4.1 Diagenetic uptake of trace elements
In fossil calculus, Mn/Ca ratios are one to three orders of magnitude greater than in dentine or enamel (Fig. 1). Because calculus is much more porous than dentine and enamel, it is more prone to diagenetic uptake of various elements. Alternatively, since the mineralogy of dental calculus is more diverse than that of enamel or dentine (Grøn, et al., 1967), it is possible that its Mn/Ca is higher than that of dentine and enamel during formation, independent of diagenetic alteration. However, fossil calculus displays Mn/Ca ratios 14 to 1,100 times higher than modern calculus (Fig. 1). Similarly, fossil calculus shows Sr/Ca ratios 0.25 to 2 times higher than those of dentine or enamel, and ~ 10 times higher than modern calculus (Fig. 2). Here too this likely reflects diagenetic alteration and uptake of Sr. The same is observed for REE, which are enriched by several orders of magnitude in fossil calculus compared to modern calculus (Table 1). Finally, U/Ca in fossil calculus is more than 5,000 times greater than in modern calculus (Table 1), illustrating that U uptake occurs within a few centuries of burial. Overall, it is clear that fossil calculus is diagenetically altered, with a significant uptake of Sr and other trace elements such as Mn and U, especially when compared to modern calculus. Below we explore how diagenetic Sr may impact the Sr isotope composition of fossil calculus.
Mn/Ca ratios in dentine are similar, lower or greater than those in enamel (Fig. 1), suggesting that it is difficult to argue for an unequivocal greater uptake of Mn in dentine compared to enamel, during diagenesis (except perhaps for sample ACAD8328). Conversely, Sr/Ca and Ba/ Ca ratios in dentine are systematically greater than in enamel (Fig. 2 and Fig. 3), illustrating the diagenetic uptake of Sr and Ba in dentine.
4.2 Strontium isotope composition of enamel, dentine and calculus
The Sr isotope values measured in different sample materials is ordered as follows: fossil calculus (in-situ) > fossil calculus (solution) > dentine ≥ enamel (Fig. 4). The shift between fossil calculus (in-situ) and enamel ranges from 0.0002 to 0.0016, and between fossil calculus (solution) and enamel ranges, from 0.0002 to 0.0014. These ranges are comparable to differences between ratios in enamel determined by in-situ vs solution (up to 0.0017; Copeland, et al., 2010). Note that the difference between fossil calculus determined in-situ vs solution (0.00002–0.0007) is less than the difference for enamel analysed in-situ vs solution in Copeland, et al. (2010). Greater 87Sr/86Sr values determined in-situ compared to solution analysis of fossil calculus could potentially result from the isobaric interference of 87Rb onto 87Sr. Rubidium is not removed during in-situ analysis, while it is removed by chromatography prior to solution analysis. This is illustrated by the 87Rb/86Sr ratios in fossil calculus analysed in solution several orders of magnitude lower than during in-situ analysis (Fig. 5). While there are no clear positive relationships between 87Sr/86Sr and 87Rb/87Sr for in-situ and solution calculus analyses, when each sample is considered separately, in-situ analyses generally show higher 87Sr/86Sr and 87Rb/87Sr values than solution analyses (except for sample ACAD8477; Fig. 6).
The shift between dentine and enamel ranges from 0.00004 to 0.001. This is less than what Copeland, et al. (2010) found in rodents (generally < 0.001 but up to 0.023), suggesting that diagenetic alteration of dentine did not significantly affect its Sr isotope composition. Greater 87Sr/86Sr values in dentine compared to enamel could result from the isobaric interference of 87Rb onto 87Sr, since dentine is more porous than enamel and thus more prone to take up metals such as Rb, following burial (and Rb is not removed during in-situ analysis). However, there is no systematic variations of the Rb/Sr ratio between dentine and enamel. Rb/Sr ratios in dentine are greater, similar or lower than those in enamel, depending on the sample (Fig. 5). For instance, in samples ACAD8328 and ACAD8817 where 87Sr/86Sr values in dentine are greater than in enamel, 87Rb/87Sr are lower in dentine compared to enamel. Alternate hypotheses are (i) that dentine records a different diet origin than enamel, or (ii) Sr with a high 87Sr/86Sr ratio is incorporated post-burial. The former is unlikely since dentine starts mineralising only shortly before enamel (Cate and Richard, 1980). For each sample, solution analysis of fossil calculus yields 87Sr/86Sr values greater than those in dentine or enamel. This observation would also support to the incorporation of Sr with a high 87Sr/86Sr ratio post-burial (since Rb is removed prior to solution analysis and high values cannot be explained by isobaric interference from 87Rb).
