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

TY - JOUR

T1 - The longevity of pockmarks – A case study from a shallow water body in northern Denmark

AU - Andresen, K. J.

AU - Dahlin, A.

AU - Kjeldsen, K. U.

AU - Røy, H.

AU - Bennike, O.

AU - Nørgaard-Pedersen, N.

AU - Seidenkrantz, M. S.

N1 - Funding Information: The research cruise to Visby Bredning in May 2017 was funded by the Danish Center for Marine Research (project ID and number: AU Limfjorden 2017, 2017-05 to KJA). We thank the captain and crew on RV Aurora for professional service and a very successful cruise. The Danish Naval Home Guard is thanked for support during the cruise in terms of operating guard vessel and acquisition of sound velocity probe data. Per Trinhammer, Egon Nørmark, Lars Rasmussen, Lasse Nygaard Eriksen, Torsten Hundebøl Hansen, Jørn Bo Jensen and Lars Rödel are thanked for their participation in the cruise and help with the geophysical data acquisition. Karina B. Oest is thanked for excellent technical assistance in geochemical analyses and Maria Nádházi is thanked for her work with the data during her master thesis. The Independent Research Fund Denmark financed the costs for radiocarbon dating and XRF scanning and photography of the gravity cores through the G-Ice project (grant no. 7014-00113B/FNU to MSS). We further acknowledge the support from IHS Markit and EIVA in terms of university academic software licenses for SeisLab Aarhus without which we could not have performed the data analysis. Steen Christensen is thanked for help with the groundwater transmissitivity map. GEUS is thanked for supporting the use of the 2007 vintage data from Visby Bredning. Funding Information: The research cruise to Visby Bredning in May 2017 was funded by the Danish Center for Marine Research (project ID and number: AU Limfjorden 2017, 2017-05 to KJA). We thank the captain and crew on RV Aurora for professional service and a very successful cruise. The Danish Naval Home Guard is thanked for support during the cruise in terms of operating guard vessel and acquisition of sound velocity probe data. Per Trinhammer, Egon N?rmark, Lars Rasmussen, Lasse Nygaard Eriksen, Torsten Hundeb?l Hansen, J?rn Bo Jensen and Lars R?del are thanked for their participation in the cruise and help with the geophysical data acquisition. Karina B. Oest is thanked for excellent technical assistance in geochemical analyses and Maria N?dh?zi is thanked for her work with the data during her master thesis. The Independent Research Fund Denmark financed the costs for radiocarbon dating and XRF scanning and photography of the gravity cores through the G-Ice project (grant no. 7014-00113B/FNU to MSS). We further acknowledge the support from IHS Markit and EIVA in terms of university academic software licenses for SeisLab Aarhus without which we could not have performed the data analysis. Steen Christensen is thanked for help with the groundwater transmissitivity map. GEUS is thanked for supporting the use of the 2007 vintage data from Visby Bredning. Publisher Copyright: © 2021 Elsevier B.V. Copyright: Copyright 2021 Elsevier B.V., All rights reserved.

PY - 2021/4

Y1 - 2021/4

N2 - 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.

AB - 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.

KW - Geohazards

KW - Marine sediment

KW - Pockmark activity

KW - Submarine groundwater discharge

KW - Subsurface fluid flow

UR - http://www.scopus.com/inward/record.url?scp=85101269093&partnerID=8YFLogxK

U2 - 10.1016/j.margeo.2021.106440

DO - 10.1016/j.margeo.2021.106440

M3 - Journal article

AN - SCOPUS:85101269093

SN - 0025-3227

VL - 434

JO - Marine Geology

JF - Marine Geology

M1 - 106440

ER -