Researchers at the University of Washington found that subsurface sea heating is the result of thawing frozen, methane which has been leaking into the atmosphere and reaching the surface. One-third of an ocean’s depth is where the water temperature is low. This is because frozen methane “ice” – a powerful greenhouse gas – has been dormant until recent times. Subsurface warming could be responsible for methane gas rising off Washington and Oregon coasts according to scientists. Over the last ten years they have seen 168 plumes, which is an ‘unreasonably high’ number. Sonar image showing methane bubbles rising off the coast of Washington. Source: University of Washington. Credit to Brendan Philip. H. Paul Johnson (a UW oceanographer and professor) said that there is an unusually large number of bubble plumes near the bottom where methane hydrate could decompose if it warmed. It is unlikely that it will be just from sediments. This appears to be from methane hydrate decomposition. Methane has historically been associated with sudden changes in Earth’s climate. Although we don’t know the role of this gas in climate change today, recent research has shown methane emission related to Arctic permafrost and Atlantic coast warming. One hundred and fourteen 168 methane plows were found at the transition level. This is more than the number of plumes that can be seen on the Oregon or Washington seafloors. Methane can act as an effective greenhouse gas. When methane bubbles reach the ocean surface they are able to enter the atmosphere. Most of deep-sea methane is consumed by marine microbes on their way to the surface. They then convert it to carbon dioxide. The deeper, more acidic and lower-oxygen environments in offshore waters result. The coastal waters eventually become contaminated by carbon dioxide as it rises along the coastline. Professor Johnson stated that “current environmental changes in Washington State and Oregon already have an impact on local biology, fisheries and food security. There are also concerns about seafloor slopes becoming unstable. Johnson explained that the frozen methane is a glue which holds in place steep slopes. There are abundant methane reserves along the Pacific Northwest coast’s continental margin. UW published a study last year showing that the ocean is heating at depths of one-third of mile meters. This was due to water which formed many years ago off Siberia. Then it made its way eastward through the Pacific Ocean. According to the study, warming of this depth would destabilize methane deposit on Cascadia’s subduction zone that stretches from Vancouver Island up to northern California. Because of the high pressure and cold temperatures at the continental margin, methane gas found on seafloor sediments forms a crystal structure with water. Methane hydrate is an ice-like liquid that reacts to temperature fluctuations and is extremely unstable. As the ocean heats up, crystals of methane hydrate begin to dissociate and leak into sediment. As a gas, some of this methane is released from sediment pores. Last year, the team that conducted the study estimated that hydrate decomposition can release about 0.1 million tons per annum into sediments along the Washington coast. This amount is approximately equivalent to methane released by the 2010 Deepwater Horizon explosion. The latest study examined evidence for bubble plumes offshore, which was observable by UW research cruises and local fishermen’s reports, as well as earlier studies. Bubble plumes at least 490feet meters (or more) high that originate from the ocean floor were included by scientists. They are more frequent at 500 metres below the surface. Their data also contained 45 plumes, which fishing boats first detected when searching for schools of fish. The bubbles are detectable by modern sonars. The results revealed that methane gas slowly releases at almost all depths on the Oregon-Washington coast. The plumes appear to be more frequent at 500 meters where methane gas would naturally decompose due to the warm seawater. Professor Johnson stated that “What we are seeing may be confirmation of the predictions made from water temperature: Methanehydrate seems to be decomposing, releasing lots of gas. It is at the critical depth of 500 meter that you see the highest concentration of methane plumes. Nobody knows for sure if these plumes come from dissociation or accumulations of frozen methane. Evan Solomon, an associate professor at UW of oceanography said that these results support the idea that methane hydrate stability is being reduced by modern-day warming. However, it is not proof that the bubbles are dissociating. Prof. Solomon currently analyzes the chemical compositions of samples of plumes that were emitted from sediments that lie along Washington’s coast 500 meters beneath the surface. These results will reveal whether or not the gas was emitted from methanehydrates. This study was supported by the US Department of Energy, and the National Science Foundation. Citation: “Analysis bubble plume distributions in order to assess methane hydrate degradation on the continental slope” H. Paul Johnson and Una K. Miller. Marie S. Salmi, Evan A. Solomon. Geochemistry, Geophysics, Geosystems, October 2015. DOI: 10. 1002/2015GC005955.
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