Seawater and Ocean Chemistry

1. Seawater and Ocean Chemistry

2. Seawater Chemistry • Water • Seawater • Salts in seawater

3. Water • Composition • Properties

4. Water is … • a chemical compound (H2O) comprising two atoms of hydrogen and one atom of oxygen, in liquid state ...

6. Two hydrogen atoms share their electrons with one oxygen atom to form the electrically polarized water molecule - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + +

7. When water freezes to ice, the angle of hydrogen bonding expands from 105° to 109°. 105° - - - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + 109° As the space taken by 27 water molecules is now used by 24 molecules, the density of ice is less than the density of water, i.e., water freezes over.

8. Hydrogen bonding of these electrically polarized molecules gives water the properties of cohesion, adhesion and dissolving power.

9. Common salt (NaCl) thus dissolves in water most readily. It is the ocean’s most abundant constituent, therefore.

10. Sodium (Na) 11 23 11 11 12 Chlorine (Cl) 17 35 17 17 18 Atomic Number Atomic Weight (or Mass) Number of Electrons Number of Protons Number of Neutrons

12. Fresh water comprises Oxygen: 857.8g (=85.8%) Hydrogen: 107.2g (=10.7%)

13. Dissolved gases (O2 and CO2 contents here) tell us much about biological activity in the ocean: 0.0 • Deep waters show increasing gas content with depth. • An oxygen minimum layer is often seen at intermediate depths. • Surface waters often have excess O2 and lack CO2 . Why? O2 0.5 CO2 Depth (km) 1.0 1.5 1.5 3.0 4.5 6.0 7.5 O2 2.0 90 94 98 102 106 110 CO2 Concentration of dissolved gases (parts per million, by weight)

14. Most of carbon from the Earth’s atmosphere eventually ended up in the ocean % of gas, by volume in Dissolved Gas atmosphere seawater Nitrogen (N2) Oxygen (O2) Carbon dioxide (CO2) 78.08% 20.95% 0.035% 48% 36% 15%

15. Major constituents of Seawater at 3.5% Salinity Constituent Water: Oxygen (O) Hydrogen (H) The most abundant ions Chloride (Cl-) Sodium (Na+) Sulfate (SO42-) Magnesium (Mg2+) Calcium (Ca2+) Potassium (K+) Bicarbonate (HCO3-) 85.8% 10.7% 1.9% 1.1% 0.3% 0.1% 0.04% 0.04% 0.01% only 2% of Cl in seawater could have come from land sources only 20% of sulfur in seawater could have come from land sources

16. Constant Proportions and Conservative Constituents 1. Some constituent salts in seawater occur in constant proportions, e.g., Thus, for average seawater with Cl = 19.2‰, salinity = 1.80655 × 19.2‰ = 34.7‰ 2. The constituents of seawater that occur in constant proportion (i.e., change very slowly) are known as conservative constituents or elements. Dissolved Salts in Seawater (‰) Chlorine in Seawater (‰) = 1.80655

17. Salt brought in by run-off from land can explain only ~2% of Cl and ~20% of S in the seawater

18. Why is seawater salty? • Hydrothermal activity on the seafloor can provide the • sources for excess Cl and S and • sinks to balance the continued enrichment in sulfates and magnesium by streams. • We should also note that, compo-sitionally, seawater has remained remarkably uniform through the earth’s history

19. Annual volcanic output x Age of the Earth Estimated existing quantity 10,000 Water Sulfur 1,000 Chlorine Quadrillion (1015) Metric Tons Carbon 100 Nitrogen 10 This comparison of the total quantities of selected substances in the oceans and atmosphere with what could have come from volcanism favors the volcanic origin of these substances. 1 Adapted from Robert Decker & Barbara Decker: VOLCANOES (W.H. Freeman, New York, 1996)

20. The case for extraterrestrial origin of the oceans Deuterium Total Hydrogen ratio • Comets are >40% water. • The deuterium/hydrogen ratio of comets and oceans overlap. Comet Halley Earth’s Oceans 0.06-0.48 ppt 0.16 ppt • During the initial 2 Ga of its history, Earth may well have received 2 x 108 to 1 x 1017 metric tons of cometary matter by way of bombardment episodes. • Comets may well have contributed significantly, therefore, to the hydrospheric mass of 1.4-1.7 x 1018 metric tons. Indeed, all this water could have been produced by either ~10% of the cometary mass or entirely by the asteroidal source if initial bombardment was of carbonaceous chondrites. Adapted from C.F. Chyba & C. Sagan in COMETS AND THE ORIGIN AND EVOLUTION OF LIFE (Ed: P.J. Thomas, C.F. Chyba & C.P. McKay; Springer-Verlag, New York, 1997).

21. A trio of frames from Polar's Visible Imaging System (VIS), taken 6 seconds apart last December 31st, captures an object rapidly descending toward northern Europe. Because the camera's filter isolates emission from hydroxyl (OH) radicals, the incoming object must have contained abundant water (the map of Earth is intended for reference).

22. Suppose • annual influx from outer space is 50-100 billion gallons of water vapor into the atmosphere, • and that Compare this to the total amount of water in the oceans = 1370x106 Km3 (volume)  109 m3/Km3  264.2 gallons/m3 = 362  1018 gallons of water • this rate has been constant through geological history (~4.5 billion years). • This amounts to 225- • 450 x 1018 gallons of • water.

23. Therefore, • there is no problem in accepting the • extraterrestrial possibility, except that • this requires • a much smaller ocean and • an appreciably weaker hydrological cycle • in the past, than at the present. • But there is no geological evidence to • support this possibility.