Acoustic and related properties of calcareous deep-sea sediments


Three expeditions to the Ontong-Java Plateau and the eastern equatorial Pacific obtained unusually good samples of calcareous sediment from box and piston cores; water depths ranged from 1600 m to 4900 m. Two sedimentary environments are represented. Dissolution, dilution, and winnowing cause reductions (with increasing water depth) of CaCO 3 , percent sand, mean grain size, sediment rigidity, and sound velocity. The best indices to predict sound velocity are percent sand, mean grain size, and the velocity ratio; this ratio varies from about 1.05 on top of the Plateau to about 1.00 at 4400 m. Hollow tests of Foraminifera act as solid particles in transmitting sound. Density and porosity are good predictors of velocity in the east Pacific but not in the Plateau area because of large amounts of hollow Foraminifera. There is no significant increase in sound velocity as CaCO 3 increases from 35 to about 75 percent; above that percent, increases in velocity are mainly due to increases in sand-size particles. Eastern Pacific sediment has higher porosities and lower densities (than the Plateau samples) because of less CaCO 3 and more biogenous silica. Biogenous silica content causes good correlation between density and CaCO 3 content in the eastern Pacific but not in the Plateau sediment; density or impedance cannot be used to determine CaCO 3 content in sediment lacking significant amounts of biogenous silica. As water depth increases from 1600 m to 4900 m, percent sand and mean grain size decrease markedly but total porosity increases only 1 to 3 percent. This is due to dissolution and breakdown of hollow tests of Foraminifera and transfer of intraparticle porosity (within the tests) to interparticle porosity between the grains. New estimates of intraparticle porosity range from 13 percent in clayey sand to zero in silty clay. New relations between the frame bulk modulus and porosity allow computations of elastic properties which indicate very small differences in bulk moduli or densities over wide ranges of grain sizes and water depths, but large changes in dynamic rigidities cause both shear and compressional wave velocities to decrease with increasing water depth.

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American Geological Institute

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