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

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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 budgets1,2. 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 carbon1. High carbon dioxide concentrations in rivers originate largely from in situ respiration of organic carbon1,2,3, but little agreement exists about the sources or turnover times of this carbon2,4,5. 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.

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Figure 1: Amazon basin and river sites sampled for carbon isotopes.
Figure 2: Distribution of 14 C and 13 C isotopes.
Figure 3: Temporal evolution of 14 C-CO 2 at four lowland sites from medium to large rivers in the Ji-Parana basin and Rio Negro.


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We thank staff at the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry for assistance with radiocarbon analyses; C. Llerena, B. Forsberg, L. Maurice-Bourgoin and J. Quintanilla for assistance with field campaigns; and I. Levin for assistance with atmospheric 14CO2 data. This work was funded by a LLNL CAMS Minigrant, US NSF DEB, NASA EOS and LBA projects, the Brazilian FAPESP agency, and a NASA ESS graduate fellowship to E.M. This work was performed, in part, under the auspices of the US Department of Energy by the University of California, LLNL.Author Contributions E.M. and A.K.A. contributed equally to all stages of this work. C.A.M. and T.A.B. supported the 14C analyses and contributed to discussions and manuscript preparation. A.V.K. collected and pre-processed many of the samples and contributed to discussions and manuscript preparation. J.I.H., P.D.Q. and J.E.R. contributed to project design and execution, including manuscript preparation.

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Correspondence to Emilio Mayorga.

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Supplementary Notes

This file includes the Supplementary Discussion, which addresses the use of δ13C of CO2 instead of δ13C-DIC, organic carbon sources, and the influence of carbonate weathering. Three Supplementary Tables list the isotopic and associated data presented in this study, and Supplementary Figure S1 describes the dominant weathering regime at each site. (DOC 350 kb)

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Mayorga, E., Aufdenkampe, A., Masiello, C. et al. Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers. Nature 436, 538–541 (2005) doi:10.1038/nature03880

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