CHAPTER 12: THE OCEAN ENVIRONMENT

1. OCEANOGRAPHY is a science composed of other sciences as applied to the study of the oceans. It includes such major branches as:
   1. physical oceanography, study of ocean currents, tides, waves and propagation of various forms of energy in the ocean;
   2. chemical oceanography, study of chemical processes and features, and their variations in the ocean;
   3. biological oceanography or marine biology, study of life in the ocean; and
   4. geological oceanography or marine geology, study of the geology of ocean basins.
   
   
2. THE VAST WORLD OCEAN: The oceans cover 71% of the total surface area of the earth. This leaves 29% land (continents, islands, and so on).
   1. In the northern hemisphere, 61% of surface areas is ocean, while in the southern hemisphere 81% is ocean. Because the northern hemisphere thus includes about two-thirds of all the land, it is sometimes called the land hemisphere and the southern hemisphere by comparison would be the water hemisphere.
   2. The Pacific Ocean is by far the largest, making up almost half the total ocean area of the earth.
   3. Also, the average depth of the oceans below sea level (3,800 m) is almost five times as great as the average height of land above sea level (840 m).
   
   
3. COMPOSITION OF SEAWATER
   1. Average seawater is 3.5 percent (%) or 35 parts per thousand (‰) by weight dissolved solids. Of this about two-thirds is sodium chloride. Because the other major dissolved components are also salts of one type or another, we customarily use the term salinity to refer to the water's total dissolved-solids content.
   2. Salinity and its variations:
      1. The usual range is from about 3.3 to 3.7%, but extremes are from less than 1.0 to over 4.5%.
      2. In surface ocean waters, salinity is controlled by the balance between rain-fall or fresh-water inflow (which lower salinity) versus evaporation (which raises salinity).
   3. Sources of Sea Salts: Most of the water in the oceans originally came from within the earth, brought up in magmas and separated during crystallization (outgassing).
      1. The dissolved salts have also been derived in part from outgassing associated with igneous activity.
      2. Another important source is weathering of exposed rock of all types.
      3. The present rate at which new salts are added is balanced by the rate at which sea-dwelling organisms consume it, plus the rate at which some materials are re-precipitated in oceanic sediments. Sea salinity has been essentially constant for billions of years.
   
   
4. RESOURCES FROM SEAWATER
   1. Ocean water is an important source for common salt, which can be obtained simply by letting a pond of seawater evaporate to dryness.
   2. Ocean water has also been processed profitably to yield magnesium and bromine.
   3. Virtually all other elements are also present in ocean water, but in such small amounts that for most the cost of extraction is would be too great in relation to other sources currently being used.
   4. Ocean water itself, after making it fresh by removing the salts through a desalination process, can be used for drinking, irrigation or other purposes. However, it costs substantially more compared to other sources such as lakes, wells, or rivers. The high cost limits the use of desalinated water to areas or projects that are willing to bear the expense.
   
   
5. THE OCEAN'S LAYERED STRUCTURE: Outside of the north and south polar regions, the ocean has three distinct layers or zones with depth.
   1. The surface mixed zone, which absorbs solar radiation and is the warmest. The mixing is due to the action of waves at the surface. Since wave action decreases with depth, they cause little or no motion or mixing below a few hundred feet from the surface. Temperatures in the surface zone can be as high as on nearby landmasses.
   2. The deep zone is cold (most of it near freezing) because sunlight does not reach it, and simply because cold water, being denser than warm water, tends to sink. Most deep water enters the deep zone in the cold polar regions, where it is cooled at the surface and then sinks.
   3. A transition zone, within which occur most of the changes in temperature and salinity, separates the top and bottom zones. The main reason for absence of this zone in the polar regions is that they are the areas where surface water exchanges with the deep zone.
   4. In the transition zone a layer of rapid temperature drop with depth is the thermocline, and a zone of rapid salinity increase with depth is the halocline. Because the ultimate governing factor in water stratification is water density, we may sometimes observe a reversed thermocline (overridden by a stronger salinity increase) or reversed halocline (overridden by a stronger temperature decrease).
   
   
6. MAPPING THE OCEAN FLOOR
   1. The voyage of the converted warship H.M.S. Challenger (1872-1876) was the first scientific expedition devoted specifically to study of all aspects of the oceans. Wireline soundings suggested the presence of mountain ranges in the middle of ocean basins.
   2. The echo sounder, now used routinely in oceanographic work, was first used on the voyage of the Meteor (1925-1927), a German expedition to assess the feasibility of "mining" gold from seawater.
   3. Several hundred vessels are used today for oceanographic studies. Notable among these are deep-diving submersibles such as Alvin, and drilling ships such as the Joides Resolution.
   4. Analyses of satellite orbits have aided detailed measurements of gravity and the true shape of sea level, now known to have bulges and depressions that reflect the density and topography of rock below.
   5. Major topographic divisions of the ocean floor are the continental margins, deep-ocean basins, and mid-ocean ridges.
   
