USE OF TOTAL GRAIN-SIZE DISTRIBUTIONS TO DEFINE BED EROSION AND TRANSPORT FOR POORLY SORTED SEDIMENT UNDERGOING SIMULATED BIOTURBATION

Without mixing the bed, current velocities greater than 16 cm were required to initiate particle movement, but because the bed became armored, this initial movement was not reflected in the total grain-size distribution of the bed. With mixing, a residual bed containing little silt and clay was produced at velocities of only a few centimeters per second. Once fines were removed from the bed, sand-sized material began to move by either traction or saltation. A curve relating current velocity to sediment transport by bed load (traction and saltation), intermittent suspension, and suspension is presented. The results of our flume experiments demonstrate how sorted sediments are presently being derived from cohesive, often overcompacted, poorly sorted glacial sediments on high-latitude continental shelves where measured current velocities are less than 20 cm (the Antarctic seafloor, for example). Furthermore, we have gained important information about the true meaning of grain-size data as it relates to current velocity. Mean grain size, sorting, and skewness reflect not only current velocity, but also the efficiency with which the bed was mixed. Attempts to reconstruct paleovelocity using only these parameters may be meaningless.