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Lecture 8: Seawater
Introduction to Oceanography Surtsey, Iceland. Wikimedia Commons, NOAA image, Public Domain, http://commons.wikimedia.org/wiki/File:Surtsey_eruption_2.jpg
Physical and chemical properties of Seawater
Playa del Rey & LAX, CA, E. Schauble, UCLA
Periodic Table figure, NASA Science Education Resource Center, Public Domain
2
Atoms • Atom: cannot be broken down into
simpler parts by chemical means • Nucleus:
– Protons (+) & Neutrons (uncharged) – Massive – Small (~10–15 m)
• Electrons (–) – Little mass – Most of the volume
(~10–10 m = 1 Å)
HeliumSvdmolen/Jeanot, Wikimedia Commons, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Atom.svg
Molecules • Substances made up of chemically bonded
atoms
Ions • Ions are atoms with net electrical charge
– Anion: negative charge (Cl–) -- extra electrons – Cation: positive charge (Na+) -- electrons removed
11+
–
–
–
– – –
–
–
– –
Na+: 11p+, 10e– Elements on the left side of the periodic table of elements tend to become positive (H, Na, Mg). Elements near the right side of the periodic table tend to become negative (O, F, Cl)
E. Schauble, UCLA
3
What kind of ions will an element form?
NASA image, Science Education Resource Center, http://serc.carleton.edu/images/usingdata/nasaimages/periodic-table.gif, Public Domain
Tend to form cations (+)
Tend to form anions (–)
Chemical Bonds • Covalent: e– shared between
atoms (i.e., H2O) • Ionic: Charges borrowed by
anions – Like Na+Cl–
• Hydrogen Bonding: in water, H has slightly + charge, which attracts negatively charged O. H in water molecules also attracts anions like Cl– O in water molecules also attracts cations like Na+ Bond Strength: Covalent > Ionic > H-bond
Na+ Cl–
+
+
e–
e– e–
e– e–
e–
e–
8+
e–
e–
e–
All images E. Schauble, UCLA
4
Water Molecule: H2O Covalent bond between O and H • Polar Molecule
– Positive “ears”-105o
– Mickey Mouse – Polar structure
• Hydrogen Bonding – Effect of polarization – ~5% as strong as covalent
bonds – tends to make molecules
clump together – i.e., condense
e–
e– e–
e–
e–
e–
e–
e–
8+
e– +
e– +
O H
H
+
+
e–
e– e–
e– e–
e–
e–
8+
e–
e–
e–
E. Schauble, UCLA ball & stick model rendered using MacMolPlt
Hydrogen bonding • Hydrogen Bonding
– Each H2O: 4 possible H-bonds
– Makes liquid water “clump” together
– Accounts for many peculiarities
• Great solvent power – Saline ocean
water • Thermal and density
properties
Qwerter/Michal Maňas, Wikimedia Commons, Creative Commons A S-A 3.0, http://en.wikipedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.jpg
5
Heat Capacity
The only common liquid with
higher heat capacity than
water is ammonia!
Substance Heat Capacity (cal/gram/oC)
Granite 0.20 Gasoline 0.50 Water 1.00 Ammonia 1.13
Photo by ..its.magic..,
Flickr, Creative Commons 2.0,
http://www.flickr.com/photos/rizielde/
3373257326/sizes/l/
Heat Capacity Examples:
TV dinner: Aluminum Foil vs. Gravy. Both are at the same temperature, but gravy has much higher heat capacity – it hurts!
At the beach: Sand vs.water
Photo by A. Lau, “are you gonna eat that”, http://www.flickr.com/photos/andreelau/186536202, Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic
6
LAX -- on coast. Strong ocean
influence. Little daily/seasonal
variation in Temp.
Omaha, NE – middle of continent.
Weaker ocean influence.
Variable Temp.
