Oxygen and sulfur mass-independent isotopic signatures in black crusts: the complementary negative Δ33S reservoir of sulfate aerosols?

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 (delta O-17, delta O-18, and Delta O-17) and sulfur (delta S-33, delta S-34, delta S-36, Delta S-33, and Delta S-36) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW-SE cross section in the Parisian basin. The delta O-18 and delta(3)4S values, ranging between 7.5% and 16.7 +/- 0.5 parts per thousand(n = 27, 2 sigma) and between -2.66 parts per thousand and 13.99 +/- 0.20 parts per thousand, respectively, show anthropogenic SO2 as the main sulfur source (from similar to 2% to 81 %, average similar to 30 %) with host-rock sulfates making the complement. This is supported by Delta O-17 values (up to 2.6 parts per thousand, on average similar to 0.86 parts per thousand), requiring > 60% of atmospheric sulfates in black crusts. Negative Delta S-33 and Delta S-36 values between -0.34 parts per thousand and 0.00 +/- 0.01 parts per thousand and between -0.76% and -0.22 +/- 0.20 parts per thousand, 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 S-33 depletion in black crust sulfates and subsequent S-33 enrichment in residual SO2. Except for a few samples, sulfate aerosols mostly have Delta S-33 values > 0 parts per thousand, and no processes can yet explain this enrichment, resulting in an inconsistent S budget: black crust sulfates could well represent the complementary negative Delta S-33 reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.