Pockmarks in Spitsbergen Fjords
Wednesday, 9. September 2009, 09:05:14
In my latest post I mentioned the open access to the Norwegian Journal of Geology. In this post I shall concentrate on a paper in their latest issue (2009 Volume 89 Nr. 1 & 2) - Pockmarks in Spitsbergen fjords.
Pockmarks are concave, crater-like features on the seafloor, generally up to several hundreds of meters in diameter and tens of meters in relief. ’Mega pockmarks’ can have diameters of more than 1.5 km and depths exceeding 150 m. The formation of pockmarks is mostly caused by the seepage of thermogenic and biogenic gases and the release of pore water. (For a discussion on biogenic versus thermogenic gas see a.o http://www.gaschem.com/determ.html ).
Forwick et al. have studied and analysed pockmarks in five selected fjords on Spitsbergen. I find that figure 8 in their paper summarises their findings quite well, so I have allowed myself to reproduce it below:
In short the pockmarks in question developed during the past ca. 11,300 years (that means after the last ice age or in other words in the Holocene), as the result of seepage of thermogenic gas and porewater. Factors controlling the distribution of pockmarks in these subpolar fjords include 1) tectonic lineaments, 2) the lithological composition and lateral outcrop of bedrock, 3) the orientation of glacial lineations and 4) exceptionally rapid deposition of debris lobes related to glacial surges.
I find it important to notice that the authors do not regard the melting of permafrost as an important factor contributing to the formation of pockmarks in Spitsbergen fjords, and they also exclude up-drifting ice detaching from the sub-seafloor as an important factor for the formation of pockmarks in the study area.
To recapitulate with reference to the figure: in general the gas is thermogenic and originating from organic-rich bedrock (4 on figure) - with pockmarks where these rocks crop out. The gas may also seep upwards through faults - with pockmarks along tectonic lineaments. Other pockmarks occur as strings in grooves of glacial lineations. More randomly orientated pockmarks occur where porewater migrate up through debris lobes (1 on figure).
Reference:
Matthias Forwick, Nicole J. Baeten & Tore O. Vorren
Pockmarks in Spitsbergen fjords
Norwegian Journal of Geology, 2009 Volume 89 Nr. 1 & 2.
I find the paper interesting on the background of the present discussion of increased release of methane in the Arctic Sea due to global warming - also “marketed” as the methane time bomb.
Here are a couple of links to articles in the media on respectively methane coming from reserves of methane hydrate beneath the sea bed and methane coming from thawing permafrost:
Pockmarks are concave, crater-like features on the seafloor, generally up to several hundreds of meters in diameter and tens of meters in relief. ’Mega pockmarks’ can have diameters of more than 1.5 km and depths exceeding 150 m. The formation of pockmarks is mostly caused by the seepage of thermogenic and biogenic gases and the release of pore water. (For a discussion on biogenic versus thermogenic gas see a.o http://www.gaschem.com/determ.html ).
Forwick et al. have studied and analysed pockmarks in five selected fjords on Spitsbergen. I find that figure 8 in their paper summarises their findings quite well, so I have allowed myself to reproduce it below:
In short the pockmarks in question developed during the past ca. 11,300 years (that means after the last ice age or in other words in the Holocene), as the result of seepage of thermogenic gas and porewater. Factors controlling the distribution of pockmarks in these subpolar fjords include 1) tectonic lineaments, 2) the lithological composition and lateral outcrop of bedrock, 3) the orientation of glacial lineations and 4) exceptionally rapid deposition of debris lobes related to glacial surges.
I find it important to notice that the authors do not regard the melting of permafrost as an important factor contributing to the formation of pockmarks in Spitsbergen fjords, and they also exclude up-drifting ice detaching from the sub-seafloor as an important factor for the formation of pockmarks in the study area.
To recapitulate with reference to the figure: in general the gas is thermogenic and originating from organic-rich bedrock (4 on figure) - with pockmarks where these rocks crop out. The gas may also seep upwards through faults - with pockmarks along tectonic lineaments. Other pockmarks occur as strings in grooves of glacial lineations. More randomly orientated pockmarks occur where porewater migrate up through debris lobes (1 on figure).
Reference:
Matthias Forwick, Nicole J. Baeten & Tore O. Vorren
Pockmarks in Spitsbergen fjords
Norwegian Journal of Geology, 2009 Volume 89 Nr. 1 & 2.
I find the paper interesting on the background of the present discussion of increased release of methane in the Arctic Sea due to global warming - also “marketed” as the methane time bomb.
Here are a couple of links to articles in the media on respectively methane coming from reserves of methane hydrate beneath the sea bed and methane coming from thawing permafrost:
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http://www.newscientist.com/article/dn17625-as-arctic-ocean-warms-megatonnes-of-methane-bubble-up.html
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http://www.columbiatribune.com/news/2009/aug/30/troubling-bubbles/
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