Climate Comparison
Pidwirny, M. (2006). "Climate Classification and Climatic Regions of the World". Fundamentals of
Physical Geography, 2nd Edition. http://www.physicalgeography.net/fundamentals/7v.html
Physical States of Matter Solid: molecules bonded in a fixed lattice
– Add energy (i.e., latent heat of fusion)… Liquid: bonded molecules but in
no fixed lattice – Add energy (i.e., latent heat of
vaporization)… Gas: Free molecules
Zeitraffer, Wikimedia Commons, CC A S-A 3.0, http://commons.wikimedia.org/wiki/File:Timelapse.GIF
NASA, Public Domain, http://ksnn.larc.nasa.gov/videos/
poolboil.mov
7
Relationship between Heat and Temperature for H2O
-40
-20
0
20
40
60
80
100
120
140
0 500 1000 1500 2000 2500 3000 3500
Temp. (ºC)
Heat (Joules/gram)
liqui
d
stea
m
ice
liquid boiling to steam
ice melting to liquid
Latent heat of vaporization
Latent heat of melting
Sensible Heat: measurable change in temperature when heat is added or subtracted (sloping lines).
Latent Heat: no temperature change with added/subtracted heat (flat lines).
E. Schauble, UCLA
Density of Pure Water • Density of fresh liquid water ≈ 1 gm/cm3
Maximum density of pure water occurs at 4o C�(but at freezing temperature in salty water, i.e. seawater)�
Density (gm/cm3)
0.90
0.92
0.94
0.96
0.98
1.00
1.02
-20 0 20 40 60 80 100
Temperature (ºC)
liquid
ice
Max density (1.0) at 4ºC
E. Schauble, UCLA
Ice much less dense
(0.92) at 0ºC
8
Density & Structure of Ice Ice density 0.92 gm/cm3
8% less than liquid water
Molecular bond angle increases to 109o
Allows H-bonds to form solid, but widely spaced, lattice
Ice is less dense than liquid! Therefore: ice floats! Unique property of water, due to H-bonds.
Materialscientist, Wikimedia Commons, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Hex_ice.GIF
Thermal Convection in Water • Cold surface water
becomes more dense
• Sinks below warmer surrounding waters Where should this
occur in the ocean?
Jberes87, YouTube, http://www.youtube.com/watch_popup?v=QBVMm9i-pvo
9
Questions?
GOES-9 movie, NASA Mesoscale Atmospheric Processes, Public Domain,
http://rsd.gsfc.nasa.gov/rsd/movies/preview.html
NASA image, Public Domain, Science Education Resource Center, http://serc.carleton.edu/images/usingdata/nasaimages/periodic-table.gif
Tend to form cations (+)
Tend to form anions (–)
Chemical Properties of Seawater
10
Water: Universal solvent (almost) Rule of solubility: like dissolves like
• Water is polar, so it tends to dissolve polar molecules and ionic salts• Non-polar stuff like oil not dissolved well
Oil-like (hydrophobic) parts of molecules in our cell membranes keep us from turning into puddles of bone soup.
Water dissolving salt, Liu and Michaelides, London Centre for Nanotechnology, UCL,
http://www.ucl.ac.uk/news/ucl-views/0803/salt500
Water: great at dissolving stuff • H-bonding in H2O like ions & polar molecules
Water combining with ions from sodium chloride
Based on illustrations by Steve Berg, Winona State U. Free license for educational use, http://course1.winona.edu/sberg/Illustr.htm
11
Salinity Dissolved Salts: Mainly Na+
and Cl– Constituents of table salt
No salt crystals in seawater Ions separated in seawater,
recombine on evaporation
Average ocean salinity: 3.5% by mass
Seawater: 96.5% water, 3.5% dissolved substances
4 x 1019kg dissolved salt Enough to cover the planet with a
80 m thick layer
Saltwater evaporation ponds, San Francisco Bay, CA. dro!d, Creative Commons A S-A 2.0, http://flickr.com/
photos/23688516@N00/364573572
Sources of Dissolved Salts 1) Weathering and alteration of
the crust Seawater chemistry doesn’t quite
