Carbon isotopes in the biosphere102907

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Carbon isotopes in the biosphere 10/29/07

• Lecture outline
• the carbon cycle
• and d13C
• 2) C fractionation
• in the terrestrial
• biosphere
• C isotopes in the
• ocean
• C isotopes in the
• atmosphere

Photo of a C3 leaf cross-section
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The Carbon Cycle
green reservoir size (1015g, Gigatons) red
fluxes (Gt/yr) blue C isotopic value
NOTE d13C always reported in PDB
Reservoirs and fluxes from Schlesinger, 1991
d13C from Heimann Maier-Reimer, 1996
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d13C and Photosynthesis
TERRESTRIAL PHOTOSYNTHESIS - theoretical
calculations predict a 4.4 kinetic
fractionation for CO2(g) moving from air through
stomata to site of photosynthesis

• C3 Pathway
• enzyme-mediated
• (RUBISCO)
• -RUBISCO fixes 1 O2
• for every 5 CO2
• Calvin cycle
• 90 of all plants
• 20-30 fractionation

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• C4 Pathway
• desert plants, some
• tropical species
• enzyme-mediated
• (PEP)
• Hatch-Slack cycle
• 10 of all plants
• 13 fractionation
• (beggars cant be
• choosers)

NOTE C4 plants still execute Calvin cycle,
but CO2 grabbing and actual carbon fixation
happening in different cells
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d13C of living organisms
Why are higher trophic organisms
progressively higher in d13C?
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d13C and CO2 in soils
Why are soil CO2 and d13C correlated?
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d13C of atmospheric CO2
What feature do they share and why?
Why do they differ?
Atmospheric biogeochemists use a global network
of flask collections to track CO2 from sources to
sinks ex most emissions are in N.H., but N-S
gradient is small therefore N.H. must be taking
up large amount of emissions
Allison, C.E. et al., TRENDS, DOE, 2003.
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d13C and CO2 for last 200 years ice core
bubbles in Siple Station, Antarctica
d13C
Suess Effect progressive depletion of
CO2 resulting from burning of isotopically light
fossil fuels 1.5 over last century
CO2
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OCEANIC PHOTOSYNTHESIS can utilize either
CO2(g) or HCO3-
0.9 equil.
7-8 equil.

• When thinking about how C isotopes move through
  the ocean, we must
• differentiate between
• inorganic C (carbonates) typically -1 to 1
  PDB
• and
• organic C typically -5 to -15 PDB
• However, the ocean, unlike the atmosphere, is NOT
  well-mixed.
• d13C of marine organisms varies because
• CO2(aq) small in warm tropical waters,
  fractionation low
• pH varies, and each inorganic DIC species has
  different a
• temperature low at poles, fractionation increases
• surface-to-deep gradients (upwelling zones have
  lower d13C(sw))

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d13C of Dissolved Inorganic Carbon (DIC) in the
ocean
Phosphate and d13C of DIC in the Pacific
Ocean. After Broecker and Peng, 1982
For info see Kroopnick, 1985
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d13C of DIC vertical and meridional gradients
ATLANTIC
PACIFIC
Kroopnick, 1985
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Central Pacific DIC and d13C of DIC
What determines the DIC of surface seawater?
What determines the d13C of surface DIC?
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Oceanic d13C on glacial-interglacial timescales
Benthic foraminifera record the d13C of the DIC
in which they grow.

• Can take cores from
• different depths
• different locations
• and reconstruct deepwater d13C
• through space and time

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Oceanic d13C on glacial- interglacial timescales
Charles et al., 1996
So South Atlantic d13C was lower during last
glacial NADW reduced! Timing of d13C
shifts look like Greenland ice!
Ninneman et al., 2002
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Long Term Carbon Cycle
Long Term Carbon Cycle
green reservoir size (1018g) red fluxes
(1018g/yr) blue C isotopic value
NOTE pre-anthropogenic values
Figure from William White, Cornell U.
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Catastrophic methane hydrate release captured in
deep-sea cores?
Jim Kennet, Clathrate Gun Hypothesis, 2002
-methane most depleted d13C (-60 for biogenic,
-40 for thermogenic) -frozen on every
continental margin, but delicate stability
depends on T and P -methane is a greenhouse gas,
can warm surface ocean, leading to more CH4
release, etc -can have medium-sized methane
hydrate release from tectonic slope failure
best example at Paleocene- Eocene Boundary (55Ma)
Norris and Rohl, 1999
Norris and Rohl, 1999
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-present-day lysocline 3700-4500m -shoaling
of lysocline to lt1500m required 4500GtC entire
fossil fuel reservoir!
Zachos et al., 2005