Sulfur Mass-independent Fractionation Research Articles

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.

Reply to comment by Cole‐Dai et al. on “Climatic impact of the long‐lasting Laki eruption: Inapplicability of mass‐independent sulfur isotope composition measurements”

Here we respond to the comments by Cole-Dai et al. [2014] on our article Schmidt et al. [2012]. Specifically, in response to section 2 of their reply, we argued in Schmidt et al. [2012] that based on previously published estimates of the volatile release height during the 1783-1784 C.E. Laki eruption, the lack of a sulfur massindependent fractionation (MIF) anomaly is expected. In other words, no previous study on Laki ever argued that this eruption emitted SO2 into altitudes >13-15 km. In section 2.3, Cole-Dai et al. [2014] argue that the nonzero Δ33S value of their Laki sample 1 may be explained by a short-lived explosive phase at Laki during which volatiles reached the stratosphere. In Schmidt et al. [2012] in section 2, we argued in agreement with Cole-Dai et al. [2014] (section 3.1) that for a MIF anomaly to be preserved, the Laki volatiles would have had to be emitted in >20 km altitude. Our main point is that eruption column heights >20 km are unlikely based on the historical accounts and plume-rise modeling for the Laki eruption [Stothers et al., 1986; Woods, 1993; Thordarson and Self, 2003]. In Schmidt et al. [2012], we argued that to deduce a short-lived climatic impact of the Laki eruption based on the lack of a MIF anomaly and the length of the sulfate deposition in Greenland ice cores may be misleading because the climatic impact will outlast the radiative forcing of the Laki aerosol cloud. Cole-Dai et al. [2014] acknowledge the latter in their reply in section 4.2. We agreewith Cole-Dai et al. [2014] in that themagnitude and length of the climatic impact during the winter of 1783-1784 depends on the altitude of the volatile release during the eruption (sections 2.3 and 4.2). However, even if we assumed that during Laki all sulfur dioxide (SO2) would have been released in the troposphere, then the aerosol cloud would still be present in the upper troposphere during March 1784, as is evident from independent model simulations of this tropospheric-only scenario [Stevenson et al., 2003]. We acknowledge that there is uncertainty on the volatile release height for Laki; however, it is worth considering that those climate model simulations that used an injection altitude between 9 km and 13 km for the Laki SO2 [Highwood and Stevenson, 2003; Oman et al., 2006a, 2006b; Schmidt et al., 2012] best match the observed temperature changes during summer of 1783 [Angell and Korshover, 1985; Brazdil et al., 2010; Briffa et al., 1998; D'Arrigo and Jacoby, 1999; Jacoby et al., 1999; Kington, 1988; Manley, 1974; Parker et al., 1992; Thordarson and Self, 2003]. Based on these model simulations, a climatic impact during the winter of 1783-1784, albeit weaker than during the climactic phases of Laki, is expected (and our argument here does not exclude the role of natural variability in contributing to the cold winter of 1783-1784 as discussed in Schmidt et al. [2012]). Therefore, we continue to argue that for high-latitude eruptions such as Laki, the applicability of sulfur isotopic measurements to interpret the climatic relevance has yet to be demonstrated. Itmay transpire that the interpretation of MIF signals for the climate-relevance of an eruption is valid and unambiguous only for short-lived explosive eruptions in the tropics. In terms of the processes producing a MIF anomaly (section 3.3 in Cole-Dai et al. [2014]), the works by Hattori et al. [2013] and Ono et al. [2013] suggest that there are remaining issues not discussed by Cole-Dai et al. [2014], for instance, self-shielding of SO2 due to high column densities typical for eruptions of Pinatubo-scale and greater, and the preservation of the MIF signature in general.

Contribution of isotopologue self‐shielding to sulfur mass‐independent fractionation during sulfur dioxide photolysis

Signatures of sulfur mass‐independent fractionation (S‐MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO2 is thought to be the most likely source for these S‐MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S‐MIF production. Laboratory SO2 photolysis experiments are carried out with a flow‐through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO2 in 0.25 to 1 bar N2 bath gas, in order to test the effect of SO2 pressure on the production of S‐MIF. Elemental sulfur products yield high δ34S values up to 140 ‰, with δ33S/δ34S of 0.59 ± 0.04 and Δ36S/Δ33S ratios of −4.6 ± 1.3 with respect to initial SO2. The magnitude of the isotope effect strongly depends on SO2 partial pressure, with larger fractionations at higher SO2 pressures, but saturates at an SO2 column density of 1018 molecules cm−2. The observed pressure dependence and δ33S/δ34S and Δ36S/Δ33S ratios are consistent with model calculations based on synthesized SO2 isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO2 pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ36S/Δ33S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.

