What is an isotope?  

The nucleus of each atom contains protons and neutrons. While the number of protons defines the element (e.g., hydrogen, carbon, etc.) and the sum of the protons and neutrons gives the atomic mass, the number of neutrons defines the isotope of that element. For example, most carbon (≈ 99 %) has 6 protons and 6 neutrons and is written as ¹²C to reflect its atomic mass. However, about 1 % of the carbon in the Earth’s biosphere has 6 protons and 7 neutrons (¹³C) forming the heavy stable isotope of this important element. Stable isotopes do not decay into other elements. In contrast, radioactive isotopes (e.g., ¹⁴C) are unstable and will decay into other elements.

The less abundant stable isotope(s) of an element have one or two additional neutrons than protons, and thus are heavier than the more common stable isotope for those elements. Both heavy and light stable isotopes participate freely in chemical reactions and in biological and geochemical processes, but the rate at which heavy and light stable isotopes react during physical or chemical reactions differs. The chemical bonds and attractive forces of atoms with heavy stable isotopes are stronger than those in the more common, lighter isotopes of an element. As a result, the heavier isotopes react more slowly than the lighter isotopes leading to isotopic separation or fractionation between reactant and product in both physical and biological reactions. Fractionation of the heavy and light stable isotopes is important because it a) produces variation in the stable isotope ratio of different element pools and b) establishes an isotope signal that can indicate the existence or magnitude of key processes involved with elemental cycling.

The stable isotope concentrations of a molecular compound or material are presented in ratio form as the molar ratio of the heavy-to-light isotopes.  Since this ratio is small, we typically present stable isotope abundances relative to an international standard using “delta” notation as:

 δX = (R_sample/R_standard - 1) x 1000, ‰

where δX is the delta value of the sample for element X (H, O, C, etc.) in parts per thousand (“per mil,” ‰) and R is the molar ratio of the heavy (less common) to light (more common) isotope in the sample and in an international standard, respectively. Organic and inorganic standard materials are obtained from the International Atomic Energy Agency (IAEA, reference materials page) and the National Institute of Standards and Technology (NIST, reference materials page) to ensure accurate measurement and reporting of isotope ratios for unknown samples and to facilitate cross-lab comparability.

What do stable isotope measurements tell us?

Isotopes integrate, indicate, record and trace fundamental ecological processes. Rapid technological advances over the past decade have greatly stimulated the use of isotope analyses by ecologists. This analytical approach is now among the most popular in ecology because of the insights provided by isotope ratios at natural abundance levels.
 

• Isotopes integrate ecological processes in space and time. The isotope ratios of plant and animal tissues and organic and inorganic compounds (including gases) in soil represent a temporal integration of significant physiological and ecological processes on the landscape. The timescale of this integration depends on the element turnover rate of the tissue or pool in question. In addition, the isotope ratios of well-mixed environmental reservoirs, such as the atmosphere, streams and aquifers, often represent an integration of source inputs to the system that extend over large spatial scales.
   
• Isotopes indicate the presence and magnitude of key ecological processes. Many ecological processes produce a distinctive isotope fingerprint. The presence or absence of such processes and even their magnitude in relation to other processes are indicated by the stable isotope ratio value relative to known background values. 
   
• Isotopes record biological responses to Earth’s changing environmental condition. For cases in which substances or residues accumulate in an incremental fashion, such as in tree rings, animal hair and ice cores, isotope ratios can be used as a record of system response to changing environmental conditions or a proxy record for environmental change.
   
• Isotopes trace the origin and movement of key elements and substances. Owing to isotopic fractionations associated with physical and biological reactions, nutrient and element pools within and among ecosystems often differ isotopically. As a result, the source(s) of essential elements and resources acquired by an organism are easily traced using isotope ratios. Strong geographic patterns in isotope signature variation provide the means to trace the movement or origin of a substance or component at landscape to continental scales.