Comparison of laboratory and in situ compressional-wave velocity measurements on sediment cores from the western North Atlantic

Abstract

Laboratory and in situ velocity measurements have been made on six piston cores taken in the western North Atlantic Ocean. Sediments from the southwestern Bermuda Rise and Greater Antilles Outer Ridge are clays having velocities ranging mostly from 1500 to 1530 m/s and velocity gradients near 1 s−1. In cores from the Nares Abyssal Plain, the clayey sediments have comparable velocities, but interbedded silty turbidites exhibit much higher values (up to 1690 m/s). Velocity gradients are slightly higher in the abyssal-plain cores. After the laboratory measurements are corrected to in situ conditions, they show reasonable agreement in average velocity and velocity gradient with in situ measurements, although the in situ velocities average 10–12 m/s higher in the clayey cores and 15–20 m/s higher in the turbidites. This difference may be caused by reduction in the dynamic frame bulk modulus and/or the dynamic shear modulus due to visually undetected coring disturbance. The profilometer used to obtain the in situ measurements does not record the fine-scale variations in velocity that were measured in the laboratory, but it accurately determines average velocities and velocity gradient. Where cores were closely spaced (2–12 km apart), inter-core correlations in lithology, velocity, and bulk properties are possible. Fluctuations in the latter two parameters are very similar in position and magnitude from core to core, suggesting either that effects of coring disturbance are small or that they are uniform in a given kind of sedimentary bed. Inter-core comparison also shows that some beds are laterally discontinuous as a result of local (less than a few kilometers) patterns of seafloor erosion and deposition.