Biogeochem - Carbon Cycle Part 3 ft Diagenesis and Catagenesis

Question 1

what reaction does the absorption of marine sediments promote?

Answer 1

condensation reaction

Question 2

What is the result of a condensation reaction? (what is released)

Answer 2

Water usually but sometimes CO2 and NH3 (ammonia) can be released

Question 3

What are the effects of condensation reactions?

Answer 3

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

Question 4

what is the undefined macromolecular organic matter called?

Answer 4

Kerogen

Question 5

What does geopolymer mean?

Answer 5

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

Question 6

Why can kerogen not be degraded

Answer 6

• it is random and complex so cannot be degraded
• it is absorbed onto the clay particels
• there is no oxygen

Question 7

What are the properties of Type 1 Kerogen

Answer 7

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

Question 8

How can the types of kerogen generally be distinguished?

Answer 8

can be distinguished by their elemental composition and the functional groups present

Question 8

What is an aliphatic?

Answer 8

an organic compound in which carbon atoms form open chains (eg alkanes)

Question 9

Characteristics of Algaenans (type 1 kerogen)

Answer 9

• high amounts of saturated and unsaturated aliphatic residues
• relatively low oxygen content
• relatively high hydrogen content

Question 10

Characteristics of Type 2 kerogen

Answer 10

• 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

Question 11

characteristics of Type 3 kerogen

Answer 11

• 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

Question 12

what are the 2 types of coals

Answer 12

Humic coals

Sapropelic coals

Question 13

Characteristics of humic coals

Answer 13

• 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

Question 14

what are the differences in oxygen and carbon content in Type 1 - Type 3 kerogen?

Answer 14

Type 1 (marine) has a higher carbon but less oxygen content

Type 3 (terrestrial) has a lower carbon but higher oxygen content

Question 15

Characteristics of sapropelic soils

Answer 15

• 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

Question 16

what is humification

Answer 16

the process of biomass turning into some type of degraded organic matter which forms kerogen

Question 17

difference between diagenesis and catagenesis

Answer 17

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

Question 18

how do temperature and pressure differ with depth to the surface level in water column

Answer 18

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!!

Question 19

What does the van Krevelen diagram show?

Answer 19

it plots organic compounds based on their O:C ratio and H:C ratio

Question 20

How does lignin develop over time? (ie from kerogen into coal?)

Answer 20

• forms kerogen
• develops into huminite
• develops into vitrinite (peat)
• develops into inertinite (coal)
• eventually forms anthracite and graphite which consist entirely of aromatic rings

Question 21

What compound is only composed of carbon?

Answer 21

Graphite

Question 22

What is the average temperature gradient in sediments?

Answer 22

30º Centigrade per km

Question 23

What effect does the increase in temperature with depth have on kerogen?

Answer 23

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)

Question 24

When the temperature increases, what breaks off from the kerogen first?

Answer 24

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.

Question 25

What temperature starts the process of ‘cracking’ kerogen?

Answer 25

at temperatures above 50-60ºC.

Question 26

How does the chemical characteristics / molecular composition of kerogen change when more molecules break off?

Answer 26

The release of alkanes reduces the hydrogen content, causing the remaining kerogen residue to mainly consist of aromatic rings

Question 27

How does oil form from kerogen cracking?

Answer 27

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.

Question 28

What is the carbon atom range of diesel

Answer 28

16-18 carbon atoms per molecule

Question 29

what is the carbon atom range of kerosene

Answer 29

10-16 carbon atoms per molecule

Question 30

What do the residues typically form?

Answer 30

Oil

Question 31

What do the aromatics typically form?

Answer 31

Coal

Question 32

What is the biggest global carbon reservoir?

Answer 32

Trapped organic carbon
- ie natural gas, coal, petroleum, bitumen, kerogen