14C AND 13C ISOTOPIC CONSTRAINTS ON INORGANIC CARBON CYCLING IN THE AMAZON RIVER SYSTEM

We analyzed the 14C and 13C isotopic composition of dissolved inorganic carbon (DIC) and organic carbon (OC) size fractions in Amazonian rivers ranging from mountain and lowland streams to the mainstem. Associated measurements include DIC, POC, DOC, and major ions. We examine variability in DIC, CO2, and DIC isotopes in the context of dominant wethering regimes. Carbonate weathering is a dominant or significant source of DIC in many rivers draining the Andes and headwaters of the Jurua and Purus in the central-western Amazon. In the majority of mid to large lowland river systems that are not influenced by carbonate weathering, DIC is very young, and its associated CO2 is generally 13C-enriched compared to bulk OC or its primary size fractions. Respiration is dominated by a labile OC fraction that is a small, unmeasured component of DOC or POC. We focus on the evolution of DIC isotopes along a couple of longitudinal transects to evaluate the role of weathering sources, in situ respiration, CO2 evasion and mixing with air, and mixing with tributaries. In lowland streams on Precambrian Shields draining highly weathered aluminosilicates, the age of DIC derived from soils ranges from contemporary to possibly several decades. In a highly deforested subbasin, enriched 13C of DIC indicates that replacement of forest by C4 pasture has had a substantial impact on metabolism and the source of DIC to rivers. From the Andean headwaters in Peru to the Amazon mainstem, DIC evolves from carbonate weathering and possibly lithospheric degassing sources in the upper Andes, through persistent carbonate weathering along 1500 km of lowland depositional environments, and gradual replacement along the mainstem by lowland-derived, younger DIC.