ESS 109C Isotope Geochemistry Notes

May 21, 2007

Carbon isotope geochemistry

1. Class notes & homeworks are available online –
   http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/
   
   
2. Carbon isotopes
1. ¹²C: 98.89%, ¹³C: 1.11%
2. Standardized indirectly to PDB (Peedee Belemnite) carbonate
3. Usually measured as CO₂, produced from McCrea reaction of carbonate (like ¹⁸O/¹⁶O) or oxidation of organic matter.
4. McCrea reaction quantitatively releases carbon, so d¹³C can sometimes be measured with greater precision than d¹⁸O in carbonates.
3. Fractionation during photosynthesis
1. Basic reaction: CO₂ + H₂O à (CH₂O)_n + O₂
2. Organic carbon is almost always ¹³C-depleted relative to CO₂.

i. Typical atmosphere CO₂, d¹³C_VPDB ≈ –8‰

ii. Typical organic matter, d¹³C_VPDB ≈ –25‰, but variable.

3. Fractionation mechanism is a combination of kinetic and quasi-equilibrium effects:

i. Diffusion of CO₂ to and through cell membranes (~pinhole diffusion). Some dependence on water stress / drought tolerance for terrestrial plants (stomatal constriction, root-leaf transport of water).

ii. Preferential reactivity of ¹²CO₂ on fixing enzymes.

iii. Subsequent synthesis of complex organic compounds

4. Net fractionation depends on photosynthetic pathway.

i. C3 plants (Calvin cycle) – most depleted. Most trees, bushes, some grasses). Largely passive CO₂ assimilation into cell

ii. C4 plants – least depleted. Tropical/warm-season grasses. CO2-pump based on pyruvate à malate conversion (uses ATP/NADPH energy). Adapted to CO₂ scarcity/water stress (cartoon from Wikipedia).