Oceans

1. Oceans

2. O1 Third rock from the Sun • 150 million km from the Sun there is a lump of rock… • It is 12750km in diameter, travelling through space at 67000 mph and spinning at 1037 mph • Inside it is a dense iron and nickel core; part solid, part liquid • Around it is a magnetic field (magnetosphere) caused by motion in this core • The outer surface is cracked into huge, slow-moving ‘tectonic plates’ between 5 and 70km thick • This surface is then surrounded by layers of; • Water – ‘hydrosphere’ • Living things – ‘biosphere’ • Air – ‘atmosphere’ • These layers are constantly moving and interacting with each other • It is the hydrosphere and the atmosphere together which enable energy to be distributed around the planet

3. The Blue Planet • 70% of the earth’s surface is covered by sea • They transport vast quantities of material and energy • The Pacific Ocean alone covers a third of the Earth’s surface • Water occupies a volume of 1.4 x 109 km3 • Detailed investigations of the deep oceans have only taken place quite recently • Surveys have shown that the oceans’ most important role is not as a store for food and chemicals • Their most important role is in controlling our climate

4. O2 Salt of the Earth • Sea salt has almost the same composition everywhere • However, the concentration does vary; • It is low wherever freshwater enters the sea; • close to estuaries, areas of high rainfall and areas with lots of icebergs • Where there is high evaporation the sea is saltier; • Hot, dry and windy areas • The two most abundant ions are Na+ and Cl- • CI3.1, CI5.1, CI2.5 (ionic chemistry revision) • Assignment 2 + 3 • CI4.5 (Energy changes in solution) • O2.1 • O2.2 • Only recently have we begun to understand in detail how the sea became salty…

5. Start here

6. O3 The smell of the sea • The distinctive smell of the seaside is caused by dimethyl sulphide (DMS) • It is formed by certain bacteria acting on dead marine algae • DMS is thought to be important in cloud formation • Clouds reflect solar radiation back into space • DMS is therefore important in climate regulation • Ass 5 • However, DMS can also be oxidised in the atmosphere (and by certain marine bacteria) • This will lead to the formation of acidic sulphur compounds and hence acid rain! • This is one source of pollution we have no control over • CI8.1 (acids and bases)(revision) • CI8.2 (strong and weak acids and pH) • O3

7. O4 The oceans – a safe carbon store? Getting rid of carbon dioxide • CO2 concentration in the atmosphere was 383ppm in 2007 • Some predict it may reach 650ppm by 2100 • Emissions must be drastically reduced even to keep it at 650ppm • This can be done in three main ways; • Use alternatives to fossil fuels • Use fossil fuels more efficiently • Capture and store CO2 • The last of these is the most radical and could involve; • Turning CO2 into something useful (as yet unknown!) • Growing more trees • Storing it in natural deep ocean trenches • Injecting it onto the sea floor – at 3500m it will liquefy at should remain undisturbed

8. The risk is that this may disturb the natural environment • Even so this may be less damaging than allowing it to be absorbed naturally by surface waters • CI7.1 (equilibrium)(revision) • CO2 ,like all gases, is most soluble at low temperatures and high pressures • It is more soluble than O2 or N2 because its polar C=O bonds allow H-bonding with water molecules • Uptake of CO2 is also speeded up by marine life such as phytoplankton • An equilibrium is set up; CO2(g) CO2(aq)

9. CO2 can also chemically react with water; CO2(aq) + H2O (l) HCO3-(aq) + H+(aq)  and HCO3-(aq) H+(aq) + CO32-(aq) • These lead to the oceans being slightly acidic • 35-50% of the CO2 we release into the atmosphere is absorbed in this way • Ocean currents cause the surface water, rich in CO2, to constantly be removed to the deep oceans • This happens in cold regions (where CO2 is most soluble) • Ass 8

10. Keeping things steady • The pH of the oceans has remained close to 8 for millions of years… • …even when the atmosphere had much more CO2 in it • CO2 in solution is a weak acid… • CI7.2 (equilibrium)(revision) • CI8.2 (weak acids and pH) • O4.1 • O4.2 • Using HA as a general acid… HA + H2O H3O+ + A- • It is the oxonium ion (H3O+) that makes the solution acidic • The equilibrium lies heavily over to the LHS • The equation can be simplified to; HA(aq) H+(aq) + A-(aq)

11. For CO2 … CO2(aq) + H2O (l) HCO3-(aq) + H+(aq) • If small amounts of alkali is added to a solution of weak acid… • …they will react with H+ • However, this is a system in equilibrium so… …system adjusts to cancel out the change… …and make more H+… …so more CO2 dissolves and… …equilibrium shifts to RHS • Therefore even when a small amount of alkali is added… • ...the pH is kept constant • The ocean is acting as a buffer solution • A buffer solution resists changes in pH despite the addition of small amounts of acid or alkali • CI8.3 (Buffer solutions) • O4.3

