Seasonally ice covered in the past, the fjords in West Spitsbergen turn into being perennially ice free in the present. This feedback to Arctic amplification of global warming changes gas fluxes at the atmosphere-ocean interface. Furthermore, in this Polar region, coupled feedbacks likely enhance Arctic amplification of global warming as numerous gas seepages provide evidence for active gas emissions at the sediment-water interface. We present a time series (2015–2017) of dissolved methane concentrations combined with hydrographic data in Adventfjorden and Tempelfjorden, two sub-fjords of Isfjorden located at the west coast of Spitsbergen. While both sub-fjords remained permanently supersaturated, we detected pronounced temporal and spatial variations in the methane excess level. Our study revealed that seasonal water transformations were key to seasonally changing methane pathways including potential sea-air flux (efflux). We suggest that a cascade of feedback processes, seasonally triggered by waterside convection and stratification, adjusts the amount of methane released and transported within fjord water. When sea ice was missing, strong winter cooling affected the methane supersaturation in contrary directions: first a drop and then a strong increase. In early winter, convective mixing favoured efflux, which reduced the supersaturation. Later in winter, the thermal convection resulted in a continuous overturning of the water column. When the thermal convection reached the bottom, sediment resuspension by turbulence increased, which in turn encouraged enhanced methane release. Subsequently transported along vertical isopycnals, methane from the bottom water reached the water-atmosphere interface. These coupled events created a steady state, simultaneously maintaining supersaturation and efflux. During the warm season, the fjord water became stratified and methane transport occurred mainly laterally in the bottom water. The seasonally changing hydrographic conditions strongly triggered the methane spreading in both sub-fjords and point to a switch between the atmosphere and ocean as main sinks in winter and summer, respectively. Upcoming variations in seasonality, i.e. warmer/cooler summer compared to colder/warmer winter will influence these pathways and the final fate of methane discharged into Arctic fjords.

过去季节性冰雪覆盖，西斯匹次卑尔根的峡湾现在变成了常年无冰。这种对北极全球变暖放大的反馈改变了大气-海洋界面的气体通量。此外，在这个极地地区，耦合反馈可能会增强北极对全球变暖的放大作用，因为大量气体渗漏为沉积物-水界面处的活性气体排放提供了证据。我们展示了一个时间序列（2015-2017）溶解甲烷浓度以及 Adventfjorden 和 Tempelfjorden 的水文数据，这两个子峡湾位于斯匹次卑尔根岛西海岸。虽然两个子峡湾保持永久过饱和，但我们检测到甲烷过量水平的显着时间和空间变化。我们的研究表明，季节性水转变是季节性变化的甲烷途径的关键，包括潜在的海气通量（流出）。我们建议由水边对流和分层季节性触发的一连串反馈过程调整了峡湾水中释放和运输的甲烷量。当海冰消失时，强烈的冬季冷却会以相反的方向影响甲烷的过饱和度：先是下降，然后是强烈增加。在初冬，对流混合有利于外排，从而降低了过饱和度。在冬季晚些时候，热对流导致水柱不断翻转。当热对流到达底部时，湍流引起的沉积物再悬浮增加，这反过来又促进了甲烷释放。随后沿着垂直等密度线运输，来自底部水的甲烷到达水-大气界面。这些耦合事件创造了一个稳定状态，同时保持过饱和和流出。在暖季，峡湾水分层，甲烷迁移主要发生在底水中。季节性变化的水文条件强烈触发了甲烷在两个子峡湾中的扩散，并表明大气和海洋在冬季和夏季分别作为主要汇发生了转换。即将到来的季节性变化，即与较冷/较暖的冬季相比，较暖/较冷的夏季将影响这些途径以及排放到北极峡湾的甲烷的最终归宿。这些耦合事件创造了一个稳定状态，同时保持过饱和和流出。在暖季，峡湾水分层，甲烷迁移主要发生在底水中。季节性变化的水文条件强烈引发了甲烷在两个子峡湾中的扩散，并表明大气和海洋在冬季和夏季分别作为主要汇发生了转换。即将到来的季节性变化，即与较冷/较暖的冬季相比，较暖/较冷的夏季将影响这些途径以及排放到北极峡湾的甲烷的最终归宿。这些耦合事件创造了一个稳定状态，同时保持过饱和和流出。在暖季，峡湾水分层，甲烷迁移主要发生在底水中。季节性变化的水文条件强烈引发了甲烷在两个子峡湾中的扩散，并表明大气和海洋在冬季和夏季分别作为主要汇发生了转换。即将到来的季节性变化，即与较冷/较暖的冬季相比，较暖/较冷的夏季将影响这些途径以及排放到北极峡湾的甲烷的最终归宿。季节性变化的水文条件强烈引发了甲烷在两个子峡湾中的扩散，并表明大气和海洋在冬季和夏季分别作为主要汇发生了转换。即将到来的季节性变化，即与较冷/较暖的冬季相比，较暖/较冷的夏季将影响这些途径以及排放到北极峡湾的甲烷的最终归宿。季节性变化的水文条件强烈引发了甲烷在两个子峡湾中的扩散，并表明大气和海洋在冬季和夏季分别作为主要汇发生了转换。即将到来的季节性变化，即与较冷/较暖的冬季相比，较暖/较冷的夏季将影响这些途径以及排放到北极峡湾的甲烷的最终归宿。