Formation mechanisms of black crusts on limestone using ³⁴S isotopic tracing

In order to use black crusts as archives of ancient air pollution, it is necessary to understand how they form and identify their growth direction. Two main processes have been proposed in the literature: outward progression by the diffusion and reaction of Ca²⁺ with SO₄^2- in the water film at the outer surface, and/or inward progression by pseudomorphosis, whereby calcite is replaced by gypsum. To clarify these mechanisms, simulation chamber experiments were designed to evaluate the growth rate of gypsum on pristine limestone slabs, and then to alter old black crusts using ³⁴S-marked SO₂ and O₃ at 100 % relative humidity (RH). NanoSIMS imaging revealed areas enriched in ³⁴S on the outer surface, suggesting an outward growth of gypsum in these experimental conditions. The extrapolation of these results to field observations is discussed. However, they are key to interpreting the pollution recorded in monument black crusts.