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