The longevity of pockmarks – A case study from a shallow water body in northern Denmark

• Published in: Marine geology 2021-04, Vol.434, p.106440, Article 106440
• Main Authors: Andresen, K.J., Dahlin, A., Kjeldsen, K.U., Røy, H., Bennike, O., Nørgaard-Pedersen, N., Seidenkrantz, M.-S.
• Format: Article
• Language: English
• Subjects: Geohazards; Marine sediment; Pockmark activity; Submarine groundwater discharge; Subsurface fluid flow
• ISSN: 0025-3227
• DOI: 10.1016/j.margeo.2021.106440

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. [Display omitted] •Freshwater pockmarks are documented in the western Limfjord, northern Denmark.•Pockmarks initially formed by expulsion of thermogenic fluids at 2 and 0.9 kyrs BP.•Further development was by continuous freshwater flow at local seepage sites.•The pockmarks grow by collapse of pockmark walls and subtle erosion at the bottom.•Pockmarks are active today and thus have a longevity of 2000 yrs.