Insights Into Sediment Gravity Flows from Man-Made Motion-Recording Boulders.

The Coordinated Canyon Experiment (CCE) was conducted in Monterey Canyon for 18 months to monitor sediment gravity flows traveling over a distance of 50 km from 285 to 1,850 mwd using an array of moorings and seafloor-resting sensors. To enable measurements of the difficult to observe dense basal part of the flow, motion-sensing 'boulder-like' Benthic Event Detectors (BED) were developed. BEDs are 0.42 m diameter spheres of 2.1 gr/cc density placed on the seafloor between 200 and 500 mwd that stored high-resolution time-stamped records of depth, acceleration, rotation, and internal temperature. A few BEDs were of cubic shape, and one was placed atop a >2 m tall tripod of >6 gr/cc density. BEDs moved in 9 of 15 sediment gravity flows during the CCE. Five BEDS moved past a mooring where bottom current velocities were recorded. Three velocities were compared: The transit velocity of the flow, the BED initial velocity, and the bottom current velocity, when available. Initial BED velocities were calculated by comparing their initial trajectories at different possible velocities with the bathymetry of the channel thalweg, which was mapped at high resolution (1m X 1m X 0.1m) six times during the CCE. Transit velocities of flows were calculated from the times of arrival of the flow to different sensors (BEDS and/or moorings). BED initial velocities were in the range 2 to 6 m/s, and were 75 ± 15% (1 SD) of the respective flow transit velocity. BEDs velocities were slower than or equal to the bottom current velocity. When moving simultaneously, the tripod moved faster than the cubic BED, which moved faster than the round BED. Total distance rotated was calculated from the integration of the rotational data. In general, the rotated distance/traveled distance ratio was the highest the first-time a BED moved, but often decreased in later moves. Temperature records from recovered BEDs show a dampened or absent tide signal after the first motion of the BED, and sudden increases when later gravity flows occurred. While not conclusive, the collective evidence from the velocities of differently shaped BEDs moving simultaneously, the rotational data, and the BED temperature suggests that during sediment gravity flows BEDs did not free-float in a bottom current but were often transported within a bottom basal layer, yet not at the front of the flow.