Lectures 5 Geosc 040
Chemical Properties of Water --The Wonder
Substance! Inputs, Outputs and Residence Time
Ocean Salinity Map
A new planet?
• Lecture Review Questions:
• TA Office Hours
Tue. 10-11, Wed. 2-3, Room 313
Deike Bld.
• Homework 1 due on Thurs. (28th)
by 11pm
• Sign up to take Quiz 1 on Feb 1
• Cell Phone Recycling
• Lecture Review Questions:
• TA Office Hours
Tue. 10-11, Wed. 2-3, Room 313
Deike Bld.
• Homework 1 due on Thurs. (28th)
by 11pm
• Sign up to take Quiz 1 on Feb 1
• Cell Phone Recycling
Jonas!
Jonas!
Jonas: Read about storms like it on the
course web site!
Hurricane Tracks
Sea Surface Temperature & Latent heat are key
factors in Hurricane development and sustainability.
Sea Surface Temperature & Latent heat are key
factors in Hurricane development and sustainability.
Warm water is the energy source for hurricanes
and storms like Jonas
energy
Warm water is the energy source
150
Vapor
Latent heat of
evaporation
540cal/gm
100
Liquid water
50
0
-50
Ice
-100
energy
0
200
400
600
Heat input (cal/gram)
800
General Classification of Water Masses
Why is ocean water stratified into these layers?
What are the characteristics of each of these regions?
Why do we care?
Density of Seawater
Salinity (S) - Temperature (T) - Density Diagram
•note dual effects of S & T
•typical range of seawater S & T
Define Sigma t :
t = (Density-1.0) x 1000
For example: seawater
with a density of 1.026
would have t = 26
We'll use this diagram to
differentiate water masses
t = (Density-1.0) x 1000
t = 21
t = 24
t = 29
More complete Salinity- Temperature- Density Diagram for
the normal range of salinity and temperature in the oceans.
You should become familiar with this diagram.
Let’s look at this in a bit more detail.
Example 1: What is
the density of 1 vs. 2?
Water masses 1 & 2
have same S (35 o/oo)
But 2 (at 4°C) is colder
(T is lower ) than 1
(20°C);
Therefore:
Water mass 2 (t = 27.7)
is more dense than
water mass 1 (t = 24.8)
1
2
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
Chemical Properties of Water --The Wonder
Substance! Inputs, Outputs and Residence Time
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/
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
ocean chemistry: A balance of inputs and outputs
Evaporation plays a big role in Surface Water Salinity
30 ppt
Surface water salinity
37 ppt
ocean chemistry: A balance of inputs and outputs
Evaporation plays a big role in Surface Water Salinity
What is this
stuff?
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
Mediterranean Sea
Suez
Canal
Sinai
Peninsula
Nile
River
Egypt
Gulf of
Suez
Red Sea
Dead Sea
Rift
Salt from the Sea
Dead Sea Salt, Evaporation! At work
Salt from the Sea
Dead Sea Salt, Evaporation! At work
Pay attention to the sequence, from the open sea,
first calcite … and eventually Halite!
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
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
t ≈ 4000 years
Residence Time: total amount divided by input rate
For Coconuts on the beach:
• 10 coconuts fall on the beach each day
• There are generally 20 coconuts on the beach
The residence time for coconuts on this beach is:
A) 1 day
B) 2 days
C) 0.5 days
D) 20 days
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.
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
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 spreading rates
and cycling of seawater through
hydrothermal systems!
Time (billions of yrs)
Ocean’s Chemistry and
The hydrologic and geochemical cycles
Outputs compete with
Inputs to shape the
chemistry of seawater
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
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
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!
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
Cold water holds
more gas in solution
Pepsi
Solubility of Gases in Seawater
A) Gases dissolve more readily
in warmer water
B) Cold freshwater can dissolve
more gas than warm
saltwater
C) Fishes need oxygen to live
and they get that by breaking
O molecules from H20
D) A and B
E) B and C
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