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

Genot, Isabelle; Au Yang, David; Martin, Erwan; Cartigny, Pierre; Legendre, Erwann; De Rafelis, Marc

To better understand the formation and the oxidation pathways leading to gypsum-forming “black crusts” and investigate their bearing on the whole atmospheric inline-formulaSO2 cycle, we measured the oxygen (inline-formulaδ17O, inline-formulaδ18O, and inline-formulaΔ17O) and sulfur (inline-formulaδ33S, inline-formulaδ34S, inline-formulaδ36S, inline-formulaΔ33S, and inline-formulaΔ36S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW–SE cross section in the Parisian basin. The inline-formulaδ18O and inline-formulaδ34S values, ranging between 7.5 ‰ and inline-formula16.7±0.5 ‰ (inline-formulan=27, inline-formula2σ) and between inline-formula−2.66 ‰ and inline-formula13.99±0.20 ‰, respectively, show anthropogenic inline-formulaSO2 as the main sulfur source (from inline-formula∼2 % to 81 %, average inline-formula∼30 %) with host-rock sulfates making the complement. This is supported by inline-formulaΔ17O values (up to 2.6 ‰, on average inline-formula∼0.86 ‰), requiring > 60 % of atmospheric sulfates in black crusts. Negative inline-formulaΔ33S and inline-formulaΔ36S values between inline-formula−0.34 ‰ and inline-formula0.00±0.01 ‰ and between inline-formula−0.76 ‰ and inline-formula M28inlinescrollmathml - normal 0.22 ± normal 0.20 64pt10ptsvg-formulamathimg1b368e87126b5ee755521a605ed743d4 acp-20-4255-2020-ie00001.svg64pt10ptacp-20-4255-2020-ie00001.png  ‰, respectively, were measured in black crust sulfates, which is typical of a magnetic isotope effect that would occur during the inline-formulaSO2 oxidation on the building stone, leading to inline-formula33S depletion in black crust sulfates and subsequent inline-formula33S enrichment in residual inline-formulaSO2. Except for a few samples, sulfate aerosols mostly have inline-formulaΔ33S 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 inline-formulaΔ33S reservoir of the sulfate aerosols, thus solving the atmospheric inline-formulaSO2 budget.

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Genot, Isabelle / Au Yang, David / Martin, Erwan / et al: Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ33S reservoir of sulfate aerosols?. 2020. Copernicus Publications.

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