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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

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

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

2

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

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

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

3

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

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

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

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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

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

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

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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

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

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

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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

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

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/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

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

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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?

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

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%

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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

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

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