Characteristics of the Sub-equatorial North-Eastern Pacific Ocean’s Abyss, with a Particular Reference to the Clarion-Clipperton Fracture Zone

Abstract

The deep seafloor, i.e. seabed areas at depths exceeding 800–1,300 m, cover about 88 % of the world ocean’s bottom. The most extensive areas represent the 3,000–6,000 depth range and include abyssal plains (depths > 4,000 m) the largest of which is the abyssal plain of the Pacific Ocean. The water column overlying it consists of a number of layers differing in their major characteristics. The most characteristic layers include that encompassing the oxygen minimum zone (OMZ, 100–1,000 m depth range in the Pacific) and the near-bottom layer, directly impinging on the seafloor. Once considered extremely stable, the near-bottom layer is now known to be prone to hydrodynamic effects such as tides and currents. The latter are generally weak, but periods of intensified current activity are not infrequent. The water column effects influencing the abyssal seafloor include also the transmission of the wind-generated surface physical energy down to the bottom (the “benthic storms”) on the one hand and sedimentation of surface-produced organic matter on the other. Both the benthic storms and organic matter deposition are known to be periodically, or episodically, intensified, thus contributing to natural environmental variability in the abyss. The Pacific abyssal plain sedimentary cover is mostly biogenic in origin. A characteristic part of the Pacific abyss is a huge (about 2 million km2) polymetallic nodule field within the NE sub-equatorial seafloor area experiencing low sedimentation rates and constrained by the Clarion and Clipperton fractures (the Clarion-Clipperton Fracture Zone, CCFZ). CCFZ extends sub-latitudinally along about 4,200 km, its surface inclining slightly westwards with depths in the east-west direction from about 4,000 to about 5,400 m. The seafloor, although generally flat, does show (particularly in the eastern part) distinct topographic features which are volcanic in origin. The relatively thin (50–200 m) sedimentary cover is formed by recent biogenic sediments (siliceous ooze). The major characteristic of the area is the presence of polymetallic nodule deposits. The nodules, occurring at abundances frequently exceeding 10 kg/m2, are mostly exposed on the sediment surface, but some are also embedded or buried in the sediment. The nodules, in addition to the occasionally occurring larger hard-rock fragments of cobalt-rich ferromanganese crusts, add to the deep-sea habitat heterogeneity and themselves constitute both a unique deep-sea habitat, of a great interest to marine ecologists, and an important mineral resource, of a great appeal to the marine mining community the size of which has been recently growing considerably.