Recombination reactions as a possible mechanism of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

High-accuracy measurement of 36SF5 + signal using an ultrahigh-resolution isotope ratio mass spectrometer

RATIONALE

The Δ36S standard deviation measured in a conventional isotope ratio mass spectrometer such as MAT 253 is ca 0.1‰ to 0.3‰. At this precision, it is difficult to resolve the origin of non-mass-dependent sulfur isotope fractionation in tropospheric sulfate aerosol and in Martian meteorites or small deviations from the canonical mass-dependent fractionation laws. Interfering ions with m/z at 131 of 36SF5 + are suggested by the community as the cause of the poor precision, but the exact ion species has not been identified or confirmed.

METHODS

Here we examined the potential interfering ions by using a Thermo Scientific ultrahigh-resolution isotope ratio mass spectrometer to measure SF6 working gas and SF6 gases converted from IAEA-S1/2/3 Ag2S reference materials.

RESULTS

We found that there are two resolvable peaks to the right of the 36SF5 + peak when a new filament was installed, which are 186WF4 2+ followed by 12C3F5 +. However, only the 12C3F5 + interference peak was observed after more than three days of filament use. 12C3F5 + is generated inside the instrument during the ionization process. Avoiding the interfering signals, we were able to achieve a Δ36S standard deviation of 0.046‰ (n = 8) for SF6 zero-enrichment and 0.069‰ (n = 8) for overall measurement start from silver sulfide IAEA-S1.

CONCLUSIONS

Aging the filament with SF6 gas can avoid the interference of 186WF4 2+. Minimizing the presence of carbon-bearing compounds and avoiding the interfering signals of 12C3F5 + from 36SF5 +, we can improve Δ36S measurement accuracy and precision, which helps to open new territories for research using quadruple sulfur isotope composition.

Vacuum ultraviolet photodissociation of sulfur dioxide and its implications for oxygen production in the early Earth's atmosphere

The emergence of molecular oxygen (O2) in the Earth's primitive atmosphere is an issue of major interest. Although the biological processes leading to its accumulation in the Earth's atmosphere are well understood, its abiotic source is still not fully established. Here, we report a new direct dissociation channel yielding S(1D) + O2(a1Δg/X3Σg-) products from vacuum ultraviolet (VUV) photodissociation of SO2 in the wavelength range between 120 and 160 nm. Experimental results show O2 production to be an important channel from SO2 VUV photodissociation, with a branching ratio of 30 ± 5% at the H Lyman-α wavelength (121.6 nm). The relatively large amounts of SO2 emitted from volcanic eruptions in the Earth's late Archaean eon imply that VUV photodissociation of SO2 could have provided a crucial additional source term in the O2 budget in the Earth's primitive atmosphere. The results could also have implications for abiotic oxygen formation on other planets with atmospheres rich in volcanically outgassed SO2.

Multiple sulphur isotope record of Paleoarchean sedimentary rocks across the Onverwacht Group, Barberton Greenstone Belt, South Africa

This study presents multiple sulphur isotope (³² S, ³³ S, ³⁴ S, ³⁶ S) data on pyrites from silicified volcano-sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF-S) anomalies is found in the cherts, with Δ³³ S ranging between -0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF-S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (-Δ³³ S and -δ³⁴ S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ³³ S and -δ³⁴ S). Pyrites from a rhyo-dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short-lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF-S. Rather, we suggest that the MIF-S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47-3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF-S exceeding >4‰ is atypical of Paleoarchean near-surface environments on the Kaapvaal Craton.

Removal of contamination in helium for precise SF6 -based Δ36 S measurements

RATIONALE

Quantifications of quadruple sulfur isotopic compositions (δ³⁴ S, Δ³³ S, and Δ³⁶ S) of sulfur-bearing compounds in nature are valuable for providing new insights into the Earth's evolution such as the crust-mantle cycle, oxygenation of atmosphere and oceans, and the origin and evolution of early life. SF₆ -based isotope ratio mass spectrometry is the most widely used method of quantification, but Δ³⁶ S measurements at high precision and accuracy have always been technically difficult due to the low abundance of ³⁶ S (~0.01%). In this paper, we identify a major source of isobaric interferences (i.e., contamination in helium carrier gas in the gas chromatography purification step) and propose a simple strategy to solve this problem.

