Sulfur Mass-independent Fractionation Research Articles

Onset of coupled atmosphere-ocean oxygenation 2.3 billion years ago.

The initial rise of molecular oxygen (O2) shortly after the Archaean-Proterozoic transition 2.5 billion years ago was more complex than the single step-change once envisioned. Sulfur mass-independent fractionation records suggest that the rise of atmospheric O2 was oscillatory, with multiple returns to an anoxic state until perhaps 2.2 billion years ago1-3. Yet few constraints exist for contemporaneous marine oxygenation dynamics, precluding a holistic understanding of planetary oxygenation. Here we report thallium (Tl) isotope ratio and redox-sensitive element data for marine shales from the Transvaal Supergroup, South Africa. Synchronous with sulfur isotope evidence of atmospheric oxygenation in the same shales3, we found lower authigenic 205Tl/203Tl ratios indicative of widespread manganese oxide burial on an oxygenated seafloor and higher redox-sensitive element abundances consistent with expanded oxygenated waters. Both signatures disappear when thesulfur isotope data indicate a brief return to an anoxic atmospheric state. Our data connect recently identified atmospheric O2 dynamics on early Earth with the marine realm, marking an important turning point in Earth's redox history away from heterogeneous and highly localized 'oasis'-style oxygenation.

SO2 Photodissociation at 193 nm Directly Forms S(3P) + O2(3Σg-): Implications for the Archean Atmosphere on Earth.

It is well-documented that photodissociation of SO2 at λ = 193 nm produces O(3Pj) + SO X(3Σ-). We provide experimental evidence of a new product channel from one-photon absorption producing S(3Pj) + O2 X(3Σg-) in 2-4% yield. We probe the reactant and all products with time-resolved photoelectron photoion coincidence spectroscopy. High-level ab initio calculations suggest that the new product channel can only occur on the ground-state potential energy surface through internal conversion from the excited state, followed by isomerization to a transient SOO intermediate. Classical trajectories on the ground-state potential energy surface with random initial conditions qualitatively reproduce the experimental yields. This unexpected photodissociation pathway may help reconcile discrepancies in sulfur mass-independent fractionation mechanisms in Earth's geologic history, which shape our understanding of the Archean atmosphere and the Great Oxygenation Event in Earth's evolution.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Sulfur mass-independent fractionation during SO2 photolysis in low-temperature/pressure atmospheres

Mass-independent fractionation of sulfur isotopes (MIF-S) has been observed in Archean sedimentary rocks and modern stratospheric sulfate aerosol (SSA). Photolysis of SO2 is known to cause significant MIF-S and could be related to the observed MIF-S. However, previous experiments with SO2 photolysis at room temperature, or at low temperatures and atmospheric pressure, did not quantitatively explain quadruple sulfur isotopic compositions (δ34S, Δ33S, and Δ36S values) of the Archean sedimentary rocks and the modern SSA. Here we describe sulfur isotopic fractionation during SO2 photolysis including isotopic self-shielding at low temperatures (down to 228 K) and low pressures (from 5.9 to 9.8 kPa) where pressure broadening of SO2 becomes negligible. Results indicate that magnitudes of sulfur isotopic fractionation factors (34ε, 33E, and 36E values, where 33E = 33ε – 1000 × [(1 + 34ε/1000)0.515 − 1] and 36E = 36ε − 1000 × [(1 + 36ε/1000)1.90 − 1]) increase with decreasing temperature, with values at 228 K being about four times those at 296 K (34ε of up to +344‰). Meanwhile, 33E/34ε and 36E/33E ratios are roughly independent of temperature over the temperature range (by approximately +0.1 and − 3.1, respectively), although the 33E/34ε ratios slightly increase with decreasing temperature, ranging from ~ + 0.08 to ~ + 0.13. A two-component mixing model involving SO2 oxidation by OH and SO2 photolysis in the stratosphere reproduces δ34S and Δ33S values of modern SSA when the contribution ratio of SO2 photolysis to SO3 production is ~20%. The contribution ratio of ~20% is consistent with a previous estimation by Whitehill et al. (2015). However, Δ33S/δ34S ratios at low temperatures are far from those of Archean sedimentary rocks, calling for an additional mechanism (or mechanisms) to explain Archean MIF-S.

