Integrated stratigraphy and astronomical tuning of lower–middle Pleistocene Montalbano Jonico section (southern Italy)

Abstract

Astronomical calibration of the lower–middle Pleistocene Montalbano Jonico section located in the Lucania Basin (Southern Italy) is presented. Previous papers widely discussed the integrated stratigraphy (calcareous nannofossils, sapropel stratigraphy, benthic and planktonic oxygen stable isotopes) and the paleoenvironmental features of this section and its potential suitability for the selection of the Middle Pleistocene Global Stratotype Section and Point (GSSP). In this study, new planktonic δ¹⁸O data, additional biostratigraphical constraints and new tephrochronology on volcaniclastic layers occurring within the studied record are reported. The new chronostratigraphic framework provides a robust base for correlation of the oxygen isotope stratigraphy for the Montalbano Jonico section with the glacial and interglacial fluctuations of the Oceanic and Mediterranean δ¹⁸O reference deep-sea records. Specifically, the lower part of the Montalbano Jonico section (Interval A) provides correlation of the planktonic and benthic δ¹⁸O cycles to Marine Isotope Stage (MIS) 36 to MIS 23. Interval A includes a distinct peak of left-coiled neogloboquadrinids, the Globoratalia crassaformis influx, the First Occurrence of Gephyrocapsa omega, and the First Common Occurrence and Last Common Occurrence of Reticulofenestra asanoi. These stratigraphical constraints support the tuning of five sapropel layers included in this part of the section to insolation cycles i-112, i-104, i-102, i-90 and i-86. The upper part of the section (Interval B), which includes the temporary disappearance (td2) of G. omega and tephra layer V5, Ar/Ar age dated at 719.5 ± 12.6 ka, is consistent with identification of MIS 22 to MIS 16 in the planktonic δ¹⁸O pattern. The δ¹⁸O time series of the whole section was reconstructed using the midpoints of individual sapropels and their correlative precession minima, visual comparison of the δ¹⁸O pattern with the record available at the Mediterranean ODP Site 975, and, in the upper part of the section, the Ar/Ar age of tephra V5. The developed astronomical tuning revealed that the Montalbano Jonico section covers an interval from 1240 ka to 645 ka. A significant change in sedimentation rate occurs between Intervals A (0.53 m/ky) and Interval B (0.91 m/ky) at about 870 ka and is consistent with a sea-level drop from a bathyal to a circalittoral environment. Bioevents recognised in the Montalbano Jonico section have been dated according to the astronomical calibration, and age assignments of tephra V1–V4 and V6–V9 are also proposed. The Montalbano Jonico section fills the gap between the top of the Vrica section and the base of the Ionian informal Middle Pleistocene stage, and represents a Mediterranean reference section for the Mid-Pleistocene transition.

Introduction

Astronomical dating of late Neogene sedimentary sequences from the Mediterranean has now been well established and forms the backbone of the Astronomical Tuned Neogene Time Scale (ATNTS2004; Lourens et al., 2004). The dating method is purely based on the calibration of sedimentary cycles such as sapropels, carbonate cycles, diatomite layers, combined with several geochemical and petrophysical climate sensitive proxies, to the computed time series of the quasi-periodic variations of Earth's orbit and axis (Hilgen, 1991a, Hilgen, 1991b, Hilgen et al., 1995, Hilgen et al., 2000a, Hilgen et al., 2000b, Hilgen et al., 2003, Hilgen and Krijgsman, 1999, Krijgsman et al., 1999, Langereis et al., 1997, Lourens et al., 1996a, Lourens et al., 1996b, Lourens et al., 1998, Lourens et al., 2001, Sierro et al., 2001, Sierro et al., 2003, Lourens, 2004, Abels et al., 2005). This time scale provides precise and accurate numerical ages, not only for the sedimentary cycles, but also for calcareous plankton events and magnetic polarity reversals recorded in the tuned sections. Accordingly, all the middle to late Neogene stages are by now defined in tuned land-based marine sections in the Mediterranean region. The Pleistocene part of the ATNTS2004 is based on a few ODP Sites (964, 969 and 967) and piston cores (RC9-181, MD84641, KC01B, KC01), with the exception of the Lower Pleistocene interval in which data from the land-based marine sequences of Vrica and Singa (Calabria, Italy) were also included. The Vrica section includes the GSSP of the Pliocene–Pleistocene boundary (Aguirre and Pasini, 1985). At present, the Calabrian Stage and the Lower/Middle and Middle/Upper Pleistocene boundaries should be regarded as informal (Cita and Castradori, 1995, Cita et al., 2006, Cita et al., 2008).

The Montalbano Jonico composite section, cropping out in Southern Italy, due to its continuity and exposure condition, together with the available detailed integrated stratigraphy, has been proposed as the reference succession for the upper portion of Calabrian Stage (Cita et al., 2008). The section can fill the gap between the top of Vrica section (according to Lourens et al., 1998 the top of Vrica section is directly above insolation cycles i-116) and the base of Ionian informal Middle Pleistocene stage. Further, the section has been considered suitable for the selection of the Middle Pleistocene GSSP (Cita and Castradori, 1995, Ciaranfi et al., 1997, Ciaranfi and D'Alessandro, 2005, Cita et al., 2006, Cita et al., 2008), although the absence of a clear paleomagnetic signal in the succession prevents the recognition of the Brunhes/Matuyama boundary, the primary criterion for the definition of GSSP. This paper presents the astronomical calibration of the composite Montalbano Jonico section based on the previously developed benthic and planktonic stable oxygen isotope stratigraphy, together with new planktonic δ¹⁸O data, additional biostratigraphical constraints and new tephrostratigraphic study of the volcaniclastic layers occurring in the studied record.