The Physical Properties of Sea Water OCEA 101

Transcription

1 The Physical Properties of Sea Water OCEA 101

2 Why should you care? - water provides the primary environment for marine organisms - properties of water significantly influence the evolution of marine organisms Copepod Synechococcus

3 The Water Molecule Water: H 2 O Hydrogen: 1 proton, 1 electron 1 st energy level can hold up to 2 electrons. 2 nd energy level can hold up to 8 electrons. Oxygen: 8 proton, 8 electrons

4 3D electron orbital wave functions

5 H atoms share their single electron with O atom to fill the 2 nd energy level. O atoms share electrons with H atoms to fill the 1 st energy level.

6 105 degrees On the whole, an H 2 O molecule is electrically neutral. The electrons spend more time around the O atom than aorund the H atom. As a result, opposite ends are oppositely charged. Polar molecule.

7 Water: Universal Solvent Covalent Bonding Polarity Excellent Solvent!

8 Oppositely charged ends of neighbouring water molecules attract forming hydrogen bonds. Hydrogen bonds are much weaker than covalent bonds. Hydrogen bonds are constantly broken and reformed mobile H 2 0 molecules in liquid phase. Leads to extraordinary properties!

9 Temperature and Heat Atoms and molecules in solids, liquids and gases are constantly in motion. Kinetic energy (KE) is energy by virtue of motion = 1/2mv 2 KE can take the form of translational KE: Vibrational KE: Rotational KE:

10 Temperature and Heat Temperature = average KE of atoms and molecules. Kelvin temperature scale: 0C=273.2K 100C=373.2K Absolute zero: 0K i.e. atoms and molecules at rest. Heat is a measure of the total KE of a substance.

11 Changes of State Latent heat is the energy required to break the hydrogen bonds

12 Specific Heat Capacity Specific heat capacity = amount of heat required to raise unit mass of substance by 1K. Variations in specific heat from place to place strongly influence the earth surface temperature.

13 Cohesion and Surface Tension Hydrogen bonding leads to structure and cohesion of molecules. Cohesion leads to surface tension which is a measure of the ability to stretch or penetrate a liquid surface. Hydrogen bonding at the water-air interface leads to ordering and a weak elastic membrane.

14 Addition of salt increases surface tension. Decreasing temp increases surface tension.

15 Viscosity Viscosity = resistance to fluid motion Water has a low viscosity which depends on temperature and salt content. Viscosity decreases with increasing temp. - tropical oceans less viscous than high latitude oceans - this influences the shape of microscopic marine organisms that populate regions of the oceans with different ocean temperatures. Viscosity increases slightly with salt content

16 Density Density = mass per unit volume Density of pure water = 1g/cm 3 at 3.98C Density increases with: - decreasing temperature - increasing salt concentration - increasing pressure (depth) Water is almost incompresible.

17 Temperature (T) dependence of density (ρ) ρ vs T for pure water

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19 Salinity (S) Salinity (S) =mass of salt per kilogram of sea water (g/kg, ppt, psu). Salts in water increase density. If S < 24.7 g/kg, ρ max reached before freezing occurs. If S=24.7 g/kg, ρ max coincides with freezing point. If S > 24.7 g/kg freezing occurs before ρ max is reached. S ~35 g/kg for ocean.

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21 Notice how different combinations of T and S yield water of the same density. T and S are useful tags for defining water masses of specific origin.

22 T-S Plots Antarctic Bottom Water (AABW) N Atlantic Deep Water (NADW) Ant Intermediate Water (AAIW) Surface Water

23 Dissolving Ability of Water Water is an excellent solvent due to its polar nature. More substances (solids and gases) dissolve in water than in any other common liquid. Polar nature aids on dissolution of salts: NaCl Na + + Cl - Ions

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25 Hydration Spheres

26 Transmission of Energy Water transmits energy in the form of heat, light and sound. Heat transfer by: - conduction - convection - radiation

27 Conduction heat transfer by molecular collision of KE transfer. Convection heat transfer by mass movement of fluid Radiation heat transfer by electromagnetic radiation.

28 Light Light is a form of electromagnetic radiation. Visible light: 390nm-760nm

29 Light Attenuation As light passes through the ocean it is scattered and absorbed by molecules, ions, suspended particles and microorganisms. Phytoplankton absorb visible light and use it for photosynthesis. All of these processes lead to attenuation of light with depth.

30 Light Attenuation Beer s Law: I z =I 0 e -kz I z =light intensity at depth z I 0 =light intensity at surface k=attenuation coefficient k depends on absorption, scattering and presence of photosynthesizing organisms; k varies with water clarity. k varies with wavelength.

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33 Irradiance Depth (m ) ~ e -Kz ~ 1%

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35 Colour Perception Variations in k lead to colour perception: - objects just below surface appear natural - objects at depth illuminated by blue light and appear dark. - blue-green colour of ocean due to scattering of blue/green light which is least attenuated. Marine organisms have evolved to take advantage of varying light conditions (eg. colour, pigment, transparency, eye size, reflective scales, luminescence).

36 A remarkable fact There is a remarkable degree of correspondence between: - the spectra of sunlight - the light transmission of water - the absorption band of various algal (phytoplankton) pigments

37 The remarkable triple complimentarity Ocean absorbs least in the visible. 460nm peak leads to a peak in primary production at depth. Sun emits most radn in visible

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39 Sound Sound travels as pressure waves. Recall that for pressure waves, sound speed increase with density of medium sound travels faster in ocean (~1500m/s) than atmosphere (~330m/s). Sound speed c=(m/ρ) 1/2 M=axial modulus (a measure of compression) ρ=density M and ρ depend on T, S and p.

40 Sound ρ as T S p M as T S p c as T S p dρ dm dρ dm dρ dm < ; < ; < dt dt ds ds dp dp

41 Sound waves are refracted by changes in sound speed.

42 Sound bends towards regions of lower speed This leads to shadow zones Sound speed minimum ~1000m due to vertical variations in T, S and p. This is the SOFAR (sound fixing and ranging) channel. Sound is important for communication and feeding in many marine mammals (whales, dolphins, porpoises) and sharks.

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