Deep ocean

Tension leg and floating platforms can easily be released and towed away for service elsewhere, which is cheap and attractive. In the case of the fixed platforms, the topside modules are removed by lift barge and taken to shore for disposal. Gravity based structures can in theory be deballasted and floated away to be re-employed or sunk in the deep ocean, and steel jackets cut and removed at an agreed depth below sea level. In some areas jackets are cleaned and placed as artificial reefs on the seabed. The... 

Deposits that can be recovered without having to use explosives or other primary energy sources to break up the material in place ate called unconsoHdated deposits. These may be found stratified or disseminated as sutficial or subsurface deposits on the continental shelf or in deep ocean basins. 

Biogenic Ma.teria.ls, Deep ocean calcareous or siUceous oo2es are sediments containing >30% of biogenic material. Foraminifera, the skeletal remains of calcareous plankton, are found extensively in deep equatorial waters above the calcium carbonate compensation depth of 4000 to 5000 m. 

Ocean Basins. Known consohdated mineral deposits in the deep ocean basins are limited to high cobalt metalliferous oxide cmsts precipitated from seawater and hydrothermal deposits of sulfide minerals which are being formed in the vicinity of ocean plate boundaries. Technology for drilling at depth in the seabeds is not advanced, and most deposits identified have been sampled only within a few centimeters of the surface. 

Fluid deposits are defined as those which can be recovered in fluid form by pumping, in solution, or as particles in a slurry. Petroleum products and Frasch process sulfur are special cases. At this time no vaUd distinction is made between resources on the continental shelf and in the deep oceans. However, deep seabed deposits of minerals which can be separated by differential solution are expected to be amenable to fluid mining methods in either environment. 

The oceans are subdivided into surface (100—1000-m) ocean and deep ocean. The zone separating the warmer, surface water from the lower, cooler layer (oceanic thermocline) is characterized by a density gradient that prevents mixing. 

Y. Nozaki, in Deep Ocean Circulation, Physical and Chemical Aspects, ed. T. Teramoto, Elsevier, Amsterdam, 1993, pp. 83-89. 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

It is significant that the earliest records of life on Earth start shortly after the period of impact frustration. Apparently life formed as soon as the conditions permitted it. Life originated from compounds produced by prebiotic organic chemistry. The source of the molecules included those produced on Earth by energetic processes such as impacts and electrical discharges as well as those that fell in from space. Whatever processes occurred, they would have had to happen either in the deep ocean or in what might have been rare regions of land and shallow water. 

Time (years) Land Land biota Organic biota Surface ocean Deep ocean... 

Many hydrologic reservoirs can be further subdivided into smaller reservoirs, each with a characteristic turnover time. For example, water resides in the Pacific Ocean longer than in the Atlantic, and the oceans surface waters cycle much more quickly than the deep ocean. Similarly, groundwater near the surface is much more active than deep reservoirs, which may cycle over thousands or millions of years, and water frozen in the soil as permafrost. Typical range in turnover times for hydrospheric reservoirs on a hillslope scale (10-10 m) are shown in Table 6-4 (estimates from Falkenmark and Chapman, 1989). Depths are estimated as typical volume averaged over the watershed area. 

The transition region between the surface and deep ocean is referred to as the thermocline. This... 

The distribution of dissolved, total particulate, and living particulate organic carbon in the surface (0-300 m) and deep ocean (>300 m) are summarized in Table 10-6. Recent analytical advances have greatly improved our understanding of the distributions of DOC in the ocean (Hedges and Lee, 1993). The important aspects of this compilation are ... 

DOC in the deep ocean gradually decreases from 48 /iM in the North Atlantic to 34 /rM in the North Pacific (Hansell and Carlson, 1998). 

Deep-ocean depletion 210pb, 230.pj Scavenging by settling particles... 

Deep ocean values increase from the Atlantic to the Pacific. 

Respiration of organic matter and dissolution of CaCOs are the main controls of the distribution of deep ocean Total CO2 and alkalinity. These reactions (and their predicted effects on DIC and alkalinity) can be represented schematically as... 

To a first approximation the deep ocean distributions shown in Fig. 10-20 can be reproduced if the particulate material dissolving in the deep sea has the ratio of 1 mol CaCOs to 4 mol organic carbon (Broecker and Peng, 1982). 

The surface ocean is depleted relative to the deep ocean in those elements fixed by organisms. 

Deep ocean concentrations increase progressively as the abyssal water flows (ages) from the North Atlantic, through the Indian Ocean to the North Pacific. 

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