Nature of sediment accumulation on the Amazon continental shelf

Abstract

Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g. 210Pb, 14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment. 210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <0.1 cm y 1 (0.1 g cm 2 y 1) nearshore, to rates as high as ∼ 10 cm y −1 (6.9 g cm 2 y −1) on the outer topset and the foreset regions (30–50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system. 14C dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 10 8 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 10 8tons y −1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.