Methane Hydrates: Southern Hydrate Ridge

...to understand the biogeochemical coupling associated with gas-hydrate formation and destruction.

Cabled infrastructure now installed at Southern Hydrate Ridge providing real-time data to shore. The main focus site is called 'Enstein's Grotto, a site where vigorous venting of methane bubbles rise several hundred meters above the seafloor. Credit: University of Washington.

An active methane hydrate seep site at Southern Hydrate Ridge. Emanating out of the cavity in the sediments are bubbles from gas hydrate dissociation within the shallow sediments. Photo credit: NSF-OOI/UW/CSSF

Methane Ice on the Seafloor

A significant amount of methane near the surface of the Earth is locked in gas hydrates buried in the shallow sediments of continental margins. The hydrates may act as a capacitor in the carbon cycle by slowly storing methane that can be suddenly released into the ocean and atmosphere during seismic events or slope failure. Southern Hydrate Ridge in the Cascadia accretionary complex, is one of the best-studied gas hydrate deposits. Significant methane seeps hosting diverse biological assemblages and formation of gas-rich hydrate deposits near the seafloor have been documented. Studies of these deposits have provided a good understanding of how gas hydrate is distributed in marine sediments and of the processes that lead to heterogeneity in distribution.

Southern Hydrate Ridge is an optimal place for the OOI Cabled Array to examine the temporal evolution of these dynamic systems, to determine the fluxes of methane from the seafloor into the ocean, and to understand the biogeochemical coupling associated with gas-hydrate formation and destruction. The area hosts multiple sites where methane gas is exiting the seafloor, forming plumes that rise several hundred meters above the seafloor. Since 2014, this site has hosted a suite of cabled instruments to monitor chemical, biological, geological and physical processes at this site. The real-time interactive capabilities of the cabled observatory are critical to studying gas-hydrate systems because many of the key processes may occur over short time scales and will require adaptive response and sampling capabilities that include fluid sampling, increases in data accumulation rates and imagery from cameras, and in situ manipulation of chemical sensors.

As outlined in the Gas-Hydrate Observatories Workshop (2007), the high power and bandwidth capability of the OOI Cabled Array is required to enable operations that include downhole seismic and/or electromagnetic studies, multiyear deployments that are needed to capture the various time scales operating in this system, and the need for real-time intervention to capture infrequent events or otherwise change experiment parameters that could not be made by passive monitoring. Significant expansion capabilites at this experimental site will provide access to a wealth of follow-on experiments that may include downhole observatories and incorporation of autonomous vehicles for repeat, high-resolution mapping and imaging of the methane seeps and deposits, associated biota, and plumes.