Organic matter remains in a “nummulite bank” from the Monte Saraceno (southern Italy)

Since the introduction of the “nummulite bank”concept, there has been a lively debate regarding its interpretation as an authochthonous structure or an allochthonous accumulation of the tests.Even if the hydrodynamic models confirmed the possibility that large B tests could be quite easily transported and accumulated far from the original life environment, some recent observations on the taxonomic composition of different banks questioned the transport and suggest in situ accumulation as a valid alternative.A new approach to the problem could come from the new techniques available for the study of the molecular fossils. A nummulite bank from the Monte Saraceno (Gargano, southern Italy) has been selected to be examined with respect to the possible content of organic matter. This nummulite bank, dominated by large B forms of Nummulites gizehensis and N. puigsecensis (SBZ 16-17, upper Lutetian-lower Bartonian, Middle Eocene), develops clinostratified thick beds, interbedded with skeletal calcarenites, with an original slope angle estimated to be about 20° (Adams et al., 2002). Selected samples were collected from both nummulite bank and skeletal calcarenite beds . To verify the possible influence of early cementation we analysed the interparticle micrite matrix to characterize its nanomorphology and geochemistry. A careful examination showed that generally the original micrite has been recrystallised into aggrading microsparite. Optical observations revealed the micrite peloidal nature and showed also high epifluorescence due to its organic matter content. The presence of organic matter remains is confirmed by the epifluorescence of nummulite tests whose pattern follows the wall microstructure and the canal system.The epifluorescent micrites, in SEM observation, display the characteristic peloidal pattern with smaller crystal size in comparison with organic matter depleted areas.The Si distribution detected by EDS analyses allows to differentiate detritic micrite from automicrite. The former is present in the skeletal calcarenites and the latter in the bank facies.The geochemical characterization of extracted organic matter has been performed through the functional group analyses by FT-IR. Infrared spectra show a strong prevalence of the aromatic fraction over the aliphatic and carboxylic ones, confirming the thermal maturation of the organic compounds. This result would exclude the presence of preserved biomarkers, but the general pattern of the chromatogram could indicate peculiar assemblages of carbonatogenic microbes. Preliminary data on the micrite preserved in the nummulite bank allow hypothesizing induced biomineralization processes probably were linked to bacterial activity. The syndepositional cementation of the micrite could have influenced the stabilization of the nummulite bank, preventing the remobilization of nummulite tests.