Thera/Santorini eruption date
The earlier 16th century BCE Mediterranean offset identified above in the Hd GOR and GrM NOC datasets is particularly relevant to a long-running controversy: the dating of the Minoan eruption of the Thera (Santorini) volcano and associated debates around its impacts and the chronology of the beginning of the Aegean Late Bronze Age (
13,
19,
54–
58). Scholarship until now has used the various versions of the NH IntCal
14C calibration curve. Although the 16th century BCE reversal in the calibration curve has long been noted as potentially creating a dating ambiguity for the Thera eruption between the later 17th century BCE and the earlier mid–16th century BCE, the weight of probability has supported the older age range in the later 17th century BCE and, thus, a date over 100 year earlier than the pre-radiocarbon archaeological estimates ~1500 BCE (
57). A recent contribution, seeking a compromise, is a proposal to revise the IntCal
14C calibration curve ~1660 to 1540 BCE based on measurements of BCP and IrO at AA, which yield older
14C ages for this interval than IntCal13 (
13). However, the identification of a specific Mediterranean offset in the earlier 16th century BCE (
Figs. 2, A and B, and
3A) indicates that the correct date will not solely derive from IntCal nor do these BCP and IrO samples reflect the typical Mediterranean growing season (see above). There is instead a specific Mediterranean context for this controversy. Thus, rather than (or in addition to) adjusting the overall IntCal calibration curve (important although this is, in general, when justified), instead, it is the effect of the Mediterranean offset at this period that is relevant. In particular, as evident in
Figs. 2 (A and B) and
3A, the Hd GOR and GrM NOC data indicate that Mediterranean
14C ages in the earlier 16th century BCE are older than the IntCal values and, thus, are potentially approximately contemporary with
14C ages for the last decades of the 17th century BCE. This could exacerbate the dating ambiguity between the later 17th century BCE and the earlier mid–16th century BCE. At the same time, the absence of elevated
14C ages for the Mediterranean Hd GOR and NOC samples ~3600 to 3555 cal B.P., contrary to the AA BCP and IrO dataset (
Fig. 3A) (
13), is also important for this topic.
We can assess the implications and remaining uncertainties. The calibrated calendar probabilities for dating the Santorini eruption following two published methods (
55,
56) can be compared with the Hd GOR scenario. If we rerun these analyses with the Hd GOR calibration dataset (as in
Fig. 2A) with its revision of the earlier mid–16th century BCE
14C values to reflect Mediterranean conditions and regional offset, we find that the results support a later 17th century BCE date range for the Thera eruption, including the entire most likely 68.2% highest posterior density (hpd) ranges (
Fig. 5, A and B; 1649 to 1617 BCE and 1680 to 1613 BCE, respectively). We may also reconsider the dating of the Thera olive branch sample, found buried in the Thera eruption pumice (
58), modeled as an ordered sequence of older to more recent wood to obtain a dating estimate for the outermost dated sample (
13,
54) against the Hd GOR dataset (
Fig. 5C). This places all the 68.2% hpd range in the 17th century BCE and most (76.6% versus 18.7%) of the 95.4% hpd range at or before 1610 BCE and hence again likely indicates a later 17th century BCE date range. Thus, the Mediterranean-relevant Hd GOR dataset does not indicate the ambiguous 17th or 16th century calibrated ranges for the Thera eruption proposed in (
13) from the AA BCP and IrO data. In the case of the olive branch, we may note one additional important element: The dates on the sample were run as LLGPC measurements at the Hd
14C laboratory (
58). Hence, there is no possible AMS
14C–to–LLGPC issue in this case. Instead, we have comparable Mediterranean Hd measurements against Mediterranean Hd measurements. The finding is most likely a later 17th century BCE date range for this sample (
19).
