Lecture 6 Geosc 040
Chemistry of SeaWater –Dissolved Gas & Density
Atmospheric Circulation & Ocean Circulation
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Home
By the Sea
Genesis
Read the Course Materials
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The Sea Around Us
The Sea Around Us
Ocean Surface Currents
Density of
Seawater
The density of seawater is determined by
temperature and salinity
Density of Seawater
Recall that Oceans have about 35 grams salt per kg water
(salinity = 35 ppt or 35 PSU) (PSU= Practical Salinity Units)
• Density of seawater increases
as
•Temperature drops
•Salinity increases
• Maximum density and freezing
point coincide for 24.7 ppt
•Note that ppt = %O
• At higher salinities seawater
reaches maximum density at
freezing point
Density of Seawater & Stratification
Surface Zone
(mixed layer)
Zone of rapid
change with
increasing depth
Pycnocline is the
zone of rapid
density change
with depth
• Temperature and salinity vary considerably in the ocean
• Because water has low viscosity and can flow, denser water masses sink
while less dense water masses rise to the surface
• Deep water masses have a rather narrow range of T and S
• Most T and S variability occurs in surface or near-surface waters
• The ocean is density stratified (this is a stable configuration)
What is the residence
time of deep water in the
large ocean basins?
500 to 2000 years!
t=
Residence time of water in
the ocean
Volume = 1.4 x 109 km3
River Influx = 3.7 x 105 km3 /yr
t = Volume / Influx
How long does it take to
cycle ocean water
through rivers and back
again?
1.4 x 109 km3
t=
3.7 x 105 km3/year
t ≈ 4000 years
Today’s in-class iClicker exercises:
A) Full credit if you answer 75% or more of
the questions
B) If there are 10 questions and you
answer at least 8 of them you’ll get full
credit for today (100%)
C) If you answer the question correctly
you’ll get a bonus point, up to a
maximum of 105% for today’s in-class
exercise
D) All of the above (this is the correct
answer, choose D!)
The Grand Geochemical Cycle
•How much time to make the ocean salty?
• about 5 x 1022 grams of dissolved solids in ocean
• rivers bring in about 2.5 x 1015 gm dissolved solids per year
•Should only take about 2 x 107 years (20 million yrs.)
to bring oceans to present salinity
Assuming:
•rivers have kept approx. same input through time
•oceans have kept approx. same composition through time
--but we know oceans are 3.8 billion yrs. old
•This confirms that there must be output of material
from ocean!!
The Grand Geochemical Cycle
Typical Element Residence Times
Cl 80 million yrs.
Na 60 million yrs.
Mg 10 million yrs.
SO4 9 million yrs.
Ca
1 million yrs.
PO4 100 thousand yrs.
Don’t worry too much about absolute numbers,
but be able to explain why Cl residence time is so
much longer than, say, that of phosphate
The Grand Geochemical Cycle
Residence time is inversely related to extent of
involvement in chemical reactions in the ocean
•Na and Cl primarily precipitate as evaporite deposits
(infrequent events over geologic history). Bio-inert
•Ca used by organisms to make CaCO3 (calcium
carbonate) skeletons
•PO4 used in biological cycle (organic matter
production)--this is a nutrient element. Biolimiting
Ocean’s Chemistry and
The hydrologic and geochemical cycles
Outputs compete with
Inputs to shape the
chemistry of seawater
Gases Dissolved in Seawater
• Gases are soluble in seawater in proportion to their
atmospheric concentration --Gas Exchange
Gas
exchange is
enhanced by
mixing and
biologic
processes
Gases in Seawater
% in atmosphere
•Nitrogen (N2)
78.08
•Oxygen (O2)
20.95
•Carbon dioxide (CO2)
0.03 (365 ppm)
•Argon, Helium, Neon (Ar, He, Ne) 0.95
For Gases Dissolved in Seawater
Solubility (amount that can be dissolved)
1) Depends on temperature and salinity of seawater
• Decreases with increasing Temperature
• Decreases with increasing Salinity
2) Helped by wind-mixing of surface layer
• Important for oxygenation of seawater and CO2 uptake
from the atmosphere
• Important for fishes and other aerobic critters. Gasp!
They need Oxygen to Breathe!
Cold water (pepsi, beer!, etc.)
holds more gas in solution
Pepsi
Solubility of gases in seawater is controlled by
temperature (and also by salinity)
These curves reflect maximum amount that can be held in
solution under these conditions
What controls variations in oxygen
and carbon dioxide with depth in the
ocean?
What controls variations in oxygen and
carbon dioxide with depth in the ocean?
Dissolved Gases in Seawater
Depth profiles of dissolved oxygen and dissolved carbon dioxide
•more oxygen in surface
waters
• less oxygen in deep
waters
Indicates consumption of
dissolved oxygen below
surface waters
•less carbon dioxide in
surface waters
•more carbon dioxide in
deep waters
Indicates a process that
creates carbon dioxide
in deep waters
What controls variations in oxygen
and carbon dioxide with depth in the
ocean?