   
7. CONTINENTAL MARGINS are the submerged edges of continents. Two types can be distinguished: active margins, which lie on plate boundaries (subduction zones, usually), and passive margins, which lie well away from a plate boundary (but were initially shaped as one side of a rift valley).
   1. Passive Continental Margins
      1. The continental shelf is almost flat (average slope 0.1°), from the shoreline to the edge of the continental slope. It amounts to submerged edge of the continent, mantled with marine sediment, and position of the shoreline dependent on sea level. Depth at the outer edge averages about 130 meters (425 feet). Width of shelves averages about 80 km, but varies greatly from one place to another.
      2. The continental slope is a drop-off at the edge of the shelf. It is not really a cliff, but with typical slope of 5° it is comparatively much steeper than a shelf. The main reason for the "drop-off" is the transition from continental crust to denser oceanic crust. Water depth at the base of the continental slope commonly is thousands of meters.
      3. The continental rise is a "levelling-off" zone that lies at the base of the continental slope. It is built by sediments accumulated at the base of the slope. Deep-sea fans are lobe-shaped sediment accumulations commonly seen at the mouths of submarine canyons (SEE below) that coalesce to give the overall wedge shape of continental rise deposits.
   2. Active Continental Margins - differ from passive margins by having:
      1. a deep-ocean trench at the edge of the shelf.
      2. an accretionary wedge, or chaotic assembage of rock material that represents "scrapings" from the subduction zone, being tectonically pushed against the trench wall.
   
   
   
8. SUBMARINE CANYONS AND TURBIDITY CURRENTS
   1. Submarine canyons are valleys cut into the edge of the shelf, extending from near shore to the face of the continental slope. A few of these canyons appear to be extensions of major rivers emptying from nearby land, and may to have formed at times when sea level was lower (as during the ice Ages). Most others have no connection with rivers from the land, and seem to have formed by strictly oceanic processes such as turbidity currents. Sediment collected at the mouth of a submarine canyon commonly makes a submarine fan.
   2. Turbidity currents are considered to be the major way in which sand and other coarse sediments are moved into deep water. Deposits created by turbidity currents are known as turbidites: they are characterized notably by graded bedding (coarse sediment at base, with successively finer materials upward).
   3. The density of water is affected by sediment content (turbidity) as well as by salinity and temperature. Turbid water can be enough denser than clear water that it will flow downhill and reach speeds up to 80 km/hr (50 mph). The resulting turbidity currents may travel for hundreds of miles before coming to rest and dropping the sediment load they carry.
   
   
9. FEATURES OF THE OCEAN BASIN FLOOR
   1. Deep-ocean trenches include the deepest parts of the ocean. The "walls" that they appear to have in some pictures are not really cliff-like (the drawings are usually exaggerated to make subtle features stand out). Trenches are associated with subduction zones, where crustal plates are driven downward. Trenches not near a continent commonly have a belt of volcanic islands located on overthrusting side. Edges of continents next to trenches also may include a volcanic belt.
   2. Abyssal Plains include the flattest topography found on earth, some areas being smoother than the ocean surface above them. Deposits on abyssal plains are dominated by the materials washed in by turbidity currents, but also include fine particles slowly settled from overlying water (organic remains plus wind-blown and ice-rafted debris from land), and chemical precipitates.
   3. Seamounts are the isolated peaks of seafloor volcanoes. Flat-topped seamounts are known as guyots, flattened by wave erosion before they were submerged into deeper water, some fringed by coral reefs before subsiding.
   
   
10. MID-OCEAN RIDGES
    1. The mid-ocean ridge system is a 70,000-km-long mountain range, with slightly more topographic relief than any present on-land mountain range. It makes up about 20% of the total surface of the earth.
    2. Mid-ocean ridges are much different from continental mountains, because of great differences in the processes going on inside. Continental mountain belts mostly are the result of crunching together and buckling of plates, whereas mid-ocean ridges mark zones of spreading, as new igneous material enters the crust from below and forces the older material aside.
    3. The crest or central axis of the ridge is marked by igneous activity. The abundance of hot rock near the surface causes high crustal heat-flow rates. Shallow-focus earthquakes are abundant, associated with the igneous activity that is pushing the crust apart. The rift valley found at the crest of a typical ridge is a strip of down-faulted rock, a result of the crustal stretching.
    4. The main reason for the elevation of mid-ocean ridges appears to be simply the contrast in density between the hot rocks near the ridge crest versus their cooler surroundings. This also fits nicely with the observation that trenches and subduction zones are mostly areas of low heat flow, where cooler, denser rocks are going down.
    
