Integrated stratigraphy and astronomical calibration of the Serravallian/Tortonian boundary section at Monte Gibliscemi (Sicily, Italy)

Abstract

Results are presented of an integrated stratigraphic (calcareous plankton biostratigraphy, cyclostratigraphy and magnetostratigraphy) study of the Serravallian=Tortonian (S=T) boundary section of Monte Gibliscemi (Sicily, Italy). Astronomical calibration of the sedimentary cycles provides absolute ages for calcareous plankton bio-events in the interval between 9.8 and 12.1 Ma. The first occurrence (FO) of Neogloboquadrina acostaensis, usually taken to delimit the S=T boundary, is dated astronomically at 11.781 Ma, pre-dating the migratory arrival of the species at low latitudes in the Atlantic by almost 2 million years. In contrast to delayed low-latitude arrival of N. acostaensis, Paragloborotalia mayeri shows a delayed low-latitude extinction of slightly more than 0.7 million years with respect to the Mediterranean (last occurrence (LO) at 10.49 Ma at Ceara Rise; LO at 11.205 Ma in the Mediterranean). The Discoaster hamatus FO, dated at 10.150 Ma, is clearly delayed with respect to the open ocean. The ages of D. kugleri first and last common occurrence (FCO and LCO), Catinaster coalitus FO, Coccolithus miopelagicus last (regular) occurrence (L(R)O) and the D. hamatus=neohamatus cross-over, however, are in good to excellent agreement with astronomically tuned ages for the same events at Ceara Rise (tropical Atlantic), suggesting that both independently established timescales are consistent with one another. The lack of a reliable magnetostratigraphy hampers a direct comparison with the geomagnetic polarity timescale of Cande and Kent (1995; CK95), but ages of calcareous nannofossil events suggests that CK95 is significantly younger over the studied time interval. Approximate astronomical ages for the polarity reversals were obtained by exporting astronomical ages of selected nannofossil events from Ceara Rise (and the Mediterranean) to eastern equatorial Pacific ODP Leg 138 Site 845, which has a reliable magnetostratigraphy. Our data from the Rio Mazzapiedi-Castellania section reveal that the base of the Tortonian stratotype corresponds almost exactly with the first regular occurrence (FRO) of N. acostaensis s.s. as defined in the present study, dated at 10.554 Ma. An extrapolated age of 11.8 Ma calculated for the top of the Serravallian stratotype indicates that there is a gap between the top of the Serravallian and the base of the Tortonian stratotype, potentially rendering all bio-events in the interval between 11.8 and 10.554 Ma suitable for delimiting the S=T boundary. Despite the tectonic deformation and the lack of a magnetostratigraphy, Gibliscemi remains a candidate to define the S=T boundary by means of the Tortonian global boundary stratotype section and point (GSSP).