First methane source under Antarctic sea ice

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The potent greenhouse gas methane rises from the sea floor in many oceans. Only Antarctica was not known to have such active methane emissions – this has now changed. In 2011, during a diving expedition along the coast of Ross Island, US researchers discovered the first signs of a freshly formed methane source on the seabed and followed its development for five years. It turned out that it took much longer than expected for methane-producing microbes to colonize this gas outlet, and the species of bacteria were also different from those otherwise known.

First methane source

Because of the large areas of sea ice around Antarctica, the seabed of the Antarctic coastal areas has hardly been explored to date. Yet it is precisely there that geoscientists suspect around a quarter of all methane deposits on earth.

“According to estimates, Antarctica could contain between 80 and 400 gigatons of carbon in the form of methane – that is a substantial proportion of the global methane reserves in marine reservoirs,” explain Andrew Thurber and his colleagues from Oregon State University.

If sea ice retreats further as a result of climate change and the sea floor becomes warmer, the potent greenhouse gas could escape from the seafloor and make a significant contribution to further heating of the atmosphere.

Discovery under the sea ice

But how likely it is that methane seeps will form along the Antarctic coasts is still unknown. It is also unknown how much of the gas escaping from the coasts reaches the water and the atmosphere. It is true that at most underwater methane sources bacteria consume a large part of the methane before it escapes into the air. But because no active methane source has yet been found in Antarctica, this has not yet been confirmed or disproved for this region – until now. This is because Thurber and his team have now discovered and investigated the first active methane release in Antarctic waters.

“This is a significant discovery that may help to close the huge gap in our understanding of the terrestrial methane cycle,” Thurber emphasizes.

Surprisingly, he and his team tracked down the source of methane at a point on the ocean floor that has been the subject of intensive research since the 1960s. It is located on the edge of the Ross Ice Shelf off the coast of Ross Island, where the active volcano Mount Erebus is located.

While up until 2010 nothing noticeable was to be seen here, this had changed by 2011

Now, divers observed an extensive whitish bacterial mat on the seafloor lying about ten metres below the sea ice cover, which extended over 70 metres in length and was about one metre wide.

“This microbial mat is a clear indication that there is a methane leakage here,” explains Thurber.

In fact, measurements showed that the water above this mat is enriched with methane and that methane is also escaping from the sediment. The researchers had thus discovered the first active methane source in Antarctica. More importantly, the discovery was made shortly after this gas escape had formed, which enabled the team to follow the development of a fresh source of methane in this region for the first time.

“It took pure luck to find an active source of methane – and we did,” says Thurber.

Surprisingly slow settlement

For their study, the researchers regularly took and analysed samples of the bacterial mat and sediment over a period of five years. They also determined on site how much methane escapes from the sea floor at this point. The result:

“According to our calculations, the methane outflow from the sediment is 5.1 liters per square meter per day,” Thurber and his team report. “This suggests that this leak is being fed by a significant underground influx of gaseous fluid.”

However, it is unknown whether the methane originates from the bacterial decomposition of organic substances underground or possibly from the nearby volcano. It also remains a mystery why this discharge forms such a long strip of the seabed. The scientists suspect, however, that the nearby volcano could play a role in this:

“The formation of volcanic cinder cones can often lead to different underground channels, which could explain the observed pattern of the eruptions,” Thurber and his colleagues say.

Investigations of the bacteria in the whitish mat revealed that the methane-rich microbes that are normally found in the area apparently settled extremely slowly at this Antarctic methane source.

“It took between one and five years for the microbial community to react to the methane rising into the sediment,” the researchers report.

This is significantly longer than at other methane sources known to date

In addition, the normally most common group of anaerobic methane-donating bacteria was not represented at all, and then only in a relatively small proportion.

“This allows two possible conclusions: There are species that do not belong to the previously known methane-eating bacteria, or else we are still at a very early stage of microbial colonisation,” said Thurber and his team.

Observations in the coming years will have to show which explanation is correct. But it already seems clear: This Antarctic methane release is unlike any previously known.

Quelle: Andrew Thurber (Oregon State University, Corvallis) et al., Proceedings of the Royal Society B – Biological Sciences, doi: 10.1098/rspb.2020.1134.

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