For many analyses of enamel and dentine in sample ACAD8327, there is a negative relationship between 87Sr/86Sr and 87Rb/86Sr (Fig. 7). This is interesting as more 87Rb should result in an overestimation of the 87Sr/86Sr ratio and thus produce a positive relationship. One of the corollaries of this observation is that correction of in-situ 87Sr/86Sr analyses for the presence of 87Rb seems to be effective. Samples ACAD8328 and ACAD8817 show the same negative relationship but enamel analyses clearly have higher Rb/Sr and lower 87Sr/86Sr values than the dentine (Fig. 7). This would imply that enamel is more diagenetically altered than dentine, which is unlikely since it is denser and it is generally accepted that enamel is more resistant to diagenesis (e.g. Reynard and Balter, 2014). However, while Ba/Ca and Sr/Ca are lower in enamel compared to dentine, this is not systematically the case for Mn/Ca ratios.
Since higher Rb/Sr ratios would generally be interpreted as pointing towards the composition of a diagenetic end-member (after ruling out isobaric interference from 87Rb), the negative relationships observed between 87Sr/86Sr and 87Rb/86Sr ratios could hint that the diagenetic end-member has a low 87Sr/86Sr value, 0.7085 or lower (Fig. 7). In absence of calculus data, this could be how fossil teeth data would have been interpreted. Nevertheless, this is somewhat lower that the expected modelled value for the York area: 0.7096–0.7117 (IQR) (Evans, et al., 2018). The closest region with values between 0.708 and 0.7085 would be about 20 km east of York (Evans, et al., 2018). If the diagenetic endmember does have a 87Sr/86Sr ratio that low, it would imply that teeth were buried 20 km east of York for a period of time (during which Sr would be uptaken) before being re-located to York.
Where the negative relationship between 87Sr/86Sr and 87Rb/86Sr is observed, it seems to converge toward that measured in the calculus by solution analysis (~ 0.7100-0.7108) (Fig. 7). Perhaps this indicates that the diagenetic end-member has a high 87Sr/86Sr ratio and, surprisingly, a low Rb/Sr ratio. Values between 0.7100 and 0.7108 could fall within the range of values expected for bioavailable Sr around York (Evans, et al., 2018). Overall, the implications are that (i) where only fossil teeth data are available, using the Rb/Sr ratio could be misleading when trying to identify a diagenetic end-member; (ii) the 87Sr/86Sr ratio of the fossil calculus is most likely dominated by Sr uptaken post-burial and is unlikely to yield useful information about the past location of an individual (unless it has been relocated post-burial); (iii) fossil calculus can be helpful in determining the Sr isotope composition of the diagenetic endmember.
A corollary from this work is that the Sr isotope composition of fossil enamel for the different samples suggests that the individuals studied were not originally from York. For samples ACAD8326, ACAD8477 and ACAD8817, combining enamel 87Sr/86Sr ratios with the Sr isoscape of the UK (Evans, et al., 2018) suggests that these individuals would have most likely originated at a minimum > 20 km east from York, or from southeastern England (Fig. 8). Interestingly, for samples ACAD8327 and ACAD8328, enamel 87Sr/86Sr ratios suggest these individuals would have originated from the Glasgow area, at the nearest, 100’s of km away from York (Fig. 8).