Amazon shelf

McKee BA, DeMaster DJ, Nittrouer CA (1984) The use of Th-234/U-238 disequilibrium to examine the fate of particle-reactive species on the Yangtze continental shelf. Earth Planet Sci Lett 68 431-42 McKee BA, DeMaster DJ, Nittrouer CA (1986) Temporal variability in the portioning of thorium between dissolved and particulate phases on the Amazon shelf Implications for the scavenging of particle-reactive species. Cont Shelf Res 6 87-106... 

Mac Kee BA, Demastre DJ, Nittrouer CA (1987) Uranium geochemistry on the Amazon Shelf Evidence for uranium release from bottom sediments. Geochim Cosmochim Acta 51 2779-2786 Mangini A, Sormtag C, Bertsch G, Muller E (1979) Evidence for a higher natural uranium content in world rivers. Natrrre 278 337-339... 

Moore WS, Astwood H, Lindstrom C (1995) Radium isotopes in coastal waters on the Amazon shelf. Geochim Cosmochim Acta 59 4285-4298... 

Figure 3. The concentration of nranium (nM) versns salinity on the Amazon Shelf with an ideal dilntion line drawn throngh the riverine and seawater end members. Removal of dissolved U is evident at salinities that range from 0 to 16. The bottom illustration shows as a function of salinity for the same waters on the Amazon Shelf. A seawater value (144 0.2) is rapidly reached at a salinity of 4 during estuarine mixing. The high turbidity zone of the water colurtm is defined by the greatest suspended particulate concentrations. Data from Swarzenski et al. (2003).

Moore WS, Demaster DJ, Smoak JM, McKee BA, Swarzenski PW (1996) Radionuclide tracers of sediment-water interactions on the Amazon shelf. Cont Shelf Res 16 645-665 Moran SB, Moore RM (1989) The distribution of colloidal aluminum and organic carbon in coastal and open ocean waters offNova Scotia. Geochim Cosmochim Acta 53 2519-2527 Nozaki Y (1991) The systematics and kinetics of U/Th decay series nuchdes in ocean water. Rev Aquatic Sci 4 75-105... 

Sullivan, K.A. and Aller, R.C. (1996) Diagenetic cycling of arsenic in Amazon shelf sediments. Geochimica et Cosmochimica Acta, 60(9), 1465-77. 

Aller, R.C., Blair, N.E., Xia, Q., and Rude, P.D. (1996) Remineralization rates, recycling and storage of carbon in Amazon shelf sediments. Cont. Shelf Res. 16, 753-786. 

DeMaster, D.J., and Pope, R.H. (1996) Nutrient dynamics in Amazon shelf waters results fromAMASSEDS. Cont. Shelf Res. 16, 263-289. 

Biogeochemical Processes on the Amazon Shelf Changes in Dissolved and Particulate Fluxes During River/Ocean Mixing... 

Water column biogeochemical processes, however, do not always leave an obvious record in the seabed. For example, Amazon shelf sediments have little biogenic silica accumulation despite extensive diatom production and abundance in estuarine surface... 

A historical perspective of the primary research that has shaped our current understanding of Amazon shelf biogeochemistry is presented initially as an overview. The primary focus of the chapter, however, is on biogeochemical cycles and related insights that have been developed during the past 5 years, many of which have come from the recendy completed AmasSeds Project (see Nittrouer and DeMaster 1996, for overview). 

Figure 17.1 Map illustrating the locations of dominant physical and biological processes on the Amazon shelf. The locations of two transects (River Mouth Transect (RMT) and Open Shelf Transect (OSTO) frequently occupied during the AmasSeds Program are shown. The Amazon shelf is defined as the shelf area between the southern extent of Amazon River influence and the border between Brazil and French Guiana. The seaward edge of the study area is the 100 m isobath.

In order to understand biogeochemical processes on the Amazon shelf, knowledge of the general physical regime is essential. The Amazon shelf, as defined in this chapter, extends from the southern limit of the Amazon River plume northward to the... 

Early research on the Amazon shelf focused largely on the magnitudes of the chemical and sediment discharges (Williams 1968, Gibbs 1972), as well as the sources of nutrients to Amazon shelf phytoplankton. 

Figure 17. 2 Schematic representation of the seasonal variations in physical forces, discharges, and suspended sediment inventory on the Amazon shelf (from Nittrouer and DeMaster 1996, and Kineke et. al. 1996). The suspended sediment inventories include the fluid mud layers, which can comprise more the 90% of the suspended material on the shelf. The timing of the 4 AmasSeds cruises are shown at the bottom of the figure. In the upper figure NBC refers to the North Brazilian Coastal Current,...

Table 17.1 Concentrations and Fluxes Used in Amazon Shelf Modeling.

Stable isotopes of carbon also have been used on the Amazon shelf to resolve terrestrial (--26 to -30 per mil) and marine ( -22.5 per mil) sources of POC (Showers and Angle... 

A summary of physical and biochemical parameters for the Amazon shelf are presented in Table 17.1. These data were used to develop the mass balance calculations described in the remainder of the paper. The rate of Amazon River sediment discharge that is cited in most studies (Meade et al. 1985) characterizes the flux at Obidos, which is hundreds of kilometers from the river mouth. The sediment fluxes that have been documented for the Amazon coastal area are the fine-grained sediment accumulation rate on the shelf (6.3 x 10 g yr, Kuehl et al. 1986, 1996) and the advection rate of particulate material northward out of the study area, primarily in nearshore environments (-1.2 X 10 g yr, Allison et al. 1995). Because some sediment may be deposited between Obidos and the river mouth, we have chosen to use the sum of the shelf... 

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