3/24/2011
Isotopes as tracers of biogeochemical processes
Scott Saleska, 2/11/11
Outline
1. Isotope Definitions and terms
a) Isotopes and isotope ratios.
b) Kinetic fractionation; thermodynamic fractionation
c) Simple illustration with the water cycle
2. CO2 isotopes in photosynthesis
a) Photosynthetic discrimination in C3 plants
b) C3 vs C4 photosynthesis and the distinction in isotopes
c) Measuring isotopic composition of a flux i.i Flux composition is not the same as concentration Fl
ii i
h
i
composition
ii. Keeling plots 1
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Radioactive (not stable)
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Nominal abundances and standards
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Simple Example – Rainfall
(note: evaporation prefers the lighter isotope, condensation the heavier)
In a closed system, isotope fractionation generates a Raleigh fractionation curve Accumulated product
note: based on conservation of mass, the accumulated end product must have exactly the same isotopic content as the initial reactant
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Raleigh fractionation/distillation curve for water vapor, with changing temperature drops to maintain saturation as vapor content of atmosphere drops
Note: direction of curve here is opposite (trending down) from previous slide because condensation prefers heavy isotope
Global d18O
IAEA
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Other examples from Biogeochemistry
CH3COOH
Methanogenesis
13Cacetate (‐28‰)
C = 25 ‐ 35‰ CO2 + CH4
13CCH4(‐50‐60‰)
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Isotope discrimination by photosynthesis
Atmospheric
CO2
stomate
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diffusion
Net biochemical fixation
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Consider two extremes
Atmospheric
CO2
Atmospheric
CO2
Stomates open to atm (ci ≈ ca) :
 = a + (b – a) *1 = b = 27‰
Stomates closed (ci ≈ 0) :
 = a + (b – a) *0 = a = 4.4 ‰
diffusion
Net biochemical fixation
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Photosynthetic Pathway
Variation
• C3 – CO2 fixation by Rubisco; PGA (phosphoglycerate –
g ) is the p
product of initial carboxylation
y
a 3 carbon sugar)
• C4 – CO2 fixation by PEP carboxylase to produce OAA
(oxaloacetate – a 4 carbon acid); C4 then transported
within the leaf, decarboxylation and re-fixation by
Rubisco.
• CAM (crassulacean acid metabolism) – CO2 fixation by
PEP carboxylase to OAA, but results in the production of
malic acid in the vacuole during the day and decarboxylation and fixation at night
Why C4 plants?
Problems with C3 photosynthesis
Increase in photorespiration - in hot dry conditions
- C3 plants conserve water by closing stomates, decreasing
intercellular [CO2]
- Competition between O2 and CO2 for Rubisco binding site
- Photorespiration increases when [O2] / [CO2] inside the leaf
increases
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Why C4 plants? CO2 limitation
-- The Compensation Point
Compensation points
f C3 and
for
d C4 diff
differ
Net Assimilation =
gross photosynthesis - respiration
http://www.steve.gb.com/science/photosynthesis_and_respiration.html
Light-use efficiency
Leaf Photosynthetic ra
ate
At temperature
optima
C4
C3
C3 in the
absence of
photorespiration
Photosynthetic Photon Flux Density (PPFD)
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QUANT
TUM YIELD OF PHOTO
OSYNTHESIS
Light-use efficiency
C3 where
photorespiration is
negligible
C4
C3
TEMPERATURE (10 TO 40oC)
C4 Photosynthesis: a 2-step biochemical process
What are consequences for isotopes?
Stomate
Mesophyll
Bundle Sheath
CO2 concentration 100
mol mol-1

C3
CO2 concentration 2000
mol mol-1

C3
Rubisco
PEP
CO2
HCO3-
CO2
PCR
(Calvin Cycle)
PEP
Carboxylase
CHO
C4
C4
(OOA)
“cost”
“benefit”
TRADE-OFF
PEP carboxylation step
added by C4 pathway
•
2 extra ATP molecules
•
Higher [CO2] at
Rubisco site
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Long-term trends in atmospheric CO2
The rise of
vascular plants
with roots
Ratio of
ancient to
modern
preindustrial
CO2
Berner, 1997
Amt of
carbon in
plant
biomass
Continuous
downward
trend over last
50 Ma
Evolution of
C4 plants
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You are what you eat (isotopically)
Example from Beer
You are what you eat (isotopically)
Example from Beer
Brooks et al (2002)
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You are what you eat (isotopically)
Example from Beer
Brooks et al (2002)
Paleo-Indicators of C4 pllant
prevalence
Apparent synchronicity in rise in C4 at sites
around the world.
Teeth of
Herbivores
Soil carbon
Teeth of
Herbivores
Ehleringer & Monson (1993)
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Size and 13C (vs PDB) of Earth’s Carbon Reservoirs
Rocks (carbonate) = 0 ‰
Rocks (organic)  -25 ‰
Volcanic CO2 = -5 ‰
Atmospheric “13C Suess Effect”
[Francey et al., 1999]
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