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Biogeochem and Ocean Atmos > Biogeochem - Chapt 6: Sediment Analysis and Diffusion > Flashcards

Biogeochem - Chapt 6: Sediment Analysis and Diffusion Flashcards

1
Q

How do you core intertidal sediment?

A

Plexiglass tubes (perspex) are driven into the sediment either by hand or using a hammer for more resistant sediments.

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2
Q

How are sediment removed from the ground? (cores less than 1m long)

A

they are sealed with a rubber bung on the trop which creates a vacuum.

this only works with smaller tubes. Larger sediment cores (>20cm) need to be dug out as the cohesiveness of the sediment is stronger than the vacuum.

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3
Q

whats the maximum core length that can be taken by using a plexiglass (perspex) tube

A

1m

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4
Q

How does vibrocoring work?

A

Aluminium liners are driven into the sediment after placing a vibrating unit on top.

Vibration transmits to the sediment particles and the weight of the core plus vibrating unit can push the liner downward between the loose sediment particles.

when the liner is in the sediment, the core liner gets cut off very close to the surface which improves the vacuum of the core.

The top-sealed aluminium tubes are remove from the sediment using a tripod and a hoist

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5
Q

what sediment types is vibro-coring suitable for?

A

all sediment types

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6
Q

Main 2 disadvantages of vibro-coring?

A

It requires some effort and equipment (eg energy generator for vibrating unit, hoist, etc.)

The tripod and hoist cant be used on very soft sediments

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7
Q

how are aluminium cores cut open?

A

Using an angle grinder

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8
Q

What is a multi-corer

A

A multi-corer takes multiple cores from deployment

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9
Q

When would you use a multi-corer?

A

When sediment underlying more than 1m of water column needs to be sampled

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10
Q

How does a multi-corer work?

A

They hodl 12 plexiglass tubes up to 1.5m in length.

The frame get lowered into the sediment surface and stops, while the inner part with the cores is pushed by the weight of the metal plates into the sediment.

When the top part of the corer is pulled out, the tubes are sealed at the top and arms swing out and close the cores from below

the metal weights must be determined as this determines how far the core is pushed into the sediment, and what comes back
- eg clay cores require less weight to obtain the sample. Too much weight could cause the liner to collapse on itself .

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11
Q

What type of corers would you use for cores up to 25m long?

A

gravity or piston corers

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12
Q

What feature to piston corers have and what is the benefit of it?

A

They have a piston. This creates a stronger vacuum so increases the change of core recovery

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13
Q

How deep can drilling ships drill?

A

over 5km deeep

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14
Q

What is a disadvantage of drilling cores and how is this overcome?

A

The sediment could be contaminated with drill fluids

This can be tested by adding chemicals and particulate as tracers to the drilling method

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15
Q

How does a Rosette with Niskin Bottles Work?

A

It collects water samples at specific depths. Each bottle can be remotely triggered to close.

It measures water depth via hydrostatic pressure as currents and potential drift can impact it

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16
Q

recovery time of a rosette with niskin bottles

A

can be lowered to 3000m depth

recovering can take 5-6hrs

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17
Q

How is porewater sampled?

A

It can be obtained by squeezing or centrifugation

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18
Q

How are anoxic sediments processed (sediment contains redox-sensitive species) ?

A

They are processed in an anoxic atmosphere under N2 to avoid re-oxidation.

Slicing and squeezing can be done in an anaerobic cabinet.

Centrifugation requires gas tight centrifuge bottles to be used

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19
Q

How does a Peeper (dialysis probe) work and when are they used?

A

An alternative to squeezing and centrifugation.

Consist of a number of sealed cells filled with distilled water. This allows an exchange of dissolved porewater and is constituents

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20
Q

Disadvantage of a peeper

A

It can take a few days to reach equilibrium for volumes less than 10ml.

For volumes larger, weeks may be needed

Peepers are usually applied by hand and so are only available or shallow water environments (divers use them).

