The variation of isotopic composition of methane (CH4) in stream sediments of Eastern Amazon

Author(s)

José Mauro Sousa de Moura

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2005-11-04

Examining board members

Marcelo Zacharias Moreira; Michael Maier Keller; Luiz Antonio Martinelli

• Advisor: Marcelo Zacharias Moreira

Abstract

The study of isotopic composition of methane (δ13CH4) provides valuable information for understanding the process of production, emission, and cycling of this gas in ecosystems. In this study, we analyzed the concentration and isotopic composition of methane trapped in gas bubbles formed in the sediment of streams. We also analyzed the isotopic composition of organic matter (OM) from the surrounding vegetation, from bottom sediments (cores), and stream shorelines. The studied sites were two “varzea" floodplain streams (Açu and Maica) and one “terra-firme" stream (Jamaraqua), located near Santarem-PA and Belterra-PA, in Eastern Amazon. Methane concentration in the Jamaraqua stream (44.9±2.0%) was significantly lower than the concentration found in the Maica (59.8%±13.2%) and Açu (76.5±16.6%) streams. Carbon and nitrogen contents and C:N ratio from the material deposited in the shorelines were higher in “terra-firme" stream (C= 16.0%, N= 1.1% and C:N= 14.7) than in “varzea" stream (C= 1.8%, N= 0.2%, and C:N= 10.4). Mean δ13C value of OM was lower in “terra-firme" stream (-29.6‰) than in “varzea" (-26.5‰). Similarly, the sediment cores at the “terra-firme" site had more OM than the “varzea" site. Mean values for δ13C, C content, N content, and CN ratio for “terra-firme" stream were: -29‰, 10.9%, 0.6% and 16.4, while in the “varzea" stream the mean values for those parameters were: -23‰, 1.0%, 0.1% and 9.0. The δ13CH4 varied seasonally and spatially among the ecosystems and this variation is probably associated with: 1) changes in the inputs of OM (C3 and C4 plants); 2) changes in fractionation processes during its decomposition; 3) methane oxidation mechanisms, and 4) environmental factors at each site. The “varzea" streams, where the CH4 tends to be more enriched in 13C during the dry season, showed higher values of δ13CH4 than “terrafirme" stream, with mean values varying from -58‰ to -55‰ at Açu stream and from - 59‰ to 52‰ at Maica stream. The Jamaraqua stream showed the largest seasonal variation in δ13CH4 ranging from -62‰ to -74‰ and tending to be depleted in 13C during the dry season. (AU)

• FAPESP's process: 03/03969-0 - Flux of radon-222 and methane in soils and sediments in Amazonia
• Grantee: José Mauro Sousa de Moura
• Support Opportunities: Scholarships in Brazil - Master