Concentration and stable carbon and hydrogen isotopes of lipid biomarkers in a thermokarst lake in Arctic Alaska

Arctic lakes and wetlands contribute a substantial amount of methane to the contemporary atmosphere, yet profound knowledge gaps remain regarding the intensity and climatic control of past methane emissions from this source. In this study, we reconstruct methane turnover and environmental conditions, including estimates of mean annual and summer temperature, from a thermokarst lake (Lake Qalluuraq) on the Arctic Coastal Plain of northern Alaska for the Holocene by using source-specific lipid biomarkers preserved in a radiocarbon-dated sediment core. Our results document a more prominent role for methane in the carbon cycle when the lake basin was an emergent fen habitat between ~12,300 and ~10,000 cal yr BP, a time period closely coinciding with the Holocene Thermal Maximum (HTM) in North Alaska. Enhanced methane turnover was stimulated by relatively warm temperatures, increased moisture, nutrient supply, and primary productivity. After ~10,000 cal yr BP, a thermokarst lake with abundant submerged mosses evolved, and through the mid-Holocene temperatures were approximately 3°C cooler. Under these conditions, organic matter decomposition was attenuated, which facilitated the accumulation of submerged mosses within a shallower Lake Qalluuraq. Reduced methane assimilation into biomass during the mid-Holocene suggests that thermokarst lakes are carbon sinks during cold periods. In the late-Holocene from ~2700 cal yr BP to the most recent time, however, temperatures and carbon deposition rose and methane oxidation intensified, indicating that more rapid organic matter decomposition and enhanced methane production could amplify climate feedback via potential methane emissions in the future.

北极湖泊与湿地对当代大气甲烷排放贡献显著，但目前学界对该源过去甲烷排放的强度及其气候调控机制仍存在显著认知空白。本研究针对阿拉斯加北部北极海岸平原的热喀斯特湖——Qalluuraq湖，依托一枚经放射性碳定年的沉积物岩芯中保存的源特异性脂质生物标志物，重建了全新世时期的甲烷周转过程与环境条件，包括年均温与夏季温度的估算值。研究结果显示，在约12300至10000 cal yr BP的时段内，该湖盆为出露的沼泽生境，此时甲烷在碳循环中发挥了更为突出的作用；这一时期与阿拉斯加北部的全新世高温期（Holocene Thermal Maximum, HTM）高度吻合。相对温暖的气温、升高的湿度、养分供给与初级生产力促进了甲烷周转速率的提升。约10000 cal yr BP之后，湖盆演变为发育大量沉水苔藓的热喀斯特湖，全新世中期的温度较此前低约3℃。在此条件下，有机质分解受到抑制，进而推动较浅的Qalluuraq湖内沉水苔藓得以堆积。全新世中期甲烷向生物量的同化作用减弱，表明热喀斯特湖在寒冷时期为碳汇。然而在约2700 cal yr BP至今的全新世晚期，温度与碳沉积量均有所上升，甲烷氧化作用增强，这意味着更快的有机质分解与增强的甲烷生成可能通过未来潜在的甲烷排放加剧气候反馈。