ABSTRACT

An examination of the relationship between acoustic, stratigraphic and physical properties of deep-sea carbonates from the eastern equatorial Pacific revealed that variations in acoustic impedance are almost entirely controlled by changes in saturated bulk density (or its inverse, porosity). Velocity changes are very small. Saturated bulk density changes are highly correlated with variations in percent calcium carbonate. High carbonate samples are dominated by high density platy carbonate material while low carbonate material is dominated by low density spiny siliceous microfossils. Stratigraphic control was achieved through: (1) comparison of the downcore variations in percent carbonate with the carbonate curves of well-dated cores (e.g., RC11-209), (2) stratigraphy of Pulleniatin coiling directions, (3) radiolarian zonation, (4) the extinction of P. lacunosa, and (5) last appearance of discoasters. Sedimentation rates in the area studied are found as 6.17 meters per million years for the last 700,000 years and 4.15 meters per million years before that time. The oxygen isotope record provided the final evidence linking the acoustic properties to the paleo-oceanographic record. Variations in saturated bulk density (or its inverse, porosity) that account for the changes in acoustic impedance are ultimately caused by climatic changes. Warm periods were times of enhanced dissolution which caused low percent carbonate and low saturated bulk density (and therefore low impedance) values. The opposite is true for cold periods. These climatic cycles can be matc ed with glacials and interglacials for the past one million years, but have also occurred before the advent of Pleistocene glaciation. Thus the acoustic record of equatorial carbonates contains a strong paleo-oceanographic signal.