Expanding-contracting Earth

The Earth was born from a giant impact at 4.56 Ga. It is generally thought that the Earth subsequently cooled, and hence shrunk, over geologic time. However, if the Earth's convection was double-layered, there must have been a peak of expansion during uni-directional cooling. We computed the expansion-contraction effect using first principles mineral physics data. The result shows a radius about 120 km larger than that of the present Earth immediately after the consolidation of the magma-ocean on the surface, and subsequent shrinkage of about 110 km in radius within about 10 m.y., followed by gradual expansion of 11 km in radius due to radiogenic heating in the lower mantle in spite of cooling in the upper mantle in the Archean. This was due to double-layered convection in the Archean with final collapse of overturn with contraction of about 8 km in radius, presumably by the end of the Archean. Since then, the Earth has gradually cooled down to reduce its radius by around 12 km. Geologic evidence supports the late Archean mantle overturn ca. 2.6 Ga, such as the global distribution of super-liquidus flood basalts on nearly all cratonic fragments (>35 examples). If our inference is correct, the surface environment of the Earth must have undergone extensive volcanism and emergence of local landmasses, because of the thin ocean cover (3–5 km thickness). Global unconformity appeared in cratonic fragments with stromatolite back to 2.9 Ga with a peak at 2.6 Ga. The global magmatism brought extensive crustal melting to yield explosive felsic volcanism to transport volcanic ash into the stratosphere during the catastrophic mantle overturn. This event seems to be recorded by sulfur mass-independent fractionation (SMIF) at 2.6 Ga. During the mantle overturn, a number of mantle plumes penetrated into the upper mantle and caused local upward doming of by ca. 2–3 km which raised local landmasses above sea-level. The consequent increase of atmospheric oxygen enabled life evolution from prokaryotes to eukaryotes by 2.1 Ga, or even earlier in the Earth history.

Sulfur mass-independent fractionation in liquid phase chemistry: UV photolysis of phenacylphenylsulfone as a case study

The sulfur isotope mass-independent fractionation (S-MIF) represents a distinct geochemical signature commonly observed in Archean rocks. Although S-MIF is generally thought to be exclusive to gas phase chemistry, several studies have suggested that liquid phase or heterogeneous chemistry may also produce S-MIF signatures.This study investigates the potential contribution of the poorly explored mass-independent effects from liquid phase sulfur chemistry. Our investigation focused on laboratory experiments of the UV photolysis of phenacylphenylsulfone as a model system. This system was chosen due to previous measurements of Δ33S (not Δ36S) by a low precision SO2 method indicating the occurrence of anomalous fractionation in 33S.The photolysis of PPS in micellar solution produced MIF of 33S in residual PPS ranging from depletion by −2.1‰ to enrichment by 6.4‰, with small mass-dependent fractionation of δ34S up to 2.1‰. While this magnitude of 33S anomaly is comparable to the range of values observed in Archean rocks, no anomaly in 36S was detected for the 32S–34S–36S system beyond analytical precision (0.19‰). These results confirm the anomalous fractionation to be caused most likely by magnetic isotope effects (MIE), affecting only the nuclear spin possessing 33S among the four stable isotopes of sulfur. The observed initial depletion of 33S in the reactant PPS was unexpected and suggests changing contributions of at least two spin-selective processes. The results of this study demonstrate that liquid phase processes, such as the photolysis of certain organic sulfur compounds, can produce anomalous 33S abundance.

Sulfur mass-independent fractionation patterns in the broadband UV photolysis of sulfur dioxide: Pressure and third body effects

The production of mass-independent fractionation (MIF) in the products of the middle/far UV photolysis of SO 2 isotopologues has been linked to large S-MIF signatures observed in the pre-2.45 Ga rock record, and provides a valuable proxy for the evolution of atmospheric O 2. The origin of such large MIF signatures in photolytic products has recently been ascribed to the optical self-shielding of SO 2 isotopologues in photon-limited regions of the lower atmosphere. To better characterize the origin of such a signal we have performed pressure-variant broadband UV photolysis experiments of SO 2 and SO 2–He mixtures. Low pressure (~ 20 Torr) photolysis of SO 2 produces extractable sulfur with large δ 34S ≈ 120–180‰, Δ 33S ~ 20‰, and Δ 36S/Δ 33S ≈ − 2.5, while near atmospheric pressures produce sulfur with Δ 33S ~ 2‰, and Δ 36S/Δ 33S ≈ − 12. Similar experiments conducted with variable pressures of helium bath gas produce a similar diminution in Δ 33S, and in both cases a Δ 36S/Δ 33S ratio that is a linear function of pressure over several orders of magnitude. We postulate a photochemical and kinetic origin for the large S-MIF observed in these static cell experiments, produced from the excited state predissociative dynamics of SO 2, but cannot expressly rule out self-shielding as a contributor to fractionations observed here.

