Biogeochem - Chapt 6: Sediment Analysis and Diffusion Flashcards
How do you core intertidal sediment?
Plexiglass tubes (perspex) are driven into the sediment either by hand or using a hammer for more resistant sediments.
How are sediment removed from the ground? (cores less than 1m long)
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.
whats the maximum core length that can be taken by using a plexiglass (perspex) tube
1m
How does vibrocoring work?
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
what sediment types is vibro-coring suitable for?
all sediment types
Main 2 disadvantages of vibro-coring?
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
how are aluminium cores cut open?
Using an angle grinder
What is a multi-corer
A multi-corer takes multiple cores from deployment
When would you use a multi-corer?
When sediment underlying more than 1m of water column needs to be sampled
How does a multi-corer work?
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 .
What type of corers would you use for cores up to 25m long?
gravity or piston corers
What feature to piston corers have and what is the benefit of it?
They have a piston. This creates a stronger vacuum so increases the change of core recovery
How deep can drilling ships drill?
over 5km deeep
What is a disadvantage of drilling cores and how is this overcome?
The sediment could be contaminated with drill fluids
This can be tested by adding chemicals and particulate as tracers to the drilling method
How does a Rosette with Niskin Bottles Work?
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
recovery time of a rosette with niskin bottles
can be lowered to 3000m depth
recovering can take 5-6hrs
How is porewater sampled?
It can be obtained by squeezing or centrifugation
How are anoxic sediments processed (sediment contains redox-sensitive species) ?
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
How does a Peeper (dialysis probe) work and when are they used?
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
Disadvantage of a peeper
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).
What is a gel film probe?
It is an improvement of peepers.
They don’t require a long equilibration time and smaller samples can be taken at more precise locations
What is a benthic flux chamber?
It was the only way to determine the uptake or release of solutes before high resolution methods
How does a benthic flux chamber work?
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
Disadvantage of benthic flux chamber
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
What does an oxygen needle electrode determine?
It determines the oxygen gradient in sediment
What is the principle behind oxygen electrodes, and what do they measure?
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
Advantage of oxygen needle electrodes
They can measure gradients at very high spatial resolution
they do not require destructive sampling like porewater analysis.
Disadvantage of oxygen needle electrodes
most electrodes are fragile and break easily
electrodes only allow one-dimensional profiles to be obtained
Give an example of what autonomous motor-driven electrodes in deep-sea sediment landers can identify
the diffusive boundary layer in the sediment profile
What compounds are electrodes limited to?
oxygen, pH, sulphide and a few others
how does an optode (optrode) work/what does it measure?
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
what do both microelectrodes and microoptodes allow for?
measuring gradients at a spatial resolution which can’t be achieve by pore water extraction and analysis
What colour are oxidised sediments?
usually brown in sandy sediments
usually dark brown in muddy sediments
What molecule has been reduced to make the sediment black?
FeS (ferrous sulphide)
Sulphide, after sulphate reduction, precipitates with reduced ferrous iron, hence the black colour
What does ‘reduced sediment’ mean?
A sediment that has low levels of oxygen
Does the redox pair O2/H2O have a positive or negative redox potential?
Positive (+0.82V)
This means that oxygen has the highest affinity for electrons
Does the redox pair SO42-/H2S have a positive or negative redox potential?
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.
In an oxidised environments, which compounds do you usually find?
O2, NO3-, MnO2, FeOOH
What compounds do you usually find in reducing environments?
H2S, Fe2+, H2
What factors can cause the mixing of water?
- wind
- inflowing rivers
- ocean currents
- convection by temperature differences (less of an effect than mechanical forces)
What develops when mixing stops?
Concentration gradients develop
Flux definition
something that constantly changes
What external factors can cause the mixing of porewater in sediments?
- tidal pumping
- bioirrigation
- bioturbation
What is the driving force behind diffusion?
Brownian motion
What is brownian motion?
the random motion of molecules, atoms or other microscopic particles.
Why is there no Brownian motion at absolute zero (0K)
because the velocity of particles is correlated to temperature
What does the diffusive flow depend on?
A concentration gradient which always flows from higher to lower concentrations
When does diffusion stop?
When equilibrium is reached
What happens if there is no net flux?
There is still the movement of particles, but the two fluxes cancel each other out
If the concentration difference reduced by 50%, what would happen to the net flow?
It would also be reduced by 50%
This is because the gradient and flux correlate.
State Ficks first law of diffusion?
substances will diffuse from areas of high concentration to lower concentration
What does the diffusion coefficient (in ficks equation) depend on?
The diffusion coefficient depends on the molecule size and temperature
it is generally larger for small molecules and increases with temperature
What is the diffusive flux dependent on?
sediment porosity
- (water content and grain size distribution)
Is the diffusive flux high or low in mud/clay sediments?
low
is the diffusive flux high or low in sandy sediments
high
what is porosity
a measure of the void spaces in a material
ie how much water the sediment can hold
What is ficks second law of diffusion?
the rate of accumulation (or depletion) of concentration within the volume is proportional to the concentration gradient.
what is the porosity of water?
1
Which direction does diffusion flow?
high to low concentrations
How does temperature impact diffusion?
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
Why is diffusion of oxygen faster in the air than in water?
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
What does it mean by “the diffusion time correlates reciprocally to the diffusion coefficient”
The larger the diffusion coefficient, the shorter the diffusion time
What impact does increasing the diffusion distance have on diffusion?
diffusion will take longer as particles can be redirected by hitting other things
In a concentration profile in sediments, what does a peak signify?
it indicates the formation of a compound
What is the result if the diffusive flux being less than the consumption?
the compound will be consumed
what is the result if the diffusive flux is higher than the consumption?
the concentration of the compound would increase over time
what does a linear profile indicate?
the compound is neither net produced nor degraded
what does a curved profile mean?
the gradient changes and the compound is either being produced or consumed
why are sulphate and sulphite profiles roughly mirror images of each other?
This is because when sulphate reduction occurs, the process responsible for sulphate consumption is sulphide production
what is the diffusive boundary layer?
it is the layer of water directly above the sediment
What happens in the diffusive boundary layer?
The layer of water is generally stationary and doesn’t get mixed
Solutes are transported across the layer by diffusion
How can the total oxygen uptake of the sediment be determined?
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
what is the equation for the diffusion time?
Diffusion time is proportional to the distance squared.
On average, how long does it take for each molecule to travel 10m?
over 1000 years
roughly how thick is the DBL
usually 0.5mm but depend on the sediment roughness
