Oxygen and sulfur mass-independent isotopic signatures in black crusts: The complementary negative I"33S reservoir of sulfate aerosols

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 (17O, 18O, and 117O) and sulfur (33S, 34S, 36S, 133S, and 136S) isotopic compositions of black crust sulfates sampled on carbonate building stones along a NW-SE cross section in the Parisian basin. The 18O and 34S values, ranging between 7.5and 16:70:5(n D 27, 2) and between2:66and 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 117O values (up to 2.6 , on average 0:86 ), requiring > 60% of atmospheric sulfates in black crusts. Negative 133S and 136S values between 0:34and 0:000:01 and between 0:76 and 0:220:20 , 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 133S 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 133S reservoir of the sulfate aerosols, thus solving the atmospheric SO2 budget.