Oceans floor zones

Deep water or zone That portion of the water column from the base of the permanent thermoHine or pycnocline to the ocean floor. 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

Vinogradov A. P., Dmitriyev L. V., and Udintsev G. B. (1971) Distribution of trace elements in crystalline rocks of rift zones a discussion on the petrology of igneous and metamorphic rocks from the ocean floor. Phil. Trans. Roy. Soc. ri 268, 487-491. 

Ruddiman W. F. and Glover L. K. (1982) Mixing of volcanic ash zones in subpolar North Atlantic sediments. In The Ocean Floor, Bruce C. Heezen Memorial Volume (eds. R. A. Scrutton and M. Taiwan ). Wiley, Chichester, NY, pp. 37-60. 

Figure 3 Marine organisms produce calcite at 4 times the rate at which the ingredients for this mineral are supplied to the sea by continental weathering and planetary outgassing. A transition zone separates the mid-depth ocean floor where calcite is largely preserved from the abyssal ocean floor where calcite is largely dissolved.

Two other metamorphic facies are formed on a regional scale and under unique circumstances. The blueschist facies forms in the low-temperature, high-pressure environment in the upper portion of a subduc-tion zone. Land-derived sediments accumulated deep on the cold ocean floor are driven into an area of high pressure during subduction of an oceanic plate. These rocks often look blue when seen in outcrops. 

In the deeper parts of the ocean floor, below the euphotic zone, no herbivores (plant eaters) can survive. However, the rain of dead organic matter from above still supports thriving bottom communities. 

The topography and structure of the ocean floor are highly variable from place to place and reflect tectonic processes within the Earth s interior. These major features are shown in Fig. 9-1. These features have varied in the past so that the ocean bottom of today is undoubtedly not like the ocean bottom of 50 Ma ago. The major topographic systems, common to all oceans, are the continental margins, the ocean-basin floors, and the oceanic ridge systems. Tectonic features such as fracture zones, plateaus, trenches, and mid-ocean ridges act to subdivide the main oceans into a larger number of smaller basins. 

From flux calculations at this Sargasso Sea station, Gagosian and Nigrelli (1979) found that a maximum of 0.05—0.3% of the sterols produced by phytoplankton in surface waters are deposited to the ocean floor. A similar calculation was done for hydrocarbons by Farrington and Tripp (1977) and found to be 0.01—1%. The sterol residence time (the average lifetime of a sterol molecule before it is metabolized) in the euphotic zone was calculated to be approximately one month, whereas the deep-water residence time value was found to be 20—150 years. This monthly turnover of surface water sterols is in contrast with that of more labile dissolved organic compounds such as amino acids whose turnover time has been estimated to be on the order of several days (Lee and Bada, 1977). 

The density of sea-water is almost independent of depth (1.040-1.045 g cm ) whereas that of liquid carbon dioxide increases with depth see Figure 3.9(b). Down to about 2500 m, liquid carbon dioxide is less dense than sea-water and tends to float upwards. At 2500-3000 m, liquid carbon dioxide is neutrally buoyant i.e., a transition zone in which it neither rises nor sinks), dependent on the sea temperature. Deeper than 3000 m, the liquid is denser than sea-water and sinks downwards to the ocean floor, where it accumulates as a lake, over which a solid layer of crystalline hydrates slowly forms as an ice-like combination of carbon dioxide and water. Within its stability range (low temperature, high pressure), solid C02-hydrate would inspire greater confidence as a permanent store than dissolved or liquid carbon dioxide, although there are few data to say how rapidly carbon dioxide would be leached out by sea-water. 

This site is located in a side valley of the Rhone catchment, in an approximately 2-km-wide zone of greenstones (i.e. metamorphic gabbros and basalts) at an altitude of about 2000 m (Figs. 1 and 9A,B). This zone represents a 120 million-years-old ocean floor. About 50% of the area is covered by local till material, on top of which a half meter thick podzol developed (Derron, 1999). The soil profile lies in a pasture area close to the upper tree limit (Lac Bleu area) and comprises four different horizons (A, E, B and C Fig. 9D). 

Seabed Treaty 1972 94 countries prohibits emplacing nuclear weapons or weapons of mass destruction on the sea bed and the ocean floor beyond the 12- mile coastal zone. Threshold Test Ban Treaty (TTBT) 1974 United States, USSR prohibits underground nuclear tests having a yield exceeding 150 kilotons. South Pacific Nuclear Free-Zone Treaty (Treaty of Rarotonga) 1985 15 countries prohibits testing, deployment, or acquisition of nuclear weapons in the South Pacific. 

---