How are benthic foraminiferal faunas influenced by cold seeps? Evidence from the Miocene of Italy

The Miocene Termina Formation of the northern Apennines (Italy) hosts small marl bodies containing a fossil megafauna that is not seen elsewhere in the formation. This fauna consists of concentrations of large lucinid clams having living representatives that occur in cold-seep settings, where they house chemosymbiotic bacteria. Together with the megafauna, a 13C-depletion of their carbonate contents suggests a cold seep-derived origin for these bodies. They belong to the cluster of mostly carbonate, Oligo–Miocene bodies outcropping in the Apennine Range, from Piedmont to southern Italy, where they are included in deep-marine units at different geological settings. The relatively soft lithology of the bodies from the Termina Formation permitted the extraction of the microfossil component, so that a taxonomically detailed investigation of benthic foraminifera was performed. Moreover, a comparison with assemblages from nearby, non-seep deposits has allowed an evaluation of their use for recognizing fossil seeps. A total of more than 200 species and subspecies of benthic foraminifera has been identified in seep bodies and surrounding non-seep deposits and allowed paleodepth estimates within the middle bathyal zone (600–1000 m). Similarly to present-day isobathyal settings, the foraminiferal diversity is high, with a major contribution by the accessory taxa that account for up to nearly 50% of the total assemblage. Based upon the microhabitat categories, the epifaunal assemblages are largely outnumbered by the infaunal ones. Our results show that significant differences do not exist between the foraminiferal assemblages recovered from seep-influenced bodies and the ones from the enclosing deposits. These results agree with the findings from present-day seep environments, for which, however, a still limited and insufficient data set is at the moment available. A further complication that should be considered is the effect of time-averaging on fossil assemblages, which would be responsible for some homogenization between foraminiferal stocks of normal marine, non-seep settings and adjacent assemblages controlled by short-term events such as the ones produced by cold seepage.