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 ( δ 13 C-CH 4 ) gas bubbles formed in the sediments of three Amazonian streams was determined over a 5-yr period. The study sites were two ‘ varzea ’ floodplain (Acu and Maica) and one ‘ terra-firme ’ (Jamaraqua) streams. The δ 13 C of sedimentary organic matter (SOM) from the surrounding vegetation and bottom sediments were also determined. The mean δ 13 C value of SOM was lower in the terra-firme (−29.6‰) than in the varzea stream (−23.8‰) as a result of less C 4 plant deposition in the former. The δ 13 C-CH 4 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 13 C-CH 4 during the high water phase covered a narrower range of values, −63 to −56‰. Contrastively, during the low water phase the δ 13 C-CH 4 values for varzea and terra-firme streams are different and are in direct opposition. At this phase, the δ 13 C-CH 4 at terra-firme stream is at least 20‰ depleted of 13 C compared to varzea 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. DOI: 10.1111/j.1600-0889.2007.00322.x