1. What’s so special about water?
Seawater
Why is the ocean salty?

2. Hydrogen bonds
Relatively weak electrostatic forces between oppositely charged ends of adjacent water molecules
The dipolar structure of the
water molecule and the
formation of hydrogen bonds
between water molecules
are responsible for many of
the unique properties of water. p

3. Properties of Water water has high surface tension
due to H bonds (e.g., rain drops)
water is a good solvent
dipolar structure weakens attraction between other molecules (e.g., NaCl: Na+ and Cl-)
p. 108

6. Salinity Why is seawater salty? What are “salts”?
Where does the salt come from?

7. Seawater Salinity: the Salt of the Ocean
There are many substances dissolved in seawater:
1. gases such as O2 and CO2,
2. nutrients required by organisms such as nitrates, phosphates, trace elements, & vitamins,
3. organic molecules, and
4. mineral salts such as Cl-, Na+, SO42-, Mg2+
Salinity = total dissolved solids
p. 114

8. Seawater Salinity: the Salt of the Ocean
many of the dissolved substances occur in ionic form
cations = positively charged ions
anions = negatively charged ions
almost all 92 naturally occurring chemical elements are found in seawater (incl. gold)
however, 6 ions account for 99.4%
the two most common ions in seawater:
chloride (Cl-): 55.1% by weight
sodium (Na+): 30.6%
together: NaCl = common table salt
p. 114

10. Seawater Salinity: the Salt of the Ocean
average seawater is 96.5% H2O and 3.5% total dissolved solids
fresh water is essentially 100% H2O with only traces of dissolved solids
mass of “salts” in seawater: 3.5 grams of salt in 100 g of seawater
= 3.5% salt (parts per hundred)
= 35.0‰ (parts per thousand)
= 35,000 ppm (parts per million)
p. 114

11. Seawater Salinity: the Salt of the Ocean
Oceanographers describe salinity with units of parts per thousand (‰)
grams of dissolved solids per kilogram of water, g/kg
average ocean salinity is ~35‰
fresh water is 0‰
salinity of the ocean typically ranges between 33‰ and 37‰
p. 114

13. Ocean surface salinity Notice any patterns?
P>E
E>P
Hypotheses to explain observations in terms of evaporation and precipitation?

14. Investigation 3.3 - Salinity
What is the salinity of the following sample of sea water?
mass of sea water sample: 630 g
mass of salt in sample: 22 g
mass of water in sample?
salinity?
22 g/630 g = 3.5% or 35‰
p. 153

15. Where do “salts” (dissolved ions) come from?
rivers contain dissolved ions (in low concentrations)
see table at bottom of p. 114
these ions are derived from the chemical breakdown (“weathering”) of rocks & soils on the continents
note: the proportion of ions in river runoff is not the same as the proportion of ions in seawater
implication: there must be other
sources of dissolved ions
What are some others sources? p

16. Dissolved solids in seawater compared to river water
Notice the four most common ions in seawater (in descending order of abundance) are Cl-, Na+, SO42-, and Mg2+, while in rivers the top four dissolved ions are HCO3-, Ca2+, SiO2, and SO42-
p. 115

17. Input Sources of dissolved “salts” (ions) in seawater:
1. chemical weathering of rocks, runoff from the land
2. volcanic emissions (e.g., CO2, H2O, HCl, SO42-)
3. hydrothermal activity on the seafloor (e.g.,
at spreading centers)
4. Fluid flow out of accretionary prisms (above trenches)
p. 115

19. Mt. St. Helens (1980)
Mt. Pinatubo (1991)

20. Black smoker at spreading center
(hydrothermal circulation through the hot, young oceanic crust)

21. Hydrothermal circulation at spreading centers: chemical reactions between hot volcanic rocks and icy cold seawater.

22. Subduction zone complex (accretionary prism) off Japan
Subduction zone complex (accretionary prism) off Japan. Fluids are squeezed out of sediments and crust due to great compression.

23. Has ocean chemistry changed over time?
Some possibilities:
Salinity has increased because more salts enter the ocean every year
Salinity has decreased because incoming river water is so dilute
Salinity probably hasn’t changed much over time because input = output “Steady State”
biogeochemical and geochemical cycles are very important in maintaining ocean chemistry steady-state!

24. Output Sinks for “salts”:
1. biological recycling (skeletons & shells, plus
biological productivity)
2. burial in sediments
3. ionic exchange between superheated seawater &
oceanic crust at the spreading centers
precipitation of metaliferous deposits in the ocean
4. subduction (recycling of crust & sediments)
p. 115

25. (Bio)geochemical cycling of fluids and dissolved ions in the surface ocean, in the sediments and ocean crust, at spreading centers and subduction zones, are very important processes controlling the stead-state of ocean salinity.

27. Summary of Oceanic Salinityp
average ocean salinity is ~35‰
lower in areas of high precipitation (P>E) or high river runoff from the land,
higher in areas of high evaporation (E>P)
all naturally occurring elements are dissolved in seawater
6 ions account for 99.4% of all dissolved solids in seawater (Cl-, Na+, SO42-, Mg2+, Ca2+, and K+)
multiple sources of dissolved “salts” in seawater (input)
chemical weathering of rocks, volcanic emissions, hydrothermal activity on the seafloor
the chemical composition of the ocean has remained relatively constant over time because input = output
geochemical and biochemical cycling of ions (biological recycling, burial in sediments, ionic exchange between seawater and hot oceanic crust, and subduction)