Stable Isotope Applications: Photosynthesis and Respiration

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Stable Isotope ApplicationsPhotosynthesis and
Respiration

• 5 November 2008

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Outline

• Carbon isotopes and the global carbon cycle
• Carbon isotope fractionation during
  photosynthesis
• Oxygen isotopes of dissolved oxygen in the ocean
• Oxygen isotope fractionation during respiration

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Goal

• Understand what processes influence the isotopic
  trends of DIC and dissolved O2 in the ocean
• Carbon isotopes are fractionated during
  photosynthesis (essentially no fractionation
  during respiration)
• Oxygen (O2) isotopes are fractionated during
  respiration, (essentially no fractionation during
  photosynthesis)

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Introduction Carbon Cycle
Carbon isotopes have been very heavily
utilized to study various aspects of the earths
carbon cycle Extremely diverse
applications -Global, regional and local
scales -Ocean, atmosphere, terrestrial
environments -Current, historic, geologic time
scales Many applications are associated with
fractionation during photosynthesis
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Earths Carbon Cycle
Rocks 20x106
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Size and d13C (vs PDB) of Earths Carbon
Reservoirs

Rocks (carbonate) 0 Rocks (organic) ? -25
Volcanic CO2 -5
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d13C in the Carbonate Rock Record
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Carbon isotope fractionation during
photosynthesis (marine)

• Two step process both fractionate
• Diffusion across cell membrane
• CO2(aq) external to CO2(aq) internal
• Epsilon -1
• Carboxylation (carbon fixation)
• CO2(aq) internal
• Epsilon -30
• Relative to CO2(aq) internal
• Why is the average composition of marine organic
  matter -22 ?
• Overall fractionation effect is the result of
  differences in the relative rates of these two
  processes

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Photosynthetic Pathways for C3 and C4 Plants
C3 Plants
C4 Plants
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Two Step Photosynthesis Model in a Leaf
Step 1
Step 2
CO2
CO2aq
CO2ext
CO2int
C6H12O6
Inside Leaf
Air
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Variations in d13C of Plankton
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Dependence of d13C on CO2aq
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Plankton d13C dependence on Growth Rate
Growth Rate
E
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L. Washington DIC and d13C
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d13C of Atmospheric CO2
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d13C Change in Surface Ocean
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O2 Cycle
Important Processes -photosynthesis,
respiration -air-water gas exchange, mixing,
circulation Photosynthesis CO2 2H2O light ?
CH2O O2 Respiration H2O O2 CH2O ? CO2
2H2O
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Evaporation / Condensation
CO2 solubility
Carbonate equilibria
Photosynthesis
Respiration
CaCO3 precipitation
Methane formation
Evaporation / Precipitation
O2 solubility
Photosynthesis
Respiration
Solubility
Nitrogen Fixation
NO3- uptake by photosynthesis
Denitrification

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KIE during Respiration
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Depth Trends in d18O-O2and O2sat at ALOHA
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d18O-O2 in Amazon Lakes and Rivers
Lakes squares Amazon R. circles
Tributaries triangles
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Summary Carbon

• Ratio of organic carbon burial to carbonate rock
  deposition determines the isotopic composition of
  different reservoirs on geologic timescales
• C isotopes are fractionated during photosynthesis
• The dominant influence is a kinetic fractionation
  which depletes the organic product (CH2O)
  relative to the source DIC
• THIS EFFECT ISOTOPICALLY ENRICHES (makes the
  delta value more positive) THE SURFACE OCEAN DIC
• Magnitude of fractionation is a function of the
  relative rate of diffusion across the cell
  membrane versus the enzymatic uptake rate of CO2
  in the cell
• d13C of plankton (organic material) ranges from
  -16 to -35 (average is -22 reflecting a
  overall fractionation of -15 relative to CO2
  aq)
• Very little fractionation during respiration (BUT
  REMINERALIZATION OF LIGHT (d13C -22 delta
  value) ORGANIC C TO DIC INFLUENCES THE ISOTOPIC
  COMPOSITION OF DIC BELOW THE SURFACE OCEAN)
• This is why d13C DIC is lower below the photic
  zone and lowest where respiration is highest
• Useful as a tracer of export production

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Summary Oxygen (O2)

• O2 is fractionated by respiration
• O2 isotopic composition evolved from
  photosynthesis reflects source water (d18O of O2
  is -23 relative to air)
• O2 of air is the standard for O2 gas (23
  relative to SMOW)
• THIS IS A POINT OF CONFUSION since SMOW is widely
  used in paleo applications
• In the surface ocean gas exchange dominates (i.e.
  isotopic composition is near d18O 0.7 relative
  to air or slightly below depending on the
  influence of photosynthetic O2)
• RESPIRATION FRACTIONATES O2 (a 0.980 e - 20
  ) isotopically ENRICHING the remaining pool of
  O2 in the heavy isotope 18O
• Respiration signal dominates below the surface
  ocean with increasing isotopic enrichment of the
  remaining O2 pool as O2 is used up and converted
  to water during respiration (i.e. the respirating
  organisms preferentially use 16O, enriching the
  remaining pool of O2 in 18O)
• Essentially no fractionation during
  photosynthesis (BUT photosynthetically evolved O2
  has a d18O of -23 which is depleted relative to
  O2 in equilibrium with the atmosphere 0.7 )
• Can be applied as a tracer of respiration,
  although somewhat problematic

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Exam

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