Words of warning about radiocarbon dating medieval human bones

Beginning in the thirteenth century and culminating approximately one hundred years later, a significant number of religious houses—circa 140—were established in Denmark by the majority of the monastic orders originally founded in Italy, France, and Germany¹. The general layout of these architectural complexes adhered strictly to the regulations of each respective order, typically comprising a church and three cloister buildings arranged around a central courtyard. Each religious house was accompanied by one or more cemeteries; in certain instances, the monastic residents possessed a burial ground of their own, whereas novices, laypeople, and townsfolk were interred elsewhere. A case in point is the Cistercian monastery at Øm in Jutland, where three distinct cemeteries have been identified^2,3,4. The most prestigious burial sites - highly sought after by the residents of the monastery and, consequently, also by affluent townsfolk willing to pay for a burial plot - were frequently located within the church, inside or adjacent to the chapter house, and along the cloister walk. For the current investigation, 27 burials have been selected from these precise locations, each demonstrating a stratigraphical or orientational relationship to the architecture. This relationship encompasses parallelism to the buildings or to other interments.

In Christian medieval Europe, meat was restricted during the lent period^5,6. However, the meat of freshwater fish was tolerated as a food source because freshwater fish were considered “cold” or “neutral” in comparison with meat from warm-blooded animals and marine fish. Freshwater fish was even in some Christian circles and cultures considered a prestigious food item⁷.

Interpretation of radiocarbon dates of organisms living in aquatic environments is not straightforward, and in the context of this Comment, this also applies to bones of humans that have consumed a significant amount of aquatic food such as fish. This Commentary is aimed at the practical application of radiocarbon dating medieval human bones, not only in Denmark but also in the rest of Europe.

The problem is caused by what is known as the “reservoir effect”. This is briefly described below and requires some explanation of the radiocarbon dating method. The method is based on the radioactive isotope ¹⁴C, which is produced in the atmosphere by cosmic radiation. After forming CO₂, it becomes part of the biosphere, including humans via the food chain; see Fig. 1. Upon death, there is no longer intake of food; from then on, the ¹⁴C-content of the remains (e.g., the buried bones) decreases due to radioactive decay. Thus, by measuring the remaining ¹⁴C-content of a human bone, its age can be determined^8,9.

This sounds simple, but there are complications such as isotopic effects (named fractionation) and calibration of the ¹⁴C measurement to calendar dates. For technical details, we refer to the extensive literature^8,10,11. An additional significant complication is the reservoir effect mentioned above, which applies to organisms whose dietary intake is not based on terrestrial reservoirs, but on aquatic reservoirs: ocean/sea, rivers, and lakes/ponds. In these reservoirs, the natural ¹⁴C-content is significantly lower than that of the terrestrial environment.

Typical numbers are shown in Fig. 1. These numbers are ballpark numbers varying from case to case. For the upper layer of the sea, the ¹⁴C content is 95% of that of the atmosphere. This is caused by upwelling of dissolved CO₂ from deep ocean water in certain regions, which is “old”. For rivers, the ¹⁴C content is about 15% less; this is caused by dissolved geologically derived carbonate, which does not contain ¹⁴C (see Mook 2006¹² and references therein). For lakes/ponds with streaming water, the effect is also approximately 15%. When the pond is closed off (like fishing ponds), the CO₂ between water and atmosphere reaches equilibrium after some time, causing the reservoir effect to disappear.

The depletion of ¹⁴C in aquatic reservoirs also applies to organisms living therein, such as fish, shells, and marine mammals. From the formula describing the radioactive decay, one can calculate that a depletion of 5% in ¹⁴C concentration corresponds to 400 years; for 15%, this is 1300 years. Hence, marine organisms that are contemporaneous with terrestrial organisms show ¹⁴C dates that are approximately 400 years older; for rivers, e.g., the river Rhine, it can be up to 1300 years older. This effect also applies to terrestrial organisms consuming aquatic food sources. For example, bones of polar bears and prehistoric Inuit people with a 100% marine diet show ¹⁴C dates which require a correction of 400 years^13,14.

The reservoir effect caused by freshwater fish consumption is much more complicated to correct for. What is missing here is the uniform 400 years, as is the case for a 100% diet based on marine fish. Freshwater reservoir offsets can vary extremely, from zero to very high numbers – numbers up to 1000 years have been observed^15,16. We note that the magnitude of the marine reservoir effect differs at various regions of the global oceans¹⁷. However, for the North Sea basin, the number is well established as 400 years^11,12,18.

Quantification of the magnitude of the reservoir effect can be done using the isotopes ¹³C and ¹⁵N of the bone collagen. Bone collagen is also the datable fraction used for the ¹⁴C measurement. The isotopes ¹³C and ¹⁵N are not radioactive but stable; their concentrations do not change over time. Hence, they can be used as time-independent tracers for the consumed food sources, which is useful for estimating the origin of ¹⁴C in the bone collagen (terrestrial, marine, or freshwater), and thus also the reservoir effect.

The stable isotope concentrations are expressed in delta values, δ¹³C and δ¹⁵N. These are defined as the ratio of rare/abundant isotope concentration in a sample relative to that of a reference material, in per mille (‰) deviation¹⁰.

Both stable isotopes provide information on the trophic level of the consumer’s bone collagen. The δ values increase for each trophic level, for example from vegetation to herbivores, and again from herbivores to carnivore^19,20. In addition, the δ values for bones of aquatic organisms show specific stable isotopic signatures. The same is true for human consumers of such organisms^21,22.

