Stable Isotopes in Archaeology: Reading Ancient Diets and Migration in Bones

In 1991, hikers discovered a frozen mummy in the Alps between Austria and Italy. Otzi the Iceman had been dead for 5,300 years. Without a single written record, scientists determined that as a child he lived in the Eisack Valley, then migrated north of the Alpine main ridge as an adult — all from isotope ratios in his tooth enamel and bone collagen. In 2013, scientists confirmed the identity of a skeleton found under a Leicester car park as King Richard III of England. Strontium and oxygen isotopes in his teeth revealed he grew up in eastern England. These stories share one technique: stable isotope analysis, in which the chemistry of a person's own body records where they lived, what they ate, and how far they traveled — silently, for thousands of years.

By the end of this lesson you will be able to: 1. Explain what stable isotopes are and how they differ from radioactive isotopes. 2. Identify which isotope ratios reveal diet (delta-13C, delta-15N) and which reveal geographic origin (87Sr/86Sr, delta-18O). 3. Interpret simple isotope data to draw conclusions about ancient diet and migration. 4. Describe the laboratory method (IRMS) used to measure isotope ratios and explain which skeletal tissues are most reliable.

Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 neutrons; Carbon-13 has 7 neutrons — same element, slightly different mass. Both are stable: they do not decay over time, unlike radioactive Carbon-14 (used in radiocarbon dating, which measures how fast it decays). Because stable isotopes have slightly different masses, they behave slightly differently in chemical reactions. Lighter isotopes react faster; heavier ones accumulate in certain compounds. This mass-dependent fractionation creates predictable isotopic signatures in different foods, water sources, and rocks — signatures that get incorporated into biological tissues as organisms eat, drink, and breathe. Key isotope ratios used in bioarchaeology: - delta-13C (Carbon-13 / Carbon-12): diet — what types of plants and animals a person ate - delta-15N (Nitrogen-15 / Nitrogen-14): diet — trophic level and protein sources - 87Sr/86Sr (Strontium isotope ratio): geographic origin — matches local geology - delta-18O (Oxygen-18 / Oxygen-16): geographic origin — reflects latitude and climate of childhood drinking water The delta notation expresses isotope ratios in parts per thousand relative to an international standard.

CARBON ISOTOPES (delta-13C) — Plants fix carbon during photosynthesis using two different biochemical pathways: - C3 plants (wheat, barley, rice, most European crops, most trees): delta-13C around -25 to -27 per mil - C4 plants (maize, sorghum, millet, sugarcane): delta-13C around -11 to -13 per mil Humans who ate mostly C3 diets — typical European farmers — show bone collagen delta-13C values around -20 per mil. People who ate large amounts of maize, like Mississippian Native Americans after AD 1000, show values closer to -12 per mil. This isotopic shift is how archaeologists have traced the spread of maize farming northward from Mesoamerica, reading the evidence directly from skeletal remains across North America. NITROGEN ISOTOPES (delta-15N) — Each step up the food chain enriches delta-15N by approximately 3 to 5 per mil (the trophic enrichment factor). Approximate ranges: plants at 0–5 per mil; herbivores at 5–10 per mil; fish eaters and carnivores at 10–15 per mil; top predators at 15–20 per mil. Result: Populations that relied heavily on marine fish (like Norse Greenlandic settlers or coastal fishing communities) show very high delta-15N values. Populations eating mostly grain and legumes show low values. Combining delta-13C and delta-15N can narrow a person's diet to a specific ecological niche.

STRONTIUM (87Sr/86Sr) — Strontium occurs naturally in rock, soil, and groundwater, and its isotope ratio varies by underlying geology. Old granite bedrock has high ratios (around 0.720); young volcanic basalt has low ratios (around 0.703). Plants absorb strontium from soil; animals get it from plants and water. The ratio locks into tooth enamel during childhood and reflects the local geology of where a child grew up. If a skeleton's tooth enamel strontium ratio matches the geology of one region but its burial site is in a different region, the individual was likely a migrant. CASE STUDY — The Amesbury Archer (found near Stonehenge, around 2300 BC) had tooth enamel strontium ratios consistent with the Alps, not Wiltshire. He is now considered one of the first continental Europeans to visit Stonehenge, possibly bringing Early Bronze Age metalworking knowledge to Britain. OXYGEN (delta-18O) — Rainfall isotope ratios vary with latitude, altitude, and distance from the coast. Drinking water reflects local precipitation, and this signature is permanently recorded in tooth enamel during childhood. For Richard III, tooth enamel oxygen matched eastern England, while later-formed bone collagen showed a dietary shift toward the high-protein intake of a royal court — delta-15N values rose markedly after he became king at age 30.

The workhorse instrument for stable isotope analysis is the Isotope Ratio Mass Spectrometer (IRMS). PROCESS FOR CARBON AND NITROGEN: 1. Tooth enamel or cortical bone is drilled, cleaned ultrasonically, and treated with weak acid to remove diagenetic carbonate. 2. Bone collagen is extracted by dissolving the mineral fraction in dilute hydrochloric acid at 70 degrees Celsius, then freeze-drying the remaining protein. 3. Dried collagen is weighed into tin capsules and combusted at 1,800 degrees Celsius in an elemental analyser, converting carbon and nitrogen into CO2 and N2 gases. 4. Those gases flow into the IRMS where a strong magnetic field separates ions by mass. The instrument measures the ratio of heavy to light isotopes with precision to within 0.1 per mil. PROCESS FOR STRONTIUM: Samples are dissolved in acid and measured on a Multi-Collector ICP-MS, which simultaneously measures all strontium isotope masses to very high precision. Accuracy: A single tooth enamel sample can place a person's childhood region within approximately 200 kilometres — sometimes far less in geologically diverse areas like the Alps.