Adrienne Hollister

• The Amazon is the largest River on earth, accounting for 15–20% of the global river freshwater discharge, and making it an important source of trace metals, nutrients and organic matter to the Atlantic Ocean. The nearby Pará River is the 5th largest river and converges to mix in the Amazon Estuary. Trace metals in the ocean (e.g., Mn, Co, Fe, Ni, Cu, Zn, Cd and Pb) act as important nutrients and/or toxins to marine organisms. However, no data exists for these trace metals in the Amazon Estuary after 1976. Therefore, it is of urgent importance to establish a baseline for trace metals in the Amazon estuary. A GEOTRACES process study (cruise GApr11) was conducted in the Amazon estuary during the wet season (April–May) of 2018. Herein we present data for dissolved trace metals and organic matter from samples collected from this cruise. Chapter 1 focuses on copper (Cu), a micronutrient and potential toxin, and its complexation to organic ligands. Chapter 2 discusses two other micronutrients, cobalt (Co) and nickel (Ni), in surface and depth samples analyzed by two different methods. Chapter 3 brings together all trace metals from this study (Al, Mn, Co, Fe, Ni, Cu, Zn, Cd, Pb and U) to calculate the fluxes from the Amazon and Pará Rivers into the Atlantic Ocean. Finally, chapter 4 describes depth profiles of bioactive metals in different size fractions. Trace metal cycling in the estuary was influenced by complex biogeochemical processes, including ligand complexation, particle adsorption-desorption, colloidal flocculation, physical mixing and biological activity. In addition, we observed distinct influences from the Amazon and Pará Rivers, which draw from distinct catchment areas. Cu was mostly conservative with respect to salinity, while Fe and Pb were highly particle reactive during early mixing and experienced the greatest estuary removal. We estimated that the Amazon and Pará Rivers account for ~21% and 18% of the global riverine Cu and Ni to the oceans.