The sources and accumulation of sedimentary organic matter in two estuaries in the Brazilian Northern coast

The origin of the organic matter was assessed in two estuaries in Eastern Amazon, Brazilian Northern Coast, under different hydrodynamic conditions. The isotopic composition (δ13C; δ15N) and the elementary ratio (C/N) were determined in the sediments, in the suspended particulate material (SPM), as well as in the phytoplankton, plants and oysters. The (δ13C) values have indicated that the sedimentary organic matter is a terrestrial-marine mix composed of terrigenous organic matter (75%) in the Pará River estuary, as well as of marine organic matter (60%) in the Mocajuba River estuary. The estimates of sources recorded through δ13C data were partially confirmed through the correlation (δ15N) versus C/N, which reflects the action of diagenetic and biological processes on the organic matter composition control. On the other hand, the high carbon content (δ13C) found in Pará River estuary samples is conclusive because it shows isotopic changes due to the anthropogenic processes in the region.