Results and discussion

The collected samples were taken primarily from injured and unhealed skulls (36 samples), where only 3 samples (Budyně 16,170 and Broumov II-13, Table 1) of injured skulls exhibited subsequent healing; samples were also taken from a tibia, rib, pelvis, fingers and femurs.

Table S1 gives an estimate of the number of skeletons (individuals) in the individual ossuaries. The largest sampled ossuaries were those in Sedlec (50,000 individuals), Kolín, Mělník and Mikulov, which contain the remains of approximately 10,000 individuals. In contrast, the smallest collections were those in Kostelec nad Ohří (50–100 individuals) and Budyně nad Ohří (160 individuals), Mnichov (50–100 individuals) and Lidéřovice (50–100 individuals). The radiometric ages of most of the samples lie in the period between 1219 and 1928. Figure 2 shows the individual samples and their radiometric ages (before present (BP) and ages (AD) based on the calibration model (Reimer et al Reference Reimer, Austin, Bard, Bayliss, Blackwell, Ramsey, Butzin, Cheng, Edwards, Friedrich, Grootes, Guilderson, Hajdas, Heaton, Hogg, Hughen, Kromer, Manning, Muscheler, Palmer, Pearson, van der Plicht, Reimer, Richards, Scott, Southon, Turney, Wacker, Adolphi, Buntgen, Capano, Fahrni, Fogtmann-Schulz, Friedrich, Kohler, Kudsk, Miyake, Olsen, Reinig, Sakamoto, Sookdeo and Talamo2020). From the age values, the definition of the individual minima and the calibration curve showing the plateau or decline in the atmospheric ¹⁴C value, it can be seen that most of the samples correspond to the fundamental climatic minima that have been observed in the northern hemisphere, i.e., the Wolf, Spörer, Maunder and Dalton minima.

Figure 2. ¹⁴C dating of deposited skeletal material from ossuaries in the Czech Republic.

The minimum and maximum ages of some samples approach the interval of the temperature minima, but do not exactly correspond to their spans. These applies to the sample from a skull from Mělník (1717–1785), the sample from a skull from Třešť (1717–1784) and the sample from a skull from Putim (1717–1782). The lower limits of the ages of these samples approach the end of the Maunder minimum (1645–1717) and the upper limits approach the beginning of the Dalton minimum (1790–1830). Because the minima themselves have variously defined spans (e.g., Eddy Reference Eddy1976a, 1976b; Jiang and Xu Reference Jiang and Xu1986; Kappas Reference Kappas2009), it can be assumed that the duration of the effect of these fluctuations lasted beyond their determined intervals.

Bones dated to the time of the Wolf minimum (1280–1350) were identified in the ossuaries in Kostelec nad Ohří (skull), Žehuň (skull), Kašperské hory (skull), Broumov (skull) and Nížkov (finger bone), Sedlec (skull), Skřípel (skull) Kotouň (skull), Malín (tibia) and Chlum sv. Máří (skull) (Figure 2). Epidemics occurring in the Wolf minimum are documented in the Zbraslav Chronical (Durynský and Žitavský, Reference Durynský and Žitavský1305–1339), but their etiology is not known. A plague epidemic appeared, originally in isolated patches, in the period from 1250 to the beginning of the 14th century; after 1312, a related plague epidemic appeared in Germany and subsequently, in 1318, in Bohemia (Kahudová Reference Kahudová1967). The large scale “plaque epidemics” on a large scale were caused by a compromised immunity of the whole population, which occurred in connection with famines. Famines are described in the Chronicle of Cosmas to the beginning of the 12th century, and in the Zbraslav Chronicle (Durynský and Žitavský, Reference Durynský and Žitavský1305–1339) almost until the middle of the 14th century.

