Characterization of the micrites in the Late Miocene vermetid carbonate bioconstructions, Salento Peninsula, Italy: Record of amicrobial/metazoan association

Small carbonate bioconstructions, composed of micrite and vermetids, occur in the Salento Peninsula (southern Italy) at the base of the reefal early Messinian Novaglie Formation. These lens-shaped bioconstructions are tens of meters in length and up to 2.5 m in thickness, and crop out patchily along the South-Eastern Salento coast line.Micrite is the most abundant component. Its origin and role in this association have been investigated by means of microfacies, epifluorescence, and organic matter analyses. Three different types of micrite were recognized: (I) not/weakly fluorescent detrital micrite with a few fine bioclasts; (II) fluorescent micrite rich in fine bioclasts; (III) fluorescent clotted peloidal micrite (automicrite). The first type of micrite, mainly present in the basal layer of the carbonate bioconstructions surrounds vermetids in life position. The type (II) and type (III) micrites occur in the overlying deposits, characterized by sub-horizontally isooriented vermetid shells. The high fluorescence of the type (II) and (III) micrite can be related to organic matter derived respectively from decaying metazoan and microbial organisms. Twofold organic matter origin was supported by FT-IR and GC-MS analyses, carried out on the extracted organic matter. Micrite (I) shows very low intensity of organic matter functional groups, confirming its abiotic origin. The spectra of the automicrite (III) are characterized by the presence of stretching C=C vibrations attributable to alkene and/or unsaturated carboxylic acids, that may be synthesized by microbes. GC-MS investigations indicate the presence of extended hopane series, straight chain saturated (C14, Cl6), monounsaturated (C16, C18), and diunsaturated Cl8-acids, diagnostic of microbial activity. Microbial communities appear to have played a prominent role in the deposition and stabilization of Salento micrite-vermetid carbonate bioconstructions. The type (III) micrite, classifiable as microbialite or automicrite, can be regarded a sort of “primary framework” of these small “bioconstructions”.