Microbial sulfur cycling during the formation of Primary Lower Gypsum in Mediterranean marginals basins

During the first phase of the Messinian Salinity Crisis, massive amounts of sulfate (SO₄^2-) have been sequestred in the form of up to 200m thick gypsum deposits (Primary Lower Gypsum) in Mediterranean marginal basins. The sulfur isotopic composition of the sulfate ion of this unit (δ³⁴S_SO4) (on average 22.3 ‰) strongly suggests that gypsum was formed by concentration of marine sulfate. Interestingly, the preservation of sulfide globules within the gypsum and marls interbeds suggests that the basin sulfate was not only involved in gypsum formation but a fraction was also reduced through microbial sulfate reduction. Moreover, filamentous fossils interpreted to be the remnants of sulfide oxidizing bacterias are entrapped in this gypsum and indicate, together with the occurrence of sulfide globules and dolomite, that an active biogeochemical sulfur cycling was active at the time of Primary Lower Gypsum deposition. To investigate the role of this active sulfur cycling in Mediterranean marginal basins, we analyzed the multiple sulfur isotopic composition of sulfate and sulfide minerals (δ³⁴S andΔ³³S)from Primary Lower Gypsum of the Vena del Gesso basin (Italy). Whereas the isotopic composition of gypsum (δ³⁴S_SO4 from 21 to 24‰ and Δ³³S_SO4 from -0.001 to 0.049‰) display very homogenous values that are close to those of the Messinian ocean (δ³⁴S_MSC ~22±0.2‰ and Δ³³S_MSC~0.039±0.015), the analyzed reduced sulfur compounds display a wide range of variability  with -36 to +9‰ in δ³⁴S and -0.017 to 0.125‰ in Δ³³S. This suggests huge hydrologically-driven redox variations during Primary Lower Gypsum deposition in the Vena del Gesso basin, possibly involving intermittent stratification of the water column and an active microbial cycling of sulfur.