Abstract
Abstract The role of boreal wetlands in driving variations in atmospheric methane (CH4) concentrations across the last deglaciation (20–10 ka) and the Holocene is debated. Most studies infer the sources of atmospheric methane via ice-core records of methane concentration and its light stable isotopic composition. However, direct evidence for variations in the methane cycle from the wetlands themselves is relatively limited. Here, we used a suite of biomarker proxies to reconstruct the methane cycle in the Chinese Hani peat across the past 16 k.y. We found two periods of enhanced methanogenesis, at ca. 15–11 ka and ca. 10–6 ka, whereas weak methanogenesis characterized the late Holocene. These periods of enhanced methanogenesis relate to periods of high/increasing temperatures, supporting a temperature control on the wetland methane cycle. We found no biomarker evidence for intense methanotrophy throughout the past 16 k.y., and, contrary to previous studies, we found no clear control of hydrology on the peatland methane cycle. Although the onset of methanogenesis at Hani at ca. 15 ka coincided with a negative shift in methane δ13C in the ice cores, there is no consistent correlation between changes in the reconstructed methane cycle of the boreal Hani peat and atmospheric CH4 concentrations.
Highlights
Methane is an important gas for atmospheric chemistry because it accounts for ∼20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases
We explored the hypothesis that more methane was emitted from boreal wetlands due to an intensified methane cycle across the last deglaciation and the Holocene
The δ13C values and abundance of diploptene at Hani provide no evidence for significant variations in methanotrophy across the past 16 k.y
Summary
Methane is an important gas for atmospheric chemistry because it accounts for ∼20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. Atmospheric methane concentrations obtained from ice cores demonstrate that across the last deglaciation (between 20 and 10 ka), concentrations doubled from ∼350 to 700 ppbv (Stauffer et al, 1988). They exponentially increased to >1850 ppbv during the past ∼150 yr. The source of the atmospheric methane increase across the last deglaciation remains intensely debated (Chappellaz et al, 1990, 1993; Kennett et al, 2000; Bock et al, 2017; Petrenko et al, 2017; Treat et al, 2019), highlighting a fundamental gap in our understanding of the methane cycle and Earth’s climate system. The main hypothesis to explain the deglacial increase revolves around wetlands, the dominant natural source of methane
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