The infection associated with famines was paleopathologically identified through the signatures of typhoid lesions on tibial planes as typhoid fever (Salmonella typhi) and was documented in the ossuaries in Nížkov, Malín, Sedlec and Kotouň. In all of them, dating pointed to the year 1345 ± 30 (Smrčka et al. Reference Smrčka, Musilová and Hořejš2020)

The bubonic plague (Yersin plague) had the greatest intensity in Europe in 1348–1351; accompanying phenomena such as high prices and hunger were recorded, not only in Bohemia, but also in neighbouring countries (Kahudová Reference Kahudová1967). The bacteria Yersinia pestis caused pandemics in waves that returned in cycles until the beginning of the 18th century (Rascovan et al. Reference Rascovan, Sjögren, Kristiansen, Nielsen, Willerslev, Deanues and Rasmussen2019; Rasmussen et al. Reference Rasmussen, Allentoft, Nielsen, Orlando, Sikora, Sjögren, Nielsen, Pedersen, Schubert, Van Dam, Kapel, Nielsen, Brunak, Avetisyan, Epimakhov, Khalyapin, Gnuni, Kriiska, Lasak, Metspalu, Moiseyev, Gromov, Pokuta, Saag, Varul, Yepiskoposyan, Sicheritz-Pontén, Foley, Lahr, Nielsen, Kristiansen and Willerslev2015). The end of the Wolf Minimum is linked to the Black Death pandemic, which killed tens of millions of people in Europe and Asia (Aberth Reference Aberth2021). However, this epidemic had significant regional variations, while in some areas we observe a decline in agriculture, in others an increase in agricultural production is documented (Izdebski et al, Reference Izdebski, Guzowski, Poniat, Masci, Palli, Vignola, Bauch, Cocozza, Fernandes, Ljungqvist, Newfield, Seim, Abel-Schaad, Alba-Sánchez, Björkman, Brauer, Brown, Czerwiński, Ejarque, Fiłoc, Florenzano, Fredh, Fyfe, Jasiunas, Kołaczek, Kouli, Kozáková, Kupryjanowicz, Lagerås, Lamentowicz, Lindbladh, López-Sáez, Luelmo-Lautenschlaeger, Marcisz, Mazier, Mensing, Mercuri, Milecka, Miras, Noryśkiewicz, Novenko, Obremska, Panajiotidis, Papadopoulou, Pędziszewska, Pérez-Díaz, Piovesan, Pluskowski, Pokorny, Poska, Reitalu, Rösch, Sadori, Sá Ferreira, Sebag, Słowiński, Stančikaitė, Stivrins, Tunno, Veski, Wacnik and Masi2022).

The cold fluctuation designated as the Spörer minimum (1400–1570) witnessed increased deposition of skeletal material in Czech and Moravian ossuaries in Kolín, Velká Losenice, Mikulov, Budyně, Hrádek u Znojma, Annín, Letařovice, Vamberk, Smečno, Kouřim, Křtěnov, Řesanice, Kotouň and Mnichov (near Karlovy Vary) (in 14 ossuaries a total of 18 dated ¹⁴C samples, Figure 2).

The coldest decade of the Spörer minimum is defined as the period after 1440 (Camenisch et al. Reference Camenisch, Keller, Salvisberg, Amann, Bauch, Blumer, Brázdil, Brönnimann, Büntgen, Campbell, Fernández-Donado, Fleitmann, Glaser, González-Rouco, Grosjean, Hoffmann, Huhtamaa, Joos, Kiss, Kotyza, Lehner, Luterbacher, Maughan, Neukom, Novy, Pribyl, Raible, Riemann, Schuh, Slavin, Werner and Wetter2016). Plant production decreased in a number of European countries as a result of long winters and frequent precipitation. This resulted in a sharp increase in prices, stagnation of trade and an increase in the number of armed conflicts (Camenisch et al. Reference Camenisch, Keller, Salvisberg, Amann, Bauch, Blumer, Brázdil, Brönnimann, Büntgen, Campbell, Fernández-Donado, Fleitmann, Glaser, González-Rouco, Grosjean, Hoffmann, Huhtamaa, Joos, Kiss, Kotyza, Lehner, Luterbacher, Maughan, Neukom, Novy, Pribyl, Raible, Riemann, Schuh, Slavin, Werner and Wetter2016). It is apparent that more people died during the Spörer minimum than in the previous century. It was similarly cold in the 16^th century when poor harvests and hunger reduced the resistance of the population to infection. In this period, plague was apparently a factor in Bohemia in the years: 1502, 1505, 1507, 1520–1521, 1530–1531, 1542, 1551–1553, 1553–1555, 1557–1558, 1561–1563, 1564, 1566–1569, 1571–1573, 1580, 1581–1582, 1584–1585, 1592 and 1597–1599. In 1582, 30,000 people died in Prague, corresponding to half of the inhabitants, while 20,000 people died in the other Czech cities (Chlumská et al. Reference Chlumská, Kábrt, Poláčková, Rozsívalová, Šplíchalová and Vojtová1965; Fialová et al. Reference Fialová, Horská, Kučera, Maur, Musil and Stloukal1998). Most influenza epidemics were followed by bacterial plague epidemics, mostly following influenza (1561 and 1584). These influenza epidemics had common symptoms—fever, stomach pains, diarrhea and nasal bleeding (Short Reference Short and Thompson1890). Major eruptions of Hekla (1510) and Etna (1510 and 1580) also occurred in this period (Sigurdsson et al. Reference Sigurdsson, Houghton, Rymer, Stix and McNutt1999).

An increased amount of skeletal material was also deposited in nine Czech and Moravian ossuaries in the Maunder minimum (1645–1715) (Figure 2). There is no doubt that a substantial portion of the increase in remains from this period occurred as a result of armed conflicts associated with the Thirty-Year War. However, this fluctuation also occurred during the coldest century of LIA (17th century) and, simultaneously, the greatest number of cold decades in the northern hemisphere occurred in this century: 1691–1700, 1601–1610, 1641–1650 (Jones et al. Reference Jones, Briffa, Barnet and Tett1998). Consequently, there was an increase in the number of bacterial infections, especially plague, and also famines (1677, 1684, 1692–1696). For example, the famine of 1692–1696 in Bohemia was less drastic than that in France, where it grew to terrifying proportions and led to social unrest (Fialová et al. Reference Fialová, Horská, Kučera, Maur, Musil and Stloukal1998). The famine was exacerbated by a locust invasion in 1693 (Mašková-Janotová and Tošnerová Reference Mašková-Janotová and Tošnerová2017). This period witnessed one of the largest eruptions of Etna in 1669 (Sigurdsson et al. Reference Sigurdsson, Houghton, Rymer, Stix and McNutt1999). Two influenza epidemics also occurred in the 17th century, in April of 1658 (Willis Reference Willis and Thompson1890) and in the autumn of 1675 (Sydenham Reference Sydenham and Thompson1890). A new kind of fever affecting the brain and nerves appeared in the summer of 1658 (Willis Reference Willis and Thompson1890). Another major eruption of Etna occurred in 1675 (Sigurdsson et al. Reference Sigurdsson, Houghton, Rymer, Stix and McNutt1999), followed by widespread bacterial dysentery infections (1677). Further influenza epidemics occurred in 1688 and 1693 (Molyneaux 1890). Plague epidemics occurred again in Bohemia in 1602, 1604, 1606–1607 to 1613, 1616, 1620, 1622–1623, 1632, 1655, 1665 and 1679–1680 and affected 70% of children under 15 years of age. Some were of only local extent, such as that in 1665, which occurred at the same time as the well-known London plague, the last one in Western Europe. However, a demographic crisis resulted from the epidemic in 1679–1680, when 12 thousand people died in Prague, i.e., one third of the inhabitants of Prague (Černý Reference Černý2014; Chlumská et al. Reference Chlumská, Kábrt, Poláčková, Rozsívalová, Šplíchalová and Vojtová1965; Fialová et al. Reference Fialová, Horská, Kučera, Maur, Musil and Stloukal1998).

