Abstract
The stable carbon isotopic composition of methane (δ13C-CH4) gas bubbles formed in the sediments of three Amazonian streamswas determined over a 5-yr period. The study sites were two ‘várzea’ floodplain (A¸cu and Maicá) and one ‘terrafirme’ (Jamaraquá) streams. The δ13C of sedimentary organic matter (SOM) from the surrounding vegetation and bottom sediments were also determined. The mean δ13C value ofSOMwas 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 δ13C-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 δ13C-CH4 during the high water phase covered a narrower range of values, -63 to -56‰. Contrastively, during the low water phase the δ13C-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.
