The Sea Around Us
Lecture 4: 21 January
Water is The Wonder Substance:
Physical & Chemical Properties
Water Cycle Jump!
Drown with Me
Porcupine Tree.
VV Brown, Shark In The Water
Thanks to Rachel B., Zach R.
Ocean Breathes Salty
Modest Mouse
• Lecture Review
Questions:
• On-line Assignment 2
is due tonight by 11pm
• Homework 1 is
available on Angel
• Cell Phone Recycling:
Benefit Relay for Life
and The Ocean!
• Book pics? (Angel
dropbox)
• Questions (Angel)
Read about The Sea Around Us
• Lecture notes and slides on the
course web site
• Your book
The WONDER SUBSTANCE: Physical & Chemical Properties
• Ice is less dense than water.
• At 0°C, density of water is 1 gm/cc; Ice is 0.917 gm/cc
• Ice has an open hexagonal structure
Water molecular structure
ice is about
IceDensity
molecularofstructure
91% of liquid water
Density of Water
• Fresh water reaches maximum density at 3.98 °C
• Density= 1,000 kg/m3 (1kg/liter)
• Density decreases
as water is heated
above 4°C
• At 20 °C, density of
pure H2O is 998.23
kg/m3
Water: The Wonder Substance
Low viscosity
•rapid flow to equalize pressure differences
High surface tension
•allows wind energy to be transmitted to sea surface
•allows cells to hold shape --and life to form
•controls the behavior of water drops
High heat capacity
•cools/warms slowly relative to land
•aids in heat retention & transport
•minimizes extremes in temperature
•helps to maintain uniform body temps
High latent heat of evaporation
•very important in heat/water transfer in atmosphere
Latent Heat and Changes of State
Latent heat of fusion (or melting)
• Heat to form or melt ice (liquid to
solid phase)
• 333 kJ/kg (80 calories/gram)
Latent heat of vaporization (or precipitation)
• Heat to vaporize (boil) a liquid or condense
liquid from a gas phase
• 2260 kJ/kg (540 calories/gram)
Evaporation of water from the surface can occur at any
temperature. However, it takes more energy to evaporate at
low T than to boil off vapor once water reaches 100°C
Heat capacity and phase changes:
ice (solid)
water (liquid)
vapor or steam (gas)
Latent Heat
Heat needed to
change phase
(from solid to
liquid, liquid to
gas, liquid to solid,
etc.)
Heat required to change
the temperature (by 1 °)
of a given mass
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
Pepsi
10
10
50
Heat input (J/kg or cal/gram)
Heat required to change
the temperature (by 1 °)
of a given mass
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
10
10
50
Heat input (J/kg or cal/gram)
Heat required to change
the temperature (by 1 °)
of a given mass
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
10
10
50
Heat input (J/kg or cal/gram)
Heat required to change
the temperature (by 1 °)
of a given mass
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
10
10
50
Heat input (J/kg or cal/gram)
Heat required to change
the temperature (by 1 °)
of a given mass
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
10
10
50
Heat input (J/kg or cal/gram)
Heat required to change
the temperature (by 1 °)
of a given mass
The green line is longer
than the yellow line!
If we add the same amount
of heat to two materials,
the one with lower heat
capacity will warm up more!
Let’s measure Heat Capacity
Temperature (°C)
Heat Capacity
Rock &
Soil
30
20
Liquid
water
10
10
50
Heat input (J/kg or cal/gram)
Today is the first in-class iClicker
exercise for credit
A) Full credit if you answer 60% or more of
the questions
B) Bonus points if you get the correct
answer for 80% of more of the
questions
C) If there are 10 questions and you
answer at least 6 of them you’ll get full
credit (100%)
D) If there are 10 questions and you
answer at least 8 of them correctly
you’ll get a 5% bonus (105%)
E) All of the above (this is the correct
answer, choose E!)
Heat capacity and phase changes:
ice (solid)
150
water (liquid)
vapor or steam (gas)
Vapor
100
vapor+ liquid
Liquid water
50
Latent Heat
Heat needed to
change phase (from
solid to liquid, liquid
to gas, liquid to solid,
etc.)
Latent heat of
vaporization or
condensation
540cal/gm
Ice + liquid
0
-50
Ice
Latent heat of
fusion or melting
80cal/gm
-100
0
200
400
600
Heat input (cal/gram)
800
Why the Sea is Salty
Seawater is essentially an NaCl solution
Average seawater salinity is 35 ppt (35 g/kg), but it varies
from place to place
30 ppt
Surface water salinity
37 ppt
Why the Sea is Salty
Why the Sea is Salty
And over the eons of time, the sea has grown ever
more bitter with the salt of the continents
Ocean Salinity
Was the Chemistry of the Ancient Oceans the Same as Today?
35 0/00
?
