HR: 11:35h
AN: OS22A-06    [Abstracts]
TI: Amazon Shelf Sediment Accumulation Rate and Subaqueous Delta Growth
AU: * Figueiredo, A G
EM: alberto@igeo.uff.br
AF: Dept. of Geology, Lagemar - UFF, Av. Litoranea, sn, Niteroi, RJ 24210-340, Brazil
AU: Nittrouer, C A
EM: nittroue@ocean.washington.edu
AF: School of Oceanography, University of Washington, School of Oceanography, University of Washington, Seattle, WA 98040, United States
AB: The Amazon drainage basin is one of the largest basins on Earth extending over 7x106 km2, and water and sediment produced are carried by the Amazon River. Sediment brought to Amazon shelf is spread on a subaqueous delta along the coast from river mouth to the Guiana coast, 1600 km to the north. Delta width varies from 200 km at the river mouth to 40 km at Cape Orange (Brazil). The subaqueous delta has topset, foreset and bottomset beds. The topset beds are found in water depths between 10 and 30-40 m, the foreset are between 30-40 m and 70 m, and bottomset are from 70 to 75-80 m. Shallow seismic data reveals that the beds of the foreset prograde oceanward at a larger rate than the topset buildup, indicating that shelf subsidence rate is less than sediment accumulation rate. The subaqueous delta progrades over carbonate-rich sandier substrate that extends to the shelf break. The substrate is molded by tidal currents into ridges normal to subaqueous-delta progradation. Subaqueous-delta sediments are sandy near the river mouth and become muddier toward north. Muddy sediments move seaward and partially fill the inter-ridge depressions. Near the river mouth, sediment magnetic susceptibility measured along cores is higher and more variable (250-550 x 106 SI), and in the distal portion is lower and less variable (280-350 x 106 SI). Despite diverse sediment types, accumulation rates based on C-14 dating in the foreset beds near the river mouth and distal portion are very similar, 5.1 mm/y and 4.3 mm/y. Accumulation rates based on Pb-210 are two orders of magnitude greater (100-600 mm/y). Particulate organic matter derived from the terrestrial basin and from marine plankton production are incorporated and buried in the delta. Biogeochemical processes transform part of the organic matter into methane gas, which can escape to the atmosphere or be trapped in sediment. High-resolution, shallow seismic data indicate a large amount of methane gas in the subaqueous delta distributed over 31,000 km2.
DE: 3020 Littoral processes
DE: 3022 Marine sediments: processes and transport
DE: 4217 Coastal processes
DE: 4235 Estuarine processes (0442)
DE: 4558 Sediment transport (1862)
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly