Seawater and Ocean Chemistry

Seawater and Ocean Chemistry

Seawater Chemistry Water Seawater Salts in seawater

Water Composition Properties

Water is a chemical compound (H 2 O) comprising two atoms of hydrogen and one atom of oxygen, in liquid state...

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

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., 105 water freezes over. When water freezes to ice, the angle of hydrogen bonding expands from 105 to 109. 109

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

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

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

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

Depth (km) Dissolved gases (O 2 and CO 2 contents here) tell us much about biological activity in the ocean: 0.0 Deep waters show O 2 increasing 0.5 gas content with depth. 1.0 CO 2 An oxygen minimum layer is often seen at 1.5 intermediate depths. 1.5 3.0 4.5 6.0 7.5 Surface O 2 waters often 2.0 have excess 90 94 98 102 106 110 CO O 2 and lack 2 Concentration of dissolved gases CO 2. (parts per million, by weight) Why?

Most of carbon from the Earth s atmosphere eventually ended up in the ocean Dissolved Gas % of gas, by volume in atmosphere seawater Nitrogen (N 2 ) Oxygen (O 2 ) Carbon dioxide (CO 2 ) 78.08% 20.95% 0.035% 48% 36% 15%

Major constituents of Seawater Constituent at 3.5% Salinity Water: Oxygen (O) Hydrogen (H) The most abundant ions Chloride (Cl ) Sodium (Na ) Sulfate (SO 4 2 ) Magnesium (Mg 2 ) Calcium (Ca 2 ) Potassium (K ) Bicarbonate (HCO 3 ) 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

Constant Proportions and Conservative Constituents 1. Some constituent salts in seawater occur in constant proportions, e.g., Dissolved Salts in Seawater ( ) = 1.80655 Chlorine in Seawater ( ) 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.

Salt brought in by runoff from land can explain only ~2% of Cl and ~20% of S in the seawater

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

Quadrillion (10 15 ) Metric Tons 10,000 1,000 100 10 1 Water Annual volcanic output x Age of the Earth Estimated existing quantity Sulfur Chlorine Carbon Nitrogen 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. Adapted from Robert Decker & Barbara Decker: VOLCANOES (W.H. Freeman, New York, 1996)

The case for extraterrestrial origin of the oceans Comets are >40% water. The deuterium/hydrogen ratio of comets and oceans overlap. Comet Halley Earth s Oceans Deuterium Total Hydrogen ratio 0.060.48 ppt 0.16 ppt During the initial 2 Ga of its history, Earth may well have received 2 x 10 8 to 1 x 10 17 metric tons of cometary matter by way of bombardment episodes. Comets may well have contributed significantly, therefore, to the hydrospheric mass of 1.41.7 x 10 18 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; SpringerVerlag, New York, 1997).

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).

Suppose annual influx from outer space is 50 100 billion gallons of water vapor into the atmosphere, and that this rate has been constant through geological history (~4.5 billion years). This amounts to 225 450 x 10 18 gallons of water. Compare this to the total amount of water in the oceans = 1370x10 6 Km 3 (volume) 10 9 m 3 /Km 3 264.2 gallons/m 3 = 362 10 18 gallons of water

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.