All over the planet, cold seeps in the ocean floor are quietly bubbling away, releasing vast quantities of methane. This methane can have significant implications for both local ecosystems and the world as a whole, as methane is a potent greenhouse gas.
Despite its importance, there’s still a lot that scientists don’t know about seafloor methane seepage and how it has changed over time.
“Trying to understand how much methane is released from the seafloor is extremely difficult, even in [the] present day,” said Davide Oppo, a geoscientist at the University of Louisiana at Lafayette. It’s notoriously difficult to study deep-ocean processes, especially those that are taking place at many different locations simultaneously. Given how challenging it is to study methane seepage now, how could scientists even begin to study it in the distant past?
Although it’s impossible to directly measure this ancient methane seepage, Oppo and his colleagues have used a clever proxy called methane-derived carbonate to assess seafloor methane release over the past 150 million years. Their research was published in Scientific Reports earlier this year.
As methane rises up through the sediment of the seafloor, some of it gets oxidized by microorganisms. Methane-derived carbonates are a by-product of this process and can stick around for a very long time. Therefore, determining how much of these methane-derived carbonates is present in the sedimentary record provides scientists with clues about historical methane seepage.
The research team compiled a database of rock records of methane-derived carbonate. By comparing how the amount of methane-derived carbonate and certain ocean characteristics changed over time, scientists could see whether there were any associations between carbonates and such characteristics as seawater sulfate concentration, sea level, deep-sea temperature, sediment flux, and organic carbon burial. These could indicate which characteristics affected seafloor methane seepage.
Scientists found that changes in sea level and organic carbon burial were closely associated with changes in methane-derived carbonate, suggesting that these may be important controllers of oceanic methane seepage.
Why Study Methane?
Historically, methane has been relatively overlooked. “When I started [working in this field], methane was just considered a minor gas,” said Paul Johnson, a professor of oceanography at the University of Washington who was not involved in the new study. “It wasn’t thought to play a big role in the environment at all, biologically or ecologically or in terms of climate change.”
Since then, however, said Johnson, “people are continually finding that methane is more and more important to the environment.” In fact, there are several different geological and biological roles that methane can play.
For example, scientists now know that methane is a major contributor to climate change: In the geological short term (in the first century after its release), methane is dozens of times more potent as a greenhouse gas than carbon dioxide.
Because methane contains carbon (its chemical formula is CH4), figuring out how much methane is released from the seafloor and what happens to it afterward is important for our overall understanding of the global carbon cycle.
On a smaller scale, understanding seafloor methane seepage is important for local geology and ecology. Oppo said that methane seepage can be associated with ocean floor instability, which could potentially affect marine infrastructure.
Oppo also said that certain chemosynthetic organisms can use methane as an energy source, so places where methane seeps out of the seafloor may be home to distinctive communities of microbes and animals.
What Comes Next?
Although these results don’t necessarily predict how seafloor methane release will change in the future, Oppo said that knowing which factors affect methane release is an essential first step toward building predictive models.
Furthermore, there are still many unanswered questions regarding oceanic methane. For example, Oppo said there is still no consensus on how much of the methane that gets released at the seafloor actually makes it to the sea surface. It will be important to figure this out if we want to understand the impacts of seafloor methane seepage on the global carbon cycle and on climate change.
—Hannah Thomasy (@HannahThomasy), Science Writer