METHODS

An SF₆ fluorination and purification system was built. Laboratory SF₆ reference gas and international Ag₂ S standard (IAEA-S1) were used as reference materials to test our method. Contamination from helium carrier gas (99.999%) was purified by a simple two-step cryogenic method to allow for accurate and precise measurements of Δ³⁶ S using the SF₆ -based isotope ratio mass spectrometry method.

RESULTS

Without proper purification of helium carrier gas, large errors in Δ³⁶ S measurements were found. Measured Δ³⁶ S values of SF₆ with trace contamination from helium were >10‰ higher than expected values. Using a newly developed purification strategy, the difference in Δ³⁶ S values of SF₆ before and after passing through the gas chromatography is less than instrumental errors (<0.2‰). Our improved method yielded an overall Δ³⁶ S precision for IAEA-S1 of 0.12‰ (n = 6). This precision is comparable to that found by other laboratories around the world.

CONCLUSION

Our simple two-step cryogenic method significantly improved the accuracy and precision of Δ³⁶ S measurements and is therefore recommended for future determination of quadruple sulfur isotopic compositions in natural samples.

Massive perturbations to atmospheric sulfur in the aftermath of the Chicxulub impact

Sulfur isotopes confirm a key role for atmospheric sulfur gases in climatic cooling, mass extinction, and the demise of dinosaurs and other global biota after the Chicxulub bolide impact at the Cretaceous–Paleogene boundary. The sulfur isotope anomalies are confined to beds containing ejecta and, in the immediately overlying sediments, are temporally unrelated to known episodes of volcanism that also bracket this event, further addressing the controversial role of the Deccan Traps in the extinction.

Sulfate aerosols have long been implicated as a primary forcing agent of climate change and mass extinction in the aftermath of the end-Cretaceous Chicxulub bolide impact. However, uncertainty remains regarding the quantity, residence time, and degree to which impact-derived sulfur transited the stratosphere, where its climatic impact would have been maximized. Here, we present evidence of mass-independent fractionation of sulfur isotopes (S-MIF) preserved in Chicxulub impact ejecta materials deposited in a marine environment in the Gulf Coastal Plain of North America. The mass anomalous sulfur is present in Cretaceous–Paleogene event deposits but also extends into Early Paleogene sediments. These measurements cannot be explained by mass conservation effects or thermochemical sulfate reduction and therefore require sulfur-bearing gases in an atmosphere substantially different from the modern. Our data cannot discriminate between potential source reaction(s) that produced the S-MIF, but stratospheric photolysis of SO₂ derived from the target rock or carbonyl sulfide produced by biomass burning are the most parsimonious explanations. Given that the ultimate fate of both of these gases is oxidation to sulfate aerosols, our data provide direct evidence for a long hypothesized primary role for sulfate aerosols in the postimpact winter and global mass extinction.

Bulk and grain-scale minor sulfur isotope data reveal complexities in the dynamics of Earth's oxygenation

The permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF–bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ³³S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed ³³S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth’s oxygenation.

