Sea Floor Methane Hydrates at Hydrate Ridge, Cascadia Margin

Abstract

The crest of Hydrate Ridge harbors a variety of gas hydrates in near-surface sediments. Hydrate formation and destruction continuously shape the ridge topography. Interstitial Cl-anomaly patterns in conjunction with video-guided sampling have established that the uppermost sediment column contains several distinct layers of gas hydrate which are exposed at the sea floor. A methaneoxidizing bacterial consortium populates the exposures of hydrate; colonies of vent macro-fauna are abundant as well. Discharge of methane from destabilized hydrate at the seafloor stimulates high rates of benthic oxygen consumption. These rates, however, vary by many orders of magnitude spatially and temporally, highlighting the need for implementing seafloor observatories at gas hydrate sites. Two types of hydrate fabrics were observed: A highly porous fabric with an estimated pore space of approx. 60 vol.-% and a massive type, with no visible pore space. Both types contain varying amounts of chloride, which need to be taken into account when estimating hydrate volumes from Cldepletion of pore waters. The porous hydrate has low bulk density, which may cause periodic release of large chunks of hydrate from the sea floor. They float to the surface and leave behind a chaotic topography of mounds and depressions. These pieces of floating hydrates constitute an important transport mechanism for methane from the seafloor directly to the atmosphere.

Review of natural gas hydrates as an energy resource: Prospects and challenges

“…However, it has been suggested that local, anomalous accumulations of gas hydrates of high concentration could exist in amounts greater than what were previously thought [11,61]. Recent field drilling programs for gas hydrate have discovered concentrated gas hydrate accumulations with examples being the marine sands in the Nankai Trough [40,62], Cascadia Margin [63,64] and Gulf of Mexico [65,66] and other types in offshore India [67], Malaysia [68] and Korea [39,69]. These concentrated hydrate deposits present a great potential for energy resource in the future.…”

Fault system and thermal regime in the vicinity of site NGHP-01-10, Krishna–Godavari basin, Bay of Bengal

“…In structural accumulation gases are transported to the GHSZ through features such as faults and mud volcanoes, e.g. northwestern Gulf of Mexico MacDonald et al, 1994;Sassen, 2000, 2001;Sassen et al, 1999Sassen et al, , 2001, Hydrate Ridge (Hovland et al, 1995;Suess et al, 1999Suess et al, , 2001Trehu et al, 1999), and Haakon Mosby (Bogdanov et al, 1999;Ginsburg et al, 1999). In stratigraphic accumulation gases are transported along permeable horizons, e.g., Blake ridge (Xu and Ruppel, 1999;Dickens et al, 1997), Gulf of Mexico minibasins (Milkov and Sassen, 2001;Pflaum et al, 1986), Nankai trough (Matsumoto et al, 2001), and Mallik (Dallimore et al, 1999).…”

Experimental verification of anomalous chloride enrichment related to methane hydrate formation in deep‐sea sediments

“…At the summit of a hydrate ridge (Ocean Drilling Program (ODP) Sites 1249 and 1250), the pore fluids were found to be highly enriched with dissolved chloride over a zone that extends from near the sediment surface [$1 mbsf (meter below sea floor)] to depths of 25 mbsf. 4 This chloride enrichment was also reported to occur in other regions, including U1328 on the Northern Cascadia Margin. 5 In efforts to obtain reasonable explanations of this phenomenon, several model studies have been carried out focusing on the exclusion and diffusion mechanism.…”

“…The viscosity of the sediment mixture was high enough that the mixture remained outside of the tubes. Also, after purging CH 4 , no invaded clay sediment was observed, because the pressure on the sediment layer and the pressure inside the tube must be equivalent. The tube details are shown in the Figure 2; the radius of the tube is 3 mm and that of the hole is 1 mm.…”

“…In equilibrium cell (1) linked with electric circuit (5, 6), we first set the three tubes with open holes (4). Subsequently, we place the sediment sample around the tubes and finally, fill the NaCl solution to possibly prevent the contact between methane gas and sediment layer.…”