Bottom Simulating Reflector Most of the oceanic occurrences of gas hydrate shown on Figure 1 as filled circles are inferred, based mainly on the appearance on marine seismic reflection profiles of an anomalous bottom-simulating reflection (BSR). This reflection coincides with the depth predicted, based on pressure-temperature considerations, as the base of the gas hydrate stability zone (Shipley et al., 1979). The base of the gas hydrate stability zone is the bottom simulating reflector, also called the BSR, which produces the reflection on seismic records. BSRs mark the interface between higher sonic velocity, hydrate-cemented sediment above and lower sonic velocity, uncemented sediment with free gas below. The seismic reflection from the base of the gas-hydrate-stability zone is generally characterized by reflection polarity reversals and negative reflection coefficients. The reflection usually mimics the seafloor form and often increases in sediment depth with increasing water depth.
Inferred Gas Hydrate in Oceanic Sediment The bottom-simulating-reflector was a key factor in detecting gas hydrate in oceanic sediment, and BSRs were used to infer gas hydrate before any gas hydrate was ever recovered from oceanic sediment. Before the Deep Sea Drilling Program (DSDP) Leg 11 (prior to 1970) in the Blake Ridge region of the Atlantic Ocean offshore from southeastern United States, geophysicists were intrigued by BSRs on seismic records from the Blake Ridge (Markl et al., 1970; Stoll, et al., 1971). The observation that some of the seismic reflections intersected bedding reflections and paralleled the seafloor was unexplained. One objective of DSDP Leg 11 was to investigate the nature of these reflectors that caused the anomalous reflections. The strongest reflection mimicked the profile of the Blake Ridge at a depth of more than 500 m below the seafloor. Core samples across the region yielded mainly methane with traces of ethane (Claypool et al., 1973). No gas hydrate was observed, and it was not until DSDP Leg 76 that gas hydrate was observed in sediment of the Blake Ridge (Kvenvolden and Barnard, 1983). The prominent BSR was at the time correlated with a break in the drilling record and with a zone of carbonate minerals (Lancelot and Ewing, 1972). Another explanation was offered by Stoll et al (1971) and Ewing and Hollister (1972) that the BSR corresponded to the isotherm that separates the gas environment from the overlying gas hydrate environment. This latter explanation was confirmed by drilling on DSDP Leg 76 and is now the accepted interpretation.
Since then, other BSRs were reported from the western North Atlantic Ocean (Tucholke et al., 1977) and the Beaufort Sea (Grantz et al., 1976). In a now classic paper, Shipley et al. (1979) described BSRs in sediments off the western and eastern coasts of the United States, in the western Gulf of Mexico, off the northern coasts of Colombia and Panama, and along the Pacific coast of Central America from Panama to Mexico. Now more than 60 sites are known worldwide where gas hydrate occurs in oceanic sediment based on BSRs and/or sample recovery (Figure 1). A compilation of 47 of these sites can be found in Kvenvolden et al. (1993).
Early Direct Observations As far as can be ascertained, the first direct observation of gas hydrate in oceanic sediment was made by Yefremova and Zhizhchenko (1974). They described crystal gas hydrates in near-surface sediment recovered from the Black Sea: "These hydrates occurred 6.5 m below the seafloor, in large cavities, as micro-crystalline aggregates resembling hoarfrost and tended to disappear before one's eyes." This work was later amplified by Kremlev and Ginsburg (1989). Thus Russian scientists were not only the first to recognize the natural occurrence of gas hydrate in permafrost regions, they were also the first to recover gas hydrate in sediment from an oceanic setting (Black Sea).
During the course of DSDP, observations of deep-sea sediment cores that released large quantities of methane suggested that gas hydrate could exist beneath the seafloor in some areas, but gas hydrate samples were never seen. In reviews of gas data from DSDP, Claypool et al. (1973; Legs 10-19) and Mclver (1974; Legs 18-23) described instances where gas evolved from core samples taken on deck. Gas evolution sometimes continued for several hours, and the pressures generated were occasionally sufficient to extrude cores from the barrel and core liner. The expanding, cooling gas often resulted in ice formation on the exposed core. The quantity and rate of gas evolution suggested gas hydrate as one possibility. In most cases the gas was methane with traces of ethane.
In was not until DSDP Leg 66, offshore from Mexico, that gas hydrate was recognized in the recovered sediments (Shipley and Didyk, 1982). This important discovery provided quantitative evidence for naturally occurring, deep-sea gas hydrate. The gas hydrate was found at three different drill sites within unconsolidated sediments, generally associated with porous zones of volcanic ash or sand layers, interbedded with mud or mudstone. Gas hydrate was also recovered in DSDP Leg 67, offshore from Guatemala (Harrison and Curiale, 1982). The gas hydrate was invariably associated with stratigraphic sequences containing high-porosity sediments. Almost pure gas hydrate was recovered from a core catcher, and another core contained gas hydrate cementing coarse vitric sands. These kinds of observations were extended on Leg 84, also offshore from Guatemala, where more gas hydrate was recovered including a 1.05 m core of almost solid gas hydrate (Kvenvolden and McDonald, 1985). In addition to these early observations, gas hydrate has also been found on the following DSDP and Ocean Drilling Program (ODP) cruises: Leg 76 (Blake Ridge, Kvenvolden and Barnard, 1983); Leg 96 (Gulf of Mexico, Pflaum, et al., 1986); Leg 112 (Offshore Peru, Kvenvolden and Kastner, 1990); Leg 127 (Japan Sea, Shipboard Scientific Party, 1990); Leg 131 (Nankai Trough, Shipboard Scientific Party, 1991); Leg 146 (Cascadia Margin, Whiticar et al., 1995); Leg 164 (Blake Ridge, Paull et al., 1998); and Leg 170 (Offshore Costa Rica, Shipboard Scientific Party, 1997).
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