> For the complete documentation index, see [llms.txt](https://taylor-lindsay.gitbook.io/tl-comps/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://taylor-lindsay.gitbook.io/tl-comps/4.-isotopes/4.1-isotope-basics.md). # 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. ####