The immense discharge of the Amazon River causes river/ocean mixing to take place out on the continental shelf instead of within a drowned river valley, as in many smaller dispersal systems (Nittrouer and DeMaster, 1996). The magnitude of this discharge can be appreciated by recognizing that the Amazon River supplies approximately 20% (6 x 1015 L yr⁻¹) of the freshwater reaching the oceans via fluvial transport and roughly 6% (1.2 x 1015 g yr⁻¹, Meade et al. 1985) of the global riverine sediment discharge. Chemical, physical, and biological processes occurring in the river/ocean mixing zone control the fates of these riverine materials, as well as the fates of substances brought onto the shelf from offshore as a result of the estuarine-like circulation. Depending on balances between transport, reaction rates, and sedimentation, the mixing zone may act as a net source, sink, or bypass conduit for chemical species in the coastal environment. For example, if Amazon River nutrients such as silicate, phosphate, or nitrate are simply removed from solution and buried as particulate biogenic debris on the adjacent shelf, the river would have little influence on global ocean nutrient budgets. In contrast, if nutrients coming down the river are not efficiently buried nearshore (as a result of minimal biological uptake or efficient recycling), then they may contribute to larger scale oceanic or atmospheric budgets of Si, P, and N (Treguer et al. 1995; Delaney, 1998). The Amazon River transports ~1015 moles yr⁻¹ of particulate organic carbon from the terrestrial environment to the ocean (Degens et al. 1991). The fate of this material (some of it from leaf litter and some of it from older, more refractory soils) is important to understand because the Amazon River/ocean mixing zone comprises a significant fraction of all deltaic depositional environments, where ~50% of the marine burial of organic matter occurs (Berner 1982, 1989, Hedges and Keil 1995; Devol et al. this volume). The Amazon River also discharges an equivalent amount of dissolved organic carbon (~1012 moles yr⁻¹), much of which is in the form of high molecular weight organic compounds (Sholkovitz et al. 1978, Degens et al. 1991).