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BIOS1004 - Building A Habitable Planet > Lecture 5 - Ocean Circulation and Chemistry > Flashcards

Lecture 5 - Ocean Circulation and Chemistry Flashcards

1
Q

How much of the Earth’s surface and volume of water do the oceans store?

A

70% of the surface.

97% of the volume of all water.

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2
Q

What is the average depth of the oceans?

A

3.7 km.

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3
Q

How does the temperature of the oceans vary with depth?

A

Depending on the location, surface waters can have very different temperatures.

Deep waters have very similar temperatures of 2-3°C.

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4
Q

What is the thermocline?

A

The zone of rapid temperature change between surface water and deep water.

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5
Q

What is the photic zone?

A

The upper layer which light can penetrate through.

Photosynthesis can occur here.

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6
Q

What conditions must be met for a planet to be habitable?

A

It must have captured enough water to produce sizeable oceans.

The water must be on the surface but not lost to space.

The water must be in the liquid phase.

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7
Q

What are the main features of the chemistry of water?

A

It is a polar molecule and can form hydrogen bonds.

It is an excellent solvent.

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8
Q

What properties do hydrogen bonds give water?

A

A higher melting and boiling point.

A greater latent heat of evaporation and fusion.

A greater specific heat capacity.

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9
Q

How does water dissolve a molecule?

A

Six water molecules surround an ion in an octahedral shape.

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10
Q

What is salinity?

A

The measure of how salty water is.

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11
Q

What is salinity defined as?

A

1.81 x concentration of chloride ions.

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12
Q

What is the average salinity of seawater?

A

35 parts per thousand.

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13
Q

How does salinity affect the freezing point of water?

A

Seawater has a lower freezing point (-1.9 °C)

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14
Q

What three factors affect the density of water?

A

Salinity

Temperature

Pressure

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15
Q

What is the maximum density of water?

A

1 g cm⁻³ at a temperature of 3.98 °C.

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16
Q

What is the density of seawater at average salinity?

A

1.0278 g cm⁻³.

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17
Q

How does temperature affect density?

A

Density decreases as temperature increases. This is because the molecules have more energy, move faster and so take up more space.

Below 3.98 °C, hydrogen bonds start to form and density decreases.

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18
Q

How does pressure affect density?

A

Pressure has a very small impact on density as liquids are difficult to compress.

Pressure increases by 1 atm per 10 m and this reduces the average sea depth by 37 m.

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19
Q

How does salinity affect density?

A

Increasing the salinity of the water increases the density.

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20
Q

How does salinity affect the behaviour of water?

A

When the salt content is less than 24.7 ppt, water behaves in the same way as fresh water as it reaches maximum density before freezing.

At 24.7 ppt, the freezing point and temperature of maximum density coincide at -1.332°C.

At greater than 24.7 ppt, the water freezes before maximum density is reached.

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21
Q

What are three surface processes that affect salinity?

A

Evaporation/Rainfall

River inflow

Ice formation/melting.

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22
Q

What is caballing?

A

When waters of identical density but different temperatures and salinities mix, the resulting mixed water will have a greater density and sink.

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23
Q

How does density change with depth?

A

Well mixed surface waters have the lowest density and then there is a rapid increase known as the pycnocline.

Deep water density increases very slightly/remains constant.

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24
Q

How does temperature change with depth?

A

Temperature decreases with depth.

Beneath the thermocline, temperature decreases very slowly/relatively constant.

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25
Q

How does salinity change with depth?

A

Salinity increases with depth.

Beneath the halocline, salinity increases very slowly/relatively constant.

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26
Q

What is the main driver of ocean circulation?

A

Density.

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27
Q

Where is the densest seawater found?

A

Off the coast of Antarctica where the water is cold and brine rejection increases the salinity.

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28
Q

What is the temperature profile in the summertime?

A

In the summertime, there is the smallest mixed layer due to the absence of strong winds and wave action. There is a shallow thermocline.

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29
Q

What is the temperature profile in autumn?

A

Surface waters cool and increased winds increase the depth of the mixed layer.

