Distribution and Characteristics of Suspended Sediment on the Amazon Shelf

Abstract

OCEANoGRAPHY.APRIL.I 99 t seven shelÊnorÍnal transects positioned between the river mouth and the French Guiana border (Fig. l). The anchor stations consist of hourly profiles repeated over a 25-h period at a specific location. Anchor stations have been occupied at inner-shelflocations across the topset beds ofthe River-Mouth and Open-Shelf Transects (see Fig. I of Nittrouer et al., this issue). Suspended-Sediment Distribution Measurements of suspended sediment throughout the water column were made using a small instrumented tripod (Sternberg et al., l99l) during the large-scale surveys and at all anchor stations. The tripod is used both as a profiler and to collect short time-series measurements while resting on the bottom. It continuously measures depth, suspended-sediment concentration (using an Optical Backscatter Sensor, OBS), current speed and direction, conductivity, and temperature , and obtains water and suspended-sediment samples at three preselected depths during each deployment. Results of the large-scale surveys show that during a time of low sediment discharge (August 1989) the shelf could be characterized by three regions: a coastal zone (<12 m water depth) which was well-mixed in temperature, salinity, and suspended sediment concentration (l-3 g l-t); a mid-shelf region (I 5-30 m depth) with a stratified water column and suspended-sediment concentrations increasing toward the bottom (-0.8 g l-r); and an outer-shelfregion (>40 m depth) characterized by near-bottom suspended sediment concentrations of approximately 0.1 g I-r. The highest nearbed concentrations were observed close to the river mouth (-5 g l-t atz: I Í1, z is height off rhe bed). During times of high sediment discharge (Feb-ruary and May 1990) dramatic differences in the concentration and distribution of suspended sediment were observed. Fluid muds (here using a general definition for fluid mud of suspended-sediment concentration > l0 g l-r; Ingliss and Allen, 1957; Krone, 1962; Wells, 1983; Faas, Drring times oÍ high sediment discharge. . dramatic differences in the concentration and distribution of suspended sediment were observed. T I HE MAJOR goals of the AmasSeds sediment transport group are to determine the dominant physical processes causing sediment transport on thg Amazon Shelf and to understand the particle ráporse to these processes. The study was motivated by preüous findings concerning such issues as: a) spatially variable sediment accumulation (from 0 to l0 cm y-t; Kuehl et al., 1986);b) extreme sediment reworking depths/surface mixed layer thicknesses ofup to 150 cm at water depths of l5 m (Kuehl et al., 1986); c) sediment dispersal (only 60-70V, of the Amazon sediment discharge is accounted for; Nittrouer et al., 1986); and d) the origin of fine-scale stratigraphy. Although dynamic physical processes control the response of the sediments, the link between them can be complicated. Therefore a comprehensive study to document physical processes and simultaneous sediment response was initiated in this complex environment. Some preliminary flndings are presented in this paper. The field program was designed to measure the fluid, flow, and suspended-sediment characteristics throughout the water column and on varying time scales (e.g. semidiurnal, fortnightly, seasonal). Measurements fall into three major categories: suspended-sediment distribution throughout the water column; boundary-layer processes; and particle characteristics. Oceanographic cruises were scheduled during each season and period of lriver discharge (see Fig. 3 of NittÍoteÍ et a/. this issue) and three of the four cruises have been completed. Cruise IV is scheduled for October-November, 199 1, during minimum river dis-char §e. The cruises consist of legs designated for large-scale (spatial) surveys and anchor-station (temporal) surveys. The large-scale surveys contain up to 50 water-column profiles spaced along 2t