Music today: Billy Ocean, “Get Outta My Dreams and Into My Car” Available by Wednesday: Find your Midterm Exam score at the class web site, under “Lectures”, April 26 Scantrons and Short Answers will be returned at Labs next week

The Properties of Sea Water What makes water so special? Why is the ocean salty?

Where’s the Water? Reservoir Volume (106 km3) Percent Ocean 1370 97.3 Ice (polar) 29 2.1 Groundwater 5 0.4 Lakes 0.1 0.01 Atmosphere 0.01 0.001 Rivers 0.001 0.0001

Where did the water in the Oceans come from? Outgassing (H2O, CO2) of the Earth from volcanoes, early in its history Sedimentary rocks as old as 3.8 billion years! A much smaller amount from comets that pass by

The Water Molecule

Water is a “Polar” Molecule Weak electrical attraction makes for “sticky” molecules This explains its unique properties: heat capacity surface tension dissolving power

Temperature vs Heat Temperature is a measure of how fast the molecules in a substance are moving Heat is a measure of how much energy has to be put into (or gotten out of) a substance to change its temperature, or “state” (solid, liquid, gas)

Sensible Heat vs Latent Heat Sensible heat is what we sense from different temperatures Latent heat is the energy needed to change state (ice to water, water to vapor)

Table 4.2 (6th edition)

Exists in three states on the planet surface water vapor ice liquid water

Water co-exists on the Earth in 3 physical states: Changes of state Water co-exists on the Earth in 3 physical states: solid liquid gas

Changes of state always occur at constant temperature The heat needed for a change of state is called latent heat

Heat and the three Physical States of Water:

But it requires more energy to do so Evaporation from lakes, oceans, rivers, etc. occurs for temperatures lower than 100 oC But it requires more energy to do so

Density of Pure Water

Consequences…

Consequences Bottom temperature of deep, cold lakes is always 4 oC. Pipes (or beer bottles) freeze and burst. Ice floats on the water surface

Water has the highest surface tension of all common liquids Surface tension - measure of how difficult it is to stretch or break the surface of a liquid Water has the highest surface tension of all common liquids

Major source of energy to power the Earth’s weather systems Energy liberated into the atmosphere (condensation) Energy removed from surface (evaporation)

Special Properties of Water Density of solid is less than liquid Melting and boiling points are very high Highest heat capacity High heat of fusion and vaporization Tremendous dissolving power

Dissolving Power of Water

Why is the Ocean Salty? Total dissolved solids (called “salinity”) About 3.5% by weight (average seawater) Usually expressed as 35 0/00 (parts per thousand, ppt) Varies geographically according to Evaporation, Precipitation, and Rivers

The Most Abundant Ions Chloride (Cl-) 19.0 g/kg Sodium (Na+) 10.6 “ Sulfate (SO42-) 2.6 “ Magnesium (Mg2+) 1.2 “ Calcium (Ca2+) 0.4 “ Potassium (K+) 0.4 “ 35.2 g/kg

Ions in Sea Water Anions are negatively charged Cl-, SO4- Cations are positively charged Na+, K+, Ca++, Mg++

Measuring Salinity Principle of Constant Proportions e.g., SO42-/Cl- is a constant, independent of salinity This means we need measure only one ion to get salinity; i.e., Cl- Today salinity is measured quickly by electrical conductivity of sea water

Where does Salinity come from? Terrigenous input (rivers, dust, ash) Hydrothermal vents Dissolving old sediments (evaporites) Steady State: Inputs equal Outputs

Weathering of Rocks H2O + CO2 ---> H2CO3 (carbonic acid) “acid” rain, pH ~4-5; environmental concerns (HCl, HF, H2SO4) Dissolves rock minerals into ions, which travel down rivers to the ocean

Residence Time How long do the various dissolved ions stay in the ocean? Depends on how “reactive”. Residence Time: The average time spent by a substance in the Ocean = Amount in Sea Rate entering or exiting

Residence Time For water entering through rivers, the residence time is about Volume = 1370x106 km3 (oceans) Flux 0.037x106 km3/yr (rivers) = 35,000 years. For Cl-, the residence time is 100 Ma! For Fe2+, the residence time is 200 yr Which is likely to obey Constant Proportions?

Adding salt lowers the freezing temperature: Seawater freezes at about -2 oC Freezing removes fresh water, leaves salt

The Hydrologic Cycle

Evaporation - Precipitation Over the oceans, evaporation exceeds precipitation The balance is restored by rain over the continents, returning water via rivers

Desalination -- fresh water from the Oceans

Salt in the Ocean and its relationship to density Average ocean salinity 35 per mil 1000 g of seawater = 965 g of water + 35 g salt If all of the water in the oceans evaporated, this amount of salt would form a layer roughly 45 m thick over the entire surface of the Earth

Ocean Surface salinities

Evaporation vs Precipitation

Which processes change the surface salinities ? saltier fresher  evaporation  precipitation  sea ice formation  sea ice melting  freshwater runoff from land

Which ocean is the saltiest?

Which ocean is Saltiest? In spite of the fact that many more big rivers empty into the Atlantic than the Pacific, the Atlantic is actually significantly saltier because of the evaporation-precipitation cycle and the Isthmus of Panama; the Indian is intermediate

Surface salinities Evaporation, precipitation, and wind patterns explain high/low salinity of Atlantic/Pacific.

Summary: Water is a polar molecule -- unique properties (melting pt, heat capacity, dissolving power, water denser than ice) Salinity is the total dissolved solids Salinity in the surface ocean varies by Evaporation - Precipitation Principle of Constant Proportions Residence Time in the Oceans