High-Resolution Double Velocity Map Imaging Photoelectron Photoion Coincidence Spectrometer for Gas-Phase Reaction Kinetics.

We present a new photoelectron photoion coincidence (PEPICO) spectrometer that combines high mass resolution of cations with independently adjustable velocity map imaging of both cations and electrons. We photoionize atoms and molecules using fixed-frequency vacuum ultraviolet radiation. Mass-resolved photoelectron spectra associated with each cation's mass-to-charge ratio can be obtained by inversion of the photoelectron image. The mass-resolved photoelectron spectra enable kinetic time-resolved probing of chemical reactions with isomeric resolution using fixed-frequency radiation sources amenable to small laboratory settings. The instrument accommodates a variety of sample delivery sources to explore a broad range of physical chemistry. To demonstrate the time-resolved capabilities of the instrument, we study the 193 nm photodissociation of SO2 via the C̃(1B2) ← X̃(1A1) transition. In addition to the well-documented O(3Pj) + SO(3Σ-) channel, we observe direct evidence for a small yield of S(3Pj) + O2(3Σg-) as a primary photodissociation product channel, which may impact sulfur mass-independent fractionation chemistry.

Experimental study on sulfur mass-independent fractionation during SO2 photochemistry and its application to Archean atmosphere

Experimental study on sulfur mass-independent fractionation during SO₂ photochemistry and its application to Archean atmosphere

New insights into the ∼ 74 ka Toba eruption from sulfur isotopes of polar ice cores

Abstract. The ∼74 ka Toba eruption was one of the largest volcanic events of the Quaternary. There is much interest in determining the impact of such a large event, particularly on the climate and hominid populations at the time. Although the Toba eruption has been identified in both land and marine archives as the Youngest Toba Tuff, its precise place in the ice core record is ambiguous. Several volcanic sulfate signals have been identified in both Antarctic and Greenland ice cores and span the Toba eruption 40Ar/39Ar age uncertainty. Here, we measure sulfur isotope compositions in Antarctic ice samples from the Dome C (EDC) and Dronning Maud Land (EDML) ice cores at high temporal resolution across 11 of these potential Toba sulfate peaks to identify candidates with sulfur mass-independent fractionation (S-MIF), indicative of an eruption whose plume reached altitudes at or above the stratospheric ozone layer. Using this method, we identify several candidate sulfate peaks that contain stratospheric sulfur. We further narrow down potential candidates based on the isotope signatures by identifying sulfate peaks that are due to a volcanic event at tropical latitudes. In one of these sulfate peaks at 73.67 ka, we find the largest ever reported magnitude of S-MIF in volcanic sulfate in polar ice, with a Δ33S value of −4.75 ‰. As there is a positive correlation between the magnitude of the S-MIF signal recorded in ice cores and eruptive plume height, this could be a likely candidate for the Toba super-eruption, with a plume top height in excess of 45 km. These results support the 73.7±0.3 ka (1σ) 40Ar/39Ar age estimate for the eruption, with ice core ages of our candidates with the largest magnitude S-MIF at 73.67 and 73.74 ka. Finally, since these candidate eruptions occurred on the transition into Greenland Stadial 20, the relative timing suggests that Toba was not the trigger for the large Northern Hemisphere cooling at this time although we cannot rule out an amplifying effect.

Multiple sulfur isotopes signature of Thermochemical Sulfate Reduction (TSR): Insights from Alpine Triassic evaporites

