Spatial and seasonal variations in the stable carbon isotopic composition of methane in stream sediments of eastern Amazonia

The stable carbon isotopic composition of methane (δ13 C-CH4) gas bubbles formed in the sediments of three Amazonian streams was determined over a 5-yr period. The study sites were two ' 'várzea' floodplain (Açu and Maicá) and one 'terra-firme' (Jamaraquá) streams. The δ 13C of sedimentary organic matter (SOM) from the surrounding vegetation and bottom sediments were also determined. The mean δ13C value of SOM was lower in the terra-firme (-29.6‰) than in the várzea stream (-23.8‰) as a result of less C4 plant deposition in the former. The δ 13C-CH4 values varied systematically both seasonally and spatially among the sites during all 5 yr of the study, in association with changes during hydrologic cycle. Overall, the variation in values of δ 13C-CH4 during the high water phase covered a narrower range of values, -63 to -56‰. Contrastively, during the low water phase the δ 13C-CH4 values for várzea and terra-firme streams are different and are in direct opposition. At this phase, the δ13C-CH4 at terra-firme stream is at least 20‰ depleted of 13C compared to várzea streams. Changes in organic matter sources, water levels and associated microbial degradation processes control the observed seasonal and spatial variations in net stable carbon isotopic composition of methane