Southern Hydrate Ridge

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The Ocean Observatories Initiative (OOI) Regional Cabled Array (RCA), operational in the NE Pacific since 2014, includes two cabled junction boxes at Southern Hydrate Ridge, a site of methane seeps at ~780 m water depth ~90 km west of Newport, OR. Located in the central Cascadia accretionary complex, Southern Hydrate Ridge is one of the most highly studied gas hydrate deposit in the oceans, with documented explosive seafloor venting, formation of gas-rich methane hydrate deposits near the seafloor, and exposed hydrate.

Previous research through the Ocean Drilling Program, by Alvin and numerous remotely-operated vehicles (ROVs),as well as by autonomous underwater vehicle (AUVs), and by myriad seismic studies provide a strong understanding of the heterogeneous distribution of gas hydrates in this area and the processes that lead to their formation and demise (e.g Suess et al., 2001; Trehu, et. al. 2004; Bangs et al., 2011′ Philip et al., 2016, Marcon et al., 2021). Subseafloor imaging using three dimensional seismic surveys have defined a focused plumbing system that the OOI used to determine deployment locations for observatory sensors (Bangs, et al. 2011).

The fluid flow system at SHR appears to be controlled by structures that cut across stratigraphic horizons, which differs from gas hydrate systems explored on the northern Cascadia margin (Trehu et al., 2004; Johnson, et al. 2003). There is also a greater abundance of coarse-grained sediments in the north (Trehu, et. al. 2004), which has a strong impact on gas hydrate deposition processes.

The now decade-long RCA-OOI time series from cabled data and annual ROV visits to SHR, coupled with data from central and northern Cascadia and other seep locations, is leading to a more comprehensive understanding of gas hydrate processes, changes over time, and the evolution of the biological community associated with these environments.

The RCA-OOI observatory includes seismometers, hydrophones, an acoustic doppler curren meter, pressure sensor, osmotic fluid samplers, seafloor cameras, and flow meters; see table below for full list. Many of the key processes associated with gas hydrate systems occur over short time scales (e.g., gas hydrate release due to small and large earthquakes, tidal pumping, – Marcon et al., 2021), so real-time data transmission and the capability for adaptive response and sampling adjustment are fundamental observatory benefits that bolster research advancements. The RCA infrastructure enables 24/7 real-time communications and power to the array of scientific instruments at Southern Hydrate Ridge.

Southern Hydrate Ridge is a core component of the OOI because the science conducted at this site spans both seafloor and water column themes that form the foundation of the OOI: e.g., Climate Variability, Ocean Circulation, and Ecosystems; Coastal Ocean Dynamics and Ecosystems; Fluid-Rock Interactions and the Sub-seafloor Biosphere, and Plate-Scale Geodynamics.

The cabled infrastructure at SHR specifically addresses 5 out of 10 OOI major science questions [as detailed in the Ocean Observatories Initiative (OOI) Scientific Objectives A Closer Look (2007), and as part of the Conceptual Network Design]. These include: 1) What is the ocean’s role in the global carbon cycle, and more specifically, gas hydrates and marine seeps? 2) How does plate-scale deformation mediate fluid flow, chemical and heat fluxes, and microbial productivity? 3) What are the forces acting on plates and plate boundaries that give rise to local and regional deformation and what is the relation between the localization of deformation and the physical structure of the coupled asthenosphere-lithosphere system? 4) How do tectonic, oceanographic, and biological processes modulate the flux of carbon into and out of the submarine gas hydrate ‘capacitor,” and are there dynamic feedbacks between the gas hydrate reservoir and other benthic, oceanic, and atmospheric processes? 5) What are the dynamics of hypoxia on continental shelves?

References:

Bangs, N. L. B., Hornbach, M.J., and Berndt, C., 2011. The mechanics of intermittent methane venting at South Hydrate Ridge inferred from 4D seismic surveying. Earth and Planetary Science Letters. 310(1‐2): 105–112, doi:10.1016/j.epsl.2011.06.022.

Johnson, J.E., Goldfinger, C., Suess, E., 2003. Geophysical constraints on the surface distribution of authigenic carbonates across the Hydrate Ridge region, Cascadia margin. Marine Geology. 202(1–2): 79-120.

Suess, E., Bohrmann, G., Rickert, D., Kuhs, W.F., Torres, M.E., Trehu, A., Linke, P., 2002. Properties and fabric of near-surface methane hydrates at Hydrate Ridge, Cascadia margin. In 4th International Conference on Gas Hydrates (pp. 740 – 744), Yokohama, Japan.

Tréhu, A., Long, P.E., Torres, M.E., Bohrmann, G., Rack, F.R., Collett, T.S., Goldberg, D.S., Milkov, A.V., Riedel, M., Schultheiss, P., Bangs, N.L., Barr, S.R., Borowski, W.S., Claypool, G.E., Delwiche, M.E., Dickens, G.R., Gracia, E., Guerin, G., Holland, M., Johnson, J.E., Lee, Y.-J., Liu, C.-S., Su, X., Teichert, B., Tomaru, H., Vanneste, M., Watanabe, M., Weinberger, J.L., 2004. Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204. Earth and Planetary Science Letters. 222(3–4): 845-862.

Tryon, M., Brown, K.M., Torres, M.E., 2002. Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, II: hydrological processes. Earth and Planetary Science Letters. 201: 541–557.