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OCN1010 Oceanography

13 September 2001

Announcements:

1. Homework 4 will be assigned on Tuesday, 18 September 2001; see format for homework

Homework No. 3 is due Thursday 20 September 2001; 

As of Monday, 17 September, I have placed four copies of Francis' Shepard's book Submarine Geology on reserved. This chapter "Origin and History of Continental Terraces is excellent and should assist you in your homework.  The library has told me that the material will be available for check out by evening of 17 September.

 see format for homework

2.  Course project literature review is due on "to be announced" ; go here to see reference format

3. Textbook Assignment

4. From the WWW

5. PPT shows, presented in class

6. Movie (mpg format, 5 megs each!) downloads (courtesy of Professor John Trefry)

9. Read Course Project page now

8. Linda Kahn, Librarian to visit class soon to assistant you on your course project

Within a week Ms Kahn, Reference Librian will present a comprehensive presentation/discussion on Florida Tech's Information Resources, Evans Library. She will speak for about 50 minutes. This is an excellent opportunity for you to ask questions regarding the searching of information for your Course Project. Please review the Course Project web page on this site. And, have questions ready for her. She plans to do a demo on two marine resources: Oil and tidal energy.

Today's Lecture Notes

Marine Sediments

Heretofore we have viewed the sea floor as being comprised of hard rock, mostly derived from magmas originally from submarine volcanoes along mid-ocean ridges; away from these ridges one can also find islands and island arcs whose origin is also volcanic.

We now turn our attention to another class of materials found on the seabed--sediments, usually particles of relatively small size (10s of microns).  Sediment particles fall relatively slowly, usually from the sea surface, to accumulate in large amounts on the seabed.  If you look at a map showing the bathymetry of the ocean floor you will discover large abyssal plains in all ocean basins.  These plains are characterized by being flat.  There flatness is due in large measure to the sediment that has settled and spread and with thickness' of greater than 100s of meters in many locations in the ocean.

What are these sediments:  Plant, animal, nonbiogenic, anthropogenic or extra-terrestrial?  The answer is all of the above.

Biogenic Sediment

Sediment of plant or animal origin is called biogenic sediment.  Generally biogenic sediments are one of  two types: calcareous or siliceous.  Calcium carbonate (CaCO₃) is the main compound present in calcareous sediment. Amorphous silica (SiO₂) is the main compound present in siliceous sediments. The bulk of biogenic sediment were originally the skeletons of microscopic plants or animals.

Coccolithophorids ("Coccoliths" are microscopic plants with skeletons made of  CaCO₃ ; foraminifera and pteropods are small animals that have skeletons made of CaCO₃ . As these plants or animals die the organic matter decomposes releasing carbon dioxide and water; the skeletal settle (sedimentation process) to the sea floor.  

Radiolaria are small animals that extract silica from seawater to form intricate skeletons usually containing spines. When a radiolariums dies its skeleton sinks to the ocean floor forming siliceous sediments.  Radiolaria date back 600 millions years.

Diatoms are small plants that also extract silica from seawater to form their skeletons.  Like radiolaria, they upon death they sink to the sea floor forming siliceous sediment.

Biogenic sediments are found in fairly well-defined geographical distributions or bands on the ocean basins.  Generally speaking we find siliceous sediments in the Antarctic region and also in the Pacific equatorial sediments.  Calcareous (i.e. calcium carbonate) sediments are absence in the deep basins but are very abundant along the mid-ocean ridges.

These peculiar distributions are due to physical-chemical factors and biological processes. 

Dissolution of Calcareous Sediment

Let's consider the physical-chemical factors first as they apply to the calcareous sediment.  First, let me review briefly three factors affecting the solubility of   CaCO₃ that is hardly shared by any other compound; these factors are pressure, temperature, and carbon dioxide. 