Ages corresponding to the Dalton minimum (1790–1830) were found for the skull samples from Třebívlice, Žehuň, Zdislavice, Plumlov, Páleček, Páleč, Velíš, Bohuslavice, Červený Kostelec, Lidéřovice, the pelvis sample from the ossuary in Náklo and the tibia sample from the ossuary in Lukavice (Figure 2). The Dalton minimum is fundamentally similar to the Maunder minimum. Nonetheless, solar activity was not as weak during the Dalton minimum and the climate was affected more by volcanic activity (Wagner and Zorita Reference Wagner and Zorita2005). The war of the Bavarian succession between Prussia and Austria took place in Bohemia at this time (Stellner Reference Stellner1998). In particular, the north-western front of this conflict with a fortified line between Ústí nad Labem and Milešovka Mountain could have affected the skeletal remains in the ossuary in Třebívlice. The other ossuaries cannot be associated with this conflict. Several influenza epidemics also occurred in the 19th century (in the spring of 1803, in June 1831, in April 1833, in January 1837, in October 1847 and from December (of 1888) to April of 1889). The influenza epidemics were followed by bacterial infections (dysentery in 1831 and cholera in 1837). The joint symptoms of the influenza epidemics in the 19th century were diarrhea, headache, nasal bleeding, loss of taste, pressure on the sternum, pneumonia or dangerous bronchial inflammation and erysipelas (Falconer Reference Falconer and Thompson1890).

Climatic fluctuations play a much more important role in the history of civilization than was thought in the past (Zhang et al. Reference Zhang, Brecke, Lee, He and Zhang2007). Especially cooling can lead to collapse of up to 80% of the agroecosystem (Zhang et al. Reference Zhang, Lee, Wang, Li, Zhang, Pei and Chen2011), which consequently loses the ability to feed the human population. During cold and wet periods, preindustrial civilization was not capable of dealing with problems connected with a shorter vegetation period, failure to ripen and impossibility of harvesting grain and drying feedstuffs and difficulty in obtaining salt as a conservation agent by evaporating sea water (Galloway 1986; Lucas Reference Lucas1930; Zhang et al. Reference Zhang, Jim, Lin, He, Wang and Lee2006). Cold and wet periods establish conditions for and initiate famines, epidemics, migration, armed conflicts and morbidity (e.g., Grolle Reference Grolle1997).

For the 1500–1800 AD period, Zhang et al. (Reference Zhang, Lee, Wang, Li, Zhang, Pei and Chen2011) compared the temperatures in the northern hemisphere determined from tree ring widths and correlated them with events in society. Mild phase 1 (1500–1559), Cold phase (1560–1660) and Mild phase 2 (1664–1800) were identified in this period, with temperature fluctuations by 0.43 σ, –0.59 σ and 0.24 σ, respectively. In these periods, they studied the ratio of grain yields to seed, the price of grain, magnitude of agricultural production, wage index, human height, number of conflicts and the number of epidemics. During the cold phase, all the indicators of growth and wellbeing decreased (the production of grain decreased by 28% and the price of grain increased by 133%). As soon as resources for a human or animal population decrease, elimination competition occurs until the group size is reduced for the remaining resources to be sufficient (Chu a Lee Reference Chu and Lee1994, Reference Chu and Lee1997). This is not different in human populations. In cold periods, the number of armed conflicts increased by 46% and there was a simultaneous 126% increase in famines and 205% increase in the number of epidemics (Zhang et al. Reference Zhang, Lee, Wang, Li, Zhang, Pei and Chen2011). Other authors have pointed out that some societies were especially sensitive to climatic fluctuations, while others were less affected and were strengthened by these negative experiences. In any case, socioeconomic bonds, the relevant institutions, culture and human behaviour are strengthened or weakened by the effects of climatic fluctuations on human society (Degroot Reference Degroot2018).

Although the correlations in these and other references need not indicate causality and connections must be sought in other demographic and anthropogenic factors, conflicts, famines and excessive mortality predominate during periods of climate change (Slavin Reference Slavin2016) and may be manifested in the deposition of skeletal remains in Bohemian and Moravian ossuaries.

The isotope compositions of carbon (δ¹³C) and nitrogen (δ¹⁵N) in the collagen of the studied bones are depicted in dependence on the various periods in Figure 3 and are shown in Table S2. Table S3 gives the collagen composition of the bones in relation to the estimated sex of the individuals. The isotopic compositions of carbon and nitrogen are similar in the groups divided according to sex. Women have an average of δ¹³C = –19.91‰, δ¹⁵N = 11.2‰ (n=3), while men have an average of δ¹³C = –19.53‰, δ¹⁵N = 11.58‰ (n=20); the bones of undetermined sex have an average of δ¹³C = –19.57‰, δ¹⁵N = 10.54‰ (n= 40). A child’s bones from the ossuary in Munich have a remote value of (δ¹³C = –18.88‰, δ¹⁵N = 15.31‰). The higher δ¹⁵N value is caused by nursing (Fuller et al. Reference Fuller, Fuller, Harris and Hedges2006). Some of the samples for carbon and nitrogen isotopic compositions were measured in replicates, so their numbers do not correspond to the number of ¹⁴C-dated bones.

