Accumulation rate and mixing of shelf sediments in the Monterey Bay National Marine Sanctuary

The distribution of excess Pb-210 in 31 sediment cores was used to determine modern (last 100 yr) mass accumulation rates and the depth of sediment mixing on the continental shelf between Pacifica and Monterey, California, USA. Apparent mass accumulation rates average 0.27 g cm(-2) yr(-1) and range from 0.42 g cm(-2) yr(-1) to 0.12 g cm(-2) yr(-1). Accumulation rates were highest at rind-shelf water depths (60-100 m) adjacent to major rivers and near the head of the Ascension submarine canyon. Cores from water depths of less than 65 m had low. uniform Pb-210 activity profiles and sandy textures. The uppermost 5-13 cm of 15 cores had uniform Pb-210 activity profiles above a region of steadily decreasing Pb-210 activity. This phenomenon was attributed to sediment mixing. The thickness of this upper layer of uniform Pb-210 activity decreased southward from 13 cm. west of Pacifica. to less than 5 cm. near Monterey Canyon. This southward decrease may be attributed to shallower bioturbation in the southern study area. Integrated excess Pb-210 activities were generally higher where sedimentation rates were high. They were also higher with increasing distance from major rivers. Thus, sedimentation rate alone does not explain the distribution of integrated excess Pb-210 in this study area. Excess Pb-210 in the seafloor is controlled by other factors such as sediment texture, the atmospheric deposition rate of Pb-210, and the residence time of sediment particles in the water column. (C) 2002 Elsevier Science B.V. All rights reserved.