CHEMICAL OCEANOGRAPHY

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Chemical Oceanography

Basically asks two questions:

1. what is the composition and distribution of materials in the oceans?

2. what processes regulate this composition?

Now let's introduce salt into the water and see how things change.

Salinity is defined as the total amount of dissolved substances in 1 kg seawater and is expressed as parts per thousand

Salinity ranges from about 33-37 ppt in the open ocean

The composition of seawater - water composes about 96.5%, dissolved materials about 3.5%

Three classes of dissolved substances can be distinguished based on their abundance.

1. major components ( >100ppm)(1 ppm = .0001%)

Cl 19,000 ppm

Na 10,500

SO4 1,350

Mg 885

Ca 400

K 380

others 140

2. minor components ( >1ppm <100ppm)

Br, C, Sr, B, Si, F

3. trace elements ( <1ppm)

N, Li, etc.

In 1865 FORSCHAMMER noted that in looking at the relative proportions of the major ions in seawater noticed that although the S%o varied from place to place, the relative proportions remained constant-This is the Rule of Constant Proportions - it is important because it means that it is not necessary to measure each ion, but only one and then calculate the concentration of all the others.

How do we collect water samples for measurement of salinity and other properties? We can use Nansen bottles. These bottles are made of metal and usually have thermometer racks attached to the outside. They are attached to the wire in an upside down position so that valves keep both ends open (this avoids implosion due to u nequal pressures). When the messenger hits the top of the bottle, the top attachment releases, a messenger below is released to go down the wire and trip the next bottle and, as the bottle flips, the valves at each end close, trapping a water sample from that depth inside. In many programs today, special "nansen-type" bottles, called Niskin bottles, are used. The are made of PVC and avoid heavy metal contamination. Often there are many of these on a rosette sampler and the bottles can be closed sequentially by electronic command from the operator on the ship. Another, faster method to measure salinity is by using electrical conductivity. To do this we use a CTD (Conductivity Temperature Depth) which can be lowere from the ship and gives continuous readings of all three parameters.

Density - mass/volume (usually expressed in gms/cc)- plays an important role in determining the behavior of substances in the oceans.

The density of ice is less than water at the same temperature (ca 9%) therefore it floats on water. Its density is greater at lower temperatures.

The density of fresh, liquid water decreases as temperature decreases until one reaches a temperature of 4 degrees. This means that in a freshwater lake, the water column will overturn twice a year and should not usually freeze solid to the bottom

As the salinity of seawater increases, the temperature of maximum density decreases as does the freezing temperature, but at 24.7 ppt the lines cross which means in higher salinity water it will freeze before it reaches its maximum density which leads to continual overturn.

1. most of the water on the earths surface is in the oceans (97%), some is in sedimentary rocks and in the sediments, but not in a free form, some is in continental ice, a little is in lakes and rivers

2. evaporation and precipitation causes a cycling of water between the oceans and the land, about 90% of the water that evaporates over the oceans rains back on the oceans, but 10% falls over land which dissolves materials and transports these dissolved materials to the oceans via rivers. This cycle is called the hydrologic cycle.

3. weathering of rocks releases water of hydration as well as the dissolved inorganics

4. new or juvenile water is input through midocean ridges and volcanoes the amounts to about 66x1015 gms/yr or about 7.3x1010 tons/yr although it sounds like a lot it amounts to only .0000208% of the oceans volume.

If we look at a comparison of river and ocean water we see that in only a few ions is there a correspondence. Some of the discrepancy is accounted for by input from volcanoes and some by removal of ions through biologcial uptake or chemical removal.

Residence Time

Residence time is defined as the average time an atom of an element spends in the ocean. The larger the residence time, the more chemically inactive the substance. For example, chloride ion has a very long residence time (100,000,000 years, while iron has a very short residence time (200 years).

Dissolved Gases

Three major gases are dissolved in the ocean: oxygen, carbon dioxide and nitrogen.

The solubility of these gases is determined by three factors:

1. temperature - as it increases, solubility decreases

2. salinity - as it increases, solubility decreases

3. pressure - as it increases, solubility increases

Atmospheric air is composed of 78% nitrogen, 21% oxygen and .03% carbon dioxide - this is a ratio 1: .027: .00038. In seawater the ratio is 1: 0.75: 0.31. This means that relative to nitrogren, the ocean is much richer carbon dioxide and somewhat richer in oxygen.

Gases that dissolve at the sea surface are distributed throughout the worlds oceans by mixing, a process called advection.

Concentrations are further modified by biological activity - particularly plants and certain bacteria.

OXYGEN - the primary source is the atmosphere and it tends to be saturated near the surface.

It is also produced as a by-product of photosynthesis at the expense of carbon dioxide (3128)

The concentration varies from 0-8 ml/l in the water column.

All photosynthesis is in the upper 200m (usually much shallower than 200m) so values tend to be higher. Below this there is no oxygen production, only utilization so levels go down

Below 1,00m there is less life, therefore utilization is less and also the deep water is formed in high latitudes where it is cold (more oxygen dissolves) and then sinks and flows toward the equator

Some areas are anoxic due to topography which prevents flushing by oxygenated water.

CARBON DIOXIDE - produced by plants and animals through respiration; used by plants in photosynthesis

It is much more soluble in seawater due to its basic pH.

Carbon dioxide is involved in a very complex equilibrium. Three different species are present: carbonic acid, bicarbonate ion and carbonate ion.

This carbonate equilibrium buffers seawater against large pH changes which would otherwise be caused by respiration at night. This is beneficial to the organisms, particularly those with carbonate shells which would dissolve in an acidic medium.

The ocean acts as a large resevoir of carbon dioxide and helps regulate the amount of atmospheric carbon dioxide - this is important because it allows incoming solar radiation to reach the earth's surface, but retains IR radiated by earth, thus acting as insulation. As the concentration of carbon dioxide in the atmosphere increases through the burning of fossil fuels, the average temperature of the earth's surface increases. This is called the Greenhouse Effect

NITROGEN - relatively inert - most important in the ocean in its dissolved form as nitrate

Nutrients - fertilizers - important because of their role in plant growth

Nutrients are brought to the surface through upwelling - a process that brings water up to the surface from depths of 300-500m. These areas are highly productive and most commercial fisheries are found in these areas. If we look at nitrate concentrations in the surface waters we that values are very low except in areas of upwelling, coastal regions or high latitude areas. If we look deeper in the water column, say 200m we see that the values are higher. This is due, in large part, to the fact that we are below the photosynthetic layer and the biological demand is very low. In deeper waters, say 1000m regeneration by bacteria replentishes nitrate levels. Similar changes can be seen in phosphate at 0m, 200m and 1000m. Silicate, important to diatom and radiolarian growths, is very similar at 0m, 200m and 1000m.

ISOTOPES - two types

1. stable - do not break down O18/O16 ratio is affected by temperature variations - by looking at shells in the sediments and knowing the temperature regime of live animals, one can make inferences about past ocean temperatures.

2. radioactive isotopes (see Earth Origin)

a) naturally occurring

b) cosmic reactions in the atmosphere

c) atomic reactions - man-made

each has a known half life and can be used to date articles or processes after one half life 1/2 of the parent will have decayed to a daughter so that the ratio of parent to daughter is 1:1

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