Isotopes in C, H, O, N and S
Most elements of biological interest (carbon, hydrogen, nitrogen, oxygen and sulfur) have two or more stable isotopes, with the lightest of these present in much greater abundance than the others. The heavy isotopes of these elements are incredibly useful as biological tracers, most notably carbon, nitrogen and sulfur, as they are abundant in the earth, the atmosphere and living things. Each has a heavy isotope (13C, 15N and 34S) with a natural abundance of ~1% or less and a light isotope (12C, 14N and 32S) that makes up most or all of the remainder.
Average terrestrial abundances of stable isotopes of major elements used in paleoenvironmental studies
Studies examining stable isotopes measure the fractionation of isotopes, usually reported as delta, a value given in parts per thousand or per mil (%o). Delta values are not absolute isotope abundances but differences between sample readings and a natural abundance standard, different for each element, which is considered delta = zero. Here is an example of calculating a delta value for carbon 13.
- Carbon – Vienna PeeDee Belemnite (VPDB) from South Carolina, USA
- Nitrogen – atmospheric air
- Hydrogen and oxygen – standard mean ocean water value (SMOW)
- Sulfur – Canyon Diablo meteorite.Delta values change based on physical and biological fractionation. Light isotopes will separate from heavier isotopes for many different reasons:
- Organic processes – biochemical processes, such as photosynthesis and bacterial reduction, usually prefer lighter isotopes because lighter isotopes generally have weaker bonds, which are preferentially broken by the mediating microorganisms. Thus, the resulting biomass is enriched in the lighter isotope compared to the heavier isotope
- Physical processes such as evaporation and diffusion separate the lighter isotopes from the heavier, as the lighter ones will preferentially evaporate or diffuse first
Stable isotope analysis is a powerful tool to attempt to reconstruct modern metabolic pathways, ancient environments and correlate rocks from different sources. In our lab, we perform isotope analysis on specific biomarkers, as well as bulk samples, to take advantage of the source-specificity of biomarkers as well as the potential for environmental reconstruction from isotopic information.
A selection of useful links and references can be found here.