The disappearance of mass-independent sulfur isotope fractionation (S-MIF) within the c. 2.3-billion-year-old (Ga) Rooihoogte Formation has been heralded as a chemostratigraphic marker of permanent atmospheric oxygenation. Reports of younger S-MIF, however, question this narrative, leaving significant uncertainties surrounding the timing, tempo, and trajectory of Earth’s oxygenation. Leveraging a new bulk quadruple S-isotope record, we return to the South African Transvaal Basin in search of support for supposed oscillations in atmospheric oxygen beyond 2.3 Ga. Here, as expected, within the Rooihoogte Formation, our data capture a collapse in Δ^3×S values and a shift from Archean-like Δ³⁶S/Δ³³S slopes to their mass-dependent counterparts. Importantly, the interrogation of a Δ³³S-exotic grain reveals extreme spatial variability, whereby atypically large Δ³³S values are separated from more typical Paleoproterozoic values by a subtle grain-housed siderophile-enriched band. This isotopic juxtaposition signals the coexistence of two sulfur pools that were able to escape diagenetic homogenization. These large Δ³³S values require an active photochemical sulfur source, fingerprinting atmospheric S-MIF production after its documented cessation elsewhere at ∼2.4 Ga. By contrast, the Δ³³S monotony observed in overlying Timeball Hill Formation, with muted Δ³³S values (<0.3‰) and predominantly mass-dependent Δ³⁶S/Δ³³S systematics, remains in stark contrast to recent reports of pronounced S-MIF within proximal formational equivalents. If reflective of atmospheric processes, these observed kilometer-scale discrepancies disclose heterogenous S-MIF delivery to the Transvaal Basin and/or poorly resolved fleeting returns to S-MIF production. Rigorous bulk and grain-scale analytical campaigns remain paramount to refine our understanding of Earth’s oxygenation and substantiate claims of post-2.3 Ga oscillations in atmospheric oxygen.

Computational refinement of the puzzling red tetrasulfur chromophore in ultramarine pigments

We investigated the cryptic red chromophore, accompanying the blue S3˙- radical in ultramarine pigments, which usually was tentatively assigned to an unspecified isomer of either neutral S4 or ionic S4˙- species. To reveal its identity, we performed the first systematic density functional studies on periodic and large cluster models of red ultramarines, considering several S4/S4˙- isomers embedded in aluminosilicate cages. For both neutral and charged tetrasulfides the most stable confined isomer is the planar C2v one. The only plausible candidate for the red chromophore among the tetrasulfur species is the planar C2v isomer of the neutral S4 molecule, which, apart from being thermodynamically preferable, strongly absorbs green light and its vibrational modes match very well with the available Raman data. The C2v-S4˙- radical, if present at all in red ultramarines, could be identified by strong absorption in the near infrared region and possibly by the slightly larger isotropic value of the g tensor than that of the S3˙- radical.

An analytical formulation of isotope fractionation due to self-shielding

Isotope fractionation due to photochemical self-shielding is believed to be responsible for the enrichment of inner solar system planetary materials in the rare isotopes of carbon, nitrogen, and oxygen relative to the Sun. Self-shielding may also contribute to sulfur isotope mass-independent fractionation in modern atmospheric sulfates, although its role in the early Earth atmosphere has not yet been convincingly established. Here, I present an analytical formulation of isotopic photodissociation rate coefficients that describe self-shielding isotope signatures for 3 and 4-isotope systems broadly representative of O and S isotopes. The analytic equations are derived for idealized molecular spectra, making an analytic formulation tractable. The idealized spectra characterize key features of actual isotopologue spectra, particularly for CO and SO₂, but are applicable to many small molecules and their isotopologues. The analytic expressions are convenient for evaluating the magnitude of isotope effects without having to pursue involved numerical solutions. More importantly, the analytic expressions illustrate the origin of particular isotope signatures, such as the previously unexplained large mass-dependent fractionation associated with photodissociation of optically-thick SO₂. The formulation presented here elucidates the origin of some of these important isotopic fractionation processes.

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

Stable isotope ratio measurements have been used as a measure of a wide variety of processes, including solar system evolution, geological formational temperatures, tracking of atmospheric gas and aerosol chemical transformation, and is the only means by which past global temperatures may be determined over long time scales. Conventionally, isotope effects derive from differences of isotopically substituted molecules in isotope vibrational energy, bond strength, velocity, gravity, and evaporation/condensation. The variations in isotope ratio, such as ¹⁸ O/¹⁶ O (δ¹⁸ O) and ¹⁷ O/¹⁶ O (δ¹⁷ O) are dependent upon mass differences with δ¹⁷ O/δ¹⁸ O=0.5, due to the relative mass differences (1 amu vs. 2 amu). Relations that do not follow this are termed mass independent and are the focus of this Minireview. In chemical reactions such as ozone formation, a δ¹⁷ O/δ¹⁸ O=1 is observed. Physical chemical models capture most parameters but differ in basic approach and are reviewed. The mass independent effect is observed in atmospheric species and used to track their chemistry at the modern and ancient Earth, Mars, and the early solar system (meteorites).

