Gypsum as an archive of biosignatures: what can we learn from the late Miocene Mediterranean salt giant?

Because of its fast growth, gypsum can rapidly entrap biogenic material and biomolecules, allowing for excellent preservation at geological time scale. The characterization of biota having thrived in the gypsum-mother brines can contribute to elucidate the paleoenvironmental conditions in the water column and at the seafloor during the formation of ancient salt giants; such reconstruction is challenged by the absence of modern analogues. A prominent example are the primary gypsum deposits that accumulated in the Mediterranean basin about 6 Ma ago, during the Messinian salinity crisis (MSC), when this basin turned into the youngest salt giant in Earth history following its partial isolation from the Atlantic Ocean. Two main types of gypsum are recognized: a) bottom-grown selenite, consisting of vertically-oriented twinned crystals and b) laminar gypsum cumulate, formed by the accumulation at the seafloor of tiny gypsum crystals nucleated in the water column, mixed with organic-rich material. Biosignatures in both types of gypsum were investigated through optical, electron and confocal laser scanning microscopy, Raman spectroscopy, and lipid biomarker analyses. Bottom grown gypsum is typified by abundant diatom remains that indicate deposition in a marine basin influenced by freshwater input. Other abundant components are filamentous microfossils corresponding to remains of sulfide oxidizing bacteria. These biogenic remains are commonly coated by microbial dolomite microcrystals and authigenic clays, which suggest that organic matter and biogenic silica underwent severe early diagenetic alteration. Planktic diatoms, calcareous nannofossils and early diagenetic dolomite are also recognized in cumulate gypsum deposits. Such composition suggests high primary productivity in the water column, inducing the export of organic matter to the seafloor and the formation of dolomite following bacterial sulfate reduction. The mechanisms promoting gypsum nucleation in the water column are instead still enigmatic. The late Miocene gypsum represents an excellent archive of biosignatures, including bacterial cells. The microbial assemblage indicates that gypsum formed in relatively deep, stratified marine basins with intermittent sulfidic bottom-water conditions, promoting intense microbially-mediated early diagenetic processes at the expenses of organic matter and biogenic silica. Such circumstances imply that the paucity of skeletal remains in the MSC sedimentary record may be the result of a taphonomic bias rather than of inhospitable hypersaline conditions induced by massive evaporation of the Mediterranean water mass.