The sulfur cycle is driven by redox processes, among which sulfate reduction is of primary importance. Sulfate is reduced to sulfide either abiotically by Thermochemical Sulfate Reduction (TSR) or biotically by Microbial Sulfate Reduction (MSR). Although these two processes occur at different temperature regimes (>100°C and <80°C, respectively), they generate similar by-products (e.g., sulfides, elemental sulfur). The 34S/32S ratio is often used as the sole criterion to identify the origin of reduced sulfur compounds, but overlaps prevent unambiguous conclusions. Contrary to MSR, the multiple sulfur isotopic signatures (δ33S, δ34S, δ36S) of natural TSR remains uncharacterized. Here, we performed multiple sulfur isotopes analyses of sulfates, sulfides, and elemental sulfur from six sites in the Alpine Triassic evaporites formation to better constrain the isotopic signatures of TSR. Unlike MSR, TSR can induce slight negative deviations (Δ33S down to −0.08‰) relative to the initial sulfate Δ33S value, which significantly discriminates between these two processes. Isotopic equilibria between anhydrite and either elemental sulfur or sulfides (pyrite or chalcopyrite) were verified according to their mass-fractionation exponents (θ33/34=0.5140 and 0.5170, respectively). Using sulfate-elemental sulfur (Δ34SSO42−-S8) or sulfate-sulfide (Δ34SSO42−-S2−) fractionation pairs and respective fractionation factors (α34) for samples that fulfilled the criteria of isotopic equilibrium, we determined the precipitation temperatures of elemental sulfur and sulfides (pyrite or chalcopyrite) to be 194 ± 14°C and 293–488°C, respectively. Interestingly, the obtained temperature of elemental sulfur precipitation corresponds exactly to the solid-liquid phase transition of native sulfur. Using Δ33S vs. δ34S and Δ33S vs. Δ36S diagrams, we are able to fully explain the isotopic signatures of disequilibrium sulfides by the mixing of sulfate with either elemental or organic sulfur in the aqueous fluid. Mixing curves allow the determination of the relative proportions of sulfate and organic and elemental sulfur, the latter being formed by the recombination of polysulfides during cooling. It appears that the sulfides' signatures are best explained by a 33% contribution of polysulfides (i.e., elemental sulfur signatures), consistent with the relative proportion of dissolved polysulfides previously measured in fluid inclusions from this formation at >200°C. Finally, no sulfur mass independent fractionation (S-MIF) is observed in this evaporitic formation, consistent with the TSR signature generated both at equilibrium and by mixing. This implies that TSR does not generate S-MIFs. Our results thus provide multiple sulfur isotopes signatures of TSR, which may be used to reliably identify this process in variable geological settings.

Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ33S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45–2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ33S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ13Corg (<−30‰) and dampened Δ33S (0.4‰ to −0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ13Corg (−33.4‰ to −19.2‰) and expanded Δ33S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ13Corg (−30‰ to −25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ33S (−0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.

In-situ quadruple sulfur isotopic compositions of pyrites in the ca. 3.2–2.72 Ga metasedimentary rocks from the Pilbara Craton, Western Australia

The sulfur mass independent fractionation (S-MIF) of Mesoarchean is distinct from the other Archean periods by the dampened magnitudes of Δ33S and a steeper slope of Δ36S/Δ33S compared with the Archean Reference Array (ARA) according to previous studies. However, samples available of this interval are limited owing to poor or no preservation. Additionally, the majority of the data were obtained by analysis of bulk rocks, thus are essentially the sulfur isotopic compositions of mixed sulfur components due to multiple stages of post-diagenesis overprints and modifications. This study systematically measured the multiple sulfur isotopic compositions of pyrites (nodular, disseminated) in ca. 3.2–2.72 Ga metasedimentary rocks from the Pilbara Craton, Western Australia, where one of the best-preserved Archean records occur, using the upgraded SHRIMP-SI, after detailed pyrite generation and genesis investigation by BSE imaging and NaOCl etching. The results show that Δ33S values of the diagenetic pyrites in these metasedimentary rocks range from −2.4 to 0.8‰. Such a range is inconsistent with the other Archean intervals and any experiment producing S-MIF so far, where the magnitude of the maximum positive Δ33S value is considerably larger than the corresponding minimum negative Δ33S value. The dampened positive Δ33S is explained in several ways from the perspective of production and preservation, including original photochemical products with larger magnitudes of negative Δ33S than positive Δ33S in a different atmospheric composition, non-zero Δ33S of initial SO2, different mechanism producing S-MIF (chain formation), and destruction of positive Δ33S by subduction. The downward Δ36S/Δ33S deviations from ARA in the diagenetic pyrites of 2.72 Ga Tumbiana Formation can be attributed to modifications induced by microbial activities. This is based on the negative (0.9Δ33S + Δ36S) values similar to biological products and the large range of negative δ34S values. The downward Δ36S/Δ33S offsets in the hydrothermally altered diagenetic pyrites of 2.78 Ga Mt. Roe Basalt are most likely mixing of primary sulfur on ARA with later sulfur disobeying ARA. The upward Δ36S/Δ33S deviations in the diagenetic pyrites of 2.93 Ga Mosquito Creek Formation likely reflect different atmospheric composition.

