SPATIAL AND TEMPORAL VARIABILITY IN AGGREGATED GRAIN-SIZE DISTRIBUTIONS, WITH IMPLICATIONS FOR SEDIMENT DYNAMICS

The grain-size distribution of bottom sediments has important implicat ions for diverse aspects of sediment dynamics, including prediction of the critical boundary shear stress and calibration of suspended sedim ent sensors. Past sampling strategies to obtain estimates of the seabe d grain-size distribution typically have not considered spatial and te mporal variability, and have been insufficient to resolve potential mi llimeter-scale vertical variations in grain size. Moreover, laboratory analyses have been predicated on chemically and/or ultrasonically dis aggregating the sediments before resolving particle diameter, therefor e the more dynamically relevant in situ grain-size spectrum is not mea sured. To test for such effects, three sites on the northern Californi a continental shelf comprising a cross-shelf transect from a sandy, in ner shelf (60 m) site, to a muddy, mid-shelf (90 m) site and a relict, outer-shelf (130 m) site Were studied, Replicate box cores were colle cted over two winter field seasons, and multiple subcores from each bo x core were vertically sectioned at 2-mm intervals. Gentle wet sieving techniques were used to determine the mass fraction in the <20, 20-44 , 45-62, 63-89, 90-179, 180-300 and >300 mu m size classes. In additio n, a lesser number of standard disaggregated grain-size analyses were performed using a Coulter Counter. Results from the sandy, inner-shelf site indicate the presence of an ephemeral fine-grained (<20 mu m) su rface layer (0-2 mm) that substantively alters the grain-size distribu tion ''seen'' by the flow. In addition, there is evidence for a progre ssive and substantial winnowing of fine-grained sediment from the surf ace layer over the course of a winter storm season. At the deeper site s, the upper 2 mm of the bed contained 5-20% more material <20 mu m th an deeper portions of the sediment column. At all three sites, the lev el of small- (10s of centimeters) and intermediate- (10s of meters) sc ale spatial variability is modest. In addition, the disaggregated grai n-size distribution al the two muddy sites is, in all cases, markedly finer than the in situ grain-size distribution. Therefore, calibration s and predictions based on a knowledge of the size distribution of the primary (i.e. disaggregated) particles could be in serious error. Cop yright (C) 1996 Elsevier Science Ltd