SONOSTRATIGRAPHY OF TROPICAL INDIAN-OCEAN GIANT PISTON CORES - TOWARD A RAPID AND HIGH-RESOLUTION TOOL FOR TRACKING DISSOLUTION CYCLES IN PLEISTOCENE CARBONATE SEDIMENTS

During the SEYMAMA expedition of the French R/V Marion Dufresne in the equatorial Indian Ocean, we retrieved giant piston cores (30-53 m long) as part of a high resolution palaeo-oceanographic and stratigraphic study of Pliocene-Pleistocene pelagic carbonates. Major changes in the compressional wave (P wave) velocity profiles recorded in these cores appear to be correlatable from the Madingley Rise (western equatorial Indian Ocean) to the southeast of the Maldives archipelago (central equatorial Indian Ocean), about 1700 km away, thus emphasizing the stratigraphic potential of acoustic records in uncemented pelagic carbonates. As expected in deep-sea carbonate deposits, changes in P-wave velocity parallel past changes in coarse fraction content(> 63 mu m) Changes in grain size appear to be mainly controlled by carbonate dissolution, as evidenced by a strong relationship between sand content and a foraminifer preservation index. Thus, in uncemented pelagic carbonates, P-wave velocities provide quick and easy to obtain qualitative information on carbonate dissolution pulses. As diagenesis takes place, however, compaction and cementation change the dynamic rigidity (mu) of the sediments and may conceal the original grain size signal. Due to the strong positive relationship between P-wave velocity and coarse fraction content in uncemented pelagic carbonates, P-wave velocity profiles can be tied to a precise chronologic framework by correlating them to the composite grain size index curve (CGSI) established by Bassinot et al. for the tropical Indian Ocean [1,2]. This composite curve has been constructed by stacking the normalized coarse fraction records from ODP Site 722 (Owen Ridge, Arabian Sea [3]) and ODP Site 758 (Ninetyeast Ridge, central equatorial Indian Ocean [4]). In these two sites, detailed delta(18)O records provide the basis for precise inter-site correlations. They ensure the accuracy of the stacking procedure, which tends to reduce most of the local grain size signals and enhances the regional signal related to carbonate dissolution pulses [1,2]. A detailed chronostratigraphy of CGSI curve was developed by correlating the delta 18O records of Sites 722 and 758 to the orbital chronology recently developed from ODP Site 677 [5]. The CGSI may be used as a reference curve for developing a sonostratigraphy in the tropical Indian Ocean.