Five-S-isotope evidence of two distinct mass-independent sulfur isotope effects and implications for the modern and Archean atmospheres

Anomalous sulfur isotopic compositions preserved in sedimentary rocks older than ∼2.5 billion years have been widely interpreted as the products of UV photolysis of sulfur dioxide in an anoxic atmosphere and used to track the history of primitive Earth and evolution of early life. In this study, we present strong observational evidence that there is an additional process that produces similar anomalous sulfur isotope signatures. This previously unknown origin not only offers a tool for quantifying the present-day atmospheric sulfur budget and evaluating its influences on climate and public health but also implies that anomalous sulfur isotopic compositions in some of the oldest rocks on Earth might have been produced in a way different from that previously thought.

The signature of mass-independent fractionation of quadruple sulfur stable isotopes (S-MIF) in Archean rocks, ice cores, and Martian meteorites provides a unique probe of the oxygen and sulfur cycles in the terrestrial and Martian paleoatmospheres. Its mechanistic origin, however, contains some uncertainties. Even for the modern atmosphere, the primary mechanism responsible for the S-MIF observed in nearly all tropospheric sulfates has not been identified. Here we present high-sensitivity measurements of a fifth sulfur isotope, stratospherically produced radiosulfur, along with all four stable sulfur isotopes in the same sulfate aerosols and a suite of chemical species to define sources and mechanisms on a field observational basis. The five-sulfur-isotope and multiple chemical species analysis approach provides strong evidence that S-MIF signatures in tropospheric sulfates are concomitantly affected by two distinct processes: an altitude-dependent positive ³³S anomaly, likely linked to stratospheric SO₂ photolysis, and a negative ³⁶S anomaly mainly associated with combustion. Our quadruple stable sulfur isotopic measurements in varying coal samples (formed in the Carboniferous, Permian, and Triassic periods) and in SO₂ emitted from combustion display normal ³³S and ³⁶S, indicating that the observed negative ³⁶S anomalies originate from a previously unknown S-MIF mechanism during combustion (likely recombination reactions) instead of coal itself. The basic chemical physics of S-MIF in both photolytic and thermal reactions and their interplay, which were not explored together in the past, may be another ingredient for providing deeper understanding of the evolution of Earth’s atmosphere and life’s origin.

Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene

Increased anthropogenic-induced aerosol concentrations over the Himalayas and Tibetan Plateau have affected regional climate, accelerated snow/glacier melting, and influenced water supply and quality in Asia. Although sulfate is a predominant chemical component in aerosols and the hydrosphere, the contributions from different sources remain contentious. Here, we report multiple sulfur isotope composition of sedimentary sulfates from a remote freshwater alpine lake near Mount Everest to reconstruct a two-century record of the atmospheric sulfur cycle. The sulfur isotopic anomaly is utilized as a probe for sulfur source apportionment and chemical transformation history. The nineteenth-century record displays a distinct sulfur isotopic signature compared with the twentieth-century record when sulfate concentrations increased. Along with other elemental measurements, the isotopic proxy suggests that the increased trend of sulfate is mainly attributed to enhancements of dust-associated sulfate aerosols and climate-induced weathering/erosion, which overprinted sulfur isotopic anomalies originating from other sources (e.g., sulfates produced in the stratosphere by photolytic oxidation processes and/or emitted from combustion) as observed in most modern tropospheric aerosols. The changes in sulfur cycling reported in this study have implications for better quantification of radiative forcing and snow/glacier melting at this climatically sensitive region and potentially other temperate glacial hydrological systems. Additionally, the unique Δ³³S-δ³⁴S pattern in the nineteenth century, a period with extensive global biomass burning, is similar to the Paleoarchean (3.6-3.2 Ga) barite record, potentially providing a deeper insight into sulfur photochemical/thermal reactions and possible volcanic influences on the Earth's earliest sulfur cycle.