Hdb Env Chem Vol. 5, Part H (2006): 71–90 DOI 10.1007/698_5_029  Springer-Verlag Berlin Heidelberg 2005 Published online: 20 October 2005 Geochemistry of the Amazon Estuary Joseph M. Smoak 1 (✉) · James M. Krest 1 · Peter W. Swarzenski 2 1 Environmental Science, University of South Florida, St. Petersburg, Florida 33701, USA smoak@stpt.usf.edu, krest@stpt.usf.edu 2 Center for Coastal and Regional Marine Studies, US Geological Survey, St. Petersburg, Florida 33701, USA pswarzen@usgs.gov 1 Introduction ................................... 72 1.1 Background setting ............................... 72 2 Chemical Processes ............................... 74 2.1 Major and Minor Elements ........................... 75 2.2 Radionuclide Tracers .............................. 78 2.2.1 Uranium ..................................... 80 2.2.2 Thorium ..................................... 81 2.2.3 Lead ....................................... 81 2.2.4 Radium ...................................... 82 2.3 Biogeochemical Cycling ............................. 83 2.3.1 Carbon ...................................... 83 2.3.2 Nitrogen ..................................... 85 2.3.3 Phosphorus ................................... 87 3 Conclusions ................................... 88 References ....................................... 88 Abstract The Amazon River supplies more freshwater to the ocean than any other river in the world. This enormous volume of freshwater forces the estuarine mixing out of the river channel and onto the continental shelf. On the continental shelf, the estuar- ine mixing occurs in a very dynamic environment unlike that of a typical estuary. The tides, the wind, and the boundary current that sweeps the continental shelf have a pro- nounced influence on the chemical and biological processes occurring within the estuary. The dynamic environment, along with the enormous supply of water, solutes and par- ticles makes the Amazon estuary unique. This chapter describes the unique features of the Amazon estuary and how these features influence the processes occurring within the estuary. Examined are the supply and cycling of major and minor elements, and the use of naturally occurring radionuclides to trace processes including water movement, scaveng- ing, sediment-water interaction, and sediment accumulation rates. The biogeochemical cycling of carbon, nitrogen, and phosphorus, and the significances of the Amazon estuary in the global mass balance of these elements are examined. Keywords Major elements · Minor elements · Nutrients · Organic matter · Radionuclides