LA REVUE GAUCHE - Left Comment

Saturday, July 30, 2022

A window of opportunity for methane to slip by nature’s filters

STOCKHOLM UNIVERSITY

Methane leaks from sea floor — IMAGE: METHANE LEAKS THROUGH THE NATURAL MICROBIAL FILTER WHEN THE METHANE TRANSPORT SUDDENLY INCREASES IN RESPONSE TO WARMING OCEANS AND SUBSEQUENT METHANE HYDRATE MELTING. THE METHANE EMISSIONS DEPEND ON THE SEDIMENT TYPE; IN FINE-GRAINED SEDIMENTS SUCH AS CLAY, FRACTURES FORM THAT LEAD TO HIGHER EMISSIONS. ILLUSTRATION: CHRISTIAN STRANNE view more
CREDIT: CHRISTIAN STRANNE

Warmer oceans can lead to large amounts of methane being released from the seabeds, which may amplify climate warming. A new study develops a method to understand the role of microorganisms in increasing emissions of methane from seabeds.

Vast reservoirs of the potent greenhouse gas methane are stored beneath the sea in a solid ice-like combination with water. This solid is known as methane hydrate. For over three decades, various concerns have been raised that warming the seafloor may cause this methane to be rapidly released, perhaps even reaching the atmosphere where it would cause further climate warming. Happily, this methane hydrate is mostly located beneath the seafloor and under hundreds of meters of seawater. Even if warming melts this methane hydrate and releases methane gas, the natural microbial filters present in the seafloor were expected to destroy most of the methane before it ever reaches the open seawater.

However, there have been some gaps in our knowledge of the relevant seafloor processes. In particular, can seafloor warming be rapid enough that methane hydrate could melt so fast that the released methane would overwhelm and ultimately bypass the natural microbial filters?
“The microbial filter layer in the sediment—we call it the ‘sulfate-methane transition’, where methane is removed—is somewhat delicate,” explains Assistant Professor Christian Stranne at the Department of Geological Sciences, Stockholm University.
“The filter layer takes many years to form and reach peak methane-consuming efficiency. The filter is a living thing, made of microorganisms that consume methane under anaerobic (no-oxygen) conditions. The filter also moves up and down within the sediment, depending on the rate at which methane is reaching it.”

In a new study, just published in Communications Earth and Environment, Stranne and colleagues from Stockholm University and Linnaeus University have combined a new model of the biological behaviour and vertical movements of this microbial filter with existing models of seafloor sediments’ physical behaviour. The physical parts of the model include processes such as how cracks form and methane can move up thorough the sediment after methane hydrates melt.

Christian Stranne explains: “Imagine that the amount of methane rising through the sediment suddenly increases, as might happen if methane hydrate begins to melt faster. It can take decades for the filter to adjust itself to consume methane at the new rate. Our new study shows that during the time that the filter is not reestablished, substantial methane can leak past the filter, and into the ocean water.”

Despite this “window of opportunity”, methane from melting hydrates that reaches the seawater faces further methane-destroying processes. These processes make it nearly impossible for substantial methane from methane hydrate melting to reach the atmosphere. However, methods as demonstrated in this study can be applied to other regions where seafloor-released methane is much shallower and is more likely to reach the atmosphere, such as the Arctic continental shelves, according to Christian Stranne.

“Methane hydrates are a massive storehouse of carbon, so it remains important to understand how they interact with ocean changes, and potentially, the atmosphere, over long and, in the case of our study, rather short timescales. We now know that there is indeed a possible process for melting methane hydrates to temporarily bypass what was previously thought to be a strong filter in the sediment,” says Christian Stranne.

The warming rate is, however, of great importance: “Our results suggest that if our oceans warm at a pace significantly lower than 1 °C per 100 years, the filter can keep up with the pace and remain highly efficient. Unfortunately, we see higher warming rates than that in some of our oceans.”

The study was funded by the Swedish Research Council, VR.

Reference: Christian Stranne, Matt O’Regan, Wei-Li Hong, Volker Brüchert, Marcelo Ketzer, Brett F. Thornton, & Martin Jakobsson, (2022) Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation, Communications Earth & Environment. https://doi.org/10.1038/s43247-022-00490-x

Contact:

Christian Stranne, e-mail christian.stranne@geo.su.se, phone +46 706476154

JOURNAL

Communications Earth & Environment

DOI

10.1038/s43247-022-00490-x

ARTICLE TITLE

Anaerobic oxidation has a minor effect on mitigating seafloor methane emissions from gas hydrate dissociation

ARTICLE PUBLICATION DATE

28-Jul-2022

New Antarctic study shows levels of ‘forever chemicals’ reaching the remote continent have been increasing

New evidence from Antarctica shows that toxic ‘fluorinated forever chemicals’ have increased markedly in the remote environment in recent decades and scientists believe CFC-replacements could be among likely sources.

