Beneath the seafloor lie vast stores of methane—a potent greenhouse gas—produced by the degradation of organic material, either by the earth’s heat or by deep-sea microorganisms. This methane is sometimes released in seeps that occur along active tectonic boundaries such as the Cascadia Subduction Zone (CSZ).
A new study led by researchers from the USGS Coastal and Marine Hazards and Resources Program employs advanced analytical techniques to better understand the spatial distribution of seeps in the CSZ.
Across the global ocean, methane seeps tend to occur in areas of specific temperature and pressure conditions on the seafloor, which vary across regions. Known as gas-hydrate stability zones, these are areas in which the pressure is high enough and the temperature low enough to keep methane in its frozen, gas hydrate form. At shallower depths, the bottom temperature increases and the pressure decreases, allowing methane hydrate to dissociate into liquid methane and water and escape through seeps.
The gas-hydrate stability zone in the CSZ is generally at around 500 meters water depth. However, methane seeps occur well outside the 500-meter zone, indicating that something besides merely water depth is influencing seep occurrence. The authors, applying spatial and statistical analyses, complemented by multibeam bathymetry and analysis of seismic reflection profiles, found that in the CSZ, the primary driver of seep occurrence appears to be tectonic and geomorphologic features on the seafloor.
The study marks a crucial first step in establishing linkages between methane seeps and submarine tectonic geomorphology in the CSZ.
“The premise was that seeps were controlled by a temperature-pressure equilibrium that dictated where methane hydrate in the seafloor dissociates from solid to liquid. But gas hydrate stability is just one piece of the puzzle,” said Rudebusch. “We wanted to investigate this more, looking in particular at the role that tectonics and geologic-geomorphic controls have in focusing subsurface fluid and providing pathways to channel the methane up to the seafloor surface.”
The study provides valuable insights into the myriad factors influencing the distribution of methane seeps in the CSZ. The ultimate goal is to develop a predictive model for seep occurrence.
“Determining where seeps occur, as well as the drivers behind their occurrence, is important for understanding the release of methane from the seafloor,” said USGS Geographer Jane Rudebusch, lead author of the study. “This is particularly important as oceans warm under climate change, because the occurrence of methane seeps is partially temperature dependent. Ocean warming can thus accelerate methane release.”
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