    
11. MARINE LIFE ZONES
    1. Availability of light - the photic zone extends from the surface of the ocean to the depth that sunlight can penetrate. This depends on factors such as sun angle and clarity of water. Longer wavelengths (reds/oranges) are absorbed first, and thus the short wavelengths (blue/violet) penetrate the deepest.
       1. The euphotic zone, generally reaching less than 100 meters beneath the surface, is the portion of the photic zone in which photosynthesis can occur.
       2. Thus the top 100 or so meters is where phytoplankton (free-floating algae/plants) can grow and establish the marine food chain or pyramid, which proceeds on to zooplankton and eventually up to top predators such as sharks, tuna or whales.
       3. Depending on water clarity, there can be enough light for vision at depths as great as 1000 meters, which is the limit of sunlight penetration. Below this is the aphotic zone, which is dark except for light produced by various bioluminescent organisms.
    2. Distance from shore - this variable connects with factors such as the effect of fresh water inflows on salinity, the flow of nutrients from land to the sea, the effects of tides and of waves.
       1. The intertidal zone lies between the high and low tide lines. Organisms that live here must be able to tolerate periods of exposure to the air, or be able to move with the tide. Wave action in this zone can be powerful enough to pound fragile, thin-shelled or delicate organisms to death.
       2. The neritic zone extends from the low tide line to the edge of the continental shelf.
    3. Depth The pelagic zone is open ocean water of any depth, inhabited by swimming or free-floating organisms. Bottom-dwelling organisms inhabit the benthic zone. The deepest, coldest part of the benthic zone is called the abyssal zone, characterized by great depth/pressure, cold (3°C) nutrient-poor water.
    
    
12. OTHER MARINE HABITATS
    1. Estuaries are partially enclosed coastal water bodies, commonly representing the "drowned" lower downstream reach of a river channel. Such an environment is thought by many to be where life first developed on earth. Estuaries are home to some of the planet's most highly bioproductive aquatic ecosystems.
    2. Coral Reefs and Atolls
       1. Reefs are built out of the remains of colonial organisms such as corals. Other carbonate-secreting organisms such as sponges, algae and shellfish can also contribute to a reef structure.
       2. Atolls - Darwin's theory of atoll formation calls for gradual subsidence of a volcanic island, so that growth of coral reefs can keep pace and maintain the ideal depth of water for their continued growth. The volcanic part of the complex eventually submerges out of sight, covered over with coral deposits. This theory was controversial, until drilling under some atolls revealed great thicknesses of coral deposits.
    
    
13. SEA-FLOOR SEDIMENTS: Continental-shelf areas receive most of the sediment eroded from land. Some are covered by 10 km or more of such deposits. The variety of material deposited on shelves is also great, because shorelines can migrate back and forth across a shelf as sea level fluctuates, or the continent uplifts and subsides. Except in trenches, sediment thickness and variety both tend to be much less in the deep ocean, where only turbidity currents can bring large amounts of land-derived sediment. The maximum known thickness of sediment in a deep ocean area other than a trench is about 1 km, and this rests on areas of old oceanic crust. The youngest oceanic crust, at mid-ocean ridges, has virtually no sediment on it.
    1. Types of Sea-floor Sediments: A special terminology is used for classifying sea-floor deposits:
       1. Terrigenous sediments are made from broken particles of preexisting rocks. This category includes turbidites, wind-blown dust, and volcanic ash.
       2. Biogenous sediment consists of the remains of oceanic plants and animals. It includes siliceous oozes from diatoms or radiolaria, calcareous oozes from a variety of calcite-secreting organisms, and phosphate deposits from some shells, also teeth, bones and scales. Cold, deep ocean water contains more carbon dioxide than warm water nearer the surface, thus is capable of dissolving calcite particles which sink into it. Thus calcareous ooze suggests shallower water depth of deposition (above the carbonate compensation depth) than siliceous, calcite-free ooze.
       3. Hydrogenous sediment is material deposited by direct precipitation from seawater. Manganese nodules are a notable example of this type of deposit, because of interest in mining them. Interest in mining stems from the fact that the U.S. has almost no manganese, which is important in steel making, while Russia and China have much of world's reserves. The nodules form in deep water, where they grow at rates of 0.0002 mm/year or less.
    2. Sea-floor Sediments and Climatic Change: Records of past global climatic conditions may be recovered from sedimentary layers deposited on the ocean floor.
       1. Climate affects the type and abundance of life in the upper ocean, thus influences what type of fossils the sediment accumulated at that time contains.
       2. Climate also affects the chemical composition of ocean water, and the rates at which some chemical reactions occur. Besides influencing marine life and resulting fossil assemblages, this also can influence deposition of some types of chemical sediments.

Last revised June 3, 2001 by MAJordan