Abstract

Geochemical and sedimentological studies have been conducted on one 6 m core and on a series of 1-3 m cores taken over nearly equal 100 km of the Hudson Estuary. Each gram (dry-weight basis) of estuarine silt contains: organic matter 30-100 mg; carbonate generally < 30 mg; quartz 250-400 mg; potassium 18-24 mg; zinc 50-550 mu g; copper 15-400 mu g; and lead 20-800 mu g. In most localities, the higher trace metal levels, as well as anthropogenic detritus ( e.g. , metalliferous slags, fly ash and coal), and reactor- and bomb-produced radionuclides ( e.g ., 137 Cs, 134 Cs 60 Co) are confined to the upper 10 cm of the sediment; but in the inner harbor area of New York they have been observed to sediment depths of 250 cm. In some areas of the inner harbor the vertical distribution of anthropogenic radionuclides indicates sedimentation rates of 5-20 cm/yr. The top 10 cm of the inner harbor sediment is highly liquefied and the top meter shows extensive turbation, although distinct sand layers and laminated zones are present. The turbation may have resulted from the release or entrapment of biochemically formed gases, as well as from mechanical mixing of sediment by organisms. 14 C analyses of organic matter in inner harbor surface sediments indicate that the major source of carbon is recent sewage, nearly all of which is discharged in this area of the estuary. The fine-grained sediments of the natural channel and subtidal bank, upstream of the inner harbor, are characterized by alternating layers of fine sandy silts and clay-rich silts on a mm to cm scale. Downstream from the Tappan Zee, sand-shell layers, 1-20 cm thick, occur at the channel surface and are interlayered with zones of laminated, fine-grained sediment at depth. Radiocarbon dating of shells and shell layers indicates a minimum net sedimentation rate of 1-3 mm/yr in this area of the estuary during the past 3000 years. In cores taken on the subtidal bank, coarse sand-shell layers are absent, but turbate zones, 1-10 cm thick, occur at the surface and are interlayered with the laminated zones at depth. One possible interpretation of the sedimentary structures is: (1) the laminated, fine-grained sediments are deposited when the Hudson transports relatively high concentrations of sediment, such as during and after severe storms or other short term events of large scale resuspension; (2) the sand-shell layers result from increased sand transport during high-energy (storm) conditions or from tidal scour under normal flow conditions; and (3) the turbate zones represent periods of slower or no deposition during normal flow conditions. Consequently, it appears that everyday tidal and estuarine processes are causing the rapid accumulation of recent "polluted" sediment in specific areas, such as the inner harbor, whereas storm deposits characterize the sedimentary record in the channel and subtidal bank environments of the river estuary, upstream of New York City. This large variation in both patterns and rates of sediment deposition in the Hudson Estuary has not been previously identified on the basis of grain-size and mineralogical analyses.

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