7. HIGH-RESOLUTION COMPRESSIONAL-WAVE VELOCITY MEASUREMENTS IN PLEISTOCENE SEDIMENTS OF THE CEARA RISE (WESTERN EQUATORIAL ATLANTIC): IMPLICATIONS FOR ORBITAL DRIVEN SEDIMENTARY CYCLES¹

J. Grützner,² F.C. Bassinot,³ and J. Mienert²

ABSTRACT

Cores from Ocean Drilling Program Sites 927 and 928, retrieved in water depths of 3315 m and 4012 m on the northern flank of the Ceara Rise, were examined to provide continuous time series of compressional-wave velocity for the Pleistocene (0-1.2 Ma), to evaluate the responses of these records to climate change in the time and frequency domain and to study the effect of carbonate dissolution on P-wave velocity changes.

Comparisons with other high-resolution sediment physical property measurements (GRAPE density, magnetic susceptibility) and sedimentological parameters (CaCO₃ content, coarse fraction) demonstrate that velocity changes at Sites 927 and 928 are the result of glacial/interglacial changes in coarse-fraction grain size, with velocity increasing as grain size increases during interglacial periods. The grain-size distribution in turn is controlled by carbonate productivity and dissolution changes. This signal, however, is diluted by abundant input of fine-grained terrigenous sediments from the Amazon Fan that contain magnetic minerals and thus lead to an inverse relationship between velocity and magnetic susceptibility.

Spectral analysis of P-wave velocity and related parameters reveals that spectral energy is concentrated at orbital frequencies in the precession, obliquity, and eccentricity band, and, therefore, confirms the potential of velocity records as a paleoceanographic tool. Marked differences between variance density spectra of the velocity and preservation index records at Site 927 indicate that velocity changes in sediments deposited well above the present lysocline—apart from the dilution effect—most probably reflect foraminifer productivity changes. At Site 928, enhanced dissolution close to the lysocline affects the velocity time series and leads to maxima of concentration in variance density between obliquity and precessional frequency bands.

¹Shackleton, N.J., Curry, W.B., Richter, C., and Bralower, T.J. (Eds.), 1997. Proc. ODP, Sci. Results, 154: College Station, TX (Ocean Drilling Program).
²GEOMAR Forschungszentrum für marine Geowissenschaften, Wischhofstraße 1-3, D-24148 Kiel, Federal Republic of Germany. jgruetzner@geomar.de
³Centre des Faibles Radioactivités, CNRS/CEA, Avenue de la Terrasse BP 1, 91198 Gif-sur-Yvette, France.