First, consider hydrostatic pressure, the weight of a column of water on an object at a specific depth.  As depth increases, hydrostatic pressure increases.  For every 10 m increase in depth (for example 100 m to 110 m), pressure increases by 1 atmosphere.  Thus, at 100 m, the pressure is 100 atmosphere (actually, the pressure is 101 atmosphere if you consider the pressure of the atmosphere). At 4000 m, the pressure is equivalent to 400 atmospheres.   The solubility of calcium carbonate increases with increasing pressure.  This is due to the negative volume change due to CaCO₃   dissolution. Because of the negative volume change, pressure favors more dissolution. So, picture, if you will, small particles of biogenic calcium carbonate sinking and "feeling" the effect of increased hydrostatic pressure. As the particles sink further and further, the solubility increases more and more---almost linearly with depth.  You might expect that very little carbonate arrives on the bottom.  This is indeed the case for the deeper parts of the sea.  I should add that the rate of the dissolution is affected by other factors and therefore the story is not entirely simple. 

Now to the role of  temperature on dissolution of CaCO₃.  The deep ocean is close to 1 deg C. The decrease in temperature with depth is not steady however.  Most of the temperature change occurs in the upper 1500 m of the water column.  While you recall that the solubility of most salts or compounds decrease with decreasing temperature, calcium carbonate behaves in the opposite way: its solubility increases with decreasing temperature! So we see now that   CaCO₃ will tend to dissolved for two reasons: increasing pressure and decreasing temperature.

Finally, the third factor: increased carbon dioxide concentration in the deeper parts of the ocean.  If you measure the concentration of dissolved carbon dioxide in the deep ocean you will find concentrations elevated over those concentrations in the surface water.  This characteristics is due to the microbial decomposition of organic matter; the organic matter is from planktonic and other debris.  The same process occurs on land and in freshwater. Soil containing organic matter (leaf litter, dead organisms) is a large source of carbon dioxide.  As bacteria consume soil litter in the presence of oxygen, carbon dioxide is produced and escapes to the atmosphere to be consumed by the plants as the cycle repeats itself.  The same process occurs in the ocean. 

But how does this process affect sediment composition vis-a-vis calcium carbonate?   Recall in high school chemistry that acidic solutions dissolve limestone (CaCO₃).  If, for example, you add hydrochloric acid to limestone, a fizz occurs.  The reaction is one of dissolving the  CaCO₃   .  In the deep ocean the carbonic acid (carbon dioxide plus water) concentration is elevated above surface or subsurface values.  The presence of the carbonic acid leads to the dissolution of calcium carbonate in the same way that hydrochloric acid dissolves limestone.

To summarize the factors leading to loss of calcium carbonate in deep sea sediments, oceanographers and marine chemists have found that increased pressure, decreased temperature, and increased carbon dioxide all act to cause dissolution of CaCO₃.

Dissolution of Siliceous Sediment

Siliceous sediments (from plants and animals) behave somewhat differently than CaCO₃ .  While increasing hydrostatic pressure (due to increasing depth) leads to dissolution, the decreasing temperature with depth leads to a decrease in the solubility of amorphous SiO₂ (opposite to calcium carbonate). Likewise the increased carbon dioxide has no significant impact on the dissolution of amorphous silica.  One characteristic that we do find with increasing depth is increasing concentration of dissolved amorphous silica (silicic acid). This increased concentration is no doubt due to dissolution of sinking biogenic silica particles as the silica tries to reach its equilibrium or solubility value.

Other Sediment Types

Besides biogenic sediments, the ocean seabed contains sediments derived from continents (terregineous source), seawater, volcanic activity, and the cosmos (outer space). 

Terrigenous Sediment

Terriegenous sediment are generally particles eroded from land-based rocks including granites and soil.  Large rivers in North America (St. Lawrence River system; Columbia River) and South America (Amazon and Rio Platt) provide very significant amounts of sediment to the ocean.  Once in the ocean sediment from these sources are transported by long-shore currents and find themselves on or near the edge of the continental slope.  Continued accumulation on the continental slope can lead to an unstable mass which, when dislodge by a seismic event, can flow as a massive "turbidity" current capable of cutting or eroding material in its path.   This process is thought responsible for submarine canyons.

Besides river-borne sediment, some terrigenous material is transported to the sea by wind and storms.  Sahara sands have been found in the Atlantic; Professor Windsor and his students have detected such sands on filters placed on the top of some Florida Tech buildings.

Summary of Main Points Discussed Today (to be developed)