Evidences of precession and obliquity orbital forcing in oxygen-18 isotope composition of Montalbano Jonico Section (Basilicata, southern Italy)

Abstract

Quantitative signal processing methods have been applied to a δ 180 profile for a land-based stratigraphic section, extending from the upper part of lower Pleistocene to the lower part of middle Pleistocene. The section is well exposed with a continuous succession of muds and muddy silts, about 400 m thick, located in the southernmost part of Bradano Trough, near Montalbano Jonico in Basilicata (south Italy). The sampled part of the section is about 240 m thick, in which a foram benthic species ( Cassidulina carinata) is continuously available for oxygen isotope ratio measurements. The aim of the data treatment is to discover how much of the Earth's orbital periodic movements, precession and obliquity, which represent the dominant periodicities in paleoclimatic variations from the base of the Pleistocene until 0.735 Myr BP, are responsible for the oscillations observed in the oxygen-18 record of the Montalbano Jonico section. A time framework of the section was constructed on the basis of calcareous nannofossil biostratigraphic analyses, preliminary magnetostratigraphic results and oxygen isotope correlation with the record from DSDP s607 (isotope data collected in the NOAA World Data Center). The resulting time-scale extends from 1.15 to 0.74 Myr. Power spectrum analysis was performed on the isotope data to illuminate the most important periodicity components of the Montalbano Jonico record. The periodic components of 41,000 and 21,000 yr are present in this record; the former associated with periodic changes in the tilt of Earth's axis and the latter with periodic changes with the precession of the equinoxes, as predicted by the astronomical theory of ice ages. They are, however, not the most important components of the power spectrum, in which a lower frequency component contains most of the variance. This low-frequency component is centered at a period around 208,000 yr. This periodicity seems not to be attributable to any known astronomical or paleoclimatic phenomenon. An attempt was made to verify if this periodicity was due to the composite effect of precession and obliquity signals together at different frequencies from their forcing frequencies. In order to investigate this effect, isotope data have been parameterized in terms of a sum of simple functions of precession and obliquity signals with unknown coefficients. The coefficients are estimated from the time series with the assumption that the best coefficients are those which minimize the ‘noise’ i.e. the difference between the data function and the precession and obliquity functions. Cross-spectra analyses were also performed on the data and the precession signal and on the data and the obliquity signal. The power spectrum of the residual ‘noise’ functions and the cross-spectra demonstrate that precession and obliquity signals are not in phase with the data at their forcing frequencies and so damp. The precession and obliquity signals were then shifted towards lower frequencies at equally spaced lags, the resultant ‘noise’ power spectra were plotted for every combination of lags of precession and obliquity. The results of this data processing demonstrate that it is possible to have a combination of precession and obliquity cyclicities that could be responsible for the signal with 208 kyr periodicity.