HR: 0800h
AN: OS51B-1298    [Abstracts]
TI: Sediment dispersal and deposition on the Waiapu River Shelf, N.Z., implications for sediment transport mechanisms and event preservation
AU: * Kniskern, T A
EM: knista@vims.edu
AF: Virginia Institute of Marine Science, 1208 Greate Rd, Gloucester Point, VA 23062 United States
AU: Kuehl, S A
EM: kuehl@vims.edu
AF: Virginia Institute of Marine Science, 1208 Greate Rd, Gloucester Point, VA 23062 United States
AB: Sediment dispersal off an extremely high-yield river, the Waiapu River, New Zealand, is examined using a suite of gravity cores collected on the adjacent highly-energetic continental shelf. High $^{210}$Pb accumulation rates are observed, coincident with high surficial clay content, on the mid-to outer-shelf. The nearshore region sediments are composed of discrete physically mixed layers, trending to high (1.5 to 3 cm yr$^{-1}$) $^{210}$Pb accumulation rates on the mid-shelf and lower accumulation rates ($<$1 cm yr$^{-1}$) on the outer-shelf and submarine canyon area. X-radiographs indicate that sediments generally either bypass the nearshore or are partially preserved in event layers. These event layers are observed less frequently on the mid- and outer-shelf interspersed with homogenous sediments or sediments that have been bioturbated. The presence of bioturbation in the sediment cores seems to correlate with distance from the Waiapu River mouth, not bathymetry, indicating that the area around the river mouth is too physically disturbed to allow benthic communities to flourish. This may also indicate that riverine deposition and dispersal mechanisms may have more impact on the shelf than wave energy. Seasonal variations in sediment depositional patterns are evident from $^7$Be geochronology. Surface sediments collected during an annual period of high river discharge (June through September) reveal that sediment deposition on the entire shelf is sparse and ranges from the nearshore to the shelf break. A contrasting pattern of deposition was observed during the stormy season (April through June) when $^7$Be was found on most of the shelf and even beyond the shelf break. High sediment accumulation rates and the energetic hydrodynamic regime of the Waiapu River shelf imply that a variety of sediment transport mechanisms deliver sediments to the shelf and slope. Furthermore, these high accumulation rates and the low observed benthic activity may allow event layers to be preserved in the sediments.
DE: 4860 Radioactivity and radioisotopes
DE: 4215 Climate and interannual variability (3309)
DE: 4219 Continental shelf processes
DE: 4558 Sediment transport
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting