Chemical and Marine Sediment Zonation Flashcards
Describe chemical zonation
It is caused by microbial element cycling.
The availability of PEDs and TEAs determines which microbes are present, and their positioning depends on the redox potential.
What are the characteristics of microbial mats?
type of zoned system
- Oldest ecosystems on earth
- Complex, self sufficient
- mm’s thick
- Grow in extreme environments so limit predators which would use for nutrition
How do microbial mats form?
1) Solid particles present in water (allow for cell attachment)
2) Biofilms develop (single species)
3) Biofilm thickens and new species introduced
4) Vertically stratified species (depends on microbial needs).
- EPS holds mats together
- EPS pools resources and buffers environmental changes
How do photosynthetic microbial mats develop in terms of microbial ecology?
1) Cyanobacteria, diatoms and microalgae dominate initially.
- These are CO2 fixers (autotrophs) and N-fixers
2) Fermentative and heterotrophic bacteria colonise
- Use organic C and N already produced
3) Aerobic heterotrophs depleat O2
- Leads to an anoxic environment
- SRB produce H2S
4) Chemolithoautotrophs join
- S-oxidisers and PSB use H2S
- end up between cyanobacteria (top) and SRB (bottom)
- protected from high O2 concentrations
Vertical order of photosynthetic microbial mat
Top to bottom:
Cyanobacteria Aerobic chemohetertrophs Chemolithotrophs Phototrophic bacteria (PSB, GSB etc.) Fermenters Sulphate reducers Methanogens
Examples of microbial mats other than photosynthetic?
Chemolithoautotrophic mat
- SRB common
Cyanobacterial mats
- distinct colour banding and lamination
Anoxygenic photosynthetic mats
- dominated by GSB, PSB, PNB
- sulphide rich environments
Examples of biosedimentary structures
Microbialites - Lithified microbial mats
Stromatolites - preserved lithified microbial mats, evidence of diurnal cycles.
Endoliths - organisms that live in pore spaces of a rock being formed. Fuses grains together.
What components are deposited on the seafloor?
Inorganic detritus
OM/ particulate organic carbon (POC)
Dissolved solutes
What is the fate of particulate organic carbon (POC) as it gets deposited on the seafloor?
- POC is degraded by aerobic chemoheterotrophs as it sinks through the water column
- POC is deposited on sea floor and respired by microbes
- Whats remains is buried into sediment. Provides PED for sub-surface microbes
- Leads to rock formation via diagenesis
- sediment is compacted
- mineral transformation, changes to pore water composition, cementation
Whats the order of process with depth in marine sediment zonation in terms of energetic favourability?
Aerobic respiration Denitrification Mn(IV) reduction Fe(III) reduction Sulphate reduction Methanogenesis
Describe the changes in metabolism with depth in marine sediment zonation
- Oxygen is depleted at the surface due to aerobic chemoheterotrophy (respiration)
- NO3- used as TEA after O2. Then gets depleted by denitrification (nitrate reduction)
- Mn-reduction produces Mn(II) which builds in pore waters. Precipitates reduced form and depletes.
- Iron is same as Mn but less energetically favourable
- Mn and Fe used up, then SO42- reduction, form HS-
- Methanogeneis occurs last, least thermodynamically favourable.
Why does methane diffuse upwards?
Methane is produced at the bottom of marine sediments.
Methane gets oxidised at sulphate-methane transition zone.
Methane is produced anaerobically and consumed anaerobically (doesn’t diffuse up very far).
Methane oxidised with sulphate:
CH4 + SO42- -> HCO3- + HS- + H2O
Definition of biomarkers
Using compounds derived from organisms to trace the occurrence of those organisms in different environments.
Describe the characteristics of lipid biomarkers
- Structural variability preserved
- Easy to determine isotopic composition
- Reflect biological adaptations to the environment
Describe the characteristics of Eukarya
- Polar, hydrophillic head
- Hydrophobic tail
- Alkali chain bonded by esters bond