Figure 3. The isotopic composition of carbon (δ¹³C) and nitrogen (δ¹⁵N) in dependence on the individual cold periods.

It is apparent in the dependences of δ¹³C and δ¹⁵N (Figure 3, Figure S2) that the data correspond to a trend of positive correlations of both variables, where lower values of δ¹³C correspond to lower δ¹⁵N, i.e., individuals that had a higher content of vegetable protein in their diets (i.e., more negative δ¹³C values) and simultaneously had lower consumption of animal proteins, which have higher δ¹⁵N values. The δ¹³C and δ¹⁵N dependencies for bone samples from each minimum are shown in Figure S2. The figure shows the tightest dependence of δ¹³C and δ¹⁵N values for samples from the Sporer minimum (correlation coefficient R = 0.8046), followed by samples from the Dalton minimum (R = 0.6041), and then by samples from the Wolf minimum (R = 0.4769). The entire data set shows a smaller correlation coefficient (R = 0.2289).

It is apparent from Figure 3 that the samples with low δ¹³C and δ¹⁵N values have carbon isotopes affected by greater consumption of plant proteins.

While δ¹³C reflects the contents of C₃ and C₄ plants in the diet (in the bones we studied, the only possible source of C₄ is millet), δ¹⁵N reflects a higher content of plant and animal proteins or the character of the animal proteins, whose δ¹⁵N reflects the position of the source in the trophy chain. The higher δ¹⁵N value in the collagen samples could be caused by greater consumption of animal proteins or by the content of proteins with higher δ¹⁵N values (e.g., animal proteins, unweaned herbivores, or fish). At the same time, some isotopic work shows that the influence of animal proteins is overestimated when studying diet composition, and the fact that increased ¹⁵N in collagen may occur due to the consumption of cereals grown in fields intensively subsidized with stable manure is underestimated (e.g. Bogaard et al. Reference Bogaard, Fraser, Heaton, Wallace, Vaiglova, Charles, Jones, Evershed, Styring, Andersen, Arbogast, Bartosiewicz, Gardeisen, Kanstrup, Maier, Marinova, Ninov, Schäfer and Stephan2013; Dreslerová et al. Reference Dreslerová, Hajnalová, Trubač, Chuman, Kočár, Kunzová and Šefrna2021).

The connection between the contents of the two isotopes in dependence on the age and correspondence to the individual cold periods is demonstrated by analysis of the main component (PCA, Figure S3) and analysis of the scatter. It follows from PCA analyses that the axis of the main components has a right-left diagonal dependence of δ¹³C vs. δ¹⁵N and the second main component is perpendicular to it (Figure S3). Subsequent analysis of the scatter demonstrated that bones from the Wolf, Spörer and Dalton periods do not have the same parameters (p = 0.0002) and thus the composition of the bones from the Wolf minimum is different from the bones from the Spörer and Dalton minima. These two minima cannot be separated on the basis of the carbon and nitrogen isotopic composition and lower consumption of animal products can be assumed at the Wolf minimum. On the other hand, changes could have occurred in agriculture during the younger cold periods, causing the increase in the δ¹⁵N value. This shift could be caused by better fertilization, which increases the δ¹⁵N value for the fertilized plants and then in the related consumers (Bogaard et al. Reference Bogaard, Heaton, Poulton and Merbach2007; Kanstrup et al. Reference Kanstrup, Thomsen, Andersen, Bogaard and Christensen2011; Szpak et al. Reference Szpak, Longstaffe, Millaire and White2012). Izdebski et al (Reference Izdebski, Guzowski, Poniat, Masci, Palli, Vignola, Bauch, Cocozza, Fernandes, Ljungqvist, Newfield, Seim, Abel-Schaad, Alba-Sánchez, Björkman, Brauer, Brown, Czerwiński, Ejarque, Fiłoc, Florenzano, Fredh, Fyfe, Jasiunas, Kołaczek, Kouli, Kozáková, Kupryjanowicz, Lagerås, Lamentowicz, Lindbladh, López-Sáez, Luelmo-Lautenschlaeger, Marcisz, Mazier, Mensing, Mercuri, Milecka, Miras, Noryśkiewicz, Novenko, Obremska, Panajiotidis, Papadopoulou, Pędziszewska, Pérez-Díaz, Piovesan, Pluskowski, Pokorny, Poska, Reitalu, Rösch, Sadori, Sá Ferreira, Sebag, Słowiński, Stančikaitė, Stivrins, Tunno, Veski, Wacnik and Masi2022) used pollen distributions showing intensive or stagnant agriculture from dated geochemical archives (lake sediments and wetlands) to demonstrate land use changes after the 1347–1352 plague epidemic (end of the Wolf Minimum). They found that while in some areas the epidemic had devastating effects on the population and its farming (central Germany, central Italy, Scandinavia) by contrast, in other parts of Europe (e.g. Bohemia, the Baltic) there was a signal (pollen content) documenting an increase in cereal production. This jump in the rural economy could be related to the observed difference between the C and N isotopic composition of collagens of individuals from the Wolf minimum and the younger minima.