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30
Q

What is the temperature profile in winter?

A

Winds and storms cause a very large mixed layer and a deep thermocline.

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31
Q

What is the temperature profile in spring?

A

The thermocline reforms due to the surface waters warming and having a lower density.

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32
Q

How are deep ocean currents initiated?

A

Ice formation and brine rejection in the polar regions causes very dense, cold, saline water.

33
Q

How is ocean circulation mapped?

A

Using tritiated water.

34
Q

What are deep water currents dictated by compared to shallow water currents?

A

Deep Water - Topography of the ocean floor.

Shallow Water - Wind direction and Coriolis force.

35
Q

What happens when waters converge?

A

Downwelling occurs.

36
Q

What happens when waters diverge?

A

Upwelling occurs.

37
Q

What are the circulation patterns in the Atlantic Ocean?

A

Water from high northern latitudes moves southwards.

The NADW floats above the Antarctic Deep Water. The ADW cannot cross the mid-Atlantic ridge.

The Mediterranean floats above the rest.

38
Q

What can ocean’s be considered to be in?

A

A steady state as the composition has remained relatively constant for a long time. This is because inputs are balanced by the outputs.

39
Q

What are inputs to the ocean?

A

River water and glacial melt water.

These will contain ions derived from weathering reactions.

Wind-blown dust is another input of dust and volcanic particulates.

40
Q

How much water do rivers and glaciers supply in a year?

A

36,000 km³.

120 mg L⁻¹ of dissolved material is carried.

41
Q

What are the five main ion removal mechanisms that occur?

A

Cation exchange in clay minerals

Sea spray

Formation of ocean sediment

Biological processes

Hydrothermal processes

42
Q

What is cation exchange in clay minerals?

A

Clay particles exchange calcium ions for sodium and potassium ions in their interlayer spaces in a saline environment.

The clay sinks slowly to ocean floor and so removes K and Na.

43
Q

How does sea spray remove ions?

A

Wind-blown sea spray deposits ions on surrounding land.

44
Q

How does the formation of ocean sediment remove ions from seawater?

A

Seawater becomes trapped within sediments as they are buried.

Evaporation of seawater in closed/semi closed basins will precipitate thick layers of dissolved minerals.

45
Q

How do biological processes remove ions?

A

When exoskeletons are formed, ions such as calcium carbonate and silica are used. These sink when the animals die.

Sulphate reducing bacteria can reduce the of sulphur in the sulphate ion forming S²⁻. Sulphide ions react with many metals to form precipitates including FeS.

46
Q

How do hydrothermal processes remove ions?

A

Seawater will be drawn into the rocks and react with them changing its composition.

Magnesium is removed effectively this way as the sink is greater than the source of magnesium.

47
Q

What is the equation for residence time?

A

Residence Time = Total Amount / Flux at which it is removed

48
Q

What factor determines the residence time of an element?

A

The reactivity of the element.

49
Q

There is a link between the residence time and the…

A

Partition Coefficient.

50
Q

What is the partition coefficient?

A

The concentration of an element in seawater divided by the mean concentration of that element in the Earth’s crust.

High partition coefficients indicate highly soluble elements.

51
Q

What is a cycle that affects the concentration of nutrient elements?

A

The biochemical cycle.

52
Q

What are conservative elements/ions?

A

Elements/ions that have a constant concentration throughout the ocean because they have large concentrations and so aren’t affected by other processes and reactions taking place.

53
Q

What are non-conservative elements/ions?

A

Elements/ions that vary in concentration depending on where they are measured.

54
Q

What are bio-limiting elements?

A

Elements that limit the growth of organisms if they are not present in sufficient concentrations.

55
Q

What are bio-unlimiting elements?

A

Elements not used by organisms and controlled only by physical processes such as ocean circulation.

56
Q

What are intermediate elements?

A

Elements that fall in between bio-limiting and bio-unlimiting. They are used by organisms but are usually present at sufficient concentrations to not limit the growth of organisms.

These include Ba²⁺, Sr²⁺ and Ca²⁺.

57
Q

What is the difference between nutrient gradient in the Pacific and the Atlantic?

