1-Introductory lecture on Chemical Oceanography

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Chemical Oceanography
Understanding the distribution, fluxes and
transformations of chemicals in the sea
The ecology of elements
and molecules
http://www.youtube.com/watch?v=rSAaiYKF0cs
Fundamental questions of Chemical Oceanography
1. What is seawater? What chemicals are dissolved in
seawater and where did they come from? Why are some
substances present at high concentrations while others are
at low concentrations? What are the consequences of these
differences?
2. How does the chemistry of the ocean affect life and how
does life affect the chemistry of seawater? How does Man
(a form of life) affect the ocean chemistry?
3. How is the ocean system connected to, and influenced by
the geosphere (the Earth’s crust) and the atmosphere?
Interdisciplinary nature of oceanography
Biology
biochemistry
Chemistry
Oceanography
biophysics
Physics
geophysics
geochemistry
Geology
Mathematics encompasses all
Adapted from Pinet, An invitation to oceanography
The Crustal-Ocean-Atmosphere Factory
(Libes, Chap 1)
“the ocean acts as a
giant stirred flowthrough reactor in
which solutes and
solids are added,
transformed, and
removed” - Libes
Libes, Fig. 1.2
A conceptual box model
Residence time of water
in the atmosphere is only
11.2 days!
The Hydrological Cycle
0.001%
Arrows
represent
fluxes
A quantitative
box model
83% of the
land-based
water is in
ice!
2.4%
Boxes
represent
reservoirs
97.5%
Residence time of water in
the ocean is ~3800 years
Percentages indicate
fraction of Earth’s
surface water
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/hyd/bdgt.rxml
The World Ocean - covers 72% of the Earth’s surface
Major ocean basins
Pacific, Atlantic, Indian, Southern
(Antarctic), Arctic
Gulfs, marginal seas, fjords, estuaries etc.
The average depth of the ocean is ~3,700 meters;
the maximum depth is about 11,000 m
The ocean is a thin skin of water on the planet.
Vertical depth averages 4 km, but the horizontal dimension
of ocean basins is 5000-10000 km.
Horizontal zones
Nearshore (estuarine, coastal, littoral)
Neritic (Shelf environments shallower than 200 m)
Pelagic (Open ocean -off the shelf break >200 m
isobath)
These zones refer to both the water column and the
underlying sediments.
For example - Nearshore sediments, pelagic sediments
etc.
Vertical zonations in the water column
Epi-pelagic
Euphotic zone
(0-200m)
Meso-pelagic
(200-1500 m)
Abysso-pelagic
(>1500 m)
Aphotic zone
Depth ranges are approximate
The ocean is a layered system- controlled by density,
which is a function of salinity and temperature
Temperature
0
50
Salinity
Density
Mixed Layer
Mixed Layer
Thermocline
500
Meters
Complex
pattern of
salinity
with depth,
depending
on water
mass
Pycnocline
restricted vertical
mixing
Horizontal
mixing occurs
along
isopycnals
(lines of
constant
density)
1000
1000
Well mixed
deep ocean
4000
Relatively
well mixed
vertically
Salinity is broadly defined as the salt content of
the water in grams per kilogram or parts per
thousand (ppt). (A more precise modern definition
will be given later).
Most ocean water falls in a narrow salinity range,
with an average salinity of 34.72 o/oo.
Most ocean water is very cold! The average
temperature is 3.5 oC.
From Libes
Sources: Open University: Seawater: its composition, properties and
behavior
Another view
Degrees latitude
Evaporation minus precipitation
Surface salinity
distribution in the
oceans is partially
driven by Net
Evaporation (EvapPrecip).
Where E-P (evaporation
– precipitation) is > zero,
salinity tends to be
high.
What causes Variations in salinity • coastal run-off, groundwater input
• evaporation/precipitation.
Latitudinal differences• high latitudes; high precip, low evap --> low
salinity
• low latitudes; high evap, high precip --> med
salinity
•Mid latitudes; high evap, low precip --> high
salinity
The density of seawater is the primary factor
governing movement and distribution of a water
mass.
 = mass/volume
Density is a function of temperature, salinity and
pressure. Seawater at 1 atm pressure with a salinity of 35 and
a temperature of 15 oC has a density of 1025.9728 kg m-3
solution (=g cm-3). This many significant figures are typically
necessary, but are tedious.
Oceanographers use a shorthand notation called
Sigma ():
 = ( -1) x 1000
Where  is the density (kg m-3)
Sigma-T (t) refers to density at 1 atm of pressure.
So for water with salinity of 35, at 15 oC and 1 atm
t = 25.9728
The density of
seawater can be
calculated from
Salinity &
Temperature and
Pressure using the
International
Equation of State
for Seawater
(I have and Excel spreadsheet
set up with this equation)
There is a new
formulation called
TEOS-10 based on
thermodynamic
principles
S= Salinity, t = temperature(oC), P = pressure (bars), v =
specific volume (m3 kg-1), K = fitting factor
Water is relatively incompressible, but,
with hydrostatic pressures of up to
1000 atmospheres the effects of
compression on temperature and
density cannot be ignored in all cases.
Because of compressive heating,
oceanographers use potential
temperature (), which is the
temperature a piece of water would
have if brought to the surface without
internal heat exchange (adiabatically).
Potential density  is the density
the water would have if brought to the
surface adiabatically (without heat loss or gain.
Hydrostatic
pressure is
the pressure
exerted by the
column of
water above.
Pressure
increases
approx. 1
atmosphere
(=~1 bar) for
every 10 m of
depth
Pressure heating
of water
becomes quite
significant at
extreme ocean
depths.
The potential
temperature is
relatively uniform
below 4000 m
Figure from Millero.
Cross section of potential temperature (θ) in the
Atlantic Ocean (Millero)
The structure of the ocean water
column is controlled by the density of
seawater.
Dense water sinks below less dense water.
The sinking of dense water will force lighter
water upward (upwelling). Freshwater plumes
float over saltier (denser) water.
Water masses - Major parcels of ocean
water have unique set of properties which set
them apart from other water masses.
Characteristics include, salinity,
temperature, (density), oxygen,
nutrients, alkalinity, pH.
Oceanographers rely heavily on use of
conservative tracers to delineate water
masses. Salinity is one of these tracers, along
with stable and radioactive isotopes, nonreactive gases, anthropogenic molecules
(freons).
Mississippi River Plume near Southwest Pass
Mississippi River Plume near Southwest Pass
View from Dauphin Island bridge showing multiple water masses and
fronts
T-S diagrams can distinguish different water masses
Temperature (oC)
Lines of constant density
Salinity
Water masses are defined by density and other characteristics
Atlantic Salinity section from WOCE 94 data set
Antarctic intermediate
water
North Atlantic Deep
Water
Antarctic
bottom water
South
Equator
North
Ocean Conveyor (Meridional Overturning) circulation
End
Surface mixed layer - Wind driven mixing
homogenizes the density in the surface layer to some
depth.
Thermocline (stratified with temperature), or
pycnocline (stratified with density). Cline = strong
gradient or rapidly changing property. Vertical density stratification
restricts vertical mixing of water, chemicals, or microplankton.
Horizontal mixing occurs along lines of constant
density (isopycnals)
Deep ocean - Weak gradients (relatively well
mixed vertically)
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