The Formation of Sulfate and Elemental Sulfur Aerosols under Varying Laboratory Conditions: Implications for Early Earth

The presence of sulfur mass-independent fractionation (S-MIF) in sediments more than 2.45 × 10(9) years old is thought to be evidence for an early anoxic atmosphere. Photolysis of sulfur dioxide (SO(2)) by UV light with λ < 220 nm has been shown in models and some initial laboratory studies to create a S-MIF; however, sulfur must leave the atmosphere in at least two chemically different forms to preserve any S-MIF signature. Two commonly cited examples of chemically different sulfur species that could have exited the atmosphere are elemental sulfur (S(8)) and sulfuric acid (H(2)SO(4)) aerosols. Here, we use real-time aerosol mass spectrometry to directly detect the sulfur-containing aerosols formed when SO(2) either photolyzes at wavelengths from 115 to 400 nm, to simulate the UV solar spectrum, or interacts with high-energy electrons, to simulate lightning. We found that sulfur-containing aerosols form under all laboratory conditions. Further, the addition of a reducing gas, in our experiments hydrogen (H(2)) or methane (CH(4)), increased the formation of S(8). With UV photolysis, formation of S(8) aerosols is highly dependent on the initial SO(2) pressure; and S(8) is only formed at a 2% SO(2) mixing ratio and greater in the absence of a reductant, and at a 0.2% SO(2) mixing ratio and greater in the presence of 1000 ppmv CH(4). We also found that organosulfur compounds are formed from the photolysis of CH(4) and moderate amounts of SO(2). The implications for sulfur aerosols on early Earth are discussed. Key Words: S-MIF-Archean atmosphere-Early Earth-Sulfur aerosols.

Atmospherically-derived mass-independent sulfur isotope signatures, and incorporation into sediments

The discovery of sulfur mass-independent fractionation (S-MIF) in Archean sediments has motivated new work on atmospheric sulfur chemistry. Laboratory experiments showed that SO 2 photolysis produces S-MIF at wavelengths both shortward of and longward of the SO 2 photodissociation wavelength of 220 nm. It has been argued that the underlying S-MIF mechanism at wavelengths < 220 nm is SO 2 self-shielding. Additional S-MIF signatures associated with SO 2 photolysis are possible due to isotopologue-dependent variations in absorption intensity and dissociation probability, which must be evaluated through new spectral measurements. Here, I claim that SO 2 photoexcitation, near-UV CS 2 photolysis, OCS photolysis, non-statistical sulfur allotrope reactions, and surface reactions during thermochemical sulfate reduction are all unlikely sources of the largest Archean S-MIF, signatures with arguments presented for each proposed source. A potential problem with the theory proposed here is that large mass-dependent fractionation (MDF) accompanies S-MIF during SO 2 photolysis. The range of δ 34S values is ∼ 100‰ in photochemically produced elemental sulfur, which exceeds the δ 34S range observed in Archean rocks by a factor of ∼ 3–5, and represents a weakness of the photochemical theory for the origin of Archean S-MIF. A combination of chemical and biogenic MDF processes may have acted to reduce the δ 34S range of SO 2 photolysis products. MDF during reactions that form elemental sulfur in the atmosphere and during aqueous phase reaction of HS − with Fe 2+ and FeS to form FeS 2 may have reduced δ 34S values by ∼ 40‰ relative to atmospheric SO. A simple mixing model suggests that a mixture of FeS 2 in sediments from both elemental sulfur (yielding pyrite with δ 34S > 0 and Δ 33S > 0) and from bacterial sulfate reduction (BSR) of BaSO 4 (yielding pyrite with δ 34S < 0 and Δ 33S < 0) may contribute to reducing δ 34S from photochemical values to the observed range in Archean pyrites.

