ESCI 555: Carbon Cycling and Carbon Sequestration

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ESCI 555 Carbon Cycling and Carbon Sequestration
Carrie Masiello masiello_at_rice.edu Andreas
Lüttge aluttge_at_rice.edu André Droxler andre_at_rice
.edu Jerry Dickens jerry_at_rice.edu
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last 500,000 y CO2
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500,000 plus now
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Recent Atmospheric CO2 trends
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CO2 changes are forcing climate
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How do the Earths organic carbon pools interact
with the atmosphere?
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Simple C cycle
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Where does the CO2 we emit go?

• per year in the 1980s
• we released 5.4 0.3 Gt
• atmospheric increase 3.3 0.1 Gt
• ocean flux -1.9 0.6 Gt
• land-atmosphere flux -0.2 0.7 Gt
• land use change 1.7 Gt
• terrestrial sink -1.9 Gt
• 1990s
• emiited more CO2
• less ocean uptake, more net biospheric uptake

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Where does the CO2 we emit go? To understand
this, we need to know what controls the movement
of carbon between the Earths reservoirs.
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Outline

• types of organic carbon
• soil OC cycling
• terrestrial biospheric OC cycling
• marine OC cycling
• intro to 13C
• summary

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What kinds of organic carbon are out there?

• carbohydrates
• glucose, cellulose very labile ( reactive)
• microbial sugars can act as glue
• lignins
• plant structural compounds
• only decomposed by specific fungi
• lipids
• fatty acids, sterols
• proteins
• rich in N
• other black carbon, pigments

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soil organic carbon

• Theoretical pools
• fast cycling (0-10 y) organic carbon, not
  associated with any minerals
• protected OC
• physically protected via ped formation
• lightly protected via charge interactions
• strongly mineral bound

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Physically protected soil carbon
ped unit of soil mm -gt cm in size held
together by sugars (fungal exudates) interior
carbon somewhat protected from decomposition
This kind of carbon decomposes rapidly when its
host soil is plowed!
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Soil mineral-carbon interactions

• Most soil C exists bound to minerals
• types of binding
• wedged within mineral lattices
• bound covalently
• bound ionically

strength of bond and increasing residence time
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Terrestrial Biomass

• Exchanges 60 Gt C annually with the atmosphere
• Land use change
• deforestation South America, Asia, Central
  America
• forest regrowth Europe, North America
• conversions between pasture, agriculture,
  unmanaged ecosystems
• 60 Gt exchange leaves an imprint on the atmosphere

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seasonal cycle, MLO
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CO2 rug
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Ocean Organic Carbon loop
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Ocean organic C loop

• Very large annual C exchanges with the atmosphere
• Essentially closed microbial loop
• River role is current wild card

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Ocean Dissolved Organic Carbon

• average 6,000 year turnover time
• chemically poorly characterized
• sources in dispute (marine? terrestrial?)
• very low lignin contents
• sugar content high (suggests marine microbial
  source)

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Introduction to Carbon Isotopes

• 12C 98.89
• 13C 1.11
• 14C 1x10-10
• notation d13C

x 1000
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Incomplete chemical reactions fractionate carbon
isotopes

• 12C bonds easier to break than 13C bonds
• 13C signatures of reservoirs
• atmosphere -8
• surface ocean -2 to 1
• marine plants -19
• terrestrial plants
• C3 oldest photosynthetic pathway -28
• C4 newer pathway -13
• CAM -11 to -27

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Plants fractionate carbon isotopes

• C3 plants majority of terrestrial
  photosynthesis
• trees, some grasses (wheat, rice)
• C4 plants about 18
• corn, some grasses
• CAM cactuses

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Plant fractionation marks the atmosphere

• 10 of the atmosphere goes through the
  terrestrial biosphere annually (60/750 Gt)
• Case 1 net biospheric uptake of CO2
• atmosphere becomes 13C heavier regionally
• Case 2 net biospheric release of CO2
• atmosphere becomes 13C lighter regionally

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photosynthesis

• respiration

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13C rug
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Where does the CO2 we emit go?

• per year in the 1980s
• we released 5.4 0.3 Gt
• atmospheric increase 3.3 0.1 Gt
• ocean flux -1.9 0.6 Gt
• land-atmosphere flux -0.2 0.7 Gt
• land use change 1.7 Gt
• terrestrial sink -1.9 Gt
• 1990s
• emiited more CO2
• less ocean uptake, more net biospheric uptake