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Methane emissions to the troposphere from the Amazon River floodplain have been determined. In total, 94 individual flux measurements were made along a 1,700-km stretch of the river during the early falling water period of the flood cycle (July–August 1985). The overall average rate of CH4 emission from wetlands was 390 mg CH4 m−2 d−1. Water surfaces covered by aquatic macrophytes had the highest emissions, 590 mg CH4 m−2 d−1, while emissions from flooded forests were 110 mg CH4 m−2 d−1 and open lake areas averaged 120 mg CH4 m−2 d−1. Ebullition was the dominant mechanism of emission, accounting for 85% of the total. Surface water dissolved oxygen was always undersaturated, while dissolved CO2 concentrations were significantly supersaturated with respect to atmospheric equilibrium. Surface water CH4 concentrations were highly supersaturated, averaging 6.4 μM. The δ13C of the CH4 varied from −42‰ to −73‰ with an apparent trend toward heavier values downstream. Extrapolation to the entire Amazon floodplain results in estimated annual emissions in the troposphere of 10 Tg yr−1 and indicates the importance of the area in the global atmospheric CH4 cycle.