_____________________________________________________________________________________________________________________________________________ Fig. 02: Localization of the study area Downtowns Santarem Belterra Legend Maica Stream Jamaraqua Stream The influence of sediment composition on isotopic variation of CH 4 in the eastern Amazonian streams José Mauro Moura 1, Christopher Martens 2, Marcelo Moreira 1, Howard Mendlovitz 2, Risonaldo Lima 3, Irene Cibelle Sampaio 4 1 Centro de Energia Nuclear na Agricultura-CENA, 2 University of North Carolina, 3 Projeto LBAECO, 4 Universidade Federal do Pará The isotopic composition of methane varies seasonally and spatially among wetland environments as a result of changes in microbial production and oxidation mechanisms. The Amazonian wetlands differ in many characteristics including vegetation, particle matter, and soil around their basins. Each of these characteristics seems to play a different role on the process by which the methane is produced. We have measured the variations in isotopic composition of sediments methane gas (δ 13 C-CH 4 ) in two streams (Jamaraqua and Maica) in Eastern Amazon. In the two years of sampling we found a strong evidence of difference between wet and dry seasons (Fig. 01) Introduction To analyze the composition of organic matter present in the sediments and vegetation that grows in these streams. To evaluate how the inputs from the varzea and primary forest affect the isotopic composition of CH 4 released at these streams. We took sediment cores, water temperature, and dissolved oxygen levels (Fig. 03) in the open water at each site during the dry season (Nov/03). The cores were sub-sampled every 5 cm, dried and processed before analyzing on a Finnigan Delta Plus mass spectrometer for organic carbon. Samples of litter and vegetation were also collected and processed in the same way. The surrounded vegetation at Jamaraqua Stream (Fig. 06) contributes to the signature of lighter  13 C values and to the great amount of organic matter found at this site. Although Maica Stream receives organic matter as input from Amazon river, it does not accumulate in the stream sediment. Since there is less amounts of organic matter at Maica this may represent the effect of a faster process of decomposition of the grass that grows at this area. We will look for seasonal difference on the isotopic composition of the sediments and sample during the high water period. We thank all of our friends in Santarem for the help during the samples collecting. We also thank the Fudanção de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and NASA for their support. Methodology Discussion Acknowledgments Objectives Study Area We worked at two different streams near Santarem-PA and Belterra-PA (Fig. 02). The Maica stream is an effluent of the Amazon River and the Jamaraqua Stream, an effluent of Tapajos River, is surrounded by a primary forest. Sampling methods Fig. 03: Core sampling and top part of a sediment sample Fig. 01: Variation of the CH 4 isotopic composition We found little differences in the water temperature and isotopic composition of leaves. Only at Maica we found the presence of grass material (Table 01). Table. 01: Temperature, Dissolved Oxygen, and  13 C of vegetation Results Fig. 09: Carbon content in the sediment of the Maica StreamFig. 08: Carbon content in the sediment of the Jamaraqua StreamFig. 07: Dissolved oxygen level in water column at Maica Stream There was an inverse correlation between the dissolved oxygen versus depth in water column at Maica (Fig. 07). This trend was not observed at Jamaraqua. The mean value of carbon content in the sediment samples of Jamaraqua was 12.0%, which is ~18-fold bigger than at Maica (Fig. 08 and 09). There was a large difference between the streams in the isotopic composition of organic matter in the sediments. The Maica stream has heavier isotopic values (p<0,05) than Jamaraqua stream (Fig. 04 and 05). Fig. 06: 3-Dimentional topography of the Jamaraqua BasinFig. 05: Isotopic composition of the sediment at Maica StreamFig. 04: Isotopic composition of the sediment at Jamaraqua Stream C 3 input