Le Huray and Schutkowski (Reference Le Huray and Schutkowski2005) provide an analysis of the δ¹³C and δ¹⁵N values of the bone collagen in the population living in the present-day Czech Republic, giving the composition of the collagen of the inhabitants of the La Tène period in Kutná Hora and Radovesice (380–150 BC; Figure S4), while Plecerová et al. (Reference Plecerová, Kaupová Drtikolová, Šmerda and Stloukal2020) describe the bones of the Moravian population living in the 5th–6th centuries AD (Figure S5), and Halffman and Velemínský (Reference Halffman and Velemínský2015) analysed the bones of the inhabitants of Great Moravia (9th–early 10th century AD) (Figure S6). Salesse et al. (Reference Salesse, Dufour, Castex, Velemínský, Santos, Kuchařová, Jun and Brůžek2013) give an analysis of the bones of individuals buried in the St. Benedict cemetery in Prague in the 15–18th century (Figure S7).

The population of La Tène period of Kutná Hora and Karlov (380–150 BC; Le Huray and Schutkowski Reference Le Huray and Schutkowski2005) exhibited differences in the nitrogen isotope compositions, especially in relation to the accessories in the graves. Individuals buried with swords, shields and spears exhibited higher δ¹⁵N values and it can thus be assumed that they consumed more animal proteins. In comparison with the sample from ossuaries and the population of the La Tène period (380–150 BC; Figure S4), a generally lower δ¹⁵N can be observed and thus a lesser consumption of animal products. At the same time, most of the samples of the La Tène period bones have higher δ¹³C values than the samples from ossuaries and several samples exhibit values of δ¹³C>–18‰, probably as a result of consumption of C₄ plants, in this case millet (Le Huray and Schutkowski Reference Le Huray and Schutkowski2005).

The bones of the Moravian Lombard population (5th-6th centuries AD) studied by Plecerová et al. (Reference Plecerová, Kaupová Drtikolová, Šmerda and Stloukal2020) are also different from the populations of the ossuaries. Samples of the bones of Lombard population have a similar δ¹³C composition to the samples from ossuaries (with the exception of three samples with δ¹³C >–18‰ documenting the consumption of C₄ plants), but lower values of δ¹⁵N (Figure S5), probably due to the effect of lower consumption of animal proteins. The greatest difference between the studied ossuary samples is apparent in comparison with the bone population of Great Moravia (9th–early 10th centuries AD) (Halffman and Velemínský Reference Halffman and Velemínský2015). The bones of the inhabitants of Great Moravia have higher δ¹³C values (mean males –17.8‰ PDB; females –17.9‰ PDB) and simultaneously lower δ¹⁵N values (mean male 10.3‰; mean female 9.7‰) compared with the bones of the studied ossuaries (Figure S6). The diet of the tested population contained the plant proteins of C₃ and C₄ plants (millet), the plant proteins of legume plants and the animal proteins of mammals, poultry and fish (Halffman and Velemínský Reference Halffman and Velemínský2015).

