Seasonal freeze-thaw modulates methane cycling in urban waters revealed by multiple isotopic constraints

Greenhouse gases are emitted from inland waters into the atmosphere during freeze-thaw events, contributing to rising atmospheric GHG concentration and warming. However, methane (CH₄) emissions from small water bodies are poorly constrained. In this study, by combining hydrogeochemical survey, conventional stable isotopes (δ¹³C-CH₄, δ¹³C-DIC/CO₂, δD-CH₄, δD-H₂O), clumped isotopes (Δ¹³CH₃D and Δ¹²CH₂D₂), and radiocarbon (Δ¹⁴C-CH₄), we present a model of the methane biogeochemical cycle of the urban water ecosystem during the freeze-thaw event. After thawing, diffusive methane emissions increased (from 95.78 mg m^-2 d^-1 to 412.17 mg m^-2 d^-1), while ebullitive methane decreased (from 66.04 mg m^-2 d^-1 to 33.46 mg m^-2 d^-1). Substantial CH₄ production was evident in the study area, and the warming potential was considerably amplified during the freeze-thaw event. Evidence from bulk and clumped isotope data indicates the dominant contribution of fermentation-driven methanogenic pathways, while the enzymatic reactions reversibility of methanogenesis and/or methanotrophy during freezing-thawing events introduced non-negligible kinetic isotope fractionation. Our radiocarbon (¹⁴C) results show that the carbon substrates (Δ¹⁴C = -231.17‰) are older than the emitted methane (via ebullition; Δ¹⁴C = -86.62‰) in the urban aquatic system, representing the preferential consumption of younger, more labile carbon by methanogenic microbial communities. The fact that methane in post-thawing (from modern to 411 yrs BP) is younger than that in pre-freezing (from modern to 3762 yrs BP) suggests a difference in the substrate availability to microbial methane metabolisms, and the ice coverage may lead to a slower turnover of old carbon sources in urban aquatic systems. This study confirmed that under the rapid advancement of global urbanization, the freezing period plays an important role in the global carbon budget and should be factored into global assessments of carbon budgets.