match river water • Na, K, Mg, Ca can be derived
from rock weathering • BUT not everything can be due to
crustal weathering alone 2) Mantle degassing
(volcanoes) H2O, CO2, HCl, N2, H2S released in volcanic gases
Bottom: Halemaumau, Hawaii, Mila Zinkova, Wikimedia Commons, Creative Commons A S-A 3.0, http://upload.wikimedia.org/wikipedia/commons/9/92/Sulfur_dioxide_emissions_from_the_Halemaumau_vent_04-14-08_1.jpg
Top: Rio Tinto, Spain, Carol Stroker(?), NASA, Public Domain, http://upload.wikimedia.org/wikipedia/commons/b/b0/Rio_tinto_river_CarolStoker_NASA_Ames_Research_Center.jpg
12
Major Constituents Most abundant dissolved elements & molecules:
Cl–, Chlorine Na+, Sodium SO4
2–, Sulfate Mg2+, Magnesium Ca2+, Calcium K+, Potassium
Major dissolved species occur in constant relative ratios in seawater
e.g., Cl/Mg mass ratio is usually 15 in seawater Implication: the oceans are mixed & stirred
Figure by Hannes Grobe, Alfred Wegener
Institute, Creative Commons A S-A 2.5,
http://commons.wikimedia.org
/wiki/File:Sea_salt-e_hg.svg
Chemical Residence Times • Residence Time: the average length of time an element spends in the ocean
• Residence time of chlorine… • Amount in ocean:
.02 kg/kg (concentration) * 1.4x1021kg (ocean mass) = ?? Kg • Rate of addition (from rivers):
~2.2x1011kg/yr • Residence time = amount/rate = ?? • Assumes long-term steady-state
€
Res. Time = Amount of element in oceanElement's rate of removal (or addition)
from the ocean
13
Chemical Residence Times Residence Time: the average length of time an element spends in the ocean
€
Res. Time = Amount of element in oceanElement's rate of removal (or addition)
from the ocean
Constituent Res. Time (yrs)
Chlorine (Cl–) 108 Sodium (Na+) 6.8 x 107 Silicon (Si) 2 x 104 Water (H2O) 4.1 x 103 Iron (Fe) 2 x 102
Chemical Residence Times Elements with shorter times aren’t well
mixed, vary place-to-place Fe, Si, CFC-11 input are examples Non-Conservative
Shorter bio/geo/seasonal residence times • Poorly soluble: Al, Ti, Fe
• Biological nutrients/products: Oxygen (respiration), Fe and P (nutrients), carbon dioxide (photosynthesis), Si (shells)
• Chemicals created by recent human activity
CFC-11 (CCl3F)
CFC-11 vs. time, Plumbago, Wikimedia Commons, CC A S-A 3.0, http://upload.wikimedia.org/wikipedia/commons/2/25/AYool_CFC-11_history.png.
CFC-11 vertical inventory, Plumbago, Wikimedia Commons, CC A S-A 3.0, http://upload.wikimedia.org/wikipedia/commons/2/20/GLODAP_invt_CFC11_AYool.png
CFC-11 vibration, E. Schauble, UCLA, http://www2.ess.ucla.edu/~schauble/MoleculeHTML/CCl3F_html/CCl3F_page.html
14
Trace Elements • Some are conservative, often these are chemically similar to
abundant conservative elements (Li+ is like Na+, Br– like Cl–) • Many trace elements behave like nutrients
– Some are necessary for life (i.e., Fe) • Some are toxic in high
concentrations Hg is fat soluble, accumulates
up the food chain From <1x10–9 g/g (seawater)
to 1x10–6 g/g (shark) – Top predators are most
likely to have high Hg: • Shark • Swordfish • King Mackerel • Tilefish ~ White (Albacore) Tuna (list from EPA, 2004)
NASA image, Science Education Resource Center, Public Domain
Biological Nutrients • N, P, Fe, Si
• More needed for organic processes or
skeletal growth than is easily available
• Consumed in photic zone (lots of biological growth) – Si used by diatoms for skeletal material
• Enriched in deep waters due to
breakdown of organic matter
• Upwelling flows transport nutrients back up to shallower waters
Image from N. Carolina Dept. of Agriculture, appears to be Public Domain,
http://www.ncagr.gov/cyber/kidswrld/plant/label.htm
