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In 2010 Tycho Brahe’s grave in the Church of Our Lady before Týn in Prague was opened by a Czech-Danish research team. Cortical femoral samples were sequestered from the Tycho Brahe (TB) and his wife Kirsten Barbara Jørgensdatter (KBJ) [1,2,3]. Collagen from thoroughly decontaminated cortical bone samples from both individuals were extracted in Prague and have now been radiocarbon dated and measured for stable isotope ratios of C and N in Groningen. The results of the analyses are listed in Table 1.

Table 1 14C dates, stable isotope ratios and relevant parameters bone collagen from Tycho Brahe and his wife
Full size table

Both samples were measured in duplicate; the table shows the averaged values. The bone samples showed excellent quality (see Additional file 1). The rationale behind the radiocarbon dating was to derive reservoir effects, age offsets between samples of terrestrial and aquatic origin. The latter are usually depleted in 14C causing apparently older radiocarbon ages. Tycho Brahe and his wife represented a relative unique opportunity, since humans with a known time of death are rarely 14C-dated. A reservoir effect was expected based on the elevated δ15N value [3,4,5].

Surprisingly, the 14C-dates for both individuals were consistent with their time of death, i.e. no significant age offset was observed. Graphs of the calibrated dates are shown in Additional file 1. At the 2-sigma level, they show a bimodal probability distribution for this time range, around 1500 and 1600 AD for both TB and KBJ. The latter is consistent with the historical time of death. The older age cannot be excluded; it shows a higher probability but that can be an artefact caused by the shape of the curve. Both dates are possible, showing either no reservoir effect, or a relatively small one of about 100 years.

Furthermore, it is seen that the δ13C and δ15N values of this study and those reported by Kacki et al. [3] are in excellent agreement. The δ13C values indicate that the individuals did not receive any appreciable part of their protein intake from the marine food chain during the last decade of their lifetime. The δ15N values, on the other hand, are strongly elevated: 14.7‰ for TB and 13.4‰ for KBJ.

In the thorough analysis by Kacki et al. [3], these δ15N values are matched by medieval samples of cod and seal. However, both cod and seal would exhibit a distinct marine character in their δ13C, which was not observed for TB and KBJ, so these food items cannot have been a major part of an important protein intake. The only other data showing comparable δ15N values are those of Bohemian freshwater fish [3].

We suggest that a plausible explanation for the peculiar combination of high δ15N values and no measurable radiocarbon reservoir effects was a diet rich in freshwater fish. Fish from freshwater systems are known to have large positive values of δ15N [3, 4, 6 and references therein]. Normally, a diet rich in freshwater fish will result in significant reservoir effects [6, 7], but that is not seen in this case. One likely combination of the observed isotope ratios is that the fish came from freshwater lakes with stagnant water, i.e. lakes with no or negligible water throughput and little erosion. In such waterbodies, CO2 equilibrium between water and atmosphere can occur by exchange [8]. The 14C-content of the dissolved bicarbonate in the lake can become equal or close to that of the atmospheric CO2. Indeed, absence of a reservoir effect has been observed under such conditions [6].

Cases where terrestrial/historic comparisons between (calibrated) 14C dates and known ages are very rare. Cook et al. [7] describe a famous prehistoric case: a pelvic human bone, embedded with an arrow made from deer bone. The deer obviously is purely terrestrial, the human is a fish consumer (also witnessed by archaeological fishing gear). This presents a direct measurement of the reservoir effect, combined with stable isotopes of the bone collagen. The averaged value for the derived reservoir effect is 455 years in this case, for the prehistoric Donau river. The Donau is a flowing river where there is no equilibrium of CO2 between water and atmosphere. The stable isotope values are comparable with those of TB and KBJ. Therefore, it is justified to expect reservoir effects of similar size for TB and KBJ.

Stagnant ponds were actually close by, both on Hven and near Prague. On the island of Hven, where Brahe and his wife spent most of their time from 1576 to 1597, Valentin Spangenberg constructed almost 60 interconnected freshwater ponds with all sort of fish [9,10,11,12,13]. From 1597 to 1598 they spent a year at the castle of Brahe’s friend Heinrich Rantzau in Wandesburg outside Hamburg, and then they moved for a while to Wittenberg, where they stayed in the former home of Philip Melanchthon. In 1599 Brahe and his wife lived for 1 year near a newly adapted observatory in a castle in Benátky nad Jizerou, 50 km from Prague. From 1600 until Brahe’s death in 1601, they lived in Prague. His wife died in Prague in 1604.

Freshwater fish farming flourished in Bohemia during the sixteenth century. It is estimated that by the end of that century there were up to 75,000 breeding ponds in Bohemia and Moravia, producing some 4000–6000 tons of freshwater fish every year [14]. For example, near the town of Třeboň in South Bohemia the Rožmberk fishpond was designed and built under guidance of Jakub Krčín, working for the Bohemian aristocratic family of Rosenberg (=Rožmberk). The average depth of this pond was ca. 6 m and it extended over almost 5 km2. It was finished in 1590, so timewise it is possible that fish from this pond were on the Brahe’s dinner table while in Prague [14, 15]. Freshwater fish were an essential part of the menu at the time of Emperor Rudolf II, where both freshwater and marine fish came to the tables of the noble [16, 17].

This means that for ca. 25 years Brahe and his wife lived rich lives with immediate access to fish from stagnant freshwater ponds. This is a sufficiently long timespan to make the major fraction of their remodeled cortical femoral bone having been turned over and recoding the special δ15N of the fish from such ponds [18, 19]. In addition to this diet, meat of young milk-fed mammals, if this would have been consumed, would also elevate the δ15N [3], while keeping an unaltered radiocarbon date.

Conclusion

Radiocarbon dating of cortical femoral tissue samples obtained from Tycho Brahe and his wife Kirsten Barbara Jørgensdatter revealed that no reservoir effects can be observed. However, the bones did show enrichment in δ15N. This led to the conclusion that whatever source caused the elevated δ15N, it must have been close to equilibrium with the atmospheric carbon reservoir. The δ13C values reduce the possibility of saltwater fish or seal as a dominant protein intake. It is suggested that the important protein intake for both Brahe and his wife for many years was freshwater fish from pools with stagnant water.