This study focusses on seafloor methane seep sites and their distribution in the northwestern part of the German North Sea. Methane seepage is a common phenomenon along marine shelfs and known to occur in the North Sea, but proof of their existence was lacking in the study area. Using a ship-based multibeam echosounder we detected a minimum of 166 flares that are indicative for free gas releases from the seafloor in the German ´Entenschnabel´ area, which are not related to morphologic expressions at the seafloor. However, a group of small pockmarks was detected lacking water column anomalies but with indications of dissolved fluid release. Spatial analysis revealed that flares were not randomly distributed but show a relation to locations of subsurface salt diapirs. Most flares were found in the vicinity of the salt diapir ‘Berta’. Dissolved methane concentrations of ~ 100 nM in bottom waters were ten times the background value in the ´Entenschnabel´ area (CH4 <10 nM), supporting the finding of enhanced seepage activity in this part of our study area. Furthermore, locations of flares were often related to narrow acoustic blanking and high amplitude reflections in sediment profiler echograms, most prominently observed at location Berta. These signatures are interpreted to result from increased gas concentrations in the sediments. Electromagnetic seabed mapping depicts local sediment conductivity anomalies below a flare cluster at Berta, which can be explained by free gas concentrations up to 2% of total volume. Nine abandoned well sites were included in our mapping campaign, but flare observations were spatially not related to these wells. Naturally seeping methane is presumably transported to the seafloor along sub-vertical faults, which have formed concurrently to the updoming salt. Due to the shallow water depths of 30 to 50 m in the study area, flares were observed to reach close to the sea surface and a slight oversaturation of surface waters in the flare-rich northeastern part of the working area indicates that at least part of the released methane is contributing to the atmospheric inventory.

中文翻译：

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德国北海海底甲烷渗透

这项研究的重点是德国北海西北部海底甲烷的渗漏部位及其分布。甲烷渗漏是沿海陆架常见的现象，已知发生在北海，但研究区域缺乏甲烷渗漏的证据。使用基于舰船的多波束回声测深仪，我们检测到至少166个火炬，这些火炬表明德国“ Entenschnabel”地区海底有游离气体释放，这与海底的形态表达无关。但是，检测到一组小麻点，没有水柱异常，但有溶解的液体释放的迹象。空间分析表明，耀斑并不是随机分布的，而是与地下盐底物的位置有关。大部分耀斑都在盐底盘“ Berta”附近发现。在“ Entenschnabel”地区（CH4 <10 nM），底水中溶解的甲烷浓度约为100 nM，是背景值的十倍，这支持了我们研究区域这一部分渗漏活动增强的发现。此外，耀斑的位置通常与沉积物廓线回波图中狭窄的消音和高振幅反射有关，在Berta位置最明显。这些特征被解释为是沉积物中气体浓度增加的结果。电磁海底图描绘了伯塔火炬群下方的局部沉积物电导率异常，这可以通过游离气体浓度高达总体积的2％来解释。我们的制图活动中包括9个废弃的井场，但火炬观测在空间上与这些井无关。天然渗入的甲烷大概是沿着次垂直断层运到海底的，而次垂直断层则同时形成于上升的盐中。由于研究区域的浅水深度为30至50 m，因此观测到耀斑接近海面，并且在耀斑丰富的工作区东北部，地表水略有过饱和，表明至少有一部分释放的甲烷有助于大气清单。