15
Questions
Seasalt evaporation and harvesting, Tavira, Portugal, Nemracc, Wikimedia Commons, Creative Commons A 3.0 Unported, http://commons.wikimedia.org/wiki/File:Salt_evaporation_pond_near_Tavira_Portugal.JPG
What controls the density of Seawater? In the ocean water density changes due to: • Temperature (Largest variability) • Salinity
(Modest variation in ocean)
Density (gm/cm3)
0.90
0.92
0.94
0.96
0.98
1.00
1.02
-20 0 20 40 60 80 100
Temperature (ºC)
liquid
ice
Max density (1.0) at 4ºC
E. Schauble, UCLA
Ice much less dense
(0.92) at 0ºC
16
Effects of Temperature & Salinity
% Salinity (grams salt/100 grams seawater)
Temp. (ºC)
Densest
Least dense
Water density at sea surface pressure, in grams/cm3
E. Schauble, UCLA, based on
Fofonoff and Millard (1983) Algorithms for computation of fundamental properties of
seawater. Unesco Tech. Pap. in Marine Sci. 44
North Atlantic Deep Water Antarctic
Bottom Water Antarctic Intermediate
Water
Physical Structure of the Oceans • Three Density Zones
– 1) Mixed Layer, 2) Pycnocline, 3) Deep Water
C
C’
American Meteorological Society, http://oceanmotion.org/images/ocean-vertical-structure_clip_image002.jpg
17
Dep
th (m
)
Tem
pera
ture
(ºC
)
Salinity (%)
Density (g/cm3)
2ºC 6ºC 10ºC 3.44% 3.46% 3.48% 3.5%
1.0258 1.0266 1.0274 1.0282
Adapted from plot of S. Atlantic (45ºS, 50ºW) CTD data at U. Southampton School of Ocean and Earth Science,
http://www.soes.soton.ac.uk/teaching/courses/oa631/ctd_plot.jpg
The ocean is layered by density
T S
Density (g/cm3)
Ocean Water: Layered by density. #1) The Mixed Layer
Top ~100 m Variable thickness
0 m - 1000 m 2% of ocean volume At surface, so is
strongly affected by wind, gas exchange with air
Sunlit Dep
th (m
)
Tem
pera
ture
(ºC
)
Salinity (%)
Density (g/cm3)
2ºC 6ºC 10ºC 3.44% 3.46% 3.48% 3.5%
1.0258 1.0266 1.0274 1.0282
Adapted from plot of S. Atlantic (45ºS, 50ºW) CTD data at U. Southampton School of Ocean and Earth Science,
http://www.soes.soton.ac.uk/teaching/courses/oa631/ctd_plot.jpg
18
Layer #2) The Pycnocline • Density gradient between
Mixed Layer and Deep Water • 18% ocean volume • Mostly due to temperature
change (deeper water is colder)
• At poles, surface water is also cold, so pycnocline caused mostly by change in salinity (I.e. halocline).
Dep
th (m
)
Tem
pera
ture
(ºC
)
Salinity (%) Density (g/cm
3)
2ºC 6ºC 10ºC 3.44% 3.46% 3.48% 3.5%
1.0258 1.0266 1.0274 1.0282
Adapted from plot of S. Atlantic (45ºS, 50ºW) CTD data at U. Southampton School of Ocean and Earth Science,
http://www.soes.soton.ac.uk/teaching/courses/oa631/ctd_plot.jpg
Layer #3) The Deep Layer • Water originates at
high latitude (cold) • Cold ~4o C waters • 80% of ocean’s
volume • Completely dark
(aphotic) and relatively unaffected by surface conditions
Dep
th (m
)
Tem
pera
ture
(ºC
)
Salinity (%)
Density (g/cm3)
2ºC 6ºC 10ºC 3.44% 3.46% 3.48% 3.5%
1.0258 1.0266 1.0274 1.0282
Adapted from plot of S. Atlantic (45ºS, 50ºW) CTD data at U. Southampton School of Ocean and Earth Science,
http://www.soes.soton.ac.uk/teaching/courses/oa631/ctd_plot.jpg
19
Thermocline • Region where
temperature changes with depth.
• Typically ~100 - 1000 m • Strong near equator
(hot surface water) • Weak at poles (surface
water almost as cold as deep water)
-1000
-900
-800
-700
-600
-500
-400
-300
-200
-100
0
0 5 10 15 20
Temperature (ºC)
Depth (m)
Plot E. Schauble, UCLA from NOAA CTD data.
34ºN
(California)
Tropics (15ºN)
Pol
ar (6
0ºS
)
Halocline
• Changing salinity instead of temperature – Sharp gradient in salinity with depth – Strongest near river mouths, regions with high
rainfall. Why?
