Where is all the water?

Transcription

1 Where is all the water? The distribution of water at the Earth's surface % of total Oceans Ice caps and glaciers 2.05 Groundwater 0.68 Lakes 0.01 Soils Atmosphere (as vapour) Rivers Biosphere Total 100 % 1

2 Water flows into the oceans and residence time Volume of the oceans: 1.37 x litres. Annual Volume Residence time Rivers = 3.74 x litres. 34,000 Rain on surface ocean = 3.8 x litres. 4,000 Hydrothermal vents = 1.7 x litres 8x 10^6 Surface waters 9-15 yrs The oceans are heated from the top, yields stability Surface layer mixed by wind thickness varies from m is a balance between wind energy and heating e.g. tropical surface waters warmer and mixed layer thinner (less wind) Polar surface waters are very cold, can get deep mixed layers 2

3 Density increases with depth, maximum change at the pycnocline Surface mixed layer is only ~ 2% of ocean volume Pycnocline is ~ 18% of ocean volume Deep waters make up ~80 % of ocean Pycnocline comes to the surface of ocean ~50-60 N, small density differences, place to make deep water Make deep water by cooling surface water but salinity must be high enough Salinity is affected by precipitation (rain) and evaporation In places where evaporation exceeds precipitation, salinity and density of surface seawater increases If precipitation exceeds evaporation, salinity and density decrease 3

4 Global Evaporation - Precipitation Data: World Ocean Atlas, 4

5 Combined effect of temperature and salinity Temperature ranges from -1.8 C to ~30 C As you cool water from 25 C to 5 C density increases from ~1.023 to ~1.028, almost dense enough for deep water Deep water formed by cooling in North Atlantic and Weddell Sea, travels through Atlantic and Indian Ocean to Pacific 5

6 Animation NOAA The concentration of tritium (3H) and 90Sr (strontium-90) in rain collected in Valencia, Ireland between 1952 and 1974 Most deposition 2 years after the ban? 6

7 Can see penetration of bomb tritium into deep ocean between 1972 and 1981 Tritium spreading into ocean Is tracing pathway of deep water formation Can use tritium to estimate deep water formation rates Excess heat in equatorial regions requires redistribution toward the poles Surface circulation moves ~50% of this excess heat 7

8 Surface currents are driven by winds, Gyres are underneath, and driven by, the bands of Trade Winds and Westerlies Wind driven currents are moving to the right of the prevailing wind because of Coriolis force 8

9 Gravity Ekman transport piles water toward the centre of the gyre Force of gravity on the piled up water creates pressure gradient away from centre of the gyre The balance between these forces is known as geostrophic balance Geostrophic "hill" is closer to the western side of the gyre because of Earth's rotation In the northern hemisphere Currents on western side of the ocean (western boundary currents) are narrow and fast flowing Currents on the eastern side of the basin are slower and more diffuse Western boundary currents are warm as they come from the Equator Eastern boundary currents are cold 9

10 Maximum in surface water salinity shows the gyres excess evaporation over precipitation results in higher surface water salinity Southern Oscillation Atmospheric pressure differential between Tahiti and Darwin, normally low pressure in Darwin, high in Tahiti Normal Low pressure High pressure El Nino El Nino high pressure in Darwin, low in Tahiti Change in pressure differential results in weakening of easterly equatorial winds 10

11 Normal conditions in the Equatorial Pacific Strong easterly winds: Pile up warm water in the western Pacific -- thermocline deep in western Pacific, shallow in eastern Pacific Winds drive equatorial upwelling Satellite image of chlorophyll abundance As thermocline is shallow in eastern Pacific upwelling brings nutrients to surface waters along the equator Nutrients promote phytoplankton growth along equator 11

12 Strong coastal winds: Cause upwelling of nutrient-rich water along West coast of South America Upwelled nutrients in coastal and equatorial regions support phytoplankton growth fisheries Onset of El Nino and the ocean's response Pressure at Darwin rises, pressure at Tahiti drops Easterly winds weaken Warm surface water surges back across central Pacific Thermocline drops in the east, rises in the west 12

13 Satellite image of sea surface temperature Normal Equatorial upwelling decreases because of lower wind strength and sea surface temperature rises as thermocline deepens in eastern Pacific El Nino Coastal upwelling continues but deeper thermocline so only warm, low nutrient water upwells phytoplankton population collapses, fisheries collapse 13

14 Thermocline moves across Pacific to the east as a sub-surface wave, another wave moves to the west. Waves reflect off Asia and return to central Pacific raising thermocline and bringing cold water near the surface again breaks the feedback loop Typical amount of salt in seawater is ~35g/litre Major ions (6) make up 99.8 % of all dissolved chemicals 14

15 Residence time of some ions in the oceans Sodium Chloride Magnesium Sulphate Potassium Calcium 68 million years 100 million years 10 million years 10 million years 7 million years 1 million years Aluminium Iron ~200 years ~ 50 years 15

16 Cyclic sea salts - Wind blown sea-spray forms aerosols containing seawater ions (Chap. 3) - A significant portion of river-transported Cl derives from these aerosols, returning them to the sea - This process removes ions in proportion to their concentration in seawater Ion exchange on river-borne clays entering the ocean - Most of the cation exchange sites on clays are occupied by Ca 2+ - Upon exposure to seawater, Ca 2+ is released and is replaced by other seawater cations, especially Na +, K +, and Mg 2+ - Most deep sea clays have higher Na +, K +, and Mg 2+ concentrations than riverine clays -- causes a net loss of these ions - Reverse Weathering, an old idea recently finding renewed interest Burial of dissolved ions (particularly Na + and Cl - ) in sediment pore waters Deposition of biogenic CaCO 3 controls Ca 2+ removal from seawater Biogenic SO 4 2- removal resulting from sulfate reduction and formation of pyrite (FeS 2 ), a secondary, authigenic mineral Evaporite minerals (salt flats, sabkhas) - Periodically in the geologic past, vast deposits of evaporite minerals formed when seawater evaporated from shallow, enclosed basins - Although limited in areal extent, this process has been important for Na +, Cl + and SO 4 2- removal from seawater during such periods Removal at hydrothermal vents - Particularly important for Mg 2+ (Mg-silicate formation), but also for SO

17 Summary of removal processes for the six major ions: - Most Na + and Cl - are removed in pore water burial, sea spray, and evaporites - Mg 2+ is largely removed in hydrothermal exchange - Ca 2+ and SO 4 2- are removed by deposition in biogenic sediments - K + is removed by exchange with clay minerals and reverse weathering Eventually, over long periods of time, ocean sediments are subducted into the Earth s mantle - Non-volatile components are melted under pressure and converted into primary silicate minerals - Volatiles are released as volcanic gases (H 2 O, CO 2, Cl 2, SO 4, etc.) What really controls residence time? The rate of removal is determined by chemical and biological processes 17