The compositions of the isotopes of carbon and nitrogen in the collagen of the tested bones is closest to the composition of the collagens of individuals buried in the St. Benedict cemetery (Prague, 15th–18th centuries AD; Salesse et al. Reference Salesse, Dufour, Castex, Velemínský, Santos, Kuchařová, Jun and Brůžek2013). The isotopic compositions of the collagens of the bones in this cemetery and in the ossuaries are depicted in Figure S7. Samples were taken from the individual graves (IG) and three mass graves (MG1, MG2 and MG3). In only one case did the samples in this work differ in their compositions of δ¹³C (IG –19.6 ‰ and MG1 –20.0 ‰), but in all cases in their values of δ¹⁵N (IG vs MG1, MG2 a MG3). The average δ¹⁵N values in the samples from IG attained 12.2‰, while MG 10.3-9.2‰. Our tested samples had average values similar to the low values of δ¹³C (mean = –19.57‰, n = 52). For example, Ogrinc and Budja (Reference Ogrinc and Budja2005) give δ¹³C for wheat and rye as –25‰. Bird et al. (Reference Bird, Haig, Ulm and Wurster2022) present values of the equivalent composition of the diets in the range of –31 to –19 ‰ for the entire set (n=1298). A value for the δ¹³C difference of 4.8‰ and simultaneously 1.9‰ in relation to the preindustrial atmosphere is common for the calculation of collagen in the diet (Bird et al. Reference Bird, Haig, Ulm and Wurster2022). This difference is manifested for both domestic fauna and humans. For example, for the Mikulčice Kostelisko–Czech Republic ((9th–early 10th c. AD) locality, Halffman and Velemínský (Reference Halffman and Velemínský2015) give an average collagen composition for cows, sheep, pigs and horses of –20.3; –20.5; –20.4 and –20.3‰. Because the δ¹³C isotopic composition of the bones varies in the range –20.3 to –18.7‰, the equivalent composition of the diet has values of –27 to –25.4 and thus corresponds to the consumption of the grains of C₃ plants, their isotopic composition is lighter compared to that of seeds (e.g., Bird et al Reference Bird, Haig, Ulm and Wurster2022).

Elevated δ¹⁵N values (mean = 11.36‰, n= 52) of the collagen of the studied bones can be explained by greater consumption of animal proteins or proteins with elevated δ¹⁵N values that are exhibited by unweaned herbivores, poultry or fish (Reitsema et al. Reference Reitsema, Kozlowski and Makowietzki2013). It was common to consume fish in medieval Europe. E.g., Reitsema et al. (Reference Reitsema, Kozlowski and Makowietzki2013) give values of δ¹⁵N for freshwater fish in the range 6.6–12.1‰ (mean 9.87‰; n = 15). Dufour et al. (Reference Dufour, Bocherens and Mariotti1999) give a range for freshwater fish of δ¹⁵N 7–14.9‰ while France (Reference France1995) states values of 4–15‰ for freshwater fish, 6–13% for estuary fish and 7–14% for anadromous fish.

After 1300, the distribution of salted and dried fish (herrings and cod) spread across European medieval Europe. They were transported from coastal areas to the interior and became a cheaper alternative to freshwater fish; they were consumed especially by the poorer members of society (Adamson Reference Adamson2004). While freshwater fish mostly do not exceed values of δ¹⁵N = 15‰, in saltwater fish this parameter can be as large as 20‰ (France Reference France1995). Where the shift in the δ¹⁵N values of the diet vs collagen is usually given as 2–5‰ (Hedges and Reynard Reference Hedges and Reynard2007; Schoeninger and DeNiro Reference Schoeninger and DeNiro1984) and the average composition in the bones investigated here reaches a value of 11.36‰ (n = 52), then it is probable that this isotopic composition could also be affected by a substantial content of fish in the diet. At the same time, it cannot be overlooked that elevated δ¹⁵N values may be related to a crop derived diet produced by the practice of intensive fertilization affecting the nitrogen isotopic composition of plant proteins.