Origin of two distinct multiple-sulfur isotope compositions of pyrite in the 2.5 Ga Klein Naute Formation, Griqualand West Basin, South Africa

Quadruple sulfur isotope ratios ( 32S / 33S/ 34S/ 36S) were measured by a high precision SF 6 method for early diagenetic pyrite in the 2.5 Ga upper Prieska facies of the Griqualand West Basin from the Agouron drill core GKP01. Measured Δ 33S (≈ δ 33S–0.515δ 34S) values range between −2.3 and +8.5‰ with no apparent meter-scale stratigraphic trend for 70 m section. It was found that early diagenetic pyrites occur in two texturally and isotopically distinct populations such that bulk rock isotope data would reflect mixing of these two sulfur pools. Nodular and layered pyrite yield mostly negative (down to −2.3‰) and relatively constant Δ 33S values over a few tens of cm scale. Disseminated pyrites, in contrast, yield positive (0.0–8.5‰) Δ 33S values that are heterogeneous in sub-cm scales. The former pyrite sulfur was likely derived from seawater sulfate through microbial sulfate reduction, and the latter preferentially sampled aqueous H 2S-S 0 reservoirs in late Archean oceans. Relatively constant Δ 33S values for layered pyrite suggest Archean sulfate reservoirs had residence times of a few million years. Most data yield a well-defined Δ 36S/Δ 33S ratios of −0.9, suggesting predominance of a single S-MIF source reaction. Four analyses of a two cm length sample (306.5 m) yield Δ 36S/Δ 33S slope of −1.6. This slope is consistent with the relationship expected for a symmetry-breaking isotope effect during the S + S 2 reaction, but deviations from the main Archean array (i.e., Δ 36S/Δ 33S = −0.9) may also result from various combinations of SO photolysis and SO 2 oxidation. High spatial resolution-high precision SF 6 measurements of Archean pyrite provide significant new insights into the origin of Archean sulfur mass-independent fractionation (S-MIF) and the late Archean sulfur cycle.

Mass‐independent fractionation of sulfur isotopes by isotope‐selective photodissociation of SO2

A quantitative understanding the origin of sulfur isotope mass‐independent fractionation (MIF) is essential to a full interpretation of the Archean sulfur geochemical record. Laboratory experiments have demonstrated that a MIF signature is present in elemental sulfur produced during SO2 photolysis, but the underlying mechanism remains unknown. Here, I report the results of atmospheric chemistry modeling of isotope‐selective photodissociation of SO2 in the 1B2 − 1A1 bands from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Self‐shielding in the rotationally‐resolved lines of 32SO2 produces MIF signatures in SO and residual SO2. A self‐shielding origin for sulfur MIF implies that the variations observed in Δ33S in Archean rocks reflect variation in atmospheric SO2 concentration, and demonstrates that MIF in terrestrial rocks can be derived from photochemistry independent of molecular symmetry.

Constraints on the Archean atmospheric oxygen and sulfur cycle from mass-independent sulfur records from Anshan-Benxi BIFs, Liaoning Province, China

The Archean atmospheric oxygen concentration and sulfur cycle was long debated. The banded iron formation (BIF) is a special type of the sedimentary formation, which has truly recorded the atmospheric and oceanic conditions at that time. In this study, the composition of multiple sulfur isotope (δ 34S/δ 33S/δ 32S) for sulfides bedded in the Archean (∼2.7 Ga) BIFs, in Anshan-Benxi area of Liaoning Province has been measured. The value of Δ33S varies from −0.89‰ to +1.21‰, which shows very obvious mass-independent fractionation (MIF) signatures. These non-zero Δ33S values indicate that the Archean sulfur cycles are different from what it is today, which have been deeply influenced by gas phase photochemical reactions. Algoma-type BIFs which are closely related to the volcanic activity have negative Δ33S value, however, Superior-type BIFs which are far away from the volcanic center have positive Δ33S value. The δ 34S varies in a large range from −22.0‰ to +11.8‰, which indicates that the bacteria reduction activity has already existed at that time, and that the oceanic sulfate concentration has at least reached 1 mmol/L in local areas. Combined with the contemporaneous existence of the hematite, magnetite and the occurrence and preservation of the sulfur MIF, it can be inferred that the Archean atmospheric oxygen level must be at 10−2–10−3 of the present atmospheric level (PAL).

Mystery of the volcanic mass‐independent sulfur isotope fractionation signature in the Antarctic ice core

We simulate the record of sulfur mass‐independent fractionation (MIF) in sulfates found in the Antarctic ice cores from the 1991 eruption of Mt. Pinatubo. Discovery of MIF in sulfur isotopes in Archean/Early Proterozoic sediments and lack of MIF in sulfur in younger rocks placed a strict upper limit on the amount of oxidants and oxygen in the ancient Earth's atmosphere prior to 2.3 Gyr ago. However, recent measurements in ice cores suggest that some non‐zero MIF in sulfate can be delivered to Antarctica following large volcanic eruptions. Current photochemical experiments do not identify the specific reaction responsible for sulfur MIF production. Here we show that a time‐dependent non‐zero sulfur MIF can be delivered to the ground in the present atmosphere if the MIF–producing process is SO3 photolysis. Fractionation from such photolysis has not been considered before, but is consistent with the available experimental data.