Given their critical impact on radiative forcing, sulfate aerosols have been extensively studied using their isotope signatures (δ³⁴S, ∆³³S, ∆³⁶S, δ¹⁸O, and ∆¹⁷O). A striking observation is that ∆³³S > 0 ‰, implying a missing reservoir in the sulfur cycle. Here, we measured ∆³³S < 0 ‰ in black crust sulfates (i.e., formed on carbonate walls) that must therefore result from distinct chemical pathway(s) compared to sulfate aerosols, and they may well represent this complementary reservoir.