The geochemical riddle of “low-salinity gypsum” deposits

Gypsum makes up about one fifth of giant salt deposits formed by evaporation seawater throughout Earth’s history. Although thermodynamic calculations and precipitation experiments predict that gypsum precipitates when the salinity evaporating attains 110 g kg−1, Mediterranean Salt Giant often bear geochemical signature from less saline water masses. Addressing this riddle is important because marine deposition continental erosion affect global carbon cycle. We investigated in marginal basins Sea during Messinian Salinity Crisis (about 6 million years ago). These low-salinity fluid inclusions isotopically light crystallization water, confirming previous published reports harbors deposits. A model constrained inclusion isotope (87Sr/86Sr, δ34SSO4, δ18OH2O, δDH2O) measurements excludes Ca2+- SO42−-enriched runoff alone provides trigger for at low salinity. propose that, concurrent with prevalent evaporative conditions SO42−-bearing runoff, biogeochemical sulfur cycle capable producing a spatially-restricted temporally-transient increase Ca2+ SO42− within benthic microbial mats, creating local chemical conductive to precipitation. This hypothesis supported presence dense packages fossils colorless bacteria several basins, together independent petrographic evidence an active same basins. Should scenario be confirmed, it would expand range environments promote deposition; also imply additional, biological coupling between calcium, cycles exists.