In order of decreasing abundance the major gases in the sea are nitrogen, oxygen,

carbon dioxide and the noble gases, argon (Ar), neon (Ne) and helium (He).

• Nitrogen and the noble gases are considered to be inert because they are chemically non-reactive.

Dissolved Gases in the Ocean

The solubility and saturation value for gases in sea water increase as temperature and salinity decrease and as pressure increases.

• Solubility is the ability of something to be dissolved and go into solution.

• Saturation value is the equilibrium amount of gas dissolved in water at an existing temperature, salinity and pressure.

– Water is undersaturated when under existing conditions it has the capacity to dissolve more gas. Gas content is below the saturation value.

P = pressure

V = volume of gas

n = number of moles of gas present

T = absolute temp. K (= C + 273.15)

R = gas constant

Henry’s Law

[G] = HG x pp(G)

[G] = concentration of gas G in solution

Pp(G) = partial pressure of the gas

HG = Henry’s law constant for gas G, this is a function of temp. and salinity

• The surface layer is usually saturated in atmospheric gases because of direct exchange with the atmosphere.

• Below the surface layer, gas content reflects relative importance of respiration, photosynthesis, decay and gases released from volcanic vents.

Oxygen tends to be abundant in the surface layer and deep layer bottom, but lowest in the pycnocline.

• Surface layer is rich in oxygen because of photosynthesis and contact with the atmosphere.

• Oxygen minimum layer occurs at about 150 to 1500m below the surface and coincides with the pycnocline.

– Sinking food particles settle into this layer and become suspended in place because of the greater density of the water below.

– The food draws large numbers of organisms which respire, consuming oxygen.

– Decay of uneaten material consumes additional oxygen.

– Density difference prevents mixing downward of oxygen-rich water from the surface or upwards from the deep layer.

• The deep layer is rich in oxygen because its water is derived from the cold surface waters which sank (convect) to the bottom. Consumption is low because there are fewer organisms and less decay consuming oxygen.

• Anoxic waters contain no oxygen and are inhabited by anaerobic organisms (bacteria).

5-6 Gases in Seawater

Apparent Oxygen Utilization (AOU)

Carbon dioxide is of major importance in controlling acidity in the sea water.

• Major sources of carbon dioxide are respiration and decay.

• Major sinks are photosynthesis and construction of carbonate shells.

• Carbon dioxide controls the acidity of sea water.– A solution is acid if it has

excess H+ (hydrogen) ions and is a base if it has excess OH- (hydroxyl) ions.

– pH measures how acid or base water is.

• - pH of 0 to 7 is acid.• - pH of 7 is neutral.• - pH of 7 to 14 is base.

5-6

– pH is related to the amount of CO2 dissolved in water because it combines with the water to produce carbonic acid which releases H+ ions.

• CO2 + H2O H2CO3 H+ + HCO3

- H+ + CO3-2

– H2CO3 is carbonic acid, HCO3- is the

bicarbonate ion and CO3-2 is the

carbonate ion.

– Changing the amount of CO2 shifts the reaction to either the right or left of the equation.

• Adding CO2 shifts the reaction to the right and produces more H+ ions making the water more acid.

• Removing CO2 shifts the reaction to the left, combining H+ ions with carbonate and bicarbonate ions reducing the acidity.

5-6 Gases in Seawater

Dissolved CO2 in water acts as a buffer, a substance that prevents large shifts in pH.Dissolution of carbonate shells in deep water results because cold water under great pressure has a high saturation value for CO2 and the additional CO2 releases more H+ ions making the water acid.Warm, shallow water is under low pressure, contains less dissolved CO2 and is less acidic. Carbonate sediments are stable and do not dissolve.

The sea surface microlayer is the water surface to a depth of a few hundred

micrometers. It is critical for the exchange (i.e., gases) between the atmosphere and the

ocean.

The Ocean Sciences: Sea Surface Microlayer

• Processes within the microlayer can be divided into the:– Biological -

bacteria and plankton are much more abundant in the layer than below.

– Photochemical effect - the interaction of ultraviolet light and organic compounds.

The Ocean Sciences: Sea Surface Microlayer

Neuston layer is the habitat of the sea surface microlayer and is inhabited by the neuston, all organisms of the microlayer.Processes that transport matter to the surface layer from below are:

Diffusion - random movement of molecules.Convection - vertical circulation resulting in the transfer of heat and matter.Bubbles - the most important process because bubbles absorb material and inject it into the air as they bursts.

Nutrients are chemicals essential for life.

• Major nutrients in the sea are compounds of nitrogen, phosphorus and silicon.

• Because of usage, nutrients are scarce at the surface and their concentrations are measured in parts per million (ppm).

• Concentration of nutrients vary greatly over time and because of this they are considered a non-conservative property of the sea.

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Trace elements occur in minute quantities and are usually measured in

parts per million (ppm) or parts per billion

(ppb). • Even in small quantities they are important

in either promoting life or killing it.

5-3 Water Molecule

He- Distribution

Iron Fertilization Experiments

Commercial Interests?

Amount of light entering the ocean depends upon the height of the sun above

the horizon and the smoothness of sea surface.

• 65% of light entering the ocean is absorbed within the first meter and converted into heat. Only 1% of light entering the ocean reaches 100m.

• Water displays the selective absorption of light with long wavelengths absorbed first and short wavelengths absorbed last.

• In the open ocean, blue light penetrates the deepest.

• In turbid coastal waters light rarely penetrates deeper than 20m. and the water appears yellow to green because particles reflect these wavelengths.

• The photic zone is the part of the water column penetrated by sunlight.

• The aphotic zone is the part of the water column below light penetration and permanently dark.

The speed of sound in water increases as salinity, temperature and pressure increase, but in the ocean, the speed of sound is mainly a function of temperature and pressure.

• Above the pycnocline increasing pressure with depth increases the speed of sound despite the gradual decrease in temperature.

• Within the pycnocline, the speed of sound decreases rapidly because of the rapid decrease in temperature and only slight increase in pressure.

• Below the pycnocline the speed of sound gradually increases because pressure continues to increase, but temperature only declines slightly.

•SOFAR Channel is located where sound speed is at a minimum. Refraction of sound waves within the channel prevents dispersion of the sound energy and sound waves travel for 1000s of kilometers within the channel.

SOund Fixing And Ranging floats (SOFAR), subsurface floats used since the mid 1970s that freely drift at prescribed pressures. These provide direct measurements of the ocean circulation by sending acoustic pulses, typically at 300 MHz, once a day which can be used to calculate their positions from their Times of Arrivals (TOAs) at listening stations moored near the SOFAR channel depth at known geographical positions.