Deep-sea carbonates: Physical property relationships and the origin of high-frequency acoustic reflectors

Abstract

A detailed survey of a small area in the eastern equatorial Pacific (136°W,4°N) was conducted with the S.I.O. Marine Physical Laboratory's Deep-Tow instrument package. A detailed examination of the relationships between acoustic, stratigraphic, and physical properties in cores from this area reveals that variations in acoustic impedance are almost entirely controlled by changes in saturated bulk density (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. Variations in saturated bulk density (porosity) that account for 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 matched with glacials and interglacials for the past one million years, but have also occurred before Pleistocene glaciation. The acoustic properties were compared with the high-resolution subbottom profiles collected at the survey site. A reflection coefficient log calculated from the cores' physical properties, reveals no interfaces with large reflection coefficients that correlate with the reflectors seen on the Deep-Tow 4-kHz seismic profile. The calculated reflection coefficients are very low (typically 10 −3–10 −5) and varied about a wavelength that is of the order of the outgoing 4-kHz pulse. Convolving the outgoing 4-kHz pulse with the reflection coefficient log generates a synthetic seismogram that very closely resembles the 4-kHz reflection profile. Varying the frequency of the outgoing pulse changes the amplitude and position of the reflectors seen on the synthetic seismograms. Thus we conclude that the reflectors seen on the 4-kHz seismic profile are not caused by discrete geologic horizons, but rather are the result of the interference of many small layers.