A sea floor observatory for long term integrated study of hydrates of thermogenic and microbial origins on the Cascadia Margin
Richard Camilli, Rick Coffin, LeRoy Dorman, Ian MacDonald, Réal Roy, Adam Schultz, George Spence, Laurenz Thomsen, Anne Trehu,
Naturally-occuring, ice-like gas hydrates form when gases (mainly microbial methane) combine with water at low temperature and high pressure. These conditions are found in the sediment section beneath deep sea continental slopes where the water depth is greater than about 600 m, including offshore Vancouver Island (N. Cascadia). Natural gas hydrates represent a very large global reservoir of natural gas, mainly methane. Globally they are estimated to contain more organic carbon than all other fossile fuel sources combined. Hydrate is important because of its potential as a large source of future clean energy, because it may have an important role in climate change (methane is a strong greenhouse gas), and because its role in seafloor stability hazard. The Canadian west coast has large and well mapped occurrences of marine hydrate in the large subduction zone accretionary sedimentary prism beneath the continental slope. The region off Vancouver Island is now one of the most comprehensively studied gas hydrate occurrences in the world. This Hydrate study can advance our understanding of the hydrate system to a new level by detailed assesment and long-term monitoring of the rising fluids and gas that form the hydrate and their stability, and the evolution of gas hydrate deposits themselves. This type of monitoring has not been possible before NEPTUNE Canada.
This proposal outlines a set of experiments at a site in Barkley Canyon, about 100 km off the west coast of Vancouver Island. This site is of great interest because hydrates of thermogenic origin are formed at the sea floor, and there is evidence of hydrocarbon gas seepage (Chapman et al., 2004). The Barkley Canyon sea floor hydrates are unique compared to hydrates recovered from shallow (8 m) piston cores at other sites nearby in the northern Cascadia Margin (Riedel et al., 2002) and from Hydrate Ridge farther south off Oregon (Suess et al., 1999), because of their thermogenic origin and high concentration of higher more complex hydrocarbons. They also may be tapping a methane source in a forearc basin rather than from the accretionary complex (Trehu et al, 2003). Consequently, the Barkley Canyon site provides the opportunity for an integrated study of both types of hydrate occurrences: sea floor hydrates related to a highly permeable fluid and gas migration channel, and also hydrates occurring throughout the stability zone that are related to the pervasive flow in sediments of low permeability. The experiments are designed to:
Barkley Canyon hydrate site: 48° 18.6846 N 126° 3.9180 W. The site is a small plateau about 1 square km and 850 m deep on the north wall of Barkley Canyon, a submarine canyon about 100 km off the west coast of Vancouver Island. The site consists of several hydrate outcrops clustered within a few 10s of metres of the central location. The hydrate is exposed as sheets up to 8 m long on the sea floor, and on the flanks of thinly-sedimented mounds about 2-3 m high. The sediment is primarily very fine grain silty mud. Near the mounds the sediment contains gas, quantities of light oil, and small hydrate fragments, and there is evidence of episodic gas emission. The seep supports extensive colonies of chemosynthetic communities consisting of several species of vesicomyid clams clustered around the hydrate mounds. Thin bacterial mats cover large portions of the hydrates and sediment on most of the mounds.
Our results from the initial survey and a subsequent visit in 2003 indicate that the site is a highly localized thermogenic gas and hydrocarbon seep (Chapman et al, 2004). The Barkley Canyon hydrates are unique compared to hydrates recovered from shallow (8 m) piston cores at other sites nearby in the northern Cascadia Margin (Riedel et al., 2002) and from Hydrate Ridge farther south off Oregon (Suess et al., 1999), which are primarily of microbial origin.