Peer-Reviewed Publication

LANCASTER UNIVERSITY

Shallow coring — IMAGE: TAKING A FIRN CORE SAMPLE view more
CREDIT: DR MARKUS FREY, BRITISH ANTARCTIC SURVEY

Known as forever chemicals because they do not break down naturally in the environment, chemicals such as perfluorocarboxylic acids (PFCAs) have a wide array of uses such as in making non-stick coatings for pans, water-repellents for clothing, and in fire-fighting foams. One of these chemicals, perfluorooctanoic acid (PFOA), bioaccumulates in foodwebs and is toxic to humans with links to impairment of the immune system and infertility.

In this new study, published by the journal Environmental Science & Technology, and led by scientists from Lancaster University along with researchers from the British Antarctic Survey and the Hereon Institute of Coastal Environmental Chemistry, Germany, firn (compacted snow) cores were taken from the extremely remote, high and icy Dronning Maud Land plateau of eastern Antarctica.

The cores, which provide a historic record between 1957 and 2017, provide evidence that levels of these chemical pollutants have shown a marked increase in the remote snowpack of Antarctica over the last few decades.

The most abundant chemical discovered by far was the shorter chain compound, perfluorobutanoic acid (PFBA). Concentrations of this chemical in the snow cores increased significantly from around the year 2000 until the core was taken in 2017.

Professor Crispin Halsall of Lancaster University, and who led the study, believes this increase can be partly explained by a switch by global chemicals manufacturers around 20 years ago from producing long-chain chemicals like PFOA to shorter-chain compounds, such as PFBA due to health concerns associated with human exposure to PFOA.

Dr Jack Garnett who conducted the chemical analysis on the snow samples, added: “The large increase in PFBA observed from the core, particularly over the last decade, suggests there is an additional global source of this chemical other than polymer production. We do know that some of the chemicals replacing the older ozone-depleting substances like CFCs and HCFCs, such as the hydrofluoroethers, are produced globally in high quantities as refrigerants but can breakdown in the atmosphere to form PFBA.

“The Montreal Protocol certainly provided huge benefits and protection to the ozone, the climate and to us all. However, the wider environmental and toxicity impact of some of these replacement chemicals is still unknown.”

PFOA shows an increase in the snow core from the mid-1980s onward, but with no evidence of a decline in more recent years to match the global industry phase out of this chemical. This indicates that production of PFOA was maintained or that volatile precursors to this chemical have remained high in the global atmosphere.

The researchers behind the study believe the chemicals are likely reaching Antarctica by the release of volatile ‘precursor’ chemicals into the atmosphere at industrial manufacturing sites. These precursors waft in the global atmosphere until they eventually degrade in the presence of sunlight to form the more persistent PFCAs.

Successive snowfall over the years has deposited these chemicals from the atmosphere resulting in a historical record of global contamination that is now trapped in the snow pack.

The results, which are consistent with modelled estimates of PFCA chemical emissions, further add to evidence that show increases in these forever chemicals in the Arctic and the Tibetan Plateau and helps provide a global picture and further understanding of how chemicals such as these are transported in the atmosphere.

Dr Anna Jones, Director of Science at the British Antarctic Survey, said: “These findings are a sobering reminder that our industrial activities have global consequences. Antarctica, so remote from industrial processes, holds this next signal of human activity arising from emissions thousands of miles away. The snow and ice of Antarctica are critical archives of our changing impact on our planet”.

Dr Markus Frey, scientist from the British Antarctic Survey and co-author of the report, said: “This is another example that despite its extreme remoteness man-made pollution does reach the Antarctic continent and is then archived in snow and ice, which allows us to establish a history of global atmospheric pollution and effectiveness of mitigation measures.”

The results are published in the paper ‘Increasing Accumulation of Perfluorocarboxylate Contaminants Revealed in an Antarctic Firn Core (1958-2017)’.

DOI: 10.1021/acs.est.2c02592

CAPTION

Logging the firn core sample

CREDIT

Dr Markus Frey, British Antarctic Survey