Plio-Pleistocene orbital periodicities in glacially influenced sediments from western pacific margin of Antarctic Peninsula

Summary. Spectral analyses of petrophysical and sedimentological data sets from Plio-Pleistocene sediments cored on the Western Antarctic continental rise during Ocean Drilling Program (Leg 178) show nonharmonic wavelength peaks, with probabilities >90%. When the wavelength peaks are normalized, they exhibit an extremely high correlation factor with predicted Earth's orbital variations for the same time interval. It is also found that both short (~ 95-125 -ky) and long (~400 -ky) eccentricity periodicities clearly emerge from the signal during the whole Pleistocene, without evident switch to obliquity at mid-Pleistocene (~ 0.9 Ma B.P.), as reported in the literature. This suggests that the lithological parameters, proxy of glacial cycles, are controlled, directly or indirectly, by astronomically-forced processes (Milankovitch cycles). Moreover, the constant presence of all orbital periods in the signal since the Lower Pleistocene and the good correlation for the last 2.6 Ma among distant coring sites, based on systematic sedimentological variations at intervals of about 140 and 370 ky, allow to extend the results at regional scale, confirming the hypothesis of the relative stability of the Antarctic ice sheet, at least since Upper Neogene and suggesting that the ice sheet in the Lower-Upper Pliocene and in the Pleistocene could be sensitive to temperature changes. Finally an age correspondence emerges between the correlated events at 1.07, 2.01 and 2.61 Ma, corresponding to Periods of particularly intense ice rafting in the studied area, and relative sea-level highs, suggesting a superimposed eustatic influence. Abstract text. The dynamics of the Antarctic ice sheet is a significant component of the global climate system, so the understanding of its driving mechanisms, still uncompletely known, is of great importance in the analysis of the Earth's climatic evolution. New insights in such dynamics came from studies on the glacial-interglacial sedimentation of upper rise sedimentary drifts off the West Pacific margin of the Antarctica Peninsula. In this sector, the irregular relief of the upper rise is due to large hemipelagic sedimentary drifts, that are separated by nine channel systems formed by turbidity fluxes. Drift sediments alternate from rapidly deposited, poorly fossiliferous silt and clays during glacial intervals, to slowly deposited biogenic muds (with only a minor terrigenous component) during interglacial times (Rebesco et al., 1998, Barker et al., 1999; Barker and Camerlenghi, 2002). Here we present the results from spectral analysis and correlation of Plio-Pleistocene sedimentological (Coarse Fraction) and petrophysical (GRAPE bulk density, Magnetic susceptibility, and Chromaticity a*) data intervals from Sites 1095, 1096 and 1101 ODP Leg 178, drilled respectively from sedimentary drift 7 and 4 (Barker et al., 1999). The spectral analysis program (Horne and Baliunas, 1986; Iorio et al., 1995; Brescia et al., 1995) originally written to deal with unevenly spaced astronomical data sets, is based on a technique which aims to detect the presence and significance of periodicities in unevenly sampled data series. In fact, as it was found in many stratigraphic data sets, the rebinding of the unevenly sampled data to equally spaced bins, and their calculation by means of a conventional periodogram, often alters the perceived frequency and significance of a periodic signal. Among all parameters in the studied intervals, only those with well defined power spectra with more than two wavelengths were considered to record a true cyclic signal (Fig.1). In order to enable comparison from the computed wavelengths expressed in sedimentary thickness (m) and orbital periods expressed in temporal units (ky), the wavelengths of each pow