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Spatial and seasonal variations in the carbon isotopic composition of methane over a five-year period in stream sediments of eastern Amazonia

Jose Mauro S Moura, Centro de Energia Nuclear na Agricultura, jmauro@cena.usp.br (Presenting)
Christopher S Martens, University of North Carolina at Chapel Hill, cmartens@email.unc.edu
Marcelo Z Moreira, Centro de Energia Nuclear na Agricultura, mmoreira@cena.usp.br
Risonaldo L Lima, Projeto LBA-ECO, risonaldo@lbasantarem.com.br
Irene Cibelle G Sampaio, Projeto LBA-ECO, cibelle@lbasantarem.com.br
Howard P Mendlovitz, University of North Carolina at Chapel Hill, mendlovi@email.unc.edu

In this study, we analyzed the isotopic composition of methane trapped in gas bubbles formed in the sediment of streams. We also analyzed the isotopic composition of sedimentary organic matter (SOM) from the surrounding vegetation, from bottom sediments (cores), and stream shorelines. The studied sites were two “várzea” floodplain streams (Açu and Maicá) and one “terra-firme” stream (Jamaraquá), located near Santarém-PA and Belterra-PA, in Eastern Amazon. Carbon and nitrogen contents and C:N ratio from the material deposited in the botton streams were higher in “terra-firme” stream (C= 14.6%, N= 0.9% and C:N= 17.4) than in “várzea” stream (C= 1.1%, N= 0.1%, and C:N= 8.8). Mean &delta13C value of SOM was lower in “terra-firme” stream (-29.6‰) than in “várzea” (-23.8‰). The &delta13C-CH4 varied seasonally and spatially among the ecosystems and this variation is probably associated with: 1) changes in the inputs of organi matter (C3 and C3 plants); 2) changes in fractionation processes during its decomposition; 3) methane oxidation mechanisms, and 4) environmental factors at each site. The “várzea” streams, where the CH4 tends to be more enriched in 13C during the dry season, showed higher values of &delta13C-CH4 than “terra-firme” stream, with mean values varying from -57.9‰ to -55.0‰ at Açu stream and from -58.5‰ to 52.8‰ at Maicá stream. The Jamaraquá stream showed the largest seasonal variation in &delta13C-CH4 ranging from -62.8‰ to -75.1‰ and tending to be depleted in 13C during the dry season

Science Theme:  TG (Trace Gases)

Session:  2C: Trace Gases and Aerosols

Presentation Type:  Oral

Abstract ID: 109

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