Seabed dynamics of the inner Amazon continental shelf: temporal and spatial variability of surficial strata

Abstract

The continental shelf southeast and northwest of the Amazon River mouth is overlain by a sediment layer of about a meter in thickness, characterized by 210Pb profiles consisting of one or more segments with nearly-uniform excess activity. Uniform 210Pb activity profiles in shelf environments commonly are a result of intense biological reworking of sediments; however, biological activity is minimal in this area. Hence, this surficial layer (SL) is a result either of physical reworking of the sediments, or of rapid (possibly episodic) deposition of sediment. Factors controlling the formation of the SL were examined during the AmasSeds project between August 1989 and October 1991. Primarily using 210Pb ( t 1/2 = 22.3 yr) activity profiles and X-radiography, the thickness and character of the SL were assessed at four locations reoccupied during four seasonal periods corresponding to various riverine and oceanographic conditions (e.g., discharge stage, wind stress); one of these stations was reoccupied during semidiurnal and fortnightly periods. Additionally, replicate sediment cores were obtained at stations in order to assess small-scale spatial variability of geochemical and sedimentological characteristics in the seabed. The SL thicknesses in replicate sediment cores obtained at each site were found to vary within ±5 cm. Normalized thickness of the SL decreased by an average of 17 cm between August 1989 and February 1990, a period of increasing wind stress during falling-to-rising river discharge. The SL thickness subsequently increased slightly (8 cm) from February to May 1990, and increased again (27 cm) between May 1990 and October 1991. The maximum SL thicknesses were observed in October, a period of low wind stress and minimal river input, suggesting that the observed seasonal changes are dictated by oceanographic conditions as well as by sediment discharge. Variations in SL thickness are reflected by changes in suspended sediment (mostly as fluid muds), as the highest suspended sediment inventories and most extensive fluid muds are observed during the period when SL thickness is at a minimum. Although a significant SL was present during all of the sampling periods, changes in the character of 210Pb profiles at two of the stations suggest that substantial, perhaps complete, removal and redeposition of the SL occurred some time between February and May. Preserved strata (i.e., below the SL) is characterized by greater evidence of bioturbation and increased sand content, but otherwise is similar to that of the SL. Long-term preservation of strata is thought to occur through episodic suspension and redeposition of the SL, with net accumulation of decimeter-thick, hiatus-bounded layers.