Reconstruction of Methane Emission Over the Past 9,100 Years From Varved Sediments in Lake Xiaolongwan, Northeastern China

Abstract Freshwater lakes have increasingly been recognized as a significant natural source of methane, a potent greenhouse gas contributing to current global warming. However, assessing methane emissions over longer timescales remains challenging. Methane‐derived carbon can be assimilated by aquatic plants through photosynthesis and later recorded in the carbon isotopic composition of their n ‐alkanes. Here, we present modern lake methane emissions and an estimation of methane production over the past 9.1 kyr from n ‐alkanes and their compound‐specific carbon isotope proxies in Maar Lake Xiaolongwan. Methane activity is high due to the decomposition of organic matter and methanogenesis under anoxic conditions in this dimictic lake. Methane production is about 0.87 mol m −2 day −1 in the hypolimnion, with an average δ 13 C value of −64.2‰. To estimate methane activity over the past 9.1 kyr, we calculated the carbon isotopic difference (Δδ 13 C S‐L ) between weighted short‐chain (C 19–21 ) and long‐chain (C 27–33 ) n ‐alkanes. We also applied a two‐endmember model to estimate the proportion of CH 4 ‐derived carbon assimilated by aquatic plants. Both Δδ 13 C S‐L and methane‐derived carbon show a rapid increase from 9.1 to 8.0 kyr BP, followed by fluctuations between 8.0 and 5.0 kyr BP; then, a quick rise from 5.0 to 4.0 kyr BP; and a decrease after 0.6 kyr BP. Methane activity over the past 9.1 kyr closely correlates with regional mean annual temperature and total nitrogen in lake sediments, both of which promote microbial methanogenesis. Natural CH 4 emissions from freshwater lakes might have contributed to the atmospheric CH 4 increase after 5.0 kyr BP.