USE OF SEDIMENTARY CLAY-MINERALS FOR THE RECONSTITUTION OF PERIODIC PALEOCLIMATIC VARIATIONS IN THE ARABIAN SEA

During the Late Neogene, the sediments of the Owen ridge (Arabian Sea) were able to record paleoclimatic variations because of tectonic stability and weak bio-turbation. These changes had a powerful influence on the western, northern and northeastern detrital supplies related to monsoon conditions and low-latitude aridity which developed during glaciary stages. In ODP Leg 117 sites, previous studies on the distribution of clay assemblages show a great variety of mineral origins and their fluctuating contribution to the Neogene sedimentation. Concurrent investigations of the sedimentological, magnetic, biogenic and geochemical properties of sediments have emphasized typical periodic sedimentary responses related to external forced functions of the earth's orbital parameters. This research investigates whether the clay fraction is able to record short-term periodicities. For this purpose we sampled site 721 with high-resolution over a short Plio-Pleistocene interval well known from magnetic susceptibility measurements. We chose the palygorskite/illite peak ratio, directly measured on the XR diffractograms, as a typical clay parameter. The fluctuations of this parameter, between 2.7 and 1.2 My, defined a time-series which has been treated by spectral analysis: autocorrelation, discrete Fourier Transform (DFI) and DFT of the autocorrelation function (DFTA). The results on the global signal (1.5 My) and on four fractions of 375 Ky, are analysed and discussed. On the spectrograms some peaks are close to the classical Milankovitch cycles of the earth's orbital parameters, but additional periods occur with high amplitude. They represent non-linear responses of the eccentricity and of the tilting cyclicities. The periodic influences change with time from a signal dominated by the precession to a signal dominated by the tilting. This shift appears at 2.4 My and reflects the transition from a dominant regional effect (monsoon) to a global and more complex effect (extension of the ice sheets). These results, which show the clay fraction to be an accurate marker of short-term paleoclimate variations, confirm some previous knowledge obtained with magnetic susceptibility data and indicate in greater detail the mode of detrital supply of the Northwest Indian Ocean. Prior to 2.4 My, the palygorskite-illite ratio data vary predominantly at the precessionnal periodicities and record the modulation of the southwest monsoon intensity and associated northwest winds. After 2.4 My the acidification of North-East Africa and Arabia (monsoonal dust areas), with expanded high-latitude ice cover, favours fibrous clay formation in well-developed pericontinental evaporitic basins. Like the illite flux, the palygorskite flux is modulated by a non-linear continental response that varied in response to the orbital obliquity periodicity.