Euxinia in the Neoarchean: The starting point for early oxygenation in a Brazilian Craton

Geochemical evidence for local redox transitions is observed 300 Ma years prior to the Great Oxygenation Event (GOE ca. 2400 Ma) and suggests evolving environmental conditions through a continuous sequence where euxinia (anoxic and sulfidic) preceded the expansion of more oxidizing conditions. Based on multiproxy data such as iron speciation (FePy/FeHR > 0.7) and Mo enrichments (varying from 3 to 64), we present one of the oldest records of euxinia from the Archean Pitangui greenstone belt (~2700 Ma), Minas Gerais, Brazil. The sulfur isotopic composition of sedimentary sulfides indicates early inputs of sulfate from oxidative weathering and an attenuation of sulfur mass independent fractionation that combined with other geochemical signatures (FeHR/FeT, Ce anomalies and rare earth element fractionation) is consistent with the expansion of oxygen availability, and suggests localized oxic conditions before the GOE.

Can organic haze and O2 plumes explain patterns of sulfur mass-independent fractionation during the Archean?

The existence of mass-independently fractionated sulfur in Archean rocks is almost universally accepted as evidence for low atmospheric O2 and O3 concentrations at that time. But the detailed patterns of the Δ33S values and of the ratios Δ33S/δ34S and Δ36S/Δ33S remain to be explained, and the mechanism for producing the mass-independent fractionation remains controversial. Here, we explore the hypothesis that the relatively low Δ33S values seen during the Mid-Archean, 2.7-3.5 Ga, were caused by the presence of organic haze produced from photolysis of methane. This haze helped shield SO2 from photolysis, while at the same time providing surfaces on which unfractionated short-chain sulfur species could condense. The evolution of oxygenic photosynthesis, and the concomitant disappearance of organic haze towards the end of the Archean allowed more negatively fractionated S4 and S8 to form, thereby generating large positive fractionations in other sulfur species, including sulfate and H2S. Reduction of this sulfate to H2S by bacteria, followed by incorporation of H2S into pyrite, produced the large positive Δ33S values observed in the Neoarchean rock record, 2.5-2.7 Ga.

Isotope shifts and band progressions in SO2 rovibrational energy levels: using quantum theory to extract rotational constants

ABSTRACTWe report the isotope shifts of the rotational constants and vibrational band progressions of the sulfur dioxide molecule (SO), for all four stable sulfur isotopes32S,33S,34S, and36S. These are extracted from exact quantum theoretical calculations of the SO rovibrational energy levels, as reported in Chem. Phys. 450–451, 59 (2015) and Chem. Phys. 461, 34 (2015) and by fitting these levels to a J-shifting (JS)-type scheme, applied to a representative set of total angular momentum (J) values. The approach used to obtain the rotational constants is unusual in that it is derived directly from the quantum theoretical framework used for the earlier calculation, which gives rise to a flexible (i.e., vibrational- and rotational-state-dependent) but symmetric rotor description. The usual rotational quantum numbers are thus replaced with a single body-fixed azimuthal rotation quantum number, K, with various strategies introduced a posteriori to address rotor asymmetry. The new model fits the numerically computed rovibrational levels well, over a fairly broad range of vibrational (v) and rotational (J) excitations. The computed rotational constants agree well with previously reported experimental values [J. Chem. Phys. 58, 265 (1973)]. The explicitly v- and J-dependent approach used here should thus prove valuable in broader contexts—e.g., for an analysis of self-shielding in sulfur mass-independent fractionation, even though the rovibrational levels themselves exhibit mass-dependent fractionation.

