Paleoenvironmental evolution in a high-stressed cold-seep system (Vicchio Marls, Miocene, northern Apennines, Italy)

Excellent exposures of Miocene seep-carbonates enclosed in marine marly sediments (Vicchio outcrops in the
northern Apennines, Italy) offer the opportunity to highlight the evolution of a fossil seep ecosystem and the
response of benthic communities to high-stressed environmental conditions. For this purpose, seep-related facies
and molluscs within carbonate bodies have been studied, coupled with benthic foraminiferal assemblages and
carbon and oxygen isotopes in the enclosing marls. The integrated planktonic foraminiferal and calcareous
nannofossil biostratigraphy has allowed us to constrain the seepage within well-calibrated bioevents and to
stress the relationships with paleoclimatic and paleoceanographic variations during the middle Miocene in the
Mediterranean area. Our biostratigraphic data indicate that the onset of the seepage approximates the Mi3b
cooling event (13.82 Ma) and the seepage system lasts for 400 kyr. The evolution of the Vicchio cold-seep system
passes through four phases: (1) The onset of the seepage, characterized by a pervasive flow of methane-rich
fluids, is inferred by δ13C depletion of marly sediments and by prevailing benthic foraminifera indicative of
suboxic conditions at the sea-floor. (2) The methane flow becomes focused causing the precipitation of wide
pinnacle-like carbonate bodies which contain giants lucinids. Enclosing marls indicate well-oxygenated conditions,
possibly enhanced by paleoceanographic variations connected to the Mi3b cooling event. (3) The appearance
of the vesicomyid Christineconcha cf. C. regab and the absence of lucinids in seep-carbonates suggest
stable methane-rich fluid emissions; higher flow rates locally favoured the flourishment of bacterial mats.
Benthic foraminifera show abundance peaks of organic matter depending taxa. (4) The reduced intensity of
methane-rich fluid flows favours the precipitation of stratiform carbonate bodies along strike; the macrofauna is
characterized by the presence of both Vesicomyidae and giant lucinids. Foraminiferal assemblages in the enclosing
marls indicate the restoration of well-oxygenated conditions.