Temporal dynamics of subglacial methane emissions revealed through continuous measurements at the margin of the Greenland Ice Sheet

Methane (CH4), produced subglacially and transported dissolved in meltwater, is released at glacier margins and contributes to atmospheric methane. Recent research expands our spatial understanding of subglacial methane production, extending beyond the large Greenland Ice Sheet to smaller mountain glaciers. However, emission patterns through an entire melt season remain poorly understood due to challenges in long-term measurements in these remote locations. Continuous measurements are crucial for accurately assessing the relation to glacial melt, total emissions, and potential future impact on the atmospheric CH4 budgets. Our study investigated the seasonal variation of dissolved methane (dCH4) export at a lateral outlet of the Isunnguata Sermia glacier in West Greenland. We used custom-built sensors for continuous measurements that were compared to on-site samples of hydrochemistry and water isotopes alongside the continuous monitoring of water levels and water temperature. The observed patterns of dCH4 concentrations reveal the influence of several interconnected processes, varying on both diurnal and seasonal scales. These processes encompass changes in the connectivity of subglacial meltwater channels to sediment pockets with CH4 production, mixing with fluctuating volumes of CH4-free supraglacial meltwater routed through englacial conduits and variations in the volume of air-filled headspaces of the drainage system following discharge fluctuations. A connection to sediment pockets is hypothesized to elevate the dCH4 concentrations measured at the glacier margin, while increased volumes of supraglacial meltwater are associated with lower dCH4 concentrations, as are larger volumes of air-filled headspaces in channels at low discharge due to degassing from the water phase. We will present how temporal variations in dCH4 concentrations link to climatic variability influencing water flow over the season and their relation to geochemical indicators and water isotopes as tracers for meltwater types. Insights gathered from three seasons of measurements highlight the limitations of discrete samples and accentuate the importance of continuous monitoring in obtaining realistic estimates of subglacial CH4 emissions through upscaling and accurately assessing climate implications.