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. Many studies investigating these chemical reactions have been conducted in the estuaries of small rivers (discharge < 10 super(14) l/y) 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 of riverine 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. 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 river systems commonly have suspended-sediment concentrations in surface waters on the order of tens of mg/l, whereas large rivers have suspended sediment concentrations on the order of hundreds of mg/l. This project examines the many chemical processes occurring near the mouth of the Amazon River that are affected by the enormous discharge of sediment particles. The specific topics studied in this portion of AmasSeds include the evaluation of: 1) the effects of suspended sediment concentrations on the biological uptake and regeneration of nutrients and carbon; 2) scavenging of particle-reactive elements (e.g., Th and Pb) from the water column; and 3) mixing and transport of Amazon shelf waters by measuring radium isotopes released from suspended sediments and the oxygen isotopic signature of different water masses. Of the four AmasSeds cruises, three have been completed (I: falling river discharge in August 1989; II: rising discharge in March 1990; and III: high discharge in May 1990). Initial results from geochemical studies of the waters at the Amazon River/ocean boundary are presented below. (DBO)