-S memory effects due to oxidative weathering after the onset of the GOE, and suggest that the current paradigm of placing the GOE at 2.33-2.32 Ga based on the last occurrence of MIF-S in South Africa should be re-evaluated.

A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis.

RATIONALE: Precise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution. METHODS: One mL of reducing solution made of HI and NaH2 PO2 was added to a septum glass tube with dry sulfate. The tube was heated at 124°C and the produced H2 S was purged with inert gas (He or N2 ) through gas-washing tubes and then collected by NaOH solution. The collected H2 S was converted into Ag2 S by adding AgNO3 solution and the co-precipitated Ag2 O was removed by adding a few drops of concentrated HNO3 . RESULTS: Within 2-3 h, a 100% yield was observed for samples with 0.2-2.5 µmol Na2 SO4 . The reduction rate was much slower for BaSO4 and a complete reduction was not observed. International sulfur reference materials, NBS-127, SO-5 and SO-6, were processed with this method, and the measured against accepted δ34 S values yielded a linear regression line which had a slope of 0.99 ± 0.01 and a R2 value of 0.998. CONCLUSIONS: The new methodology is easy to handle and allows us to process multiple samples at a time. It has also demonstrated good reproducibility in terms of H2 S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.

A simple and reliable anion-exchange resin method for sulfate extraction and purification suitable for multiple O- and S-isotope measurements.

RATIONALE: The O- and S-isotope compositions of sulfates can be used as key tracers of the fate and sink of sulfate in both terrestrial and extra-terrestrial environments. However, their application remains limited in those geological systems where sulfate occurs in low concentrations. Here we present a simple and reliable method to extract, purify and concentrate sulfate from natural samples. The method allows us to take into account the separation of nitrate, which is known to be an issue in O-isotope analysis. METHODS: The separation and concentration of sulfate from other anions in any aqueous solution are performed within a few hours via anion-exchange resin. The possible O- (δ18 O and Δ17 O) and S- (δ34 S, Δ33 S and Δ36 S) isotope exchanges, fractionations and/or contaminations are for the first time monitored during the whole procedure using initial O- and S-mass-dependent and mass-independent sulfate solutions. RESULTS: After elution in HCl, pure sulfate is fully retrieved and precipitated into BaSO4 , which is suitable for O- and S-isotopic measurements using established techniques. The analysis of retrieved barite presents no variation within 2σ uncertainties: ±0.5‰ and ±0.1‰ in O- (δ18 O, Δ17 O) and ±0.2‰, ±0.02‰ and ±0.09‰ in S- (δ34 S, Δ33 S and Δ36 S) isotope ratios, respectively. CONCLUSIONS: This study shows that the resin method for sulfate extraction and purification, in addition to being cheap, simple and quick, is applicable for the measurements of all O- and S-isotopic ratios in sulfates (including the Δ17 O, Δ33 S and Δ36 S values). Therefore, this method can be easily used for a high range of natural samples in which sulfate occurs in low concentration including aerosols, ice cores, sediments, volcanic deposits, (paleo)soils and rainwater, and thus it can be a key to our understanding of the sulfur cycle on Earth. Copyright © 2016 John Wiley & Sons, Ltd.

Multiple sulfur isotope evidence for massive oceanic sulfate depletion in the aftermath of Snowball Earth.

The terminal Neoproterozoic Era (850-542 Ma) is characterized by the most pronounced positive sulfur isotope ((34)S/(32)S) excursions in Earth's history, with strong variability and maximum values averaging δ(34)S∼+38‰. These excursions have been mostly interpreted in the framework of steady-state models, in which ocean sulfate concentrations do not fluctuate (that is, sulfate input equals sulfate output). Such models imply a large pyrite burial increase together with a dramatic fluctuation in the isotope composition of marine sulfate inputs, and/or a change in microbial sulfur metabolisms. Here, using multiple sulfur isotopes ((33)S/(32)S, (34)S/(32)S and (36)S/(32)S ratios) of carbonate-associated sulfate, we demonstrate that the steady-state assumption does not hold in the aftermath of the Marinoan Snowball Earth glaciation. The data attest instead to the most impressive event of oceanic sulfate drawdown in Earth's history, driven by an increased pyrite burial, which may have contributed to the Neoproterozoic oxygenation of the oceans and atmosphere.

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere.

Sulfur isotopic anomalies (∆(33)S and ∆(36)S) have been used to trace the redox evolution of the Precambrian atmosphere and to document the photochemistry and transport properties of the modern atmosphere. Recently, it was shown that modern sulfate aerosols formed in an oxidizing atmosphere can display important isotopic anomalies, thus questioning the significance of Archean sulfate deposits. Here, we performed in situ 4S-isotope measurements of 3.2- and 3.5-billion-year (Ga)-old sulfates. This in situ approach allows us to investigate the diversity of Archean sulfate texture and mineralogy with unprecedented resolution and from then on to deconvolute the ocean and atmosphere Archean sulfur cycle. A striking feature of our data is a bimodal distribution of δ(34)S values at ∼+5‰ and +9‰, which is matched by modern sulfate aerosols. The peak at +5‰ represents barite of different ages and host-rock lithology showing a wide range of ∆(33)S between -1.77‰ and +0.24‰. These barites are interpreted as primary volcanic emissions formed by SO2 photochemical processes with variable contribution of carbonyl sulfide (OCS) shielding in an evolving volcanic plume. The δ(34)S peak at +9‰ is associated with non-(33)S-anomalous barites displaying negative ∆(36)S values, which are best interpreted as volcanic sulfate aerosols formed from OCS photolysis. Our findings confirm the occurrence of a volcanic photochemical pathway specific to the early reduced atmosphere but identify variability within the Archean sulfate isotope record that suggests persistence throughout Earth history of photochemical reactions characteristic of the present-day stratosphere.

Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria.

Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe3O4) nanoparticles. Although they may be among the earliest microorganisms capable of biomineralization on Earth, identifying their activity in ancient sedimentary rocks remains challenging because of the lack of a reliable biosignature. We determined Fe isotope fractionations by the magnetotactic bacterium Magnetospirillum magneticum AMB-1. The AMB-1 strain produced magnetite strongly depleted in heavy Fe isotopes, by 1.5 to 2.5 per mil relative to the initial growth medium. Moreover, we observed mass-independent isotope fractionations in (57)Fe during magnetite biomineralization but not in even Fe isotopes ((54)Fe, (56)Fe, and (58)Fe), highlighting a magnetic isotope effect. This Fe isotope anomaly provides a potential biosignature for the identification of magnetite produced by magnetotactic bacteria in the geological record.

Improved analysis of micro- and nanomole-scale sulfur multi-isotope compositions by gas source isotope ratio mass spectrometry.

RATIONALE: Multiple sulfur isotope compositions are usually measured on relatively large samples (in the range of micromoles); however, sometimes only small amounts are available and thus it is necessary to analyze small (sub-micromole) samples. We report an improved method to measure multiple sulfur isotope compositions: δ(33) S, δ(34) S and δ(36) S values on the SF6 molecule (m/z 127, 128, 129, 131) for quantities down to 0.1 micromole, and δ(33) S and δ(34) S values for quantities down to 20 nanomoles. METHODS: Multiple sulfur isotope analyses including fluorination and purification of two international Ag2 S standards, IAEA-S1 and IAEA-S3, were carried out at various low concentrations on a dual-inlet isotope ratio mass spectrometer using a microvolume and modified resistor capacities. RESULTS: The analyses yielded a narrow range of δ(34) S values vs CDT (the international standard), with an overall standard deviation of ±0.2 ‰, which was within the range of certified values. This demonstrates the feasibility of determining both Δ(33) S and Δ(36) S values on the sub-micromole scale, and Δ(33) S values on the nanomole scale with similar accuracy to conventional dual-inlet analyses. CONCLUSIONS: The analysis of the three S-isotope ratios on the SF6 molecule using the so-called conventional fluorination method and dual-inlet ion ratio mass spectrometry is reliable for sample sizes down to ~20 nanomoles. Despite being close to the theoretical limits for maintaining the viscous flow regime of gas in the capillary, errors were not limited by counting statistics, but probably relate to sample gas purification. Copyright © 2016 John Wiley & Sons, Ltd.

Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec).

Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4-2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered some of the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions (δ(33)S, δ(34)S, and δ(36)S) of the >3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions (Δ(33)S up to +2.2‰) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in (33)S/(32)S, (34)S/(32)S, and (36)S/(32)S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earth's formation, a common mechanism for S-MIF production was established in the atmosphere.

Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets.

Achondrite meteorites have anomalous enrichments in (33)S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying (33)S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the (33)S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous (33)S depletions in IIIF iron meteorites (<-0.02 per mil), and (33)S enrichments in other magmatic iron meteorite groups. The (33)S depletions support the idea that differentiated planetesimals inherited sulfur that was photochemically derived from gases in the early inner solar system (<∼2 AU), and that bulk inner solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content.

Can marine protected areas enhance both economic and biological situations?

This paper investigates impacts of the creation of Marine Protected Areas (MPAs), in both economic and biological perspectives. The economic indicator is defined as the sum of discounted benefits derived from exploitation of the resource in the fishery sector, assumed to be optimally managed. The biological indicator is taken as the stock density of the resource. The basic fishery model (C.W. Clark, Mathematical Bioeconomics: The Optimal Management of Renewable Resources, second ed., John Wiley and Sons, New York, 1990) will serve as a convenient benchmark in comparing results with those that are derived from a model of two patchy populations (cf. R. Hannesson, Marine reserves: what would they accomplish, Mar. Resour. Econ. 13 (1998) 159). In the latter, a crucial characteristic is the migration coefficient with describes biological linkages between protected and unprotected areas. A set of situations where both economic and biological criteria are enhanced, after introducing a MPA, is presented. These results are obtained with the help of numerical simulations.

Early Proterozoic ultrahigh pressure metamorphism: evidence from microdiamonds.

Microdiamonds from the Akluilâk minette dykes (Nunavut, Canada) are similar to diamonds formed in subducted metamorphic rocks. High concentrations of unaggregated nitrogen and positive delta(15)N suggest that the microdiamonds formed within rocks subducted to ultrahigh pressures before being sampled by the minette magma 1.8 billion years ago. This ultrahigh pressure metamorphism in North America, probably related to the Trans-Hudson orogen (about 2 billion years ago), extends the occurrence of ultrahigh pressure metamorphism from 0.6 billion years to before 1.8 billion years ago and suggests that Phanerozoic-type subductions were active by the Early Proterozoic.

Diamond genesis, seismic structure, and evolution of the Kaapvaal-Zimbabwe craton.

The lithospheric mantle beneath the Kaapvaal-Zimbabwe craton of southern Africa shows variations in seismic P-wave velocity at depths within the diamond stability field that correlate with differences in the composition of diamonds and their syngenetic inclusions. Middle Archean mantle depletion events initiated craton keel formation and early harzburgitic diamond formation. Late Archean accretionary events involving an oceanic lithosphere component stabilized the craton and contributed a younger Archean generation of eclogitic diamonds. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the Archean diamond suite.