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Stable Isotopes

Stable Isotopes. Elements composed of various isotopes Hydrogen = 1 H, 2 H, 3 H (radioactive) Oxygen = 16 O, 17 O (exceedingly rare), 18 O Carbon = 12 C, 13 C, 14 C (radioactive) The stable (non-radioactive) isotopes change abundances during chemical reactions

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Stable Isotopes

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  1. Stable Isotopes • Elements composed of various isotopes • Hydrogen =1H, 2H, 3H (radioactive) • Oxygen = 16O, 17O (exceedingly rare), 18O • Carbon = 12C, 13C, 14C (radioactive) • The stable (non-radioactive) isotopes change abundances during chemical reactions • Typically heavy isotope goes in lower energy phase

  2. Uses – very important tool • Change in abundance called “fractionation” • Measurements of fractionation can describe a lot of information about processes • Here consider H and O, since that is water composition

  3. Curvature of flight depends on mass Mass spectrometer Ionize gas Accelerate ions Focus Ions Sample gas Reference gas 12C16O16O = 44 13C16O16O = 45 12C18O16O = 46

  4. Introduction to Stable Isotopes and Rayleigh Fractionation • Derive on board

  5. Isotopic composition of rain • Fractionation factor function of T • At high T, e = 0 (no fractionation) • e becomes increasingly larger at colder T • Rainout from vapor caused by decreasing T • Increasing latitude • Increasing elevation

  6. Isotopic evolution during rainout Rainout as move inland/up mountain – f decreases, colder T increases a Change in e because of change in T

  7. Isotopic composition of rain • H isotopes behave similarly to O isotopes • Empirical evidence shows d18O and dD or rain covary linearly • Called meteoric water line • Slope = 8 • Intercept = 10

  8. Global Meteoric Water Line dD intercept = +10‰ Slope = 8

  9. Meteoric water line originates from evolution of water vapor: • Most vapor originates from tropical evaporation • Rainout at high latitudes or elevation • The slope of the line (8) and intercept (+10) originates from difference in fractionation of O and H

  10. Rainout • Assume condensation follows Rayleigh distillation • (derive on board)

  11. Evaporation • Rainout is equilibrium process • Evaporation is kinetic • Not equilibrium • Vapor is isotopically lighter than expected from equilibrium calculations • Kinetic effect stronger for 18O than D

  12. Equilibrium evaporation from ocean at 25º C • Equilibrium: d18O should be -9.4‰, Kinetic: (observed) is -13‰ (around 35% excess 18O) • Equilibrium: dDshould be -85‰, Kinetic: (observed) is -95‰ (around 10% excess D)

  13. Depletion is more for 18O than for D • This depletion makes a D excess: • From the global meteoric water line, d = 10‰

  14. Locally may vary from meteoric water line • Results from more intense kinetic fractionation • If higher excess • may be evaporation in low humidity area • Evaporation from inland water already fractionated • Requires developing a “local” meteoric water line

  15. Temperature effects • Coastal areas – linear relationship between d and T: • T vsd18O: • T vsdD: • Where tc is average yearly T • Empirically derived (Dansgaard, 1964) • Comes from T dependence of a

  16. Temperature control alters seasonal, latitudinal, and elevation affects on isotopes of precipitation • Continental effect - Isotopic compositions lighter • colder temperatures required for rainout • Evaporation of inland water makes vapor isotopically lighter

  17. Seasonal effects • The Pas, Canada (high latitude, continental) • Addis Ababa, Ethiopia (low latitude) • Stanley, Falkland Islands (high latitude, oceanic)

  18. Global d18O in precipitation • Continental effect • Latitudinal effect • Altitude effect = -0.1 to -0.5‰/1 m altitude

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