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 (δ13C-CH4) gas bubbles formed in the sediments of three Amazonian streamswas determined over a 5-yr period. The study sites were two ‘v´arzea’ floodplain (A¸cu and Maic´a) and one ‘terrafirme’ (Jamaraqu´a) 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 v´arzea 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 v´arzea 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 v´arzea 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