A

The Pacific has a much greater nutrient gradient.

This is because the water is older and has collected more of the nutrient ‘rain’. NO₃⁻ and PO₄³⁻ concentrations are double and silicic acid concentrations are 10 times greater.

58
Q

What does the relative proportion of nutrients reflect?

A

The relative proportion of the organisms living the oceans. This is known as the Redfield ratio.

59
Q

What is the Redfield ratio?

A

C:N:P - 106:16:1.

This is the ratio of nutrients delivered to deep sea sediments where bacteria regenerate inorganic nutrients.

60
Q

What happens during the Ocean Carbon Cycle?

A

Carbon dioxide in the atmosphere dissolves into the surface of the oceans to form bicarbonate ions.

Bicarbonate ions are used in photosynthesis during the daytime and carbon is converted to organic forms.

Once organisms die, they sink through the water column.

61
Q

Why is only a small proportion of the carbon locked away in sediments?

A

Most of the carbon is recycled in the surface waters.

62
Q

How do the oceans act as both a sink and source of carbon dioxide?

A

Marine phytoplankton take up carbon dioxide (the Biological pump).

Carbon dioxide is more soluble in cold water (the Solubility pump).

63
Q

What factors control carbon dioxide exchange between the atmosphere and the ocean?

A

pH, temperature and biological activity.

64
Q

What happens as part of the Ocean Nitrogen Cycle?

A

As nitrogen gas is insoluble, it can only enter the ocean if it is fixed by blue-green algae.

Nitrate ions and ammonium ions are generated and are used to make proteins. When the organisms die, the nitrate and ammonium ions are regenerated.

65
Q

What happens as part of the ocean silicon cycle?

A

Silicon slowly dissolves out of sediments and rocks to create the silicate anion (SiO₄²⁻).

This can then be used by diatoms and radiolarian to form their exoskeletons.

Burial in sediments is the main removal mechanism.

66
Q

What are nitrate, phosphate and silicate ions used for?

A

Nitrate and Phosphate - Soft parts of organisms

Silicate - Formation of exoskeletons.

67
Q

Describe the oxygen profile.

A

Oxygen concentrations are high at the surface due to exchange with the atmosphere and high levels of photosynthesis.

This initially decreases with depth as photosynthesis decreases and respiring organisms are present.

Oxygen concentrations start to increase again as cold water can hold more oxygen than warm water.

68
Q

Describe the phosphate profile.

A

Almost no phosphate is found at the surface as the organisms have used most of it up.

The concentration then rapidly increases due to its presence in waste materials excreted by organisms and decomposition in the deep oceans.

This very slowly decreases with depth.

69
Q

How does the phosphate concentration change with latitude?

A

Further south there is a higher concentration of phosphate.

70
Q

Describe the nitrate profile.

A

The exact same as the phosphate profile for all the same reasons.

71
Q

How do ion concentrations vary between oceans?

A

The Atlantic Ocean has lower concentrations than the Indian and Pacific Ocean as deep water is formed in the Atlantic and hasn’t had chance to pick up more nutrients.

72
Q

Describe the silica profile?

A

There is very little silica at the surface due to its demand by organisms.

However, concentrations continue to increase with depth as its main source is the sediments and rocks from the ocean floor.

73
Q

What is a trace element?

A

An element with a concentration of less than 1 mg L⁻¹.

74
Q

What is the caesium profile dominated by?

A

Salinity.

75
Q

What is surface enrichment?

A

A process where some trace elements are enriched in surface waters as their main source is wind-blown material.

76
Q

What is photic zone depletion?

A

Trace elements such as Cu²⁺ are used by animals in photosynthesis and so are depleted.

77
Q

Where do elements released from hydrothermal vents accumulate?

A

In the mid-depths as they pump out hot water which rises and cools before spreading out.

78
Q

Where are elements that are being degassed found?

A

Typically at the mid depths.

79
Q

Where are elements that are derived from rocks found?

A

They are enriched in the deep waters which is a process called bottom water enrichment.

BIOS1004 - Building A Habitable Planet (9 decks)

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