“We were surprised to suddenly find a crater landscape in an area that used to be a flat expanse of sand”, says Knut Krämer, first author of the article and PhD student at MARUM – Center for Marine Environmental Sciences at the University of Bremen. Based on detailed surveys, the study describes a dramatic transformation in the Helgoland Reef region about 45 kilometers northwest of Helgoland. Until July 2015, this area was characterized by an almost flat and featureless sea floor. More recent mapping in November 2015 revealed a bottom strewn with depressions the size of tennis courts. Subsequent cruises with the research vessel Heincke in August and September 2016 confirmed that these craters covered an area of around 915 square kilometers, more than twice the area of the State of Bremen. Each square kilometer can contain up to 1,200 craters. Based on the associated high concentrations of methane in the sediments, the craters were identified as pockmarks.
Bacteria generate methane
The term pockmark is used to designate characteristic craters on sea beds that are formed by the release of liquids or gases from the sub-seafloor. They can be found worldwide in different types of water bodies, including lakes, rivers, estuaries, and in coastal to deep ocean waters. In clay-rich sediments and under low current and wave conditions, they can sometimes persist over centuries and remain as evidence of past gas releases.
In shallow coastal waters with sandy bottoms, under the forcing of tidal currents and waves, the craters are quickly erased, and therefore have been rarely observed before. Prior to the postglacial sea level rise, however, the near-coastal regions in particular were often wetlands and thus rich in organic material. Methane is commonly produced by the bacterial breakdown of this material. The gas can then accumulate underneath impermeable layers below the sea floor. Methane released into the atmosphere acts as a greenhouse gas approximately 25 times more effective than carbon dioxide (CO2).
Measurements by the research team have shown that around 6.9 million cubic meters of sediment were displaced as a result of the methane ejection – sand that would fill 200,000 standard shipping containers. “The total amount of methane released is difficult to estimate. We do not know exactly how the gas was distributed in the substrate prior to the release,” explains Knut Krämer. “But even a conservative estimate suggests an amount of around 5,000 metric tons. This would be equivalent to about two-thirds of the previously assumed annual emission of the entire North Sea.”
The sea floor is altered by currents and waves
Krämer and his co-authors presume that the trigger for the pockmark ejections was a series of storms with waves up to seven meters high and periods of around ten seconds that caused large pressure fluctuations in the sea bed. These acted like a pump on the gas stored there. The sea floor eventually yielded to the gas pressure and the gas escaped into the water column, dragging sediment with it. This was then redeposited on the lee side of the current or wave, producing a characteristic pattern of craters and mounds.
“This study is an excellent example of cooperation among various institutes that are involved in coastal research,” says PD Dr. Christian Winter, chief scientist of the survey expedition and leader of the Coastal Dynamics working group at MARUM. “We obtain measurements together on the German research ships and combine the expertise of different disciplines.”
The Helgoland Reef pockmarks are the first to be observed of this form in the German Bight. Knut Krämer surmises that “the frequency of triggering storm waves suggests that this could be a recurring phenomenon that has been previously overlooked”. Detection of the relatively shallow craters has only become possible through recent advances in the development of highly accurate multibeam echosounders. It is also assumed that the craters, located in mobile, sandy sediments, will quickly be leveled again by waves and currents as soon as no more methane is being released.
Compared to methane emissions caused by humans, the amount from the pockmark field discovered here is small. It is equal to only 0.5 per cent of the annual anthropogenic methane emissions by Germany. It is believed, however, that coastal regions worldwide with rich methane occurrences are in a similar state of instability. It is therefore possible that highly dynamic coastal regions have been overlooked as an important contributor to the global methane budget, says Knut Krämer. “We hope that our article will help to stimulate scientific discussion and further investigations of these kinds of methane sources.”
Knut Krämer, Peter Holler, Gabriel Herbst, Alexander Bratek, Soeren Ahmerkamp, Andreas Neumann, Alexander Bartholomä, Justus E.E. van Beusekom, Moritz Holtappels und Christian Winter: Abrupt emergence of a large pockmark ﬁeld in the German Bight, southeastern North Sea. Scientific Reports 7, 2017; DOI: 10.1038/s41598-017-05536-1
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