Introduction to Carbon Cycle

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Introduction to 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 Mainly focused on the CO2
cycle. -photosynthesis, respiration, carbonate
formation/dissolution, air-sea CO2 exchange
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Earths Carbon Cycle
Pre-anthropogenic C flux in black Anthropogenic C
flux in red (1980s)
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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Equilibrium Fractionation effects for DIC species
(CO2aq, HCO3- and CO3)
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d13C of Fossil Fuels Consumed Globally
Natural gas 44 Oil 27 Coal 24
Why has the d13C of fossil fuels been decreasing?
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d13C of Atmospheric CO2
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d13C vs CO2 Concentration in Air
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Atmospheric CO2 Budget (1990s)
(Gtons or Pg C yr-1) Fossil Fuel
Combustion 6.30.4   Atmospheric CO2
Accumulation 3.20.1   Missing CO2
3.10.4 Net Ocean Uptake 2.00.5 Net
Terrestrial Biota Uptake (by difference) 1.10.7
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Decadal Change in the DIC and d13C in the surface
waters of the subtropical N. Pacific
C.D. Keeling et al, in GLOBAL BIOGEOCHEMICAL
CYCLES, VOL. 18, GB4006, 2004.
10
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d13C of Terrestrial Plants
C3 -273 C4 -132
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Seasonal Atmospheric CO2 and d13C
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d13C of Seasonal Atmospheric CO2 Changes
d13Closs -24.80.6 (Pt. Barrow)
-20.30.9 (M. Loa) -17.50.6
(C. Grim)
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Photosynthetic Pathways for C3 and C4 Plants
C3 Plants
C4 Plants
(PEP Phosphenol Pyruvate)
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Photosynthetic Pathway for C3 Plants
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Photosynthetic Pathway for C4 Plants
the bicarbonate ion (HCO3) rather than CO2 reacts
with PEP
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Fractionation Factors during Photosynthesis
a Klight/Kheavy
Sensitivity of anet to pCO2int
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Two Step Photosynthesis Model in a Leaf
Step 1
Step 2
CO2(gas)
CO2(aq)
CO2ext
CO2int
C6H12O6
Air Outside
Inside Stomata
Within Leaf
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Sensitivity of d13C of C4 plants to
pCO2int/pCO2ext
d13Cplantd13CCO2ext a (b4b3f
a)pCO2int/pCO2ext
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Altitude Effects on d13C of plants
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Within Tree d13C Variations
A A
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Recycling of CO2 in the Forest
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Plant d13C Fractionation Effect(predicted by
ecosystem model)
e ()
(Scholze et al., Geo Res Lett, 2003)
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Variations in d13C of Plankton
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d13C of Organic Compounds in Plants
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Plankton utilization of HCO3 rather than CO2
J. Berry in Stable Isotopes in Ecological
Research (1989)
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Plankton d13C dependence on CO2 Supply
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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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Plant Respiration Pathways and KIEs
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d13C of CO2 Released at Night in Forest
d13C of CO2 released (intercept) -27.60.6
d13C of Amazon trees -27.62 d13C of Amazon
leaves -30.12 o/oo
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Diurnal Cycle of d13C of DIC in a Coral Reef
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d13C in the Carbonate Rock Record
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Correlation between d13C-DIC and Phosphate
Photosynthesis in surface layer and respiration
below 200m primarily control PO4 and d13C of DIC.
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d13C-Carbonate in Santa Barbara Basin Sediments
Spikes of low d13C in carbonate sediments
interpreted as methane hydrate release A 3
decrease implies an addition of CO2 to increase
DIC pool by 5, which equals 1800 Pg C
(estimate of current reserves is 500-2500 Pg) How
does CH4 released wind up in DIC pool?
(Kennett et al, 2000)
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Global Methane Budget
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Methane Isotope Mixing Diagram
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Rivers in the Amazon Basin
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d13C and Chemical Composition of Organic Matter
in Rivers in the Amazon Basin
? represents the lignin/carbon for organic
material. Lignins are produced only by vascular
plants. Coarse and fine suspended OM were
analyzed. The inset indicates same
characteristics for possible source material
(leaves, wood, grasses)
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Downriver and Discharge Trends in d13C of DIC and
OM
CPOC is coarse OM and FPOC is fine OM suspended
in river
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Container Ship Cruise Track and pCO2 Trend
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d13C and Air-Sea d13C Disequilibrium Trends
d13C disequilibrium d13Cequil
d13Cmeasd Observed d13C is more enriched than
expected at equilibrium
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Calculated Rates of NCP, Supply and Air-Sea CO2
flux
The d13C disequilibrium based estimates of NCP
and DIC supply indicate that the higher CO2 gas
evasion rate at the equator is a result of NCP
not keeping up with DIC supply. In contrast, in
the subtropics the NCP rate is close to the DIC
supply rate which yields a small air-sea CO2
flux.