Stable Isotopes

by

Eric Bartelink

• LAST REVIEWED: 15 January 2020
• LAST MODIFIED: 15 January 2020
• DOI: 10.1093/obo/9780199766567-0153

Introduction

Stable isotope analysis refers broadly to a series of chemical methods used to study phenomena such as earth systems, the hydrological cycle, and ecology. The use of stable isotopes in anthropology traces its beginnings to the late 1970s. Originally focused on geological, hydrological, and ecological problems, stable isotope analysis now is a major component of the anthropologist’s toolkit. Current applications within anthropology draw from the core scientific disciplines but adapt these methods for specific research questions within archaeology, bioarchaeology, paleoanthropology, and forensic anthropology. One major area of study is dietary reconstruction, which focuses on isotope analysis of human skeletons to examine temporal and regional dietary patterns, as well as variation associated with sex, age, mortuary patterns, and social status. More specialized approaches examine weaning and childhood diet patterns using serial sections of tooth enamel and dentin. Another major area of isotope research in anthropology is the study of past residence patterns (e.g., migration and mobility). Some stable isotopes record information about the local water and geology, and thus provide information regarding the place of origin or travel history of an individual. By examining multiple isotopes in different tissues, anthropologists can reconstruct changes in residence patterns. For example, isotopes in enamel bioapatite (mineral fraction) do not change throughout life, so teeth provide a record of where a person lived when a particular tooth formed. Bone collagen (organic fraction) and bioapatite (mineral fraction), in contrast, are constantly replaced through remodeling; thus, if a person moved to a new geographic region, the isotope values in their bones will shift toward the new location over time. By comparing the isotopes in teeth versus bones, anthropologists can detect whether a person was local or nonlocal to the area where they died. In instances where hair or nails are preserved (e.g., mummies, modern forensic cases), serial sections of these tissues can be used to provide a detailed record of diet or travel history in the months prior to death. More recently, forensic anthropologists have begun to examine stable isotopes in modern human remains to aid with medicolegal identification. These methods are especially useful when a decedent is nonlocal to the area where they died (for example, a recent traveler or foreign-born individual).

Definition

Stables isotopes are forms of the same element that have the same number of protons and electrons but a different number of neutrons in the atom’s nucleus. Isotopes exist in both stable and unstable (radioactive) forms. Stable isotopes of the same element (e.g., ¹³C and ¹²C) have different atomic weights and therefore travel at slightly different rates in chemical reactions. In instances where a reaction is incomplete (e.g., photosynthesis), the product will show differential incorporation of the heavy and light isotopes relative to the substrate, a process known as isotopic fractionation. Fractionation results in the enrichment of one isotope relative to another isotope in physical and chemical reactions. This process is what creates variation in stable isotope ratios in nature. Stable isotopes are incorporated into body tissues, such as bones, teeth, hair, and nails, through consumed foods and imbibed water and do not change over time. Michener and Lajtha 2008 provides a useful overview of the basic definitions and processes involved with stable isotope analysis.

• Michener, Robert H., and Kate Lajtha. 2008. Stable isotopes in ecology and environmental science. 2d ed. Malden, MA: Blackwell.

  Provides basic terminology and a discussion of general processes involved in the study of stable isotopes in nature.