BIOMAGNETOSTRATIGRAPHIC CORRELATIONS FROM LEG 107 IN THE TYRRHENIAN SEA

Correlations of biostratigraphic datums to the geomagnetic reversal time scale (GRTS) at Leg 107 sites provide a means of correlating these datums to sections outside the Mediterranean. Unfortunately, poor recovery and core deformation due to rotary drilling at Sites 651, 652, and 654 severely hampered efforts to acquire detailed magnetostratigraphies and biostratigraphies. However, many biostratigraphic markers could be correlated to the GRTS, including those close to the Miocene/Pliocene and Tortonian/Messinian boundaries. These boundaries are interpreted to occur in Chrons 3r and 3B, respectively (chron nomenclature after Cox, 1982). Comparison of the correlation of Plio-Pleistocene calcareous plankton biostratigraphic events to the GRTS in the Mediterranean and in the open oceans indicates that many events are broadly synchronous between the two environments. The outstanding exception is the first occurrence of Globorotalia margaritae which is delayed in the Mediterranean by about 1 m.y. INTRODUCTION The standard chronostratigraphic units of the late Neogene were introduced with reference to marine land sections in Italy, where all the stratotype sections are located. Correlation of paleontological and other stratigraphic events from the Mediterranean Neogene to the open ocean is therefore critical to global Neogene stratigraphy, and has not been satisfactorily accomplished. The problem of direct biostratigraphic correlation from the Mediterranean to the open ocean is hindered by the provincial distribution of the Mediterranean foraminiferal assemblages and, to a lesser extent, the nannofossil assemblages. In addition, some biostratigraphic markers are apparently immigrants into the Mediterranean, their first occurrences being delayed with respect to their appearance in the open ocean. Rio et al. (1984) have analyzed the ranking and spacing of Pliocene and lower Pleistocene calcareous plankton biostratigraphic events in the Mediterranean, comparing them with the succession of events in the open ocean using Shaw Diagram techniques. This approach has led to an improved evaluation of the absolute age of Plio-Pleistocene biostratigraphic events within the Mediterranean, and has facilitated their global correlation. However, the key to further progress is magnetostratigraphy. Geomagnetic polarity reversals are the only frequent synchronous events recorded in sedimentary sequences, and for the Plio-Pleistocene the absolute ages of the reversal boundaries are well constrained (e.g., Berggren et al., 1985; Cox, 1982). Our goal in this paper is to review the available correlations of Neogene biostratigraphic events to the geomagnetic reversal time scale (GRTS), and in so doing make an evaluation of the timing of biostratigraphic events, and assess their usefulness for correlation between the Mediterranean and the open ocean. Until a few years ago, there were no reliable magnetostratigraphies from the Mediterranean Neogene. The compilation of Ryan et al. (1974) was based on the controversial, and now obsolete, magnetostratigraphies of Ryan and Flood (1972) and 1 Kastens, K. A. , Mascle, J., et al., 1990. Proc. ODP, Sci. Results, 107: College Station, TX (Ocean Drilling Program). 2 Department of Geology, University of Florida, Gainesville, FL 32611. 3 Istituto di Geologia, Universita di Parma, 43100, Parma, Italy. 4 Istituto di Geologia, Universita di Palermo, 90100 Palermo, Italy. 5 Laboratoire de Stratigraphic et de Paleocologie, Universite de Provence, 13331 Marseille, France. Nakagawa et al. (1974, 1977). The first useful magnetostratigraphy from the Mediterranean Neogene was that of Tauxe et al. (1983a) who provided a magnetostratigraphy for the Plio-Pleistocene type section at Vrica (Calabria). Subsequently, Zijderveld et al. (1986) and Channell et al. (1988) obtained high quality magnetostratigraphic control for the lower Pliocene in Calabria (Italy), and Langereis et al. (1984) have obtained a high quality, albeit controversial, magnetostratigraphy from the late Miocene in Crete. Leg 107 provided an opportunity to obtain sorely needed additional magnetostratigraphic control for the Mediterranean Neogene. MATERIALS AND METHODS Magnetostratigraphy The Leg 107 magnetostratigraphies are almost entirely based on rotary cores. The priorities of Leg 107 were such that only Sites 650 and 653 (Fig. 1) were drilled using the APC/XCB technique. The sedimentary sequence recovered at Site 650 was largely Pleistocene in age, and Site 653 did not yield a magnetostratigraphy due to adverse magnetic properties at this Site (Channell, Hawthorne, and Torii, this volume). The quality of the magnetostratigraphic records at Sites 651, 652, and 654 (Fig. 1) is severely compromised by the use of the rotary drilling technique. Not only does rotary drilling result in poor recovery and core disturbance, but also gives rise to a steeply inclined magnetic overprint, apparently imparted by the drill-string, which is not easily removed. The overprint has sufficiently high coercivity that the shipboard long-core cryogenic magnetometer/demagnetizer could not be used to resolve the primary component, as the peak allowable alternating demagnetizing field (8 mT) was insufficient to obliterate the overprint. The polarity determinations are based on inclination data from thermally demagnetized subsamples (see Channell, Torii, and Hawthorne, this volume). Biostratigraphy The biostratigraphy of Leg 107 cores was carried out by a number of independent workers. In our summary diagrams (Figs. 2-5), we do not attempt to reconcile discrepancies but rather to illustrate where these discrepancies occur. Muller (this volume) utilized the zonation of Martini (1971). The nannofossil data for Sites 650 and 651 (Figs. 2 and 3) are based entirely on the work of Muller (this volume). For Sites 652 and 654 (Figs. 4 and 5), both the Martini (1971) zonation (Muller, this volume), and the alternative zonation of Raffi and Rio (1979) emended by Rio et al. (this volume) are presented (Glacon et al., this volume). The nannofossil events for Sites 652 and 654 (Figs. 4 and 5) are from Rio (in Glacon et al., this volume). Comparison of Columns A and B in Figures 4 and 5 with the theoretical correlation of the two zonal schemes (Fig. 6) illustrates the magnitude of the discrepancies. These are discussed below.