Southern Hydrate Ridge, located about 90 km offshore

Oregon Coast The Oregon Coast is a coastal region of the U.S. state of Oregon. It is bordered by the Pacific Ocean to its west and the Oregon Coast Range to the east, and stretches approximately from the California state border in the south to the Columbia ...

, is an active methane seeps site located on the southern portion of Hydrate Ridge. It extends 25 km in length and 15 km across, trending north-northeast-south-southwest at the depth of approximately 800 m.Liu, & Flemings. (2006). Passing gas through the hydrate stability zone at southern Hydrate Ridge, offshore Oregon. ''Earth and Planetary Science Letters,'' ''241''(1-2), 211-226. Southern Hydrate Ridge has been the site of numerous submersible dives with the human occupied ''Alvin'' submarine, extensive visits by numerous robotic vehicles including ''the Canadian ROV ROPOS'', ''Jason (US National Deep Submersible Facility)'', and ''Tiburon'' (MBARI), and time-series geophysical studies that document changes in the subsurface distribution of methane. It is also a key site of the National Science Foundations Regional Cabled Array that is part of the

Ocean Observatories Initiative The Ocean Observatories Initiative (OOI) is a National Science Foundation (NSF)br>Major Research Facilitycomposed of a network of science-driven ocean observing platforms and sensors ( ocean observatories) in the Atlantic and Pacific Oceans. Thi ...

(OOI), which includes eight types of cabled instruments streaming live data back to shore 24/7/365 at the speed of light, as well as uncabled instruments. Location of SHR

Geological background

The geologic history of the Southern Hydrate Ridge has been reconstructed through

seismic imaging Geophysical imaging (also known as geophysical tomography) is a minimally destructive geophysical technique that investigates the subsurface of a terrestrial planet. Geophysical imaging is a noninvasive imaging technique with a high parametrical ...

, which provides constraints on the origin of

methane ice Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...

deposits found in this region. Hydrate Ridge is in a region where faults along the Cascadia Margin transition from seaward-verging to landward-verging. This fault reorientation corresponds to the transition from

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

accretion Accretion may refer to: Science * Accretion (astrophysics), the formation of planets and other bodies by collection of material through gravity * Accretion (meteorology), the process by which water vapor in clouds forms water droplets around nucl ...

to subduction in this active accretionary margin. Seaward-verging

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s characterize the ridge deformation front, extending down to ~7 km beneath the

summit A summit is a point on a surface that is higher in elevation than all points immediately adjacent to it. The topography, topographic terms acme, apex, peak (mountain peak), and zenith are synonymous. The term (mountain top) is generally used ...

. Initiation of the uplift of Southern Hydrate Ridge is predicted to have initiated about 1 million years ago.

Sedimentary characteristics

Clay-rich sediments have been found at the Southern Hydrate Ridge. These sediments are from

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

in age, and composed of 29%

smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

, 31%

illite Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...

, and 40% (

chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...

kaolinite Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...

) on average. Underlying the Pleistocene-Holocene

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

is the late-

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58accretionary material, composed of 38% smectite, 27% illite, and 35% (chlorite + kaolinite). A thick

permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...

zone of coarse-grained

turbidite A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing Turbidites were ...

s underlies the sediments. Located along the Cascadia accretionary margin,Philip, B., Denny, A., Solomon, E., & Kelley, D. (2016). Time‐series measurements of bubble plume variability and water column methane distribution above Southern Hydrate Ridge, Oregon. ''Geochemistry, Geophysics, Geosystems,17''(3), 1182-1196. sediment build-up in this region is driven by two

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

-related processes: #Scraping of sediments off of the subducting Juan de Fuca plate onto the overlying

North American plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacific ...

, and #Underplating of subducted sediments onto the overlying plate. Continuous duplexing and underplating of sediment has caused thickening of sediments through uplifting. Furthermore, compaction and dewatering in this region has led to increased local pore pressure.

Methane Ice Methane clathrate (CH4·5.75H2O) or (8CH4·46H2O), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate, is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amou ...
at Southern Hydrate Ridge

Methane ice at Southern Hydrate Ridge has been found within the shallow sediments, and more rarely exposed on the seafloor. Because Southern Hydrate Ridge is located on the upper

continental slope A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

, the regional hydrate stability zone (RHSZ), which is controlled by the sediment pore pressure and temperature,Bangs, N. L., Musgrave, R. J., & Tréhu, A. M. (2005). Upward shifts in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon. ''Journal of Geophysical Research: Solid Earth'', ''110''(B3). is very shallow. As organic material in the sediments is utilized by microbes, producing methane saturation within the sediment pores, methane ice forms within the RHSZ. The base of the RHSZ marks the transition from methane-ice-rich sediment, to clay sediments. Due to the impedance contrast between RHSZ and the underlying sediments, the depth of RHSZ can be detected using seismic imaging techniques.

Associated microbially-mediated carbonate formations

Methane hydrate formation is associated with extensive

authigenic Authigenesis is the process whereby a mineral or sedimentary rock deposit is generated where it is found or observed. Such deposits are described as authigenic. Authigenic sedimentary minerals form during sedimentation by precipitation or recrys ...

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...

