W10 - carbon cycling

Question 1

difference between organic and inorganic carbon

Answer 1

Organic
* Derived from living things
Inorganic
* Derived from mineral sources i.e., carbonate rocks

Question 2

Order carbon stores from largest amount of carbon to smallest

Answer 2

Ocean
Fossil fuels
Terrestrial
Atmosphere
Land biota

Question 3

What is the largest terrestrial carbon store?

Answer 3

soil
* The SOC pool is approximately twice the atmospheric C pool and three times the vegetation pool
* In the UK, soil C storage exceeds vegetation storage by over 80x! (Milne and Brown, 1997)
o Soils 9383 MtC
o Vegetation 114 MtC

Question 4

What is particulate carbon?

Answer 4

• Sediment from the land surface (> 0.45 µm)
• Spatial variation in soil carbon content

Question 5

How does POC get into the stream?

Answer 5

• Erosion and transportation
• Freeze-thaw/ desiccation
• Importance of rainfall (intensities)
• Overland flow (sheetwash, rills, gullies) > further detachment

Question 6

What is dissolved carbon?

Answer 6

• By-product of decomposition of organic materials
• Accumulates in pore spaces of soils and sediments
• < 0.45 µm

Question 7

What are the DOC controlling factors?

Answer 7

Factors relating to organic matter availability:
* Vegetation
* Climate
Factors relating to decomposition:
* Oxygen availability
* Temperature
Antecedent Conditions

Question 8

What happens to the fluvial carbon?

Answer 8

Overbank deposition
In stream processing
Lakes and reservoirs

Question 9

What is in stream processing?

Answer 9

Larger C molecules broken down into smaller ones
* Photodegradation – light
* Biodegradation – decomposers (temperature)
* Other processes i.e. Birch effect
Evasion
* Oxygen present > CO2
* Limited oxygen > CH4

Question 10

What do we need to know to quantify POC?

Answer 10

1. amount of sediment in the water (SSC, mg/l)
2. Organic carbon content of the sediment (OC%)
3. Discharge in m3/s

Question 11

How do we measure POC Flux (gC/h)

Answer 11

amount of sediment in water (SSC, mg/l) * organic carbon content of the sediment (OC%) * Discharge (m3/s)

Question 12

How do we find out the amount of sediment in the water (SSC, mg/l)?

Answer 12

Filtration (lab)

Question 13

How do we find out the organic carbon content of the sediment (OC%)?

Answer 13

Loss on ignition (lab)

Question 14

What do we need to know to find out DOC flux?

Answer 14

Amount of dissolved carbon in water (mg/l) and discharge

Question 15

What are the dissolved Carbon: Lab Methods

Answer 15

• DOC largely comprises organic acids which produce water colour
• We can quantify colour through measured absorbance at different wavelengths using a spectrophotometer
• Typically there is a good relation between absorbance at 400 nm and dissolved carbon
• Variation according to the quality of the dissolved carbon…some organic acids are colourless so needs calibration against direct measurement.
• Shimadzu TOC analyser (solids and dissolved phases). Accurate but time consuming and expensive

Question 16

What are microplastics?

Answer 16

• Microplastics – particles <5 mm across
• Abundant in ecosystems and food webs
• Transported to remote environments
• The amount of plastic added to existing waste each year — whether carefully disposed of in sealed landfills or strewn across land and sea — could more than double from 188 million tonnes in 2016 to 380 million tonnes in 2040 (Lau et al. 2020 in Science).

Question 17

Where are microplastics stored?

Answer 17

on the bed – a key habitat and a zone of intense biological activity

Question 18

relationship between microplastics and flooding

Answer 18

Floods flush microplastics from channel beds and wash them downstream – reset floods
We could estimate the microplastic load washed away by the winter floods – down to the ocean?
* Microplastics are found in all parts of the catchment
* Rapid turnover of the channel bed microplastic load
* Some reaches are very heavily contaminated
* Channel beds will clean themselves
* Varied microplastic assemblages
* Microplastic hotspots can form rapidly

Question 19

Microplastics in the arctic:

Answer 19

• Plastic pollution is now pervasive in the Arctic, even in areas with no apparent human activity, such as the deep seafloor.
• Although some pollution is from local sources — distant regions are a substantial source, as plastic is carried from lower latitudes to the Arctic by ocean currents, atmospheric transport and rivers.

Question 20

sources of microplastics

Answer 20

Domestic wastewater, road runoff, trade effluents, agriculture?