Salinity limited methane cycling in lacustrine settings during the Paleocene–Eocene Thermal Maximum

Abstract Inland aquatic ecosystems are the largest natural source of greenhouse gas methane (CH4) release to the atmosphere. Although the temperature dependence of CH4 dynamics in freshwater systems is well documented, CH4 cycling in salt-rich inland waters and its response to rapid global warming remain poorly understood, particularly during past greenhouse climates. Here, we use the carbon isotopic composition of lipid biomarkers to reconstruct CH4 cycling in a saline lake during the Paleocene–Eocene Thermal Maximum (PETM; ca. 56 Ma), a geological analog for future warming. Our results suggest that, in contrast to the high temperature sensitivity reported for contemporaneous freshwater wetlands, microbial CH4 cycling in the saline lacustrine system of the Jianghan Basin (central China) showed a muted response to rapid greenhouse warming during the PETM. The high salinity and sulfate concentrations, combined with limited available substrates, may have inhibited methanogenesis and subsequent CH4 emissions at the ecosystem level. Our findings suggest that widespread salinization could restrict CH4 dynamics in inland aquatic ecosystems and affect large-scale greenhouse gas feedbacks to climate warming.