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21
Q

What is a gel film probe?

A

It is an improvement of peepers.

They don’t require a long equilibration time and smaller samples can be taken at more precise locations

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22
Q

What is a benthic flux chamber?

A

It was the only way to determine the uptake or release of solutes before high resolution methods

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23
Q

How does a benthic flux chamber work?

A

The chamber is an enclosed space above the sediment, separating the sediment from the surrounding water

Over time, differences in sediment concentration create changes in the concentration inside the enclosed spaces

By isolating the water above the sediment and measuring the changes in concentration over a known area, scientists can estimate the movement (flux) of substances into and out of the sediment

The measured values can be adjusted to account for any production and consumption of the solute of interest in the bottom waters above the sediment-water interface

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24
Q

Disadvantage of benthic flux chamber

A

gives total amount of uptake or release, it doesn’t tell you where in the compound (for example) oxygen is consumed

they give no information about the shape or profile or how deep the compound may be present

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25
Q

What does an oxygen needle electrode determine?

A

It determines the oxygen gradient in sediment

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26
Q

What is the principle behind oxygen electrodes, and what do they measure?

A

Oxygen electrodes are amperometric
electrodes measuring an electric current
caused by the chemical reduction of
oxygen. The current is proportional to
the oxygen consumption.

other electrodes called measure a potential between an electrode and a reference electrode (potentiometric electrodes). Most commonly used are sulphide and pH electrodes

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27
Q

Advantage of oxygen needle electrodes

A

They can measure gradients at very high spatial resolution

they do not require destructive sampling like porewater analysis.

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28
Q

Disadvantage of oxygen needle electrodes

A

most electrodes are fragile and break easily

electrodes only allow one-dimensional profiles to be obtained

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29
Q

Give an example of what autonomous motor-driven electrodes in deep-sea sediment landers can identify

A

the diffusive boundary layer in the sediment profile

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30
Q

What compounds are electrodes limited to?

A

oxygen, pH, sulphide and a few others

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31
Q

how does an optode (optrode) work/what does it measure?

A

They are based on the presence of a dye which fluoresces upon interaction with a target molecule

they have been designed to detecht molecular oxygen, CO2, pH, chlorophyll fluroesence, and ammonium

32
Q

what do both microelectrodes and microoptodes allow for?

A

measuring gradients at a spatial resolution which can’t be achieve by pore water extraction and analysis

33
Q

What colour are oxidised sediments?

A

usually brown in sandy sediments

usually dark brown in muddy sediments

34
Q

What molecule has been reduced to make the sediment black?

A

FeS (ferrous sulphide)

Sulphide, after sulphate reduction, precipitates with reduced ferrous iron, hence the black colour

35
Q

What does ‘reduced sediment’ mean?

A

A sediment that has low levels of oxygen

36
Q

Does the redox pair O2/H2O have a positive or negative redox potential?

A

Positive (+0.82V)

This means that oxygen has the highest affinity for electrons

37
Q

Does the redox pair SO42-/H2S have a positive or negative redox potential?

A

Negative (-0.218)

They have an oxidised form with a relatively low affinity for electrons. Which means the reduced part (H2S) will easily be oxidised.

38
Q

In an oxidised environments, which compounds do you usually find?

A

O2, NO3-, MnO2, FeOOH

39
Q

What compounds do you usually find in reducing environments?

A

H2S, Fe2+, H2

40
Q

What factors can cause the mixing of water?

A
  • wind
  • inflowing rivers
  • ocean currents
  • convection by temperature differences (less of an effect than mechanical forces)
41
Q

What develops when mixing stops?

A

Concentration gradients develop

42
Q

Flux definition

A

something that constantly changes

43
Q

What external factors can cause the mixing of porewater in sediments?

A
  • tidal pumping
  • bioirrigation
  • bioturbation
44
Q

What is the driving force behind diffusion?

