The potent greenhouse gas methane rises from the sea floor in many oceans. Until now, such active methane emissions were only known from the Antarctic – this has now changed. In 2011, US researchers discovered the first signs of a freshly formed methane source on the sea floor during a diving expedition on the Ross Island coast and followed their development for five years. It turned out that it took far longer than expected for methane-consuming microbes to colonize this gas outlet, and the types of bacteria were also different than usual.
Due to the large sea ice areas around the Antarctic, the seabed of the Antarctic coastal areas has hardly been researched. But it is precisely there that geoscientists suspect around a quarter of all methane deposits on earth. “Antarctica is estimated to contain between 80 and 400 gigatons of carbon in the form of methane – a substantial portion of the world’s methane reservoirs in marine reservoirs,” said Andrew Thurber and his colleagues from Oregon State University. If the sea ice continues to retreat as a result of climate change and the seabed warms up, the potent greenhouse gas could escape from the seabed and make a significant contribution to further heating up the atmosphere.
Discovery under the sea ice
However, it is not yet known how likely it is that methane emissions will form along the Antarctic coast. As well as how much of the gas escaping there reaches the water and the atmosphere. In most of the submarine methane sources, bacteria consume a large part of the methane before it escapes into the air. However, because no active methane source has yet been found in the Antarctic, this has not been confirmed or refuted for this region – until now. Because Thurber and his team have now discovered and investigated the first active methane leak in Antarctic waters. “This is a significant discovery that can help close the big gap in our understanding of the earth’s methane cycle,” Thurber emphasizes. Surprisingly, he and his team tracked down the methane source at a point on the sea floor that had been intensively researched since the 1960s. It is located on the edge of the Ross Ice Shelf off the coast of Ross Island, on which the active volcano Mount Erebus is located.
While there was nothing noticeable at this point until 2010, this had changed by 2011. Now divers observed an extensive whitish bacterial mat on the sea bed around ten meters below the sea ice cover, which extended over 70 meters in length and was around one meter wide. “This microbial mat is a clear sign that there is a methane leak here,” explains Thurber. In fact, measurements showed that the water above this mat is enriched with methane and that methane also escapes from the sediment. The researchers had thus discovered the first active methane source in the Antarctic. More importantly, the discovery was made shortly after this gas leak had formed, which allowed the team to monitor the development of a fresh methane source in this region for the first time. “It was pure luck to find an active source of methane – and we had it,” says Thurber.
Surprisingly slow settlement
For their study, the researchers regularly took and analyzed samples of the bacterial mat and sediment over the course of five years. They also determined on site how much methane escapes from the ocean floor at this point. The result: “According to our calculations, the methane outflow from the sediment is 5.1 liters per square meter and day,” report Thurber and his team. “This suggests that this outlet is fed by a significant underground inflow of gas-containing liquid.” However, it is unknown whether the methane comes from the bacterial degradation of organic substances in the underground or possibly from the nearby volcano. Also why this outlet forms such a long strip on the sea floor remains puzzling for the time being. However, the scientists suspect that the nearby volcano could play a role in this: “The formation of volcanic cinder cones can often lead to different underground channels that could explain the observed pattern of discharges,” says Thurber and his colleagues.
The examinations of the bacteria in the whitish mat showed that the otherwise common methane-consuming microbes apparently settled extremely slowly on this Antarctic source of methane. “It took between one and five years for the microbial community to respond to the rise of methane in the sediment,” the researchers report. That is significantly longer than at other previously known methane sources. In addition, the usually most commonly represented group of anaerobic methane-consuming bacteria was not present at all, then only in a relatively small proportion. “This allows two possible conclusions: there are species that are not among the methane teachers known to date, or we are at a very early stage of microbial colonization,” stated Thurber and his team. Observations in the coming years must now show which explanation applies. But it already seems clear that this Antarctic methane leak is different from all previously known ones.
Source: Andrew Thurber (Oregon State University, Corvallis) et al., Proceedings of the Royal Society B – Biological Sciences, doi: 10.1098 / rspb.2020.1134