VUV pressure-broadening in sulfur dioxide

In the pre-oxygenated ancient Earth atmosphere, the lack of O3 absorption allowed ultraviolet photodissociation of numerous molecules in the troposphere and lower stratosphere. For molecules with narrow line-type absorption spectra, optically thick columns would have produced isotope fractionation due to self-shielding of the most abundant isotopologues. In the lower atmosphere pressure broadening would modify, and in some cases, eliminate these isotope signatures. Shielding is particularly important for quantifying or constraining photolysis-derived isotope effects, such as those believed to explain the sulfur mass-independent fractionation in Archean sedimentary rocks. Here, we report pressure broadening coefficients for natural abundance SO2 in theC˜1B2←X˜1A1band system at 215 nm. For gas bath pressures up to 750 mbar, we find broadening coefficients of 0.30 ± 0.03 cm−1 atm−1 and 0.40 ± 0.04 cm−1 atm−1 for N2 and CO2, respectively. These broadening coefficients are ∼30% larger than SO2 broadening coefficients previously measured in the B˜−X˜ bands at 308 nm. Because of the highly congested nature of the C˜−X˜ bands, pressure broadening in the early Earth troposphere will cause line profile overlap that will diminish the self-shielding-derived mass-independent isotope fractionation for optically thick SO2 columns. Thus, non-explosive volcanic eruptions may not have left a signature of SO2 self-shielding in the ancient sedimentary rock record.

Photochemistry of Sulfur Dioxide and the Origin of Mass-Independent Isotope Fractionation in Earth's Atmosphere

Archean sulfide and sulfate minerals commonly exhibit anomalous ratios among four stable sulfur isotopes, 32S, 33S, 34S, and 36S. These anomalous relationships, referred to as sulfur mass-independent fractionation (S-MIF), provide strong evidence for an early anoxic atmosphere. Correlated variations among three isotope ratios (δ33S, δ34S, and δ36S) can be observed in rocks throughout the Archean and are a key clue toward identifying the source reaction of S-MIF. Studies to investigate the origin of Archean S-MIF so far have primarily focused on the photochemistry of sulfur dioxide (SO2). Photolysis of SO2 at wavelengths &lt;220 nm and photoexcitation at 240–340 nm both yield large-magnitude S-MIF. Proposed mechanisms of S-MIF include isotopologue-dependent self-shielding, cross-sectional amplitudes, and vibronic coupling during intersystem crossing. This review discusses the emerging picture of the physical origins of S-MIF and their implications for the chemistry of the early Earth's atmosphere.

Multiple sulfur isotope geochemistry of Dharwar Supergroup, Southern India: Late Archean record of changing atmospheric chemistry

Earth's tectonic and climatic systems may have changed fundamentally before the Great Oxidation Event (GOE) at about 2.3 Ga. Sulfur Mass Independent Fractionation (S-MIF) has demonstrated that Earth's atmosphere was virtually oxygen-free before the GOE. During 3.0 to 2.4 Ga, the change in Δ33S and Δ36S signals may reflect the perturbation of atmospheric chemistry, though the mechanisms of the change are uncertain. Here, we reported multiple sulfur isotopic studies of Archean volcano-sedimentary sequences of the Dharwar Supergroup, distributed in the Chitradurga Schist Belt (CSB), Southern India. New field mapping and zircon U–Pb dating allows us to reconstruct detailed lithostratigraphy of the Dharwar Supergroup. The lower unit consists of post-3.0 Ga conglomerate, stromatolitic carbonate, siliciclastics with diamictite, chert/BIF and pillowed basalt in ascending order, all of which are older than the 2676 Ma dacite dyke that had intruded into the lower unit. The upper unit unconformably overlies the pillow basalts at the top of the lower unit, and consists of conglomerate/sandstone with ∼2600 Ma detrital zircons, komatiitic basalt, BIF and siliciclastic sequence with mafic volcanics. Sulfur isotope analysis of extracted sulfides shows MIF signals (ΔS33>+1‰) with clear ΔS33–ΔS36 correlations. The lower group of the Dharwar Supergroup shows a ΔS36/ΔS33 slope of −1.48, the middle group shows −1.16 and −1.07, and the upper group shows −0.94. Reassessment of all the Archean S-MIF records from sedimentary rocks indicates that the ΔS36/ΔS33 slope systematically changed during the Archean period. The observed trend in the Indian section is similar to those of its Pilbara–Kaapvaal equivalents, thus it could reflect a global atmospheric signature. Moreover, the isotopic trend seems to correlate with mid-Archean glaciation. Thus, the ΔS36/ΔS33 slope could be a useful tracer for atmospheric chemistry and its link with climate change before the GOE.

