Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59....7s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 1, pp.7-18
Other
11
Scientific paper
The size distribution of U was examined in surface waters of the Amazon shelf. Water samples were collected during a low discharge river stage across a broad salinity gradient (0.3-35.4%) and fractionated by planar filtration and tangential-flow ultrafiltration into (1) solution (U s , <10,000 MW; ~1-10 nm), (2) colloidal (U c 10,000 MW-0.4 m), (3) dissolved (U d , <0.4 m), and (4) particulate (U p , >0.4 m) phases. Concentrations of colloidal U comprise up to 92% of the dissolved U fraction at the river mouth and attain highest values (~0.45 g/L) in the productive, biogenic region of the Amazon shelf (salinities above ~20%). U d and U c distributions are highly nonconservative relative to ideal dilution of river water and seawater, indicating extensive removal at salinities below ~10%. The distribution of U s also shows some nonconservative behavior, yet removal, if any, is minimal. Saltwater-induced precipitation and aggregation of riverine colloidal material is most likely the dominant mechanism of U removal in the low salinity, terrigenous region of the Amazon shelf. There is evidence of a substantial colloidal U input (~245% of the riverine U c flux into surface waters above 5%. Such U c enrichment most likely is the result of colloidal U-rich porewater diffusion/advection from the seabed and fluid muds or shelf-wide particle-colloid disaggregation. Removal of solution and dissolved phase U via a colloidal intermediate and U c aggregation was examined in terms of coagulation theory. The highly reactive nature of all U phases on the Amazon shelf suggests that remobilization and fractionation of U may also occur in other river-influenced coastal environments.
Booth James G.
McKee Brent A.
Swarzenski Peter W.
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