Transcript Document 7485282

Dissolved Gases in the Ocean
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.
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 oxygenrich 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)
5-6
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
Gases in Seawater
– 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.
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 Ocean Sciences: Sea
Surface Microlayer
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.
• 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.
5-3
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 nonconservative property of the sea.
5-3
Water Molecule
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.
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.