The remaining caveat is the issue of whether there is, in addition, a typical AMS
14C–to–LLGPC offset that we should take into account when considering AMS
14C dates. As discussed above, the evidence is mixed. Nonetheless, consideration above of some Mediterranean AMS
14C cases suggested that perhaps an additional factor of ≤10
14C years might apply to the LLGPC Hd GOR dataset versus AMS
14C dates. These comparisons especially including OxA AMS
14C data are particularly relevant to the Thera case since OxA AMS
14C data, or the demonstrated very comparable Vienna (VERA) AMS
14C data (
23,
46,
55), comprise 79% and 75% of the two datasets (
55,
56) for the Thera volcanic destruction level. If we rerun the analyses in
Fig. 5 (A and B) with an additional hypothetical +10
14C years adjustment to the Hd GOR dataset, then we do not find a substantial change (
Fig. 5, D and E). The dating probabilities still continue to indicate the later 17th century BCE as the most likely date range. To effect substantive change, a putative AMS to LLGPC offset would need to be rather larger. If it were to reach around 15
14C years, then the date of the Santorini eruption starts to become more ambiguous. There is still greater probability in the later 17th century BCE, but moderate probability now lies in the earlier mid–16th century BCE (
Fig. 5, F and G). Only if the hypothetical adjustment is 20 or 25
14C years, does the dating probability switch to indicating that an earlier mid–16th century BCE date range is more likely (
Fig. 5, H to K). However, despite some data suggesting larger AMS
14C–to–LLGPC differences of around this level (
10,
12,
13), the Mediterranean cases reviewed above only suggest a difference of about half this level (or less), other comparisons are mixed, and some are even close to zero (see above). Much of the current observed variation is as likely to relate to interlaboratory variations in methods and instruments [an ever-present issue (
8,
11,
48)], something only more evident as AMS
14C approaches the precision of LLGPC and LSC datasets. The conclusion at present is that more work is needed to clarify and quantify the status of any typical AMS
14C–to–LLGPC
14C offset on comparable samples, that is, an offset that is common across multiple AMS
14C laboratories and not cases of individual interlaboratory variations (up and down) within an overall range of values. In the case of the Thera olive branch sample, this avoids any possible AMS
14C–to–LLGPC issue since it was measured at Hd using LLGPC (
58). Here, comparison with the Mediterranean relevant Hd GOR dataset indicates a most likely later 17th century BCE date range (
Fig. 5C), which is consistent with the analysis of sets of AMS
14C data against the Hd GOR dataset (
Fig. 5, A and B). This suggests the reality of an additional AMS
14C–to–LLGPC contribution of no more than about ≤10
14C years, as discussed above.
The situation could change if future work can, to the contrary, robustly demonstrate a much larger standard AMS
14C–to–LLGPC offset. We also need to better define (and enlarge the database concerning) the Mediterranean offset independent of these questions of interlaboratory and intermethod variations. Already, we can likely set the parameters of the extreme alternative scenario with respect to the Santorini eruption case using the available data and the same models (
13,
54–
56,
58). The BCP record of (
13) likely exhibits a maximum alternative case for a revised AMS
14C IntCal summer NH baseline (
Fig. 6, A to C) (
19). We can then, in addition, consider the possible relevance of the positive average offset of ~21
14C years between the Mediterranean and NH in the period ~3550 to 3486 cal B.P. (1601 to 1537 BCE) as identified from the comparison of the Hd datasets (
Fig. 2B) and apply this adjustment to the AA BCP data (
Fig. 6, D to F). In this hypothetical experiment, we treat the remainder of the period ~3600 to 3450 cal B.P. (1651 to 1501 BCE) as not being substantively offset (
Figs. 2B and
3A). The probability distributions in
Fig. 6 illustrate the extreme alternative dating scenarios. The published AA BCP record (
Fig. 6, A to C) creates an ambiguous situation: A clear probability region remains in the late 17th century BCE to early 16th century BCE, but there is also considerable probability in the mid–16th century BCE. However, were the approximate range of the AA BCP data to form a new IntCal baseline and a Mediterranean positive offset to also apply in addition (a logical implication, but needing empirical testing), then the dating probability starts more strongly to support an early to mid–16th century BCE date. Such a scenario [or the hypothetical large LLGPC to AMS
14C adjustments considered in
Fig. 5 (H to K)], interestingly, does not, however, return us to the traditional “low” archaeological chronology (
19,
57). Instead, it would point to a possible alternative between the current “high” and low scenarios: a shortened “compromise early chronology” for the date of the Santorini eruption and the earlier Aegean Late Bronze Age (
19,
56). Even at the extreme hypothetical adjustment range (
Figs. 5, H to K, and
6, A to F), the traditional date range of the Santorini eruption ~1500 BCE (
57), or any date after ~1530 BCE, appears highly improbable.
Overall, our findings, both the periods of positive
14C offsets that we focus on in this paper, as well as the instances of periods of negative offsets noted above, in addition to other indications of similar Mediterranean or seasonal
14C offsets (
8,
14–
19), highlight the relevance of these recurring offset elements to high-resolution absolute dating for Old World prehistory. This topic assumes greater and greater importance as
14C dating and chronometric aspirations become ever more accurate and precise. We may draw two key conclusions. First, there is now clearly a need going forward for the development of a detailed consensus Mediterranean
14C time series to secure an appropriate closely defined Old World archaeological time scale. No simple static adjustment is possible as a satisfactory solution for the recurring, periodic offsets observed in both the BCE and CE windows reported in works to date. Second, it is necessary to establish greater temporal and spatial delineation of seasonal/growing season variations for accurate high-precision
14C dating worldwide.