ESS 109C Isotope Geochemistry Notes

May 24, 2007

 

Carbon isotope geochemistry

 

  1. Class notes & homeworks are available online Ð
                            http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/

  2. Geologic records of d13C in CO2: Carbonates
    1. As with oxygen, ubiquitous precipitation of CaCO3 can record ancient carbon cycles.
    2. Modern ocean Ð well mixed, dissolved inorganic carbon in the oceans is in rapid communication with atmospheric CO2

                                                     i.     Higher d13C in planktonic (near-surface) carbonate than benthic carbonate.  Why?

    1. Ancient oceans Ð d13C variation indicates changes in mass-balance of reduced, oxidized carbon sinks.

                                                     i.     Low - d13C excursions due to respiration of organic carbon, shutdown of productivity?

    1. Carbon isotope composition of carbonates is more durable than oxygen isotope composition of carbonates.

                                                     i.     Water is 89% oxygen, but <1% carbon.

                                                      ii.     Carbonate is 48% oxygen, 12% carbon.

                                                        iii.     A given mass of water will be able to affect the isotopic composition of much more oxygen than carbon in a carbonate.

                                                       iv.     High-quality Paleozoic & Precambrian C-isotope records.

                                                      v.     O-isotope records controversial.


Figure from Veizer et al. (1999) Chem. Geol., v. 161, p. 59-88.


  1. Nitrogen isotope geochemistry (nutrient tracers)
    1. 14N: 99.63%, 15N: 0.37%
    2. Standardized to atmospheric N2
    3. Usually analyzed as N2, prepared by pyrolysis