The temperature probes have been developed at Oregon State University by A. Schultz, his engineer Phil Taylor, and A. Trehu. Each sediment probe comprises a Ti lance just over 1 m long, with an array of fifteen sensitive and accurate thin film resistance temperature device (RTD) sensors along the length of the probe. The probes include internal data logging with up to 1 GB capacity and rechargeable internal power to enable operations in the event of interruption of NEPTUNE network operations. A small Ti pressure case contains control, power and real-time telemetry hardware and firmare for integration with NEPTUNE over TCP-IP/ethernet and RS-232. The probes measure temperatures with 24-bit resolution, i.e. resolution well below the absolute accuracy of 2 millidegrees. A thermal pulse generator provides an in situ calibration for the thermal conductivity of the sediments. The probes are deployed in the sediment to measure the temperature profile in the near-bottom water column and into the top m of sediment. A total of 12 probes is planned, two at each of 6 hydrate mounds, in order to obtain an adequate spatial sample of the conditions in different fluid flow rates over the seep site.
NRL has used a methane sensor, METS, from ASD Sensortechnik Gmgh (Germany). The METS sensor specifications list an operational depth range from 0-2000 m, temperature range of 0-40 oC, and a methane concentration range of 50 nmol/l – 10 micromol/l. The methane sensor is a semi- conductor (metal oxide) that works on the principle of hydrocarbon adsorption.
The digital imaging system combines a 5.4 megapixel digital camera (Nikon Coolpix 5400) with a stepper motor, solid-state controller/interface, and integrated strobe, which are housed in a tempered glass tube. Interfacing to the NEPTUNE array will provide optional surveillance routines in the event of rapid change.
The sea floor array consists of 20-4C sensors, each sensor consists of a 3- component geophone and a hydrophone. The sensor separation is 25 m, so that the array spans the entire length of the hydrocarbon seep field. A repeated operation implosive sound source will be used with the array.
The robot crawler is an internet operated vehicle (IOV) with the capability to move along the seafloor by video control and to carry out detailed investigations on fluid –and particle fluxes in the benthic boundary layer. The Crawler is small (≈ 80x50x30 cm) and capable of carrying a scientific payload of up to 50 kg. It will be connected via suspended internet/power cable to the node,which can be up to 100 m away; and will be equipped with:
The North Cascadia Hydrate Research Team is a multidisciplinary research group that covers the fields of geophysics, geology, geochemistry, and micro-biology. It has been in place since the early 1990s, and consists of scientists from several Canadian (UVic, GSC, UofT, Dalhousie-U), and international institutions (Naval Research Lab, SCRIPPS, UofBremen, UofWashington, USGS). The team has been extremely successful over the past 15 years in establishing the North Cascadia region as one of the primary sites world wide for hydrate research. More than 20 collaborative research cruises have been carried out in this period, and the research has generated over 25 papers and presentations. The two complementary Neptune hydrate proposals provide the opportunity for the group to continue its outstanding research, and to build new partnerships with other researchers.
The Barkley Canyon hydrate site was discovered and studied during a series of three collaborative research cruises between UVic (R. Chapman) and NRL (R. Coffin) that used the ROPOS submersible to survey the site and characterize the geochemistry of the hydrocarbons (Chapman et al., 2004; Pohlman et al., 2005). For this NEPTUNE proposal a group of collaborating scientists was formed that integrates new expertise in developing, implementing and operating sensor systems for experimental work on the sea floor into the hydrate research team. The scientists include researchers from the International University of Bremen (L. Thomsen, V. Karpen: robot crawler, biology, geochemistry); Oregon State University (A. Schultz and A. Trehu: Temperature probes); UVic (R. Chapman, G. Spence: seismo-acoustics; R. Roy: microbiology), GSC (Drs. M. Riedel, R. Hyndman, S. Dallimore, D. Mosher: geophysics), NRL (R. Coffin: geochemistry), Texas A&M University (I. MacDonald: Digital cameras; benthic environment), SCRIPPS (L. Dorman: seismoacoustics) and Woods Hole Oceanographic Institution (R. Camilli: AUV survey and geochemistry).