Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ<sup>33</sup>S reservoir of sulfate aerosols?

Abstract

Abstract. To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric SO2 cycle, we measured the oxygen (δ17O, δ18O, and Δ17O) and sulfur (δ33S, δ34S, δ36S, Δ33S, and Δ36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The δ18O and δ34S values, ranging between 7.5 ‰ and 16.7±0.5 ‰ (n=27, 2σ) and between −2.66 ‰ and 13.99±0.20 ‰, respectively, show anthropogenic SO2 as the main sulfur source (from ∼2 % to 81 %, average ∼30 %) with host-rock sulfates making the complement. This is supported by Δ17O values (up to 2.6 ‰, on average ∼0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Negative Δ33S and Δ36S values between −0.34 ‰ and 0.00±0.01 ‰ and between −0.76 ‰ and -0.22±0.20 ‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the SO2 oxidation on the building stone, leading to 33S depletion in black crust sulfates and subsequent 33S enrichment in residual SO2. Except for a few samples, sulfate aerosols mostly have Δ33S values > 0 ‰, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative Δ33S reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.

Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

“…The origins of Δ 33 S anomalies appear more complex. Apart from several hypotheses including stratospheric influences, combustion, and coal itself as discussed above for Δ 36 S, recent studies argued that heterogeneous formation of sulfate on mineral dust aerosol surfaces may also lead to Δ 33 S anomalies. − …”

“…16,17 A magnetic isotope effect was further speculated given the decoupled Δ 33 S/Δ 36 S relationship, but the deep mechanism is not explicitly discussed. 16,17 Magnetic isotope effects that arise from spin-selective reactions are usually linked to radical pair reactions. 54,55 It remains unclear where the radical pair reaction is involved in the formation of black crust sulfate.…”

“…Overall, the lack of correlation between Δ 33 S and all tracers investigated in this study is not consistent with any previously proposed mechanism, complicating the search of origins of Δ 33 S. There may be multiple processes resulting in Δ 33 S anomalies and current observation studies using a simple Pearson correlation analysis are not able to unambiguously distinguish them. 10,11,15,19,36−39 ) and from combustion experiments, 53 black crust sulfates, 16,17 pyrites from Mesoarchean volcanic lake sediments 61 and late Ordovician sedimentary rocks, 7 and sulfur in Martian meteorites. 9 The reference slopes for Archean sediments (−0.9 to −1.5) 4 and modern stratospheric aerosols (−1.56) 60 33 S and Δ 36 S anomalies in modern sulfate aerosols are decoupled and affected by at least two different processes.…”

“…Based on the possible production of S-MIF in sulfur recombination reactions that may occur in gaseous thermochemical reactions such as combustion and volcanism, it is further argued that similar processes may occur in Archean, providing additional non-photochemical S-MIF processes that may need to be considered in interpreting Archean data. , This idea was preliminarily tested by physical chemistry experiments and atmospheric chemistry models . Subsequent multiple sulfur isotope measurements of modern sulfate aerosols however found that Δ 33 S anomalies are also correlated with mineral dusts. − It was preliminarily proposed that heterogeneous sulfur reactions account for such Δ 33 S anomalies, but Δ 33 S signs among different follow-up studies are different. − A deep mechanistic understanding on the possible chemical reaction that may lead to Δ 33 S anomalies is useful for using Δ 33 S to identify and quantify any missing sulfate formation pathway in the modern atmosphere with profound implications for air pollution and climate change. − …”

South-hemispheric marine aerosol Hg and S isotope compositions reveal different oxidation pathways

“…Together with the fact that stratospheric-tropospheric mass flux exchanges are low in the tropics (30°N-30°S) with values varying from 0 to 20 kg s −1 km −2 [50] based on a lapse-rate tropopause method (comparatively, the highest mass flux is 600 kg s −1 km −2 in 30°N-60°N [50]), this again suggests that upper tropospheric or stratospheric Hg and S inputs would not be the sole mechanism responsible for the Δ 200 Hg and Δ 33 S excursions. To explain the S-MIF in sulfate aerosols varying from -0.60‰ to -0.50‰, photooxidation of SO 2 in the presence of mineral dust was recently suggested [29,51]. Although the implicated mechanism remains unconstrained yet, this hypothesis may account for both the higher SO / Ca 4 2 2+ enrichment factor used as a geochemical tracer for dust (see Supplementary Information) and the difference of Δ 33 S between the O/R trip between Shanghai and Australia, as dust storms frequently occur from September to December in Australia [52].…”