Aus dem Meeresboden gast Methan aus. Das is ein natürlicher Prozess, den hat es immer schon gegeben. Wir wirkt sich die Klimaerwärmung darauf aus und welchen Einfluss hat dies wiederum auf den Klimawandel? Wir haben uns an dieser Stelle bereits mehrfach mit dem Phänomen beschäftigt:
- Geomar: Methanquellen vor Spitzbergen verringern Treibhauseffekt (siehe hierzu auch USGS-Artikel vom 8. Mai 2017)
- Nicht neu: Es blubbert Methan aus dem Meeresboden
- Amerikanischer Geologischer Dienst gibt Entwarnung: Methanhydrat im arktischen Polarmeer stellt keine große Klimagefahr dar
- Eine kleine Sensation: Aus dem arktischen Meeresboden ausgasendes Methan gelangt nicht in die Atmosphäre
- Was macht eigentlich…das Methan? Methanaustritte vor Spitzbergen nicht durch Klimawandel bedingt
Was gibt es Neues zum Thema aus der Aktis? Vor Spitzbergen hat die Universität Tromsö jetzt Methanmessstellen im Meer platziert:
Methane observatories successfully deployed in the Arctic
Mysteries still abound about methane release from the ocean floor. Two state of the art observatories have been deployed offshore Svalbard this summer, to try and unveil the secrets of natural release of the climate gas.
It is not only the space agencies that launch landers with sensors to far away places . Marine science institutions have a lot of unknown ground to cover in their quest for knowledge. Also they are depending on groundbreaking lander and sensor technology to lead them to that knowledge. CAGE recently deployed two observatories on the site of the methane seeps in the Arctic Ocean. Kongsberg Maritime built the two observatories that are now comfortably placed on the ocean floor in two locations offshore Svalbard. These are the sites where flares of gas bubbles have been observed, indicating release of methane gas to the water column. The observatories are placed at the depth of 90 meters and 240 meters respectively. “The launch went perfectly,” says chief scientist on the cruise, Dr. Anna Silyakova.
Methane is a potent climate gas that can amplify the global warming if released into the atmosphere. However, there is still a lot to be learned about the release of methane from the ocean floor, and what happens to it in the water column. Does it get dispersed with the currents? Do bacteria consume it? Or is it released in the atmosphere? The observatories include several instruments which will monitor methane release from the seabed to the water column as well as CO2, ocean acidification and circulation. The data from these observatories will provide knowledge that will help understand processes related to climate change. The observatories will stay put in their locations, collecting crucial data for a full year. Aboard the the ship was also the freelance photojournalist Randall Hyman, who made this nice video.
Neben organischem, biogenem Methan tritt jedoch auch abiotisches CH4 aus dem Meeresboden aus, das aus magmatischen Quellen des Erdmantels stammt. Pressemitteilung CAGE – Center for Arctic Gas Hydrate, Climate and Environment aus dem April 2015 (via WUWT):
New source of methane discovered in the Arctic Ocean
Methane, a highly effective greenhouse gas, is usually produced by decomposition of organic material, a complex process involving bacteria and microbes. But there is another type of methane that can appear under specific circumstances: Abiotic methane is formed by chemical reactions in the oceanic crust beneath the seafloor. New findings show that deep water gas hydrates, icy substances in the sediments that trap huge amounts of the methane, can be a reservoir for abiotic methane. One such reservoir was recently discovered on the ultraslow spreading Knipovich ridge, in the deep Fram Strait of the Arctic Ocean. The study suggests that abiotic methane could supply vast systems of methane hydrate throughout the Arctic. The study was conducted by scientists at Centre for Arctic Gas Hydrate, Environment and Climate (CAGE) at UiT The Arctic Univeristy of Norway. The results were recently published in Geology online and will be featured in the journal’s May issue.
“Current geophysical data from the flank of this ultraslow spreading ridge shows that the Arctic environment is ideal for this type of methane production. ” says Joel Johnson associate professor at the University of New Hampshire (USA), lead author, and visiting scholar at CAGE. This is a previously undescribed process of hydrate formation; most of the known methane hydrates in the world are fueled by methane from the decomposition of organic matter. “It is estimated that up to 15 000 gigatonnes of carbon may be stored in the form of hydrates in the ocean floor, but this estimate is not accounting for abiotic methane. So there is probably much more.” says co-author and CAGE director Jürgen Mienert.
Life on Mars?
NASA has recently discovered traces of methane on the surface of Mars, which led to speculations that there once was life on our neighboring planet. But an abiotic origin cannot be ruled out yet. On Earth it forms through a process called serpentinization. “Serpentinization occurs when seawater reacts with hot mantle rocks exhumed along large faults within the seafloor. These only form in slow to ultraslow spreading seafloor crust. The optimal temperature range for serpentinization of ocean crust is 200 – 350 degrees Celsius.” says Johnson. Methane produced by serpentinization can escape through cracks and faults, and end up at the ocean floor. But in the Knipovich Ridge it is trapped as gas hydrate in the sediments. How is it possible that relatively warm gas becomes this icy substance? “In other known settings the abiotic methane escapes into the ocean, where it potentially influences ocean chemistry. But if the pressure is high enough, and the subseafloor temperature is cold enough, the gas gets trapped in a hydrate structure below the sea floor. This is the case at Knipovich Ridge, where sediments cap the ocean crust at water depths up to 2000 meters. ” says Johnson.
