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Pockmarks in the Witch Ground Basin, Central North Sea

Pockmarks in the Witch Ground Basin, Central North Sea
Pockmarks in the Witch Ground Basin, Central North Sea

Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2-D and 3-D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km 2 . The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (>6 m deep, >250 m long, and >75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9–3.1 m deep, 26–140 m long, and 14–57 m wide) and are limited to the soft, fine-grained sediments of the Witch Ground Formation and possibly sourced by compaction-related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present-day fluxes but require an analysis of the pockmark forming processes through geological time.

fluid flow, glaciology, hydrocarbon, quaternary geology, sedimentology, seepage
1525-2027
Böttner, Christoph
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Berndt, Christian
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Reinardy, Benedict T.I.
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Geersen, Jacob
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Karstens, Jens
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Bull, Jonathan M.
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Callow, Ben J.
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Lichtschlag, Anna
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Schmidt, Mark
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Elger, Judith
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Schramm, Bettina
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Haeckel, Matthias
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Böttner, Christoph
e35e2e9c-3e87-48b2-a5bb-a68cd68266be
Berndt, Christian
d6db3f62-9891-4e8a-9210-b3aa6a8a4c22
Reinardy, Benedict T.I.
85d329c2-631a-48c1-afc5-7e0eac7963da
Geersen, Jacob
abcf5f76-3608-4322-ab54-7bfb8dfcaf2d
Karstens, Jens
ffdc8de3-f74f-4ff0-91b0-fe02fdf6f12e
Bull, Jonathan M.
974037fd-544b-458f-98cc-ce8eca89e3c8
Callow, Ben J.
19f1a5fe-cabd-4c49-a84f-dc7de0e3c462
Lichtschlag, Anna
be1568d9-cc63-4f85-bd38-a93dfd7e245f
Schmidt, Mark
c97b9449-688b-438b-9afb-b2ded834d22c
Elger, Judith
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Schramm, Bettina
1c3bc4c4-4939-4982-89f5-14c6d06a2ffc
Haeckel, Matthias
8b05c8db-8426-4270-88e1-80d3bd38df76

Böttner, Christoph, Berndt, Christian, Reinardy, Benedict T.I., Geersen, Jacob, Karstens, Jens, Bull, Jonathan M., Callow, Ben J., Lichtschlag, Anna, Schmidt, Mark, Elger, Judith, Schramm, Bettina and Haeckel, Matthias (2019) Pockmarks in the Witch Ground Basin, Central North Sea. Geochemistry, Geophysics, Geosystems. (doi:10.1029/2018GC008068).

Record type: Article

Abstract

Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2-D and 3-D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km 2 . The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (>6 m deep, >250 m long, and >75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9–3.1 m deep, 26–140 m long, and 14–57 m wide) and are limited to the soft, fine-grained sediments of the Witch Ground Formation and possibly sourced by compaction-related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present-day fluxes but require an analysis of the pockmark forming processes through geological time.

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Accepted/In Press date: 11 March 2019
e-pub ahead of print date: 14 March 2019
Additional Information: An edited version of this paper was published by AGU. Copyright 2019 American Geophysical Union
Keywords: fluid flow, glaciology, hydrocarbon, quaternary geology, sedimentology, seepage

Identifiers

Local EPrints ID: 430515
URI: https://eprints.soton.ac.uk/id/eprint/430515
ISSN: 1525-2027
PURE UUID: b4fe18ee-3dde-4d77-a662-61450b1014f6
ORCID for Ben J. Callow: ORCID iD orcid.org/0000-0003-2296-1702

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Date deposited: 02 May 2019 16:30
Last modified: 03 Dec 2019 05:02

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Contributors

Author: Christoph Böttner
Author: Christian Berndt
Author: Benedict T.I. Reinardy
Author: Jacob Geersen
Author: Jens Karstens
Author: Ben J. Callow ORCID iD
Author: Anna Lichtschlag
Author: Mark Schmidt
Author: Judith Elger
Author: Bettina Schramm
Author: Matthias Haeckel

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