. These carbonate deposits are associated with the local

chemosynthetic In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic compounds (e.g., hydrogen gas, hydro ...

communities such as sulfide-oxidizing

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

, mussels, vesicomyid clams, snails and tube worms (although tube worms are not observed at Southern Hydrate Ridge).Boetius, & Suess. (2004). Hydrate Ridge: A natural laboratory for the study of microbial life fueled by methane from near-surface gas hydrates. ''Chemical Geology,'' ''205''(3), 291-310. Migration and egress of methane-rich fluids and microbial interactions can lead to the formation of chemoherms through anaerobic oxidation of methane. At Southern Hydrate Ridge, in addition to a gentle rampart of authigenic carbonate cobbles that rims the main seep site, there is a 60-m tall massive carbonate deposit called the Pinnacle. Uranium-thorium dating of carbonate material from the Pinnacle indicates that the Pinnacle is between ~ 7,000 and 11,000 years old.

Methane venting: spatial and temporal discontinuity

Methane venting includes the release of methane in the form of fluid and gases from methane seeps as methane ice dissociates. Due to the narrow RHSZ at the upper continental slope, methane ice at Southern Hydrate Ridge is metastable such that changes in seafloor temperature and

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

may lead to destabilization of methane ice and the disassociation into fluid and gas.Ruppel, C. D. (2011) Methane Hydrates and Contemporary Climate Change. ''Nature Education Knowledge'' 3(10):29 Methane venting at Southern Hydrate Ridge has been observed to be transient and episodicRiedel M., M. Scherwath, M. Römer, M. Veloso, M. Heesemann, & G.D. Spence. (2018). Distributed natural gas venting offshore along the Cascadia margin. ''Nature Communications,'' ''9''(1), 1-14. with temporal variations of hours to days.Daigle, H., Bangs, N., & Dugan, B. (2011). Transient hydraulic fracturing and gas release in methane hydrate settings: A case study from southern Hydrate Ridge. ''Geochemistry, Geophysics, Geosystems,12''(12), N/a. This area is characterized by multiple sites of venting. which is thought to reflect different

fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

networks. While active venting may maintain open fracture networks, fractures may also be filled by hydrates when there is no venting. As venting reactivates, a new fracture system may be created. While temporal and spatial variations in venting have been observed at this seep site, the local venting rate has been found to varyi over six orders of magnitude: the controls are still not well understood. New instrumentation at this site, including cabled multibeam sonar systems developed by the University of Bremen, now image the entire seep area of Southern Hydrate Ridge, scanning for plumes every two hours. An overview sonar and quantification sonar at the main study site "Einsteins Grotto", are providing new insights into the temporal, spatial and intensity of the plumes and quantification of methane flux from this highly dynamic environment.

Significance

Release of methane from marine seep sites into the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

may have been a factor for past

climate warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

events, such as the Paleocene-Eocene Thermal Maximum (PETM). It is estimated that there are Gigatons of carbon trapped as methane in margin environments and the release of methane from seeps is thought to be responsible for 5 to 10% of the global

atmospheric methane Atmospheric methane is the methane present in Earth's atmosphere. Atmospheric methane concentrations are of interest because it is one of the most potent greenhouse gases in Earth's atmosphere. Atmospheric methane is rising. The 20-year glob ...

Scientific investigation

Since the discovery of methane seeps and novel microbial and macro

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

at Hydrate Ridge in 1986, the Southern Hydrate Ridge has become an extensive study site. Currently, it is one of the study sites under the OOI Regional Cabled Array.{{Cite web, url=https://interactiveoceans.washington.edu/story/Southern_Hydrate_Ridge_, title=Southern Hydrate Ridge, website=interactiveoceans.washington.edu, access-date=2018-10-16 Infrastructure, including a diverse suite of instruments, was installed and became fully operational in 2014. Sensors that are currently at this site include: *

Pressure Sensor A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually act ...

measures the pressure exerted by the overlying water column at the seafloor and is installed to study the impacts of lunar tides on methane release. *

Current Meter A current meter is an oceanographic device for flow measurement by mechanical, tilt, acoustical or electrical means. Different reference frames In physics, one distinguishes different reference frames depending on where the observer is located, ...

measures the current velocity and temperature of the water using acoustic signals. *

Acoustic Doppler Current Profiler An acoustic Doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar, used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water col ...

(ADCP) measures the current velocity of the water profile in the region using acoustic signals. This instrument is installed by the OOI for understanding the local fluxes of heat, mass and momentum. An example of such application is the study of bubble plume evolution over time.Philip, B., Kelley, D., Solomon, E., & Delaney, J. (2016). Monitoring methane emissions at Southern Hydrate Ridge using an OOI Cabled Array Acoustic Doppler Current Profiler. ''OCEANS 2016 MTS/IEEE Monterey,'' 1-5. *

Digital Still Camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile devices ...

records the changes in seafloor morphology and biology, as well as methane plumes. This is important to understand how the local system and

biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...

evolves through time. *

Mass Spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

measures the dissolved gas concentration, which is important for understanding the local

biogeochemical Biogeochemistry is the scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the cryosphere, t ...

processes and quantification of methane release from the seafloor. *Low-frequency

Hydrophone A hydrophone ( grc, ὕδωρ + φωνή, , water + sound) is a microphone designed to be used underwater for recording or listening to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electric potenti ...

records sound waves that propagate through the water column for examination of seismic activity. *Bottom Ocean

Seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

s detect

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

activity local and at a regional scale. At Southern Hydrate Ridge, there is currently one broadband seismometer with an accelerator, and three short-period seismometers (for examination local seismic events that may provide insights into the fracture distribution in the subsurface). *'Osmo' Fluid Sampler samples the fluid coming issuing from the seep sites through drawing fluid into a capillary tube-like tubing. *Benthic Flow Sensors measures the fluid flow rates into and out of the sediment, which are important for determining the local methane and sulfide flux into the ocean.

References

Clathrate hydrates