Carbon influence on metal distribution in sediment of Amazonian macrotidal estuaries of northeastern Brazil.

The present work aims to observe the spatial distribution of metals associated with carbon forms (fraction < 2 mm) in surface sediments of two macrotidal estuaries, São Marcos Bay and Anil River Estuary, which are located within the transition region between the Amazonian and the semi-arid northeast regions. Grain size, metal content (Al, Fe, Mn, Cu, Pb, Cr, Zn, and Ni), organic matter, and calcium carbonate content were determined. Grain size analyses showed the predominance of the sand-sized fraction < 2 mm due to the local hydrodynamic conditions. Anil River Estuary sediments exhibited high organic matter content due to both the mangrove outwelling and domestic sewage discharge. They also presented high calcium carbonate content as a result of abundant remnants of gastropod shells. Organic matter acted as the primary geochemical carrier for most metals in both estuaries, while calcium carbonate acted as the secondary carrier. Enrichment factors indicated Mn sediment contamination in São Marcos Bay and Fe, Pb, and Zn contamination in the Anil River Estuary. These results also suggest that São Marcos Bay is influenced by harbor activities, mostly ore shipment, whereas Anil River Estuary sediments are enriched in these metals as a result of domestic and hospital effluents reaching the urbanized drainage basin.