Stable for two million years
Another peculiarity about this ridge is that because it is so slowly spreading, it is covered in sediments deposited by fast moving ocean currents of the Fram Strait. The sediments contain the hydrate reservoir, and have been doing so for about 2 million years. ” This is a relatively young ocean ridge, close to the continental margin. It is covered with sediments that were deposited in a geologically speaking short time period -during the last two to three million years. These sediments help keep the methane trapped in the sea floor.” says Stefan Bünz of CAGE, also a co-author on the paper. Bünz says that there are many places in the Arctic Ocean with a similar tectonic setting as the Knipovich ridge, suggesting that similar gas hydrate systems may be trapping this type of methane along the more than 1000 km long Gakkel Ridge of the central Arctic Ocean. The Geology paper states that such active tectonic environments may not only provide an additional source of methane for gas hydrate, but serve as a newly identified and stable tectonic setting for the long-term storage of methane carbon in deep-marine sediments.
Need to drill
The reservoir was identified using CAGE’s high resolution 3D seismic technology aboard research ressel Helmer Hanssen. Now the authors of the paper wish to sample the hydrates 140 meters below the ocean floor, and decipher their gas composition. Knipovich Ridge is the most promising location on the planet where such samples can be taken, and one of the two locations where sampling of gas hydrates from abiotic methane is possible. ” We think that the processes that created this abiotic methane have been very active in the past. It is however not a very active site for methane release today. But hydrates under the sediment, enable us to take a closer look at the creation of abiotic methane through the gas composition of previously formed hydrate.” says Jürgen Mienert who is exploring possibilities for a drilling campaign along ultra-slow spreading Arctic ridges in the future.
Übertreibungen hinsichtlich des Erwärmungseffekes waren beim Methan leider lange eher die Regel als die Ausnahme. Mittlerweile reduzieren Forscher die früheren Visionen jedoch wieder zurück auf ein reales Normalmaß. Beispiel Stranne et al., die im August 2016 in den Geophysical Research Letters Probleme mit der Dynamik der Modelle publik machten:
Overestimating climate warming-induced methane gas escape from the seafloor by neglecting multiphase flow dynamics
Continental margins host large quantities of methane stored partly as hydrates in sediments. Release of methane through hydrate dissociation is implicated as a possible feedback mechanism to climate change. Large-scale estimates of future warming-induced methane release are commonly based on a hydrate stability approach that omits dynamic processes. Here we use the multiphase flow model TOUGH + hydrate (T + H) to quantitatively investigate how dynamic processes affect dissociation rates and methane release. The simulations involve shallow, 20–100 m thick hydrate deposits, forced by a bottom water temperature increase of 0.03°C yr−1 over 100 years. We show that on a centennial time scale, the hydrate stability approach can overestimate gas escape quantities by orders of magnitude. Our results indicate a time lag of > 40 years between the onset of warming and gas escape, meaning that recent climate warming may soon be manifested as widespread gas seepages along the world’s continental margins.
Interessant auch eine Arbeit von Carolyn Ruppel und John Kessler aus dem Februar 2017 in den Reviews of Geophysics. Es gäbe noch keine überzeugenden Anzeichen dafür, dass Methan aus Gashydraten derzeit in die Atmosphäre ströme. Abstract:
The interaction of climate change and methane hydrates
Gas hydrate, a frozen, naturally-occurring, and highly-concentrated form of methane, sequesters significant carbon in the global system and is stable only over a range of low-temperature and moderate-pressure conditions. Gas hydrate is widespread in the sediments of marine continental margins and permafrost areas, locations where ocean and atmospheric warming may perturb the hydrate stability field and lead to release of the sequestered methane into the overlying sediments and soils. Methane and methane-derived carbon that escape from sediments and soils and reach the atmosphere could exacerbate greenhouse warming. The synergy between warming climate and gas hydrate dissociation feeds a popular perception that global warming could drive catastrophic methane releases from the contemporary gas hydrate reservoir. Appropriate evaluation of the two sides of the climate-methane hydrate synergy requires assessing direct and indirect observational data related to gas hydrate dissociation phenomena and numerical models that track the interaction of gas hydrates/methane with the ocean and/or atmosphere. Methane hydrate is likely undergoing dissociation now on global upper continental slopes and on continental shelves that ring the Arctic Ocean. Many factors—the depth of the gas hydrates in sediments, strong sediment and water column sinks, and the inability of bubbles emitted at the seafloor to deliver methane to the sea-air interface in most cases—mitigate the impact of gas hydrate dissociation on atmospheric greenhouse gas concentrations though. There is no conclusive proof that hydrate-derived methane is reaching the atmosphere now, but more observational data and improved numerical models will better characterize the climate-hydrate synergy in the future.