Seasonal variations in the ¹³C-CH₄ of Amazon floodplain waters

Abstract

Measurements of the stable carbon isotopic composition of methane (CH₄) trapped in bubbles in the sediments and CH₄ emitted to the atmosphere from wetland areas of the Amazon River floodplain are reported for different portions of the annual flood cycle. The δ¹³C_CH4 varied from — 75‰ to — 42‰. Of the three characteristic floodplain habitats, the most ¹³C-enriched CH₄ was found consistently in the floating meadow environments (avg. — 52‰), while the most ¹³C-depleted CH₄ was observed in the open water areas (avg. — 63‰). CH₄ bubbles were also significantly more enriched during rising water than at other times. The enrichment of ¹³C in macrophyte beds probably results from decomposition of organic matter associated with the C-4 grass Paspalum, the principal meadow plant. Overall, the CH₄ emitted to the atmosphere from the Amazon is somewhat more enriched in ¹³C than other global wetland sources.