14C and 13C Constraints on CO2 Cycling in Pristine and Deforested Lowland Amazonian Rivers

"Lowland" rivers in the Amazon are those that, unlike the Amazon mainstem, do not drain the Andean cordillera and are not directly influenced by sediments recently eroded from the Andes. The Brazilian Amazon is predominantly drained by lowland watersheds. We examined the sources and fate of CO2 and Dissolved Inorganic Carbon (DIC) in a range of small to large lowland rivers across the region. In particular, we focused on the Ji-Parana basin, which spans the deforestation arc in the state of Rondonia. Carbonate weathering appears to be a strong source of DIC in the western lowland basins (Jurua and Purus); however, carbonate-derived DIC in these systems is completely flushed out downstream through outgassing and mixing with contemporary respiratory CO2 inputs and carbonate-free tributaries. DIC exported from carbonate-free lowland soils appers to be predominantly contemporary, but some regions in the Ji-Parana headwaters export CO2 with a mean age of several decades. In that system, replacement of C3 forests with C4 pastures appears to exert a strong influence on riverine carbon cycling from small to large rivers. Inputs from riparian grasses may play a disproportionately important role. Finally, we found tentative evidence that anoxic conditions in wetlands may result in mineralization of previously protected, aged organic matter, leading to significant aging in riverine DIC.