20
Pycnocline • Depth interval
with strong vertical density gradient
• Caused by thermocline & halocline D
epth
(m)
Tem
pera
ture
(ºC
)
Salinity (%) Density (g/cm
3)
2ºC 6ºC 10ºC 3.44% 3.46% 3.48% 3.5%
1.0258 1.0266 1.0274 1.0282
Adapted from plot of S. Atlantic (45ºS, 50ºW) CTD data at U. Southampton School of Ocean and Earth Science,
http://www.soes.soton.ac.uk/teaching/courses/oa631/ctd_plot.jpg
Questions
0
200
400
600
800
1000
1200
0 10 20 30
Temperature (ºC)
0
200
400
600
800
1000
1200
34 34.5 35 35.5
Salinity (g/103g)
0
200
400
600
800
1000
1200
0 50 100 150 200 250
Dissolved O2 (10–6moles/103g)
Pre
ssur
e (1
04 k
g/m
/sec
2 ) --
ro
ughl
y eq
uiva
lent
to m
eter
s de
pth
CTD data from ALOHA station, Hawaii, July 7, 1997
21
Dissolved Gases in the Ocean • Atmospheric gases
dissolved in seawater – Mainly N2, O2 – CO2
• Relative Solubilities: – Gases are most soluble in
COLD water • Polar waters: cold, rough
waters = gas rich • Less soluble in salty
water (“salting out”) • Not quite the same process
as Mentos+Diet Coke
Photo by JD (Kinchan1), Creative Commons Attribution-NonCommercial-
NoDerivs 2.0 Generic http://www.flickr.com/photos/jdbaskin/
5334126513
Photo by Michael Murphy, Wikimedia Commons, GFDL/Creative Commons-BY-SA 3.0, http://
commons.wikimedia.org/wiki/File:Diet_Coke_Mentos.jpg
Dissolved Gases in the Ocean
Gas Atmosphere (Volume %)
Dissolved in Ocean (Volume %)
Nitrogen (N2) 78.08% 48%
Oxygen (O2) 20.95% 36%
Carbon dioxide (CO2) 0.039% 15%
22
Oxygen (O2) • Produced in the photic zone (top 200 m)
where photosynthesis occurs Also dissolves from
atmosphere Consumed below
photic zone by Animal respiration Bacterial oxidation
of organic detrital matter
Mainly at sea floor • Oxygen minimum
in region below photic zone (200 - 1000 m) – Also depleted
bottom water zone
Plot from Station ALOHA, N. of Hawaii, from Dore et al. (2009) PNAS doi: 10.1073/pnas.0906044106
Carbon Dioxide • Like N2 and O2, dissolves from the atmosphere
at the ocean surface • Also produced by respiration (digestion) of
organic matter • Consumed by photosynthesis • CO2 combines chemically with H2O
– VERY soluble in seawater---1000x solubility of nitrogen or oxygen
€
CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3− ⇔ 2H+ + CO3
2–
Carbonic Acid Bicarbonateion
Carbonateion
23
Carbon Dioxide
• > 90% stored in bicarbonate ions, HCO3-
– At 10o C, Salinity = 3.43% and pH = 8.0:
• Consumed in photic zone (photosynthesis) • Produced by respiration, decomposition of organic matter
CO2 (HCO3)– (CO3)2– 1% 94% 5%
Photosynthesis • Plants and phytoplankton make simple
organic compounds (sugars) from H2O, CO2 and light energy – Energy stored in compounds – O2 formed as byproduct – Occurs in the photic zone
€
6H2O + 6CO2 + sunlight⇔ C6H12O6 + 6O2
PHOTOSYNTHESIS
RESPIRATION
Light CO2
O2 Sugar
Photo by Wikiwatcher1, Wikimedia Commons, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Seaweed_Rocks2_wiki.jpg
24
Respiration
• Plants and animals oxidize sugars to release energy – Water and carbon dioxide are by products – Occurs throughout the water column
€
6H2O + 6CO2 + sunlight⇔ C6H12O6 + 6O2
PHOTOSYNTHESIS
RESPIRATION
O2 a
nd C
O2 v
s. D
epth
ORGANICDECAY
LOW T, HIGH P:�HIGH CO2
SOLUBILITY
Respiration
Photosynthesis
Plot from Station ALOHA, N. of Hawaii, from Dore et al. (2009) PNAS doi: 10.1073/pnas.0906044106
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