ESS 109C Isotope Geochemistry Notes

May 23, 2007

 

Carbon isotope geochemistry

 

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

  2. Other carbon reservoirs:
    1. Limestone (CaCO3) slightly ÒheavyÓ relative to CO2, typically ~0ä on VPDB scale.
    2. Volcanic CO2 emissions (Å bulk Earth?) typically around Ð5ä on VPDB scale.


  1. Historical records of d13C of CO2: What is going on here?
    1. d13C of CO2 oscillates with a 1-year period, suggesting a seasonal intensity photosynthetic production (= net fixation of CO2). Each tick on the x-axis indicates the January mean. Which months have the most intense productivity?
    2. Seasonality is much stronger in the northern hemisphere than in the souther hemisphere.
    3. Long term trend: CO2 is becoming 13C-depleted over time Ð correlated with the increase in CO2 in the atmosphere

                                                     i.     Good evidence that excess CO2 comes from oxidation of organic material.

                                                      ii.     Decline of atmospheric O/N also consistent with burning/decomposition of organic carbon.

                                                        iii.     Volcanic emission, degassing of limestone NOT CONSISTENT with drop in d13C of CO2.

  1. 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.