Microbial mediated micritic cements in Upper Pleistocene (MIS 5e) mid-latitude shallow marine carbonate deposits (Taranto Gulf, south Italy)

Warmer conditions and higher sea level than at present are inferred for the Last Interglacial (MIS 5e 135–116 ky), that is regarded as a good analog of near-future Earth climate in response to the global warming. MIS 5e deposits outcropping along the Taranto Gulf are mainly constituted of few meters thick bioclastic calcarenite layers, representing a locally vegetated infralittoral sandy bottom, characterized by relatively high-energy bottom conditions. They are affected by very limited post depositional diagenesis that allowed to study early marine cementation processes. Clasts mostly consist in medium to coarse sandy size skeletons and fragments of skeletons of a variegated biota together with mainly large mollusks shells that inhabited the sediment and a very minor amount of siliciclastic. Micritization variably affect most of the bioclasts while cements show a microcrystalline texture with various micro-morphologies and fabrics: not-isopachous aphanitic and filamentous rims, vacuolar peloidal meniscus, aphanitic micro-mounds, and aphanitic porosity-filling matrix. Despite different fabrics, cements are constituted by sub-micron sized anhedral to nonospheroidal crystals of low Mg calcite, mixed with a minor amount of irregular platy crystals of saponite. All fabrics of cement are rich of mineralized filamentous, tubular and sub-spherical bacteria bodies that implies the synsedimentary presence of a microbial community forming an epilithic to endolytic biofilm that stabilized the incoherent sediment, and mediated the early precipitation of the cements. A similar process of microbial induced micritic cementation has been observed in tropical carbonate platform realms, that could suggest similar climate conditions for the mid-latitude studied deposits.