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Biogenic oozes- review- important (we will revisit later)
Summary -- general distribution
RESOURCES OF THE SEA FLOOR
Continental margin
Deep-sea floor
Law of the Sea Treaty
1. Production in surface waters:
Biological productivity controlled by nutrients -- N and P
"Upwelling" brings nutrients to surface waters
Productivity high in upwelling zones
2. Dissolution in deep waters
Deep waters "undersaturated" -- particles tend to dissolve
"Carbonate Compensation Depth" (CCD)
Calcareous oozes absent below CCD
Atlantic: ~ 4,000 m
Pacific: ~ 500 - 1,500 m
Siliceous particles dissolve more slowly; nutrients more important
than depth
3. Dilution by other sediments
High input of terrigenous sediment
"Dilutes" biogenous components to < 30 %
SUMMARY -- Distribution of sediment on the sea floor (modern sediments)
Economic considerations determine profitability/viability of each type
The distribution of biogeneous sediments is controlled by three important processes:
1. Production in surface waters: The growth of marine algae (which are the base of the oceanic food chain) is controlled principally by the availability of two critical nutrient elements -- N (nitrogen) and P (phosphorus). These critical nutrients are mostly supplied to surface waters by deep waters that "upwell" to the surface. Therefore, biological productivity is high in areas of strong vertical upwelling -- along the equator, in certain coastal regions, and in the Southern Ocean around Antarctica.
2. Dissolution in deep waters: Deep-ocean waters are undersaturated in calcium carbonate and opalline silica. Therefore, biogenic particles tend to dissolve as they settle through the water column and as they sit on sea floor. This effect is more pronounced for calcareous sediments. In fact, calcareous oozes are absent below a certain depth called the "Carbonate Compensation Depth, or CCD. The depth of the CCD varies from ocean to ocean. It occurs at 4,000 m in the Atlantic. The CCD is shallower in the Pacific, at depths of 500 - 1,500 m. Siliceous particles dissolve more slowly as they sink and are not limited in distribution by depth as much. Nutrient supply more important in controlling the distribution of siliceous sediments.
3. Dilution: Calcareous and siliceous components can
be diluted to
< 30 % of the total sediment (and therfore not qualify as a
"biogenous ooze") in regions where the input of terrigenous
sediment is very high. This occurs along continental margins --
surface productivity is high and dissolution is minimal, but biogenous
oozes don't occur because of the high influx of terrigenous sediment
from continents
SUMMARY -- Distribution of sediment on the sea floor (modern sediments)
Terrigenous: - - continental margins and adjacent abyssal
plains.
Manganese nodules: - - deep basins, especially the Pacific.
Red Clay: - - deep ocean regions where not diluted by biogenic
particles.
Calcareous oozes: - - wide-spread in relatively shallow
areas of the deep sea.
Siliceous oozes: - - polar and equatorial bands where nutrients
are supplied to surface waters by vertical upwelling.
Economic considerations dictate whether a potentially
useful material is developed as a resource (whether on land or
in the oceans). Questions that must be addressed for a potential
marine resource are:
(1) Is there a market for the material?
(2) Will extra costs of offshore production be offset by profits?
Resources of the Continental Margin
Building materials - - Bulk materials such as sand and gravel as well as CaCO3 shell deposits (for cement) represents the largest off-shore operation in terms of gross production. But the success of such ventures is tied to the proximity of materials to markets. Thus, the most cost-efficient operation are in bays and along beaches, and along the inner continental shelves near major markets.
Phosphate deposits - - The element phosphorus is important component of fertilizer. Phosphates deposits are recovered from the sea floor. These occur as calcium-phosphate muds, sands, and nodules on some continental shelves and slopes.
Hydrocarbons (fossil fuels) - - Petroleum (liquid) and
natural gasare the most valuable resource of the sea bed. One-third
of the estimated global reserves of oil and gas are in continental
margin sediments.
At present, offshore recovery hydrocarbons is going on in the
USA (Gulf Coast, California, north slope of Alaska), the North
Sea, the Arabian/Persian Gulf, Indonesia, and Australia. Many
other areas of potential production have not yet been explored.
How do economically important hydrocarbon deposits form?
Sediment of planktonic organic matter is deposited. (This is only a very small fraction of organic matter produced in surface waters -- the rest is eaten and decomposed in marine food chains.)
Rapid accumulation of sediment on margins preserves deposited organic matter from decomposition.
The organic matter is altered to liquid and gaseous hydrocarbons as sediment is buried to high temperatures (>100 C).
Continental margins will always be the target of offshore recovery
because:
Drilling is in relatively shallow waters, the costs are less.
Preservation of organic matter in deep-sea pelagic sediments is
poor.
Sulfur - - in its elemental form is an important industrial chemical. Elemental sulfur is often found on the top of subsurface geologic structures called "salt domes." Salt domes often occur along continental margins and adjacent on-land coastal areas, like the Gulf Coast of USA and Mexico. The salt (mostly halite) was deposited during the initial stages of continental rifting. In addition to a source for elemental sulfur, salt domes are also natural "traps" for petroleum and natural gas.
Resources of the Deep-Sea Floor:
Manganese nodules - - are potentially important sources
for the metals Mn, Co, Cu, Ni. There has already breen considerable
investment in locating concentrations of manganese nodules ion
the seas floor and developing technologies for processesing nodules
as sea. But there are currently limitations to the development
of manganese-nodule mining:
(a) Depressed global markets for metals.
(b) High cost of development.
(b) Ownership of "found" deposits on the deep-sea floor.
Recent Gold Mining Activity- Black Smoker "Chimney" Deposits -- An australian company recently developed plans to mine gold from a vast deposit of minerals formed by "black smokers" on the sea floor. Plans include blasting the "chimney" deposits with explosives, recovering the fragments by dredging, then refining these ores on-shore.
"Law of the Sea" Treaty (United Nations, 1982)
was developed to address the question of ownwership and to
regulate development of seabed resources. Some of its provisions
are:
200-mile Exclusive Economic Zone. In that area, countries have
the exclusive right to develop all marine resources (fisheries
included). Unfortunately, there is considerable overlap between
EEZ's, and consequently disputes between nations about who owns
what (e.g., USA vs Canada and Cuba
UN Seabed Authority regulates deep-sea mining; profits shared
among all countries.
The USA and other developed countries did not agree to profit-sharing; they have licensed their own multinational companies for development.
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