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

Article metrics

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

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.

References

  1. 1

    Richey, J. E., Melack, J. M., Aufdenkampe, A. K., Ballester, M. V. & Hess, L. L. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2 . Nature 416, 617–620 (2002)

  2. 2

    Cole, J. J. & Caraco, N. F. Carbon in catchments: Connecting terrestrial carbon losses with aquatic metabolism. Mar. Freshwat. Res. 52, 101–110 (2001)

  3. 3

    Mulholland, P. J. et al. Inter-biome comparison of factors controlling stream metabolism. Freshwat. Biol. 46, 1503–1517 (2001)

  4. 4

    Raymond, P. A. & Hopkinson, C. S. Ecosystem modulation of dissolved carbon age in a temperate marsh-dominated estuary. Ecosystems 6, 694–705 (2003)

  5. 5

    Raymond, P. A. & Bauer, J. E. Riverine export of aged terrestrial organic matter to the North Atlantic Ocean. Nature 409, 497–500 (2001)

  6. 6

    Jones, J. B. & Mulholland, P. J. Carbon dioxide variation in a hardwood forest stream: An integrative measure of whole catchment soil respiration. Ecosystems 1, 183–196 (1998)

  7. 7

    Quay, P. D. et al. Carbon cycling in the Amazon River: Implications from the 13C compositions of particles and solutes. Limnol. Oceanogr. 37, 857–871 (1992)

  8. 8

    Devol, A. H. & Hedges, J. I. in The Biogeochemistry of the Amazon Basin (eds McClain, M. E., Victoria, R. L. & Richey, J. E.) 275–306 (Oxford Univ. Press, New York, 2001)

  9. 9

    Hedges, J. I. et al. Organic carbon-14 in the Amazon River system. Science 231, 1129–1131 (1986)

  10. 10

    Clark, I. & Fritz, P. Environmental Isotopes in Hydrogeology (Lewis Publishers, Boca Raton, 1997)

  11. 11

    Stuiver, M. & Polach, H. A. Discussion: reporting of 14C data. Radiocarbon 19, 355–363 (1977)

  12. 12

    Levin, I. & Hesshaimer, V. Radiocarbon—A unique tracer of global carbon cycle dynamics. Radiocarbon 42, 69–80 (2000)

  13. 13

    Randerson, J. T., Enting, I. G., Schuur, E. A. G., Caldeira, K. & Fung, I. Y. Seasonal and latitudinal variability of troposphere Δ14CO2: Post bomb contributions from fossil fuels, oceans, the stratosphere, and the terrestrial biosphere. Glob. Biogeochem. Cycles 16, doi:10.1029/2002GB001876 (2002)

  14. 14

    Levin, I. & Kromer, B. The tropospheric 14CO2 level in mid-latitudes of the Northern Hemisphere (1959–2003). Radiocarbon 46, 1261–1272 (2004)

  15. 15

    Stallard, R. F. & Edmond, J. M. Geochemistry of the Amazon. 2. The influence of geology and weathering environment on the dissolved load. J. Geophys. Res. 88, 9671–9688 (1983)

  16. 16

    Melack, J. M. & Forsberg, B. R. in The Biogeochemistry of the Amazon Basin (eds McClain, M. E., Victoria, R. L. & Richey, J. E.) 235–274 (Oxford Univ. Press, New York, 2001)

  17. 17

    Kerrick, D. M. Present and past nonanthropogenic CO2 degassing from the solid earth. Rev. Geophysics 39, 565–585 (2001)

  18. 18

    Körner, C., Farquhar, G. D. & Wong, S. C. Carbon isotope discrimination by plants follows latitudinal and altitudinal trends. Oecologia 88, 30–40 (1991)

  19. 19

    Hedges, J. I. et al. Organic matter in Bolivian tributaries of the Amazon River: A comparison to the lower mainstem. Limnol. Oceanogr. 45, 1449–1466 (2000)

  20. 20

    Aufdenkampe, A. K., Hedges, J. I., Richey, J. E., Krusche, A. V. & Llerena, C. A. Sorptive fractionation of dissolved organic nitrogen and amino acids onto fine sediments within the Amazon Basin. Limnol. Oceanogr. 46, 1921–1935 (2001)

  21. 21

    Gibbs, R. J. The geochemistry of the Amazon River system: Part I. The factors that control the salinity and the composition and concentration of the suspended solids. Geol. Soc. Am. Bull. 78, 1203–1232 (1967)

  22. 22

    Trumbore, S. E. Age of soil organic matter and soil respiration: Radiocarbon constraints on belowground C dynamics. Ecol. Appl. 10, 399–411 (2000)

  23. 23

    Grimm, N. B. et al. Merging aquatic and terrestrial perspectives of nutrient biogeochemistry. Oecologia 442, 485–501 (2003)

  24. 24

    Bernardes, M. C. et al. Riverine organic matter composition as a function of land use changes, Southwest Amazon. Ecol. Appl. 14, S263–S279 (2004)

  25. 25

    Aufdenkampe, A. K. et al. Organic matter in the Peruvian headwaters of the Amazon: A comparison to Bolivian tributaries and the lowland Amazon mainstem. Org. Geochem. (in the press)

  26. 26

    Vogel, J. S., Nelson, D. E. & Southon, J. R. 14C background levels in an accelerator mass spectrometry system. Radiocarbon 29, 323–333 (1987)

  27. 27

    Zhang, J., Quay, P. D. & Wilbur, D. O. Carbon isotope fractionation during gas-water exchange and dissolution of CO2 . Geochim. Cosmochim. Acta 59, 107–114 (1995)

  28. 28

    Gesch, D. B., Verdin, K. L. & Greenlee, S. K. New land surface digital elevation model covers the Earth. Eos 80, 70–71 (1999)

  29. 29

    Mayorga, E., Logsdon, M. G., Ballester, M. V. R. & Richey, J. E. Estimating cell-to-cell land surface flow paths from digital channel networks, with an application to the Amazon basin. J. Hydrol. (in the press)

  30. 30

    Hedges, J. I. et al. Compositions and fluxes of particulate organic material in the Amazon River. Limnol. Oceanogr. 31, 717–738 (1986)

Download references

Acknowledgements

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.

Author information

Correspondence to Emilio Mayorga.

Ethics declarations

Competing interests

Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

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)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

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

Download citation

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.