ESS 109C Isotope Geochemistry Notes

May 14, 2007

 

Oxygen and hydrogen isotope geochemistry

 

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

  2. Oxygen and hydrogen isotopes
    1. O: 16O 99.76%, 17O 0.04%, 18O 0.20%

                                                                  i.      18O/16O usually measured (d18O)

                                                                    ii.      d17O Å 0.52 * d18O, with some variability

    1. H: 1H 99.984% 2H (=D) 0.016%
    2. Both systems standardized to SMOW (standard mean ocean water)
    3. d18O of carbonates sometime standardized to PDB, like d13C
  1. Fractionation in the hydrologic cycle
    1. At equilibrium, liquid water is enriched in both 18O and D relative to vapor

                                                                  i.      At 20¼C, alÐv(18O) = 1.0098

                                                                    ii.      al-v(D) = 1.084

    1. Implication for the hydrological cycle Ð

                                                                  i.      Atmospheric water vapor is isotopically light.

                                                                    ii.      What is the isotopic behavior of precipitation?

    1. Batch equilibrium fractionation

                                                                  i.      Product and reactant remain in contact Ð liquid-vapor exchange

                                                                    ii.      R(l)/R(v) = alÐv(18O) = 1.0098; d18Ol - d18Ov Å 9.8ä

                                                                      iii.      For a closed system with d18Ol(i) = 0, as condensation proceeds,
d18Ov*Xv + d18Ol*Xl = 0              (Isotopic mass balance)

Xv is the mole fraction of vapor. Xl is the mole fraction of liquid

then d18Ov*Xv + (9.8 + d18Ov)*(1 Ð Xv) Å 0

d18Ov Å Ð9.8*(1 Ð Xv) = Ð9.8*Xl
d18Ol Å 9.8 Ð 9.8*(1 Ð Xv) = 9.8 * Xv = 9.8*(1 Ð Xl)

1.      As more liquid condenses, residual liquid becomes 18O-poor, liquid becomes more like original vapor!

    1. Rayleigh fractionation (distillation)

                                                                  i.      Product removed from contact Ð no back-exchange with reactant

                                                                    ii.      R(l)/R(v) = alÐv(18O) = 1.0098 for each incremental liquid droplet produced

                                                                      iii.      Then R(v) = R0(v)*Äa-1, where Ä is fraction of vapor remaining (analogous to Xv)

Incremental water droplet R(ldroplet) = aR0(v)*Äa-1

Converting to d-unitsÉ

d18Odroplet = [d18Ov(i) + 1000]a*Äa-1 Ð 1000

What happens as Ä ˆ 0?

    1. EarthÕs hydrological cycle close to Rayleigh distillation

                                                                  i.      Most water vapor forms by evaporation of near-tropical ocean water.

                                                                    ii.      Little post-evaporation exchange between vapor and ocean

                                                                      iii.      Condensed water quickly removed as precipitation, or re-evaporated.

                                                                     iv.      dD strongly correlates with d18O, but varies more strongly (a further from 1).
dD Å 7.96 x d18O + 8.9

                                                                   v.      Main control on amount of vapor remaining is temperature

1.      Equator Ð pole gradients
d18O(annual ave) Å 0.7*T(annual ave) Ð 13.6

2.      Elevation gradients (rain shadow)

                                                                     vi.      Glacial/interglacial variation