Pockmarks or crater-like depressions formed due to fluid venting, is a well-known geological phenomenon. On a global scale, pockmarks expel a significant amount of fluids and greenhouse gasses to the marine environment and the atmosphere. Despite pockmarks being well-studied, the longevity and history of activity are rarely addressed. In this study, we analyse more than twenty nearshore seafloor depressions of varying morphology and size in the shallow waters of the western Limfjord in northern Denmark. By using high-resolution seismo-acoustic and sediment core data, we characterize the morphology and sediments of the pockmarks. We further use ¹⁴C-dating and pore water analyses to address the timing and activity of the depressions. Our results show that the depressions coincide with submarine groundwater discharge areas and hence represent freshwater pockmarks. The pockmarks most likely formed initially due to two eruptive events of thermogenic light oil/gas ca. 2000 and 900 yrs. BP, both triggered by relative sea-level fall. Freshwater seepage continues to sustain and develop the pockmark morphology and we show that pockmarks grow by collapse of pockmark walls and amalgamation of discrete erosive seepage sites at the bottom. The episodic expulsions of thermogenic fluids and the continuous seepage of freshwater probably prolonged the longevity of the studied pockmarks and further demonstrates the complex nature of pockmark formation and activity. While thermogenic and biogenic gas escape to the surface, on the global scale, is relevant for the greenhouse gas budget, the outflow of groundwater which may or may not be polluted is relevant with respect to the geochemistry, biota and productivity of the receiving marine environment especially in shallow water settings.

由于流体排泄而形成的麻点或坑状凹陷是一种众所周知的地质现象。在全球范围内，麻子会将大量的液体和温室气体排放到海洋环境和大气中。尽管对麻子的研究得到了很好的研究，但很少能解决其寿命和活动历史。在这项研究中，我们分析了丹麦北部利姆峡湾西部浅水区中二十多种形态和大小各异的近岸海底凹陷。通过使用高分辨率地震声波和沉积物岩心数据，我们可以表征麻点的形态和沉积物。我们进一步使用¹⁴C约会和孔隙水分析可解决凹陷的时间和活动。我们的结果表明，这些凹陷与海底地下水排放区一致，因此代表了淡水麻点。痘痕最有可能最初是由于大约两次产热轻质油/气的爆发事件形成的。2000和900年。BP，均由相对海平面下降触发。淡水渗漏继续维持和发展麻点形态，我们显示麻点由于麻点壁的塌陷和底部离散侵蚀性渗漏点的合并而增长。生热流体的偶发驱逐和淡水的持续渗漏可能延长了所研究痘痕的寿命，并进一步证明了痘痕形成和活性的复杂性。