Methane eruptions on the sea floor

Methane eruptions on the sea floor - Science article: New evidence, new methods

E M B A R G O : February 20th, 8 p.m.

Methane is a highly potent greenhouse gas. In its solid, ice form, called methane hydrate, it is stored in large amounts below the sea floor. Some signs indicate that there have been repeated intense undersea methane emissions over the course of the Earth's history. Often, these emissions appeared to be linked to climate changes, species extinctions, and biological cataclysms both in the oceans and on continents. In the coming issue of the journal Science, Prof. Kai-Uwe Hinrichs from the DFG-Research Center Ocean Margins Bremen/Germany and his colleagues from Woods Hole Oceanographic Institution (WHOI) present new insights related to methane emissions in the recent past. According to Hinrichs, who was only recently appointed to the Bremen research center, multiple rapid, intensive eruptions occurred in the warmer phases of the last ice age on the North American Pacific coast. Undersea melting of methane ice and landslides could have been the causes of these events, which resulted in oxygen depletion in the ocean.

The research team investigated marine deposits from the Santa Barbara Basin that were drilled by the international Ocean Drilling Program (ODP) off the coast of southern California. "There was already indirect evidence for methane eruptions in this area, but viable alternative hypotheses existed as well. It was time to search for unambiguous evidence," says Prof. Hinrichs. "So we looked for fossil remnants of bacteria that would have flourished under conditions of high methane levels." So-called biomarkers, molecular fossils of ancient bacterial communities, gave the researchers the key to their objective. "In particular, the measurements in a 44,100 year-old sediment layer provided evidence for an abrupt, catastrophic release of methane, presumably trapped as hydrate below the sea floor," says the Bremen geochemist. "Only in this layer, did we detect molecular tracers diagnostic of bacteria that could survive without oxygen but not without methane as an energy source." The oxygen content in the deepest compartments of the basin remained low for about a decade after the eruption. Then, only as oxygen again became abundant in the deep-sea water could the oxygen-consuming bacteria reclaim their habitat.

In addition, the biomarker findings indicate that there have been multiple intense methane outbreaks in the Santa Barbara Basin area over the past 60,000 years, especially in the milder phases of the last cold period, which ended 12,000 years ago. In comparison, however, the eruption 44,100 years ago was particularly massive. The American scientist James Kennett has estimated that this event released enormous amounts of methane gas of which 90 million tons were consumed by bacteria in the ocean.

"The amounts of methane presumably released 44,100 years ago call for a mechanism different from those underlying the emissions at warmer periods, i.e. slow decomposition of methane hydrate triggered by warming of bottom waters. The sudden release of these enormous quantities was probably caused by sliding of a huge sediment package, easing pressure on the methane deposits and releasing the greenhouse gas," states Kai-Uwe Hinrichs.

According to current estimates, there is about 10,000 billion tons of methane carbon stored beneath the ocean and on continents. For reference, the greenhouse effect resulting from human activities over the past 150 years was produced by the release of "only" 210 billion tons of carbon (in the form of carbon dioxide). If only a small fraction of the stored methane were to escape into the atmosphere, the resulting greenhouse effect would be enormous.

"Our biomarkers indicate the portion of methane that was consumed in the ocean by bacteria, but don't tell us how much methane escaped into the atmosphere. This is a totally different question," Hinrichs asserts. "Nevertheless, our results demonstrate how drastically ocean margin regions can react to relatively minor environmental changes."


Information / Interviews / Photos:
Albert Gerdes
DFG-Research Center Ocean Margins
University of Bremen
- Public Relations -
Tel. +49 - 421 - 218-7761
email:
www.oceanmargins.de

 Brennendes_Eis.pdf