For both the stable and radioactive isotopes, the isotopic history for human bones with significant fish consumption quickly becomes complicated, in particular for mixed diets (terrestrial and fish from both marine and freshwater resources). Special models have been developed to quantify the size of the reservoir effect and use that to provide the most accurate correction possible for the ¹⁴C date^11,23. Note that this modelled ¹⁴C date is not the so-called conventional ¹⁴C date. The latter is defined based on conventional values for reference materials and the half-life of ¹⁴C, expressed in BP¹⁰. Because the convention was made for terrestrial samples, a reservoir effect needs to be subtracted from the BP number (similar to the 400 years for the marine case). For the modelled date, the reservoir effect is included and should therefore not be expressed in BP.

Both dates, either in BP (and subtraction of the reservoir effect) or modelled, need one final processing step: they need to be calibrated into calendar ages. Calibration is needed because the production of ¹⁴C in the atmosphere is not constant through time; it depends on the strength of the cosmic ray flux, which in turn depends on the strength of the Earth’s magnetic field and variations in the solar cosmic ray flux. Both are not constant. These variations are known by comparing ¹⁴C dates with absolute dates obtained from other chronological methods, in particular dendrochronology. Calibration curves are available and updated regularly, transforming ¹⁴C dates into calendar ages^17,24.

When excavating monastic burial grounds, and in many other archaeological situations as well, radiocarbon dating of human bones is in demand. But in this Comment, we would like to raise concerns about the feasibility of getting reliable radiocarbon dates from medieval human skeletons.

We have selected 27 human bone samples from well-preserved long bones (femora or tibia), all of which were previously excavated in clean monastic environments as described above. The collagen fraction has been radiocarbon dated at the AMS facility at Kiel University, Germany. The stable isotopes of the collagen from the same samples were measured at Bradford University, United Kingdom. The collective data set is listed in Table 1.

The δ¹³C of the bone collagen ranges from −18.4 to −20.6‰, which indicates relatively small reservoir effects from the marine food chain. However, the δ¹⁵N values are all raised significantly above 8‰, indicating a substantial intake of freshwater protein.

The results of the calibrated radiocarbon dates not corrected for reservoir effects are shown in Fig. 2. The terrestrial calibration curve IntCal20 has been used²⁴.

An extraordinary date is IIIB9 from Svendborg Franciscan Friary, which is dated to at least a hundred years before the erection of the friary, which is of course highly unlikely. The same applies to 810C28C from Viborg Maria, a Canons Regular of Saint Augustine, where the highest 2σ-end is dated to 976, more than 200 years prior to the erection of the monastery; and the same problem appears for A257, A260, and 6.K from the Cistercian Monastery at Øm in Central Jutland.

The obvious explanation is that these six individuals consumed a relatively high proportion of freshwater fish protein in their diet, which is confirmed by their high δ¹⁵N value (Table 1). This corresponds to 22% of the individuals in this study.

These “words of warning” do not just apply to the Danish medieval monastery burials, which are used here only as an example to illustrate the problem. The reservoir effect is observed worldwide in both human and faunal bones - historical, protohistorical, and prehistorical. Among the first publications describing the offset between terrestrial and aquatic samples were Henrik Tauber (1979)¹⁴ and Ingrid Olsson (1983)¹⁸. Other well-known examples from (proto)history are the “lost king” Richard III from England and excavated bones from Dorestad in the Netherlands, destroyed by a Viking raid in the 9^th Century.

The ¹⁴C date for King Richard, who died in battle, is too old; this can be explained by a diet of fish, as models result in a small but significant reservoir effect (Bayliss and Marshall, 2022, 5:2, 69-73⁸). The same applies to Dorestad, where a series of bones show a reservoir effect of about 250 years²³. Dorestad is located at the Rhine. Also, medieval relics from Belgium show varying reservoir effects²⁵, similar to the monastic bones from Denmark shown in Table 1 and Fig. 2.

Significant prehistoric examples are Mesolithic bones from Hardinxveld in the Netherlands²⁶, and the Iron Gates Gorge in the Balkans²⁷. The latter includes direct proof: bone arrows made from ungulates penetrated human bones, suggesting they are more or less contemporaneous. The ungulate obviously had a 100% terrestrial diet, but the human bones are ca. 400-500 years older, suggesting that the difference is being caused by a reservoir effect. Stable isotopes measured in human bones indeed show a significant freshwater dietary component originating from the Danube. Many examples (including model calculations and ample references) are discussed in the recent reservoir effect article by van der Plicht and Streurman²³. Even more recently, Russia revealed many bones showing reservoir effects from the Bronze Age, causing major adaptations in the existing chronologies²⁸.

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Authors and Affiliations

1. Institute of Physics, Chemistry and Pharmacy, University of Southern Denmark, Odense M, Denmark

   Kaare Lund Rasmussen

2. Center for Isotope Research, Groningen, The Netherlands

   Johannes van der Plicht

3. ADBOU, Section of Anthropology, Department of Forensic Medicine, University of Southern Denmark, Odense M, Denmark

   Jesper Lier Boldsen

Contributions

Conceptualisation: K.L.R., J.L.B. Sampling: K.L.R. Interpretation of the chemical analyses: K.L.R., JvdP. Historical context: K.L.R., J.L.B., Jvd.P. Writing and editing: K.L.R., Jvd.P. All authors have approved the final version.

Corresponding author

Correspondence to Kaare Lund Rasmussen.

Competing interests

Kaare Lund Rasmussen is an Associate Editor for npj Heritage Science, but was not involved in the editorial review of, or the decision to publish this article. All other authors declare no competing interests.

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