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

Significant warmer conditions and higher eustatic sea level than at present are inferred for the Last Interglacial (MIS 5e 135–116 ky BP), that is generally regarded as a good analog of near-future Earth climate developments in response to the projected global warming. MIS 5e coastal deposits outcropping along the Taranto Gulf are mainly constituted of few meters thick bioclastic calcarenite layers, interbedded with clay. Calcarenites represent a locally vegetated infralittoral sandy bottom, characterized by a lateral transition from a relatively high-energy level to more sheltered bottom conditions. They are affected by very limited post depositional diagenesis, consisting of local dissolution and neomorphism, that allowed to study early marine cementation processes. Clasts mostly consist of medium to coarse sandy size skeletons and fragments of skeletons of a variegated biota together with mainly large molluscs shells that inhabited the sediment and a very minor amount of siliciclastic. Micritization variably affect most of the bioclasts while cements are constituted by 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. Cements are constituted by sub-micron sized anhedral to nanospheroidal crystals of calcite, with 4-6 moles % of Mg, 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 of various dimensions. This implies the presence of a microbial community forming a syn-sedimentary to post-sedimentary epilithic to endolytic biofilm that stabilized the incoherent sediment and mediated the early precipitation of cements. A very similar process of microbial induced micritic cementation of carbonate sand has been observed in tropical carbonate platform realms, suggesting similar environmental and climate conditions for the mid-latitude studied deposits.