Alex V. Krusche

Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers

Rivers are generally supersaturated with respect to carbon dioxide, resulting in large gas evasion fluxes that can be a significant component of regional net carbon budgets. Amazonian rivers were recently shown to outgas more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon, but little agreement exists about the sources or turnover times of this carbon. Here we present results of an extensive survey of the carbon isotope composition (13C and 14C) of dissolved inorganic carbon and three size-fractions of organic carbon across the Amazonian river system. We find that respiration of contemporary organic matter (less than five years old) originating on land and near rivers is the dominant source of excess carbon dioxide that drives outgassing in medium to large rivers, although we find that bulk organic carbon fractions transported by these rivers range from tens to thousands of years in age. We therefore suggest that a small, rapidly cycling pool of organic carbon is responsible for the large carbon fluxes from land to water to atmosphere in the humid tropics.

DINÂMICA DO CARBONO INORGÂNICO DISSOLVIDO NOS RIOS TELES PIRES E CRISTALINO NA BACIA AMAZÔNICA

The rivers are extremely important in the carbon cycle, either inthe CO2 out gassing flows or carbon exporters to the oceans. Directmeasuring of CO2 exchanges between the terrestrial ecosystems and theatmosphere should consider the aquatic systems flows, not overestimatingthe carbon terrestrial accumulation. The Amazon basin has the largest andmore complex system of fresh water in the world, so that it holds animportant in the global carbon cycle. The objective of this work was toevaluate, during the period between September 2004 to August 2005, thedynamics of the Total Dissolved Inorganic Carbon (DIC) and fractions(dissolved CO2 + HCO3 - + CO32 -), in the waters of Teles Pires andCristalino rivers, components of the Amazon basin, in the district of AltaFloresta, MT. Fortnightly were determined DIC concentration, pH andtemperature of the rivers. Based in those results and applying thethermodynamic balance equations, it was possible to estimate the pCO2and the carbonate system fractions. The rivers presented a seasonal variationin the pH values, being closer to the neutrality in the drought period. Theconcentration of HCO3- was significantly larger in the drought than duringthe flood season. Both studied rivers were oversaturated in CO2 related tothe atmosphere.