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The Orca Basin is a mid-slope, silled, mini-basin in the northern Gulf of Mexico some 300 km southwest of the Mississippi River mouth on the Louisiana continental slope. It is unique amongst the mini-basins in this area, in containing a large brine pool of anoxic salt brine. The pool is approximately 123 km2 (47 sq mi) in area and up to 220 m (720 ft) deep under 2,400 m (7,900 ft) depth of Gulf water and is derived from dissolution of underlying Jurassic age Louann Salt. With a volume of 13.3 km3 (3.2 cu mi) the pool results from the dissolution of about 3.62 billion tonnes of the Louann Salt bed into seawater. The basin owes its shape to ongoing salt tectonics and is surrounded by salt diapirs.
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Map of Orca Basin
Gas hydrates were detected in a number of cores collected in the Orca basin during Leg 96 of the Deep Sea Drilling Program (DSDP). The cores were recovered from a water depth of 2,412 m (7,913 ft) at Holes 618 and 618A, with first evidence of gas hydrate occurring in Hole 618. Hydrates were observed in the top section of Core 618-4 at 85 fbsf (26 mbsf) in gray mud and consisted of a few white crystals of a few millimeters in diameter. At Hole 618A, gas hydrates were observed in both Cores 618A-2 and 618-3 in the 62-121 fbsf (19-37 mbsf) range, with hydrates distributed throughout Core 618A-3. The hydrates ranged in size from a few millimeters to possibly a centimeter in diameter and were white.
Based on light δ13C values, the origin of the hydrate gas is biogenic. Researchers also noted that some of the hydrates appeared to occur in the sandy layers of the cores. In contrast to other gas hydrate occurrences in the Gulf of Mexico, the gas hydrate was found within a mini-basin instead of on the fractured and faulted rim of the mini-basin. It was also noted that the depth of gas hydrate occurrence coincides with the presence of black organic and/or pyrite-rich mud.
Importance of Orca basin as a study site
The only recorded recovery of gas hydrates in the Gulf of Mexico from depths greater than 66 fbsf (20 mbsf) occurred at DSDP Site 618 in Orca basin. The recovery of biogenic methane hydrate from Orca basin is also significant due to the high salinity values, which at the sediment/water interface were nearly as high as those found in the Red Sea (where salinity values are 240-260 percent). The values decreased rapidly with depth to about 98 fbsf (30 mbsf) before becoming constant (48-56 percent). The hydrate recovered from both sites in Orca Basin were in the range of 85-121 fbsf (26-37 mbsf) and are physical evidence of the decreased salinity levels.