Sulfur mass-independent fractionation in subsurface fracture waters indicates a long-standing sulfur cycle in Precambrian rocks

The discovery of hydrogen-rich waters preserved below the Earth's surface in Precambrian rocks worldwide expands our understanding of the habitability of the terrestrial subsurface. Many deep microbial ecosystems in these waters survive by coupling hydrogen oxidation to sulfate reduction. Hydrogen originates from water–rock reactions including serpentinization and radiolytic decomposition of water induced by decay of radioactive elements in the host rocks. The origin of dissolved sulfate, however, remains unknown. Here we report, from anoxic saline fracture waters ∼2.4 km below surface in the Canadian Shield, a sulfur mass-independent fractionation signal in dissolved sulfate. We demonstrate that this sulfate most likely originates from oxidation of sulfide minerals in the Archaean host rocks through the action of dissolved oxidants (for example, HO· and H2O2) themselves derived from radiolysis of water, thereby providing a coherent long-term mechanism capable of supplying both an essential electron donor (H2) and a complementary acceptor (sulfate) for the deep biosphere.

Sulfur isotope fractionation by broadband UV radiation to optically thin SO2 under reducing atmosphere

Photochemical mechanisms of Sulfur Mass-Independent Fractionation (S-MIF) are still poorly understood. Previous laboratory experiments have indicated that the S-MIF depends largely on the spectrum of the incident light source and the partial pressure of SO2, though the basic character of the Archean S-MIF (ΔS36/ΔS33=∼−1) has never been reproduced. We have conducted new photochemical experiments at low pSO2 (1–10 Pa) conditions under the presence of CO and found a reasonable mechanism to reproduce the Δ36S/Δ33S slope about −1. As previously suggested (Ono et al., 2013), the low pSO2 is key to studying the self-shielding effect within a range of realistic atmospheric conditions. Also, reducing conditions are critical for simulating the O2-poor atmosphere, whereas photolysis of pure SO2 provides excess O atoms that significantly change the overall chemistry. Our experimental results confirmed that significant S-MIF (ΔS36/ΔS33=−2.4) can be produced by self-shielding in the SO2 photolysis band (185–220 nm), even if the SO2 column density is as low as 1016 molecules cm−2. Thus, photolysis within a volcanic plume of ∼0.1 ppm SO2 is capable of producing a large S-MIF signature. The isotopic fractionations originating from the different absorption cross sections of SO2 isotopologues (i.e. wavelength dependent effect; without self-shielding) are only minor (potentially up to +4‰ for Δ33S). Under reducing conditions, however, another S-MIF signal with Δ36S/Δ33S ratio of ∼+0.7 is produced due to collision-induced intersystem crossing (ISC) from singlet to triplet states of SO2 (Whitehill et al., 2013), and should also be transferred into the final product that is responsible for changing the Δ36S/Δ33S slope. Based on a photochemical model of the S–O–C system with the two S-MIF-yielding reactions, the largest S-MIF observed in the late Archean Mt. McRae Fm. (ΔS33=+9.4‰, ΔS36=−7.5‰) can be reproduced by solar UV irradiation of a SO2 column of ∼6.4×1016 molecules cm−2 with a sufficiently high concentration of reducing gasses (∼2% CO or CH4) where the ISC-derived MIF contributes ∼3% through the photoexcitation channel initiated in the 240–340 nm region. Our work shows that a photochemical model considering the two major S-MIF-yielding reactions (SO2+hν→SO+O and SO21+M→SO23+M) can explain the behavior of the S-MIF observed in laboratory experiments. The combination of the two effects is more important under reducing condition and should be considered to interpret the geological record.

