MICROBIAL BIOSIGNATURES PRESERVED IN ANCIENT GYPSUM AND THEIR PALEOENVIRONMENTAL IMPLICATIONS: A TALE FROM THE LATE MIOCENE MEDITERRANEAN SALT GIANT (Invited Presentation)

Thick gypsum deposits formed in the Mediterranean during the Late Miocene Messinian salinity crisis (5.97-5.33 Ma), when this basin was turned into the youngest salt giant in Earth history. These successions comprise different gypsum lithofacies, including both large (cm to dm-sized) twinned selenite crystals and cumulate deposits. The former consist of bottom-grown crystals and display an internal lamination in the re-entrant angle of the twins, marked by the repetition of millimetre-thick turbid laminae, rich in clay and biogenic material and limpid laminae in which the latter is scarce or absent. Cumulate deposits are formed by alternating gypsum-rich and organic-rich laminae with abundant dolomite microcrystals of microbial origin. Although Late Miocene gypsum is generally considered as the product of hypersaline and shallow water environments recent studies suggested that it may reflect deeper settings and low salinity mother waters. To investigate paleonvironmental conditions under which gypsum formed, we studied its microbiological content through optical microscopy, SEM/EDS analyses, Raman spectroscopy and lipid biomarker analyses. The bottom-grown crystals contain abundant and well-preserved remains of nano-sized planktic diatoms that point to a relatively deep marine basin influenced by fluvial discharge and high primary productivity in the upper water column. Other abundant components are benthic filamentous microfossils, of sulfide-oxidizing bacteria. These prokaryotes can live in a wide range of water depths, and inhabit different sedimentary environments including stratified basins in which bacterial sulfate reduction produces hydrogen sulfide in organic-rich and oxygen-depleted sediments. No fossils of aquatic organisms have been observed in cumulate deposits. However, the dolomite microcrystals observed in these deposits indicate widespread seafloor anoxia, promoting an increased sedimentation of organic matter in turn favouring bacterial sulfate reduction and precipitation of early diagenetic dolomite. These conditions were favoured by intense water column stratification. The Late Miocene gypsum represents an archive of well-preserved microbial biosignatures, including bacterial cells. Its study reveals that microbial sulfur cycling was a dominant biogeochemical process in the gypsum forming brine.