Geochemistry of organic matter by multi-proxy analyses and temperature sensitivity of methanogenesis in clearwater Amazonian lake sediments

Despite the relevance of bottom sediments from clearwater floodplain lakes in organic matter preservation and degradation, there is a scarcity of data regarding the composition of molecular biomarkers and responses of benthic methanogenesis in Amazonian lakes in global warming scenarios. Here, surface sediments from a small clearwater floodplain lake (Jacund ' a Lake) in the Brazilian Amazon (Tapaj ' os River) were assessed in a combined approach between molecular biomarker composition and the sensitivity of methanogenesis under two experi-mental temperatures: 25 degrees C as control vs. + 4 degrees C as an Intergovernmental Panel on Climate Change (IPCC) global warming scenario for 2081-2100. Lipid biomarker composition (n-alkanes, sterols) in a 12-cm sediment core revealed a strong predominance of terrestrial-derived organic matter over aquatic contributions, indicating more refractory preserved organic matter, while aquatic organic matter represents substrates to biological degrada-tion. Moreover, alkanoic acids and microbial triterpenoids indicated an intense microbial contribution and elevated organic matter processing. Compound specific isotopic analysis confirmed the presence of methano-trophic microorganisms (diploptene delta 13C = -53.5 parts per thousand) and an uptake of depleted 13C in aquatic organic matter. Diagenetic transformation of biomarkers also indicated a highly anoxic depositional environment throughout the core. Methanogenesis in the upper 15-cm sediment layer under 25 degrees C (39.3 mg C-CH4 m- 2 d-1) was similar to higher rates reported in Amazonian lakes and increased about 60% in the treatment when the temperature increased by 4 degrees C. Despite the predominant terrestrial sources and subsequent refractory composition of organic matter, our findings integrating biomarkers and metabolic processes support that biomass stocks in clearwater lake sediments of the Amazon basin may be sensitive to global warming.

Automated summary

This study investigates the sediment composition and methanogenesis in a clearwater floodplain lake in the Brazilian Amazon. The findings suggest that the biomass stocks in these lakes may be sensitive to global warming due to the predominance of terrestrial organic matter and intense microbial contribution.