23. MIOCENE EVOLUTION OF CARBONATE SEDIMENTATION AT THE CEARA RISE: A MULTIVARIATE DATA/PROXY APPROACH¹

T. A. King,² W.G. Ellis, Jr.,²,³ D.W. Murray,⁴ N.J. Shackleton,⁵ and S. Harris⁶

ABSTRACT

Synthetic records of percent CaCO₃, carbonate mass accumulation, and noncarbonate (terrigenous) mass accumulation are developed for late Neogene portions of Ceara Rise Sites 925-929. Shipboard measurements of natural gamma emissions and magnetic susceptibility, collected at 3- to 10-cm intervals at each site, are combined with equally high resolution measurements of digital color reflectance to estimate lithologic variations over the whole sequence. Where magnetic susceptibility and natural gamma were measured in adjacent holes, the between-hole meters composite depth (mcd) scale was refined so that all data from a given site could be combined. A linear regression model is developed that predicts percent CaCO₃ from high-resolution measurements of stacked natural gamma emissions, stacked magnetic susceptibility, and digital color reflectance. Over 1300 measurements of percent CaCO₃ were made to develop a robust regression equation for each site. An orbitally calibrated chronology for Sites 925-929 relates all data to a common temporal framework. Carbonate and noncarbonate (terrigenous) mass accumulation rates are estimated from 5 to 14 Ma at Sites 925-929. These data are in turn used to evaluate variations in the lysocline from the Miocene to Holocene as a function of depth and time. Over the depth transect, CaCO₃ fluxes during the Miocene undergo significant changes, most likely owing to development and intensification of North Atlantic Deep Water during the late Miocene. Terrigenous fluxes show a long-term trend of increasing accumulation in response to Andean uplift and late Miocene development of the Amazon drainage into the Atlantic Ocean.

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).
²Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A. tking@gsosunl.gso.uri.edu
³Current address: Corning School of Ocean Studies, Maine Maritime Academy, Castine, ME 04420, U.S.A.
⁴Department of Geological Sciences, Brown University, Providence, RI 02912, U.S.A.
⁵Subdepartment for Quaternary Research, Godwin Laboratory, University of Cambridge, Cambridge CB2 3RS, United Kingdom.
⁶College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, U.S.A.