Biogeochem - Carbon Cycle Part 3 ft Diagenesis and Catagenesis Flashcards
what reaction does the absorption of marine sediments promote?
condensation reaction
What is the result of a condensation reaction? (what is released)
Water usually but sometimes CO2 and NH3 (ammonia) can be released
What are the effects of condensation reactions?
Formation of large macromolecules (geopolymers)
–> the polymers are formed randomly on sediment particles so are called geopolymers
Organic matter loses oxygen and nitrogen resulting in higher H and C content
what is the undefined macromolecular organic matter called?
Kerogen
What does geopolymer mean?
It indicated that biological entities (organisms, enzymes etc) are not involved in the process
molecules in geopolymers are randomly connected to anything that allows condensation to occur
Why can kerogen not be degraded
- it is random and complex so cannot be degraded
- it is absorbed onto the clay particels
- there is no oxygen
What are the properties of Type 1 Kerogen
marine kerogen
- eg something you’d find in a sapropel
most of its weight is carbon
relatively rare
lots of lipid material and long chains of aliphatics (alkanes and waxes)
low content of aromatics (high oil formation potential)
major contribution of algal (algaenan) and bacterial organic matter
How can the types of kerogen generally be distinguished?
can be distinguished by their elemental composition and the functional groups present
What is an aliphatic?
an organic compound in which carbon atoms form open chains (eg alkanes)
Characteristics of Algaenans (type 1 kerogen)
- high amounts of saturated and unsaturated aliphatic residues
- relatively low oxygen content
- relatively high hydrogen content
Characteristics of Type 2 kerogen
- more common
- intermediate properties between type 1 and type 3
- can be terrestrial or marine
- if marine, there is a signification contribution of allochthonous (terrestrial) carbon
characteristics of Type 3 kerogen
- more common
- high O:C ratio and low H:C ratio
- significant contribution of aromatics (particularly polyaromatics)
- minor amounts of aliphatics (alkanes)
- originated mostly from higher plants (terrestrial)
- high coal formation potential
what are the 2 types of coals
Humic coals
Sapropelic coals
Characteristics of humic coals
- Most coals of western and central to N Europe are humic
- Formed mainly from plant remains, stratified, bright, have gone through peat stage involving humification (formation of humic material)
- Kerogen III
what are the differences in oxygen and carbon content in Type 1 - Type 3 kerogen?
Type 1 (marine) has a higher carbon but less oxygen content
Type 3 (terrestrial) has a lower carbon but higher oxygen content
Characteristics of sapropelic soils
- Formed from grained organic muds in shallow waters
- Not stratified
- Do not go though peat stage
- Follow diagenetic path of hydrogen-rich kerogens
- Generally also contain allochthonous (terrestrial) material
- Kerogen I
what is humification
the process of biomass turning into some type of degraded organic matter which forms kerogen
difference between diagenesis and catagenesis
Diagenesis
- peatification and early coalification (biochemical stage)
- mostly biological agents
- FROM HUMUS TO PEAT
Catagenesis
- late coalification (geochemical stage)
- does not involve organisms
- elevated temperature and pressure
- FROM PEAT TO COAL
how do temperature and pressure differ with depth to the surface level in water column
In the water column, 10m depth = increase in 1 bar pressure (ie reading of 2 bar)
In sediments, temperature increases with depth.
- at 1000m depth, temp is 30º CENTRIGRADE!!
What does the van Krevelen diagram show?
it plots organic compounds based on their O:C ratio and H:C ratio
How does lignin develop over time? (ie from kerogen into coal?)
- forms kerogen
- develops into huminite
- develops into vitrinite (peat)
- develops into inertinite (coal)
- eventually forms anthracite and graphite which consist entirely of aromatic rings
What compound is only composed of carbon?
Graphite
What is the average temperature gradient in sediments?
30º Centigrade per km
What effect does the increase in temperature with depth have on kerogen?
A higher temperatures, the atoms have more energy to move around and can lead to the breakage of hydrocarbon chains (usually alkanes) from the kerogen (cracking)
When the temperature increases, what breaks off from the kerogen first?
At ‘lower’ temperatures, long hydrocarbons break off first
With increasing temperature and thus the kinetic energy, the chain length of the released hydrocarbon molecules decreases.
What temperature starts the process of ‘cracking’ kerogen?
at temperatures above 50-60ºC.
How does the chemical characteristics / molecular composition of kerogen change when more molecules break off?
The release of alkanes reduces the hydrogen content, causing the remaining kerogen residue to mainly consist of aromatic rings
How does oil form from kerogen cracking?
When the volatile hydrocarbons are released (alkanes) from kerogen, they slowly move upwards through the sediment.
When then hit an impermeable layer (eg consolidated carbonate), they can accumulate and form a carbon reservoir.
What is the carbon atom range of diesel
16-18 carbon atoms per molecule
what is the carbon atom range of kerosene
10-16 carbon atoms per molecule
What do the residues typically form?
Oil
What do the aromatics typically form?
Coal
What is the biggest global carbon reservoir?
Trapped organic carbon
- ie natural gas, coal, petroleum, bitumen, kerogen
