Cryptic sulfur cycling during the formation of giant gypsum deposits

Salt giants are large-scale, basin-wide deposits formed sporadically in the geological past, from the early Paleozoic to the late Cenozoic. Their role as sinks for seawater dissolved ions is well known, however the biogeochemical conditions that accompany salt giant formation and their effects on carbon cycling remain poorly constrained. Here we show that massive gypsum deposits of the Mediterranean salt giant – the youngest salt giant on Earth – formed in a particularly dynamic biogeochemical environment controlled by orbitally-driven climate oscillations at the precessional scale. Using multiple sulfur isotopes combined with a steady-state sulfur cycle model, we show that, prior to gypsum precipitation, more than 80% of its constituting sulfate was first microbially reduced into sulfide, possibly stored as elemental sulfur, and then almost completely microbially reoxidized back to sulfate. This “cryptic” sulfur cycling contemporaneous to gypsum precipitation implies both negligible net sulfate consumption and sulfide production, despite a significant benthic flux of organic carbon remineralized through microbial sulfate reduction. This is the first known evidence of cryptic sulfur cycling in the geological past.