Geochemical Processes Occurring in the Waters at the Amazon River/Ocean Boundary

Abstract

GEOCHEMICAL REACTIONS occurring in estuarine waters can significantly affect the riverine flux of nutrients, minor elements, and trace elements to the ocean. Biological uptake, adsorption/desorption reactions, coagulation, as well as precipitation of iron hydroxides and high-molecular-weight organic acids, all can affect the chemical flux to the marine environment (e.g., Boyle et al., 1977; Sholkovitz et aL, 1978; DeMaster et aL, 1986). Many studies investigating these chemical reactions have been conducted in the estuaries of small rivers (discharge < 1014 1 y-l) because of simplicity in modeling the hydrographic regime or because of logistical reasons. Geochemical studies in these small estuarine systems have provided valuable insight to the chemical reactions that occur during the mixing ofriverine and oceanic waters. However, much of the chemical flux from rivers enters the ocean via large dispersal systems. For example, the Amazon River alone supplies 17% of all the dissolved silica reaching the marine environment via rivers (Livingstone, 1963). Estuarine processes occurring in the more frequently studied small dispersal systems are extrapolated to those occurring in the large dispersal systems, but the validity of this extrapolation is unclear. Suspended-sediment concentration is one of the important factors that differs between small and large dispersal systems. Small fiver systems commonly have suspended-sediment concentrations in surface waters on the order of tens of mg 1 a, whereas large rivers (i.e., the Amazon River or Yangtze River) have suspended sediment concentrations on the order of hundreds of mg 1-1.

The balance between silica production and silica dissolution in the sea: Insights from Monterey Bay, California, applied to the global data set

“…Eight regional estimates are available representing a range of ocean environments, including such widely differing systems as the Sargasso Sea, the Peru upwelling system, and the Antarctic Circumpolar Current (ACC). Two data sets were excluded from the analysis: the study by Nelson and Goering (1977) off northwest Africa and the data from the turbid waters of the Amazon River plume presented by DeMaster et al (1991). In both studies, the reported # D : # P ratios have a high likelihood of reflecting the characteristics of allochthonous silica rather than being representative of the rate of recycling of biogenic silica produced and retained within the euphotic zone.…”

Diatom growth and productivity in an oligo-trophic midocean gyre: A 3-yr record from the Sargasso Sea near Bermuda

“…Nelson and Goering 1978;Brzezinski and Nelson 1989), as is the observation that silica production continues at undiminished rates through the night (e.g. Brzezinski and Nelson 1989;DeMaster et al 1991). The detectable silica production at 160 m, in particular, means that our vertically integrated silica production rates in the upper 160 m (discussed below) are minimum estimates for the upper water column.…”

“…Ratios of silica dissolution to silica production exceeding 50% have been measured isotopically in surface waters off northwest Africa (Nelson and Goering 1977b), in the plume of the Amazon River (DeMaster et al 1991), in the Ross Sea (Nelson et al 1991), and in a Gulf Stream warm-core ring (Brzezinski and Nelson 1989). In all of those studies, surface-layer biogenic silica concentrations were at least 40 times higher than those typical of the BATS site during nonbloom periods, permitting silica dissolution rates to be measured directly by isotope-dilution methods.…”

“…Rates of biogenic silica production have been measured and rates of diatom growth and carbon production estimated for a reasonably wide variety of marine systems via tracer incubation studies using the stable isotope "OSi (e.g. Nelson and Smith 1986;DeMaster et al 1991) and the radioisotope 32Si (e.g. Treguer et al 1991).…”

Monsoon related variations in Zaire (Congo) sediment load and influence of fluvial silicate supply on marine productivity in the east equatorial Atlantic during the last 200,000 years

“…In the time domain, changes in lateral supply of oceanic nutrients and upwelling intensity controlled both the extent of primary production and also total opal production off the Zaire because the river plume production underlies the same limitation by marine nitrate and phosphate levels controlling the oceanic productivity in the entire eastern Angola Basin. This situation off the Zaire seems to be different from the Amazon River mouth, where recent high opal productivity in coastal waters has been attributed mainly to river-borne nutrients [DeMaster et al, 1983[DeMaster et al, , 1991. Investigations of late Quaternary opal MAR here and off other major rivers distributing high amounts of silica to the ocean should reveal whether changes in fluctuations of river-induced opal production are controlled by oceanic nutrient concentrations in general, or if this is a situation unique to the Zaire River mouth.…”