4.1 Isotope Basics

Most isotope info sourced from URI OCG 550 with Kelton McMahon

Definitions

Isotope - atoms of the same element with different numbers of neutrons, differ in mass but have essentialy the same chemical reactivity

• Stable - do not undergo radioactive decay, but do undergo fractionation

• Radioactive - unstable isotopes that will decay to other elements by shedding their neutrons / protons to achieve stability

Fractionation - the alteration of the distribution of stable isotopes as a result of chemical, physical, or biological processes

• Kinetic - Unidirectional reactions due to differences in reaction rate of molecules or atoms containing different masses - light isotope usually reacts faster - includes transport or diffusion processes dealing with flux

  • sometimes there are multiple products, which still have to conserve mass

• Equilibrium - Isotope exchange reactions driven by changes in vibrational energies of molecules, forward and back reactions reach thermodynamic equilibrium - the heavy isotope accumulates where it is held strongest - phase changes

  • alpha (fractionation rate) decrease with temp

  • fractionation highest for light elements

• Rayleigh fractionation - products are isolated from reactants immediately after formation, leading to characteristic isotope pattern of evolving phases

  

  As you increase up a mountain slope, the delta18O in clouds becomes increasingly negative. the 18O is being captured in the water / condensation and clouds are getting isotopically lighter. the gradient that is created is called rayleigh fractionation
• Conservation of mass - reactant and product must preserve original epsilon when 100% reacted

Most important Isotopes (for me) - O and H are typically associated with environmental fractionation and gradients, while N and C are more important for biological processes

• 1H vs 2H

• 12C vs 13C

• 14N vs 15N

• 16O vs 18O

Notation

• Delta Notation - Allows us to compare between samples by standardizing them to international standards. where R is the heavy/light isotope ratio, units are permil ‰.

  • positive values = enrichment

  • negative values = depletion

• Big Delta Notation - Isotope separation
• Epsilon - Isotope Enrichment
• Alpha notation - Isotope Fractionation

  • very temperature dependent

• Turnover time - T

References

McMahon, Hamady & Thorrold 2013

Provides a thorough background on ecogeochemistry and the uses of isotopes in marine ecosystems.