The Monterey Event in the Mediterranean platform to basin transition. The Guadagnolo Formation (Miocene, Prenestini Mountains, Central Apennines)

The stratigraphic evolution of the Guadagnolo Fm, developed on the transition between the Latium-Abruzzi Platform and the Umbria Sabina Basin (central Apennines), was studied through facies and stable isotope analysis. Calcareous nannofossil biostratigraphy allows to identify the Burdigalian-Langhian boundary, constraining the sections to the lower to middle Miocene. This interval consists of marly deposits forming the intermediate member of the formation dominated by sponge spicules, molluscs, bryozoans, benthic and planktonic foraminifers. This member is deposited in a distal outer ramp where the abundance of siliceous sponges has been attributed to the high seawater fertility coinciding with the Monterey event, enhanced by regional nutrient flux due to continental runoff and volcanism. The upper member is Serravallian and consists of bryozoan-dominated cross-bedded calcarenites deposited in a shallower environment of the outer ramp. A positive carbon isotope excursion was correlated with the Monterey event recorded in the Central Apennines carbonate ramps and from ODP sites. Five carbon maxima belonging to the Monterey Carbon Isotope Excursion have been identified. Four carbon isotope peaks fall in the spongolitic member, one in the upper member. The Carbon Maxima recorded within the spongolitic member show an attenuate isotope excursion compared with the pelagic and the carbonate platforms record. The weaker signal is due to the fractionation effect of primary producers of the photic zone and to the sponge release of 12C-enriched CO2 deriving from the oxidation of organic matter. Conversely, the largest positive carbon isotope excursion is recorded in the upper member, representative of a shallower environment, and it is due to the absence of sponges and the increased contribution of photosynthetic biota. The shoaling of the depositional environment is related to the global cooling recorded after 14 Ma, as evident also from the δ18O positive shift at the base of the upper member of the Guadagnolo Fm.