A

Brownian motion

45
Q

What is brownian motion?

A

the random motion of molecules, atoms or other microscopic particles.

46
Q

Why is there no Brownian motion at absolute zero (0K)

A

because the velocity of particles is correlated to temperature

47
Q

What does the diffusive flow depend on?

A

A concentration gradient which always flows from higher to lower concentrations

48
Q

When does diffusion stop?

A

When equilibrium is reached

49
Q

What happens if there is no net flux?

A

There is still the movement of particles, but the two fluxes cancel each other out

50
Q

If the concentration difference reduced by 50%, what would happen to the net flow?

A

It would also be reduced by 50%

This is because the gradient and flux correlate.

51
Q

State Ficks first law of diffusion?

A

substances will diffuse from areas of high concentration to lower concentration

52
Q

What does the diffusion coefficient (in ficks equation) depend on?

A

The diffusion coefficient depends on the molecule size and temperature

it is generally larger for small molecules and increases with temperature

53
Q

What is the diffusive flux dependent on?

A

sediment porosity
- (water content and grain size distribution)

54
Q

Is the diffusive flux high or low in mud/clay sediments?

A

low

55
Q

is the diffusive flux high or low in sandy sediments

A

high

56
Q

what is porosity

A

a measure of the void spaces in a material

ie how much water the sediment can hold

57
Q

What is ficks second law of diffusion?

A

the rate of accumulation (or depletion) of concentration within the volume is proportional to the concentration gradient.

57
Q

what is the porosity of water?

A

1

58
Q

Which direction does diffusion flow?

A

high to low concentrations

59
Q

How does temperature impact diffusion?

A

At higher temperatures, the kinetic energy of molecules is higher meaning they move faster.

This means they require less time to cover the same distance.

Eventhough the diffusion movement is random, stochastically the net movement will be faster

60
Q

Why is diffusion of oxygen faster in the air than in water?

A

Gases are less dense than water. This means that even though the particles are more spread out and have fewer collisions, the molecules travel a greater distance between collisions, meaning they have more energy

Water is more dense than gas, resulting in more frequent collisions. But, the molecules travel over smaller distances (resulting in having less energy), causing diffusion to take longer in water

61
Q

What does it mean by “the diffusion time correlates reciprocally to the diffusion coefficient”

A

The larger the diffusion coefficient, the shorter the diffusion time

62
Q

What impact does increasing the diffusion distance have on diffusion?

A

diffusion will take longer as particles can be redirected by hitting other things

63
Q

In a concentration profile in sediments, what does a peak signify?

A

it indicates the formation of a compound

64
Q

What is the result if the diffusive flux being less than the consumption?

A

the compound will be consumed

65
Q

what is the result if the diffusive flux is higher than the consumption?

A

the concentration of the compound would increase over time

66
Q

what does a linear profile indicate?

A

the compound is neither net produced nor degraded

67
Q

what does a curved profile mean?

A

the gradient changes and the compound is either being produced or consumed

68
Q

why are sulphate and sulphite profiles roughly mirror images of each other?

A

This is because when sulphate reduction occurs, the process responsible for sulphate consumption is sulphide production

69
Q

what is the diffusive boundary layer?

A

it is the layer of water directly above the sediment

70
Q

What happens in the diffusive boundary layer?

A

The layer of water is generally stationary and doesn’t get mixed

Solutes are transported across the layer by diffusion

71
Q

How can the total oxygen uptake of the sediment be determined?

A

All oxygen diffusing into the sediment diffuses across the DBL

This means only the diffusive flux of oxygen across the DBL needs to be determined

72
Q

what is the equation for the diffusion time?

A

Diffusion time is proportional to the distance squared.

73
Q

On average, how long does it take for each molecule to travel 10m?

A

over 1000 years

74
Q

roughly how thick is the DBL

A

usually 0.5mm but depend on the sediment roughness

75
Q
A

Biogeochem and Ocean Atmos (14 decks)

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