ESS 109C Isotope Geochemistry Notes

May 2, 2007

 

Cosmogenic Nuclides

 

  1. Class notes & homeworks are available online Ð
                            http://www2.ess.ucla.edu/~schauble/Isotope_geochemistry/
  2. 3rd homework due Wednesday (5-2-07).

  3. 14C dating
    1. Preferential fixation of 12C by photosynthesis (lowers initial 14C/12C rel. to atmosphere)

                                                     i.     Have so far assumed that isotopes have the same chemistry in natural systems, and are not separated from each other.

                                                      ii.     However, isotopes of light elements like C actually do separate slightly in natural processes, typically on the order of ä, or per mil.

                                                        iii.     Photosynthetically fixed carbon typically ~2-4% (20-40ä) depleted in 14C relative to air.

                                                       iv.     Can correct by using appropriate standard, and by monitoring 13C/12C, which should track 14C/12C (with 1/2 the amplitude).

                                                      v.     Will come back to carbon isotope separation (fractionation) later.

    1. Variability of production of 14C (Initial 14C/12C vs. time)

                                                     i.     Variation is small, expressed in units of parts per thousand, or per mil.
D14C(t) = [(14C/12C)t/(14C/12C)std Ð 1]x1000

                                                      ii.     Secular variation in recent past (500 years) has three major causes:

      iii.                                                      Bomb carbon: Atmospheric testing of H-bombs (1952-1963) - injects 14C into the atmosphere. Fig from McGee et al. 2004, GCA 68:2509-2516.

      iv.                                                      Suess effect: Burning of fossil fuels (& manufacture of concrete, ~1800 - present) release ÒdeadÓ 14C-free carbon, diluting the atmospheric reservoir. Fig. From Stuiver and Quay, 1981, EPSL 53:349-362.

                                                      v.     Solar activity anomalies: ÒMaunder minimumÓ Ð (1645-1715), period of reduced sunspots appears to correlate with increased 14C production

                                                       vi.     Accurate calendar ages must account for these variabilities. Rapid oscillations on ~century timescales can lead to ambiguous ages (i.e., 14C/12C in a sample may be consistent with formation at two or more periods in time, including older, 14C-rich times and younger 14C-poor times).

                                                         vii.     Chronology depends on accurate 14C-production records. Still not complete beyond ~20 kyr.