Time (billions of yrs)
Surface water salinity
Was the Chemistry of the Ancient Oceans the Same as Today?
Ocean Salinity
35 0/00
Time (billions of yrs)
Surface water salinity
Note the
attraction of
oppositely
charged ends of
water molecules
for one another
Seawater is essentially an NaCl solution (saltwater)
Cl-, Na+, S04-2, Mg+2, Ca+2, K+ >99% of salt in sea water
HC03-2, Br-, Sr-2, B+2, F- (with these, 99.99%)
http://www.webelements.com/
Seawater is essentially an NaCl solution
Average seawater salinity is 35 ppt or 35 g/kg.
Relative abundance of ions in seawater, in rank order:
Cl, Na, SO4,
Mg, Ca, K (these
make up >99% of
the salt in
seawater)
HCO3, Br, Sr,
B, F (with these
>99.99% of the salt
in seawater)
All other elements occur at very low concentrations (ppm to ppb: 10-6 to 10-9)
Charges must balance, therefore:
Charge associated with cations: Na+, Mg+2, Ca+2, K+
Must equal charge associated with anions: Cl-, SO4-2
Major ions in seawater keep “constant proportion,” regardless
of salinity
• Except near river outlets
(near coastal regions)
• Salinity (o/oo) ~1.81 x
Chlorinity (o/oo)
And in soils
But rivers are not the only important input
Ocean Chemistry is influenced by Erosion
and Weathering of the land
Rivers vs. Other Sources
•Difference in chemical
compositions between rivers and
ocean
--reflects sedimentation
(precipitation) processes
--other inputs/exchanges, such
as basalt-seawater reactions at
midocean ridges
For example,
exchange of
Magnesium (Mg)
in seawater
for Ca in ocean
crust supplies
excess Calcium
Oceans: Chemical Inputs
-rivers (weathering)
-volcanic gases: HCl, SO2, CO2
-interaction of seawater
with seafloor, e.g., hot
basalt associated with
Hydrothermal Circulation,
this is a source of Ca and K
Note: A volume of water
equal to the entire ocean is
circulated through seafloor
material (crust) ~ every 10
m.y.
Ocean Chemistry is
influenced by:
A. water interacting with
rocks (Earth’s crust) at
the mid-ocean ridges
B. Evaporation of seawater
C. River water
D. Erosion and weathering
of the land
E. All of the above.
Why the Sea is Salty
Seawater is essentially an NaCl solution
Average seawater salinity is 35 ppt (35 g/kg), but it varies
from place to place
30 ppt
Surface water salinity
37 ppt
Can we explain ocean chemistry using
the inputs of rivers alone?
atmosphere
rock
ocean
weathering
rivers
We’ve already examined why water is a powerful
Solvent, now let’s look at the whole picture
Ocean Chemistry and the Geochemical Cycle
The Ocean has Both Inputs and Outputs
Outputs include:
1--sea salt aerosols
2--biogenic sediments (biological processes); deposited on
ocean floor (CaCO3, SiO2)
3--inorganic sediments (precipitates, evaporites; adsorption)
4--interaction of seawater with hot basalt (Mg and SO4 "sink”)
Outputs compete with
Inputs to shape the
chemistry of seawater
Outputs: Seawater Evaporation in isolated basins.
These sediments (“evaporites”) provide a record of
seawater chemistry
Salt from the Sea
Dead Sea Salt, Evaporation! At work
The Grand Geochemical Cycle: Residence time
Let’s consider:
The average time that a substance remains dissolved in seawater
We call this the “residence time” of an element or substance
Residence Time (yrs.) =
Total amount in seawater (kg)
Input rate (kg/yr)
where Input rate= average concentration in rivers
(kg/km3) x river discharge (km3/yr)
We will see how this works: first for water, then for total salt, and,
finally, for some individual elements. These calculations give us insights
into how the system works
Residence time of water in
the ocean
Volume = 1.4 x 109 km3
How long does it
take to cycle
ocean water
through rivers
and back again?
River Influx = 3.7 x 105 km3 /yr
t = Volume / Influx
1.4 x 109 km3
t = 3.7 x 105 km3
t = 4000 years
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
--think about it!
•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. SO4 9 million yrs.
Na 60 million yrs. Ca
1 million yrs.
Mg 10 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
Was the Chemistry of the Ancient Oceans the Same as Today?
35 0/00
Ocean Salinity
We can use ancient evaporite deposits
to tell us how ocean chemistry
changed through time (different
sequence of minerals precipitated)
We also can use the chemistry of
“brine” inclusions in the evaporites
to constrain elemental ratios of
major elements in seawater through
time.
New data suggest that ocean
chemistry has changed a bit
through time. Perhaps this reflects
changes in ocean basin spreading
rates and cycling of seawater
through hydrothermal systems!
Time (billions of yrs)