Dissolved Gases in Seawater
Depth profiles of dissolved oxygen and dissolved carbon dioxide
Indicates:
Photosynthesis
Oxygen production in
surface waters
Consumption of oxygen
below surface waters
Respiration
Indicates:
CO2 consumption in
surface waters
Production of CO2 in
deep waters
Zone of Light
Penetration
(the Photic Zone)
Zone of Light Penetration
(the Photic Zone)
Shorter wavelengths of visible light
(e.g. blue) penetrate deeper than
longer wavelengths (e.g. red)
The Energy Cycle
Photosynthesis
Consumers
Hey, did you
Consumers
notice that blue
light penetrates
nutrients
deeper?
Note that photosynthesis (and formation of plant organic matter) requires
duh…
sunlight and nutrients
anybody
Organic matter is consumed by animals and plants (respiration),
supporting
their growth
knows that
Nutrients must be “recycled” (excreted by animals, “regenerated” by
bacteria) to be reused by plants
Energy from the Sun!
Is that the end of the story?
Did you know?
http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
Earth’s average surface temperature is 15°C
The moon’s average surface temperature is
more than 30°C colder than Earth’s.
Why?
It’s all about the Atmosphere
The Role of the Atmosphere in Earth’s Temperature
The “greenhouse effect” is the
higher temperature that the Earth
experiences because certain
gases in the atmosphere (water
vapor, carbon dioxide, nitrous
oxide, and methane, for example)
trap and re-radiate energy.
Without these gases, heat (longwave radiation) would escape
back into space and Earth’s
average temperature would be
about 33ºC colder. These gases
are loosely referred to as
“greenhouse” gases.
Go to this website to see an animation that illustrates the “greenhouse effect.”
Ignore, for a moment the pronouncements at the end of the animation--we’ll get
to that later.
http://earthguide.ucsd.edu/earthguide/diagrams/greenhouse/
Average Sea Surface Temperatures
Cold near the poles
Strong
Temperature
gradient
Hot tropics
What Causes this Temperature Pattern?
-2° C
Sea Surface Temperature
30° C
Wind and Ocean Currents are driven by heat imbalance
Radiation Balance for the Earth
N. Pole
The Sun heats
Earth more at
the equator
than at the
poles!
equator
S. Pole
Fig. 6.12
Sun’s energy (solar heat) is
radiated to Earth and received
unevenly
Solar energy
received at any
location on Earth
varies with latitude,
because the sun angle
changes with
latitude.
It’s easier to get
a tan at the
equator than at
the north pole!
Sun angle varies with
seasons because
Earth’s axis of
rotation is tilted
relative to the plane
of our orbit around
Sun.
Sun’s energy (solar heat) is
radiated to Earth and received
unevenly
EARTH’s ORBIT (elliptical)
Effects of
Earth’s
Axial Tilt-A. Northern
Hemisphere
summer
(solstice)
occurs when
tilt is towards
the sun.
Elliptical
The Earth is actually
orbit causes
closer to the sun in
about 3.5%
January
variation too.
SEASONALITY Systematic variation in solar energy receipt on a
yearly basis is produced by Earth’s axial tilt and
orbit around the sun.
Radiation balance for Earth
Earth’s radiation balance is approximately at “Steady state,” which
means that it doesn’t change much from one year to the next
A Balanced Budget means:
Outgoing Radiation is approx. equal to Incoming Radiation
If the radiation budget were not balanced, Earth would either warm up or cool off over long
periods of time!
This indicates that Earth must re-radiate energy equal to the amount that it receives from
the sun.
The previous slides
provided a global
summary of Earth’s
radiation budget.
But solar energy
input is not evenly
distributed across
the Earth’s surface.
At high latitude
outgoing longwave
rad. exceeds
incoming solar (see
figure)
This creates zones
of surplus and
deficiency
The Earth radiates energy in the form of infrared (long-wave) radiation,
which is emitted in proportion to the Earth’s surface temperature
Question:
Why don’t the
tropics boil and the
poles freeze over?
Answer:
Heat Transfer from the tropics to
the poles!
Because a temperature gradient is created from low (warmer) to high
(colder) latitude, heat must be transferred to compensate for the highlatitude thermal deficit.
But how is this heat transfer accomplished?
The Ocean-Atmosphere Connection, Winds & Surface Currents
GLOBAL ATMOSPHERIC CIRCULATION (WINDS)
Large Scale Winds Transfer Heat, Note Air Pressure Zones
Average Sea Surface Temperatures
Cold poles
Strong Temperature gradient
Hot tropics
So, the Temperature Pattern is
Determined by Solar Energy Receipt
but Must be Modified by Heat Transport
\
The Pattern of Surface Water Ocean Circulation
Ocean Currents Transfer Heat
Ocean Chemistry is a
balance between inputs
and outputs:
A) This is true for the
surface water, but not for
the whole ocean
B) False
C) True
D) None of the above