Mass Fractionation Laws, Mass-Independent Effects, and Isotopic Anomalies

Isotopic variations usually follow mass-dependent fractionation, meaning that the relative variations in isotopic ratios scale with the difference in mass of the isotopes involved (e.g., δ17O ≈ 0.5×δ18O). In detail, however, the mass dependence of isotopic variations is not always the same, and different natural processes can define distinct slopes in three-isotope diagrams. These variations are subtle, but improvements in analytical capabilities now allow precise measurement of these effects and make it possible to draw inferences about the natural processes that caused them (e.g., reaction kinetics versus equilibrium isotope exchange). Some elements, in some sample types, do not conform to the regularities of mass-dependent fractionation. Oxygen and sulfur display a rich phenomenology of mass-independent fractionation, documented in the laboratory, in the rock record, and in the modern atmosphere. Oxygen in meteorites shows isotopic variations that follow a slope-one line (δ17O ≈ δ18O) whose origin may be associated with CO photodissociation. Sulfur mass-independent fractionation in ancient sediments provides the tightest constraint on the oxygen partial pressure in the Archean and the timing of Earth's surface oxygenation. Heavier elements also show departures from mass fractionation that can be ascribed to exotic effects associated with chemical reactions such as magnetic effects (e.g., Hg) or the nuclear field shift effect (e.g., U or Tl). Some isotopic variations in meteorites and their constituents cannot be related to the terrestrial composition by any known process, including radiogenic, nucleogenic, and cosmogenic effects. Those variations have a nucleosynthetic origin, reflecting the fact that the products of stellar nucleosynthesis were not fully homogenized when the Solar System formed. Those anomalies are found at all scales, from nanometer-sized presolar grains to bulk terrestrial planets. They can be used to learn about stellar nucleosynthesis, mixing in the solar nebula, and genetic relationships between planetary bodies (e.g., the origin of the Moon). They can also confound interpretations based on dating techniques (e.g., 146Sm-142Nd) when they are misidentified as isotopic variations of radiogenic origin. To summarize, there is a world to explore outside of mass-dependent fractionation whose impact is promised to expand as analytical capabilities to measure ever-subtler isotopic anomalies on ever-smaller samples continue to improve.

Modeling the signature of sulfur mass-independent fractionation produced in the Archean atmosphere

Minor sulfur isotope anomalies indicate the absence of O2 from the Archean atmosphere. A rich dataset showing large variations in magnitude and sign of Δ33S and Δ36S, preserved in both sulfates and sulfides, suggests that further constraints on Archean atmospheric chemistry are possible. We review previous quantitative constraints on atmospheric Δ33S production, and suggest that a new approach is needed. We added sulfur species containing 33S and 34S to a 1-D photochemical model and describe the numerical methodology needed to ensure accurate prediction of the magnitude and sign of Δ33S produced by and deposited from the Archean atmosphere. This methodology can test multiple MIF-S formation mechanisms subject to a variety of proposed atmospheric compositions, yielding Δ33S predictions that can be compared to the rock record. We systematically test SO2 isotopologue absorption effects in SO2 photolysis (Danielache et al., 2008), one of the primary proposed mechanisms for Δ33S formation. We find that differential absorption through the Danielache et al. (2008) cross sections is capable of altering predicted Δ33S as a function of multiple atmospheric variables, including trace O2 concentration, total sulfur flux, CO2 content, and the presence of hydrocarbons, but find a limited role for OCS and H2S. Under all realistic conditions, the Danielache et al. (2008) cross sections yield Δ33S predictions at odds with the geologic record, implying that additional pathways for sulfur MIF formation exist and/or the cross sections have significant errors. The methodology presented here will allow for quantitative constraints on the Archean atmosphere beyond the absence of O2, as soon as additional experimental measurements of MIF-S producing processes become available.