12. As you have seen in CI8.3; a simple buffer forms this eqm... HA(aq) H+(aq) + A-(aq) • This requires a supply of HA and A- • In the case of the oceans; HA is CO2 and A- is HCO3- • Processes involving shells and limestone rocks can provide an almost limitless supply of HCO3- • Looking back at the three reactions discussed so far; CO2(g) CO2(aq) CO2(aq) + H2O (l) CO3-(aq) + H+(aq) HCO3-(aq) H+(aq) + CO32-(aq) • Many marine organisms use the carbonate ions to make shells • This again affects the eqm and encourages CO2 to dissolve • This is what led to the fall in atmospheric CO2 levels as our Earth’s atmosphere evolved

13. CaCO3 is insoluble in sea water and so is suitable for shells • However, it is very slightly soluble in pure water (it is a sparingly soluble solid). An equilibrium is set up; CaCO3(s) Ca2+(aq) + CO32-(aq) • CaCO3 is more soluble deeper in the ocean (there are no shells on the deep ocean floor) • Assignment 9, 10 • If we look at all the equilibria together we can see that, as CO2 levels rise, equilibria will cause CaCO3 to dissolve…

14. O5 The global central heating system • When the Sun’s rays reach the Earth they can be: • Reflected (30%) • Absorbed by the atmosphere (23%) • Absorbed by the land and oceans (47%) • The Earth is surrounded by fluids (water and gas) • Temperature differences set up currents in the oceans and atmosphere • These spread out the heat from the Sun more evenly • Not only does warm water circulate but it also evaporates… • Energy is taken in for evaporation • It must therefore be released when water condenses • The tropics are therefore cooled by evaporation and higher latitudes are warmed by condensation • Northern Europe is warmer than places in Canada of the same latitude because winds and warm water currents travel NE across the Atlantic from the tropics and the Caribbean. • CI4.3 (entropy revision) • CI4.4 (energy and entropy)

15. Energy in the clouds • When water evaporates, the intermolecular bonds (H bonds) must be broken. • This requires energy - measured by enthalpy of vaporisation (∆Hvap) • When clouds form, water condenses so an equal quantity of energy is released (-∆Hvap) • Evaporation and condensation affect temperatures by; • Taking energy from low to high latitudes • Warming the land through the condensation of water • CI5.4 + CI5.5 (H-bonding) • O5.1 • ∆Hvap of water is +2260 kJ/kg; for propanone it is +520kJ/kg • If the water on our planet was replaced by propanone; each kg of rain would only release a quarter of the energy or… • Four times as much ‘propanone rain’ would have to fall to maintain present temperatures

16. Warm water from the west • Western Europe is also kept warmer in winter by an ocean ‘conveyor belt’ • These surface currents (the Gulf Stream and North Atlantic Drift) carry warm water up from the tropics • The specific heat capacity tells us how much energy a substance will release as its temperature drops; • Water : 4.18 J/g/K • Propanone : 2.18 J/g/K • Water is an excellent liquid for transporting energy • A propanone sea would provide us with half as much warmth • O5.2

17. Heavy water? • Water is denser when it is colder and when it is saltier • Melted snow and ice from Greenland creates two cold water currents • This water is… • cold, • of low salinity (due to their origin) • denser • The water in the Gulf Stream is… • warmer • of higher salinity (due to evaporation) • less dense • When these three currents meet, the result is a mass of water which is denser than the currents which feed it. • It therefore sinks

18. The result of this is a deep, cold water current which follows the gulf stream but in the opposite direction.   • This deep ocean current and another from the Antarctic transport cold salty water around the Earth. • Deep ocean currents transport 20x more water than all the world’s rivers but the water moves slowly and may take 1000 years to resurface. • Any materials dissolved in it are also removed for a long time • We know these currents are important in controlling the Earth’s climate but we cannot yet predict how they may change and affect us

19. On and off • The deep ocean current may have stopped during the last ice age, 18000 years ago. • The warm currents may also have stopped causing temperatures in Britain to drop by about 6-8°C • At the Earth warmed, ice melted causing the water to become much less salty • It therefore couldn’t sink and so the deep ocean currents (and the Gulf Stream) stayed “switched off” • The ice in Northern Europe took nearly 1000 years to melt • What if global warming caused Greenland’s ice to melt? • Low salinity water pours into the N Atlantic. • Low salinity water doesn't sink so deep water currents might shut down. • N Europe (including the UK) might actually become colder!!