Tree-ring records constructed from ancient wooden timbers can provide calendar-dated frameworks to underpin archaeological and palaeoenvironmental chronologies beyond the reach of written evidence. They can provide securely dated records of construction, abandonment, and trade across different cultural regions while simultaneously providing calendar-dated, annual resolution records of contemporary climatic variability. As such, they represent an invaluable resource for studies of past human and environmental interactions and for the resolution of complex chronological issues. However, for certain key geographic regions and time periods, the only tree-ring records preserved are not calendar dated to the exact year but rather, 'float' in time, dated with less precision and accuracy by radiocarbon wiggle-match dating (a dating method that uses the non-linear relationship between Carbon¹⁴ age and calendar age to match the shape of a series of closely sequentially spaced Carbon¹⁴ dates with the Carbon¹⁴ calibration curve). While this approach can produce excellent results for certain time periods, limitations of the method include multiyear error ranges and the fact that calibrated date ranges may shift forward or backward in time depending on which iteration of the international radiocarbon calibration curve is used for calibration. The full benefits of the annually derived tree-ring record for establishing rigid archaeological chronologies for cultural interaction plus the impacts of climatic or geological events on ancient civilizations can be fully realized only by securely fixing such records in a precise and accurate calendar-dated range.
In a paper published in the Proceedings of the National Academy of Sciences of the United States of America on 30 March 2020, Charlotte Pearson of the Laboratory of Tree-Ring Research, Geosciences, and the School of Anthropology at the University of Arizona, Matthew Salzer, also of the Laboratory of Tree-Ring Research at the University of Arizona, Lukas Wacker of Ion Beam Physics at Eidgenössische Technische Hochschule Zurich, Peter Brewer, again of the Laboratory of Tree-Ring Research at the University of Arizona, Adam Sookdeo, also of Ion Beam Physics at Eidgenössische Technische Hochschule Zurich, and of the Chronos ¹⁴Carbon-Cycle Facility at the University of New South Wales, and Peter Kuniholm, once again of the Laboratory of Tree-Ring Research, and the School of Anthropology at the University of Arizona, present the results of an attempt to resolve a Bronze Age floating tree-ring record in the Eastern Mediterranean, using using timbers taken from a chamber surrounding the grave of a predecessor of King Midas in the Phrygian capital city of Gordion (modern day Yassihöyük, Turkey).
Map to show the proximity of the Gordion site to Thera, the main direction of fallout of the Thera ash, and other locations. Pearson et al. (2020).
This record is one of a group of interlocking tree-ring series from the ancient East Mediterranean, which when first published as a dated sequence, ncluded wooden timbers from 22 archaeological sites in central Anatolia (Turkey) spanning the years from approximately 2220 to 718 BC. The Gordion part of this sequence was subsequently redated multiple times, with each redate necessitating a reevaluation of the associated archaeological evidence. Aside from being the key to dating a number of critical archaeological sites in the East Mediterranean, the tree-ring series from Gordion has an extra relevance in that it is the only tree-ring record from the ancient Mediterranean that fully spans the period during which all scholars would agree that the Minoan eruption of Thera occurred. This event provides a pivotal marker horizon through which the chronologies of ancient Egypt, the Levant, Greece, and Anatolia could be linked. Dating this tree-ring series to a fixed point in time rather than a shifting calibrated range would, therefore, offer significant new opportunities for dating the eruption and the synchronization point that it offers because it is possible that the tree rings hold an anatomical or chemical marker for the event, which could be used to further refine the dating. This is particularly important as radiocarbon dating for Thera is impeded by a plateau in the radiocarbon calibration curve between about 1620 and 1540 BC.
In particular, if a chemical response related to environmental changes brought about by the eruption could be identified in the wood, as has been observed in both the lake environment at Gölhisar and in the Speleothem record (deposits of secondary minerals that can be dated from their isotope content) in Sofular cave on the Black Sea coast, then it might be possible to suggest a more exact date for the event. While there are many factors that can lead to disturbances in the anatomy of tree rings, there are only a few that can lead to major chemical changes in the environment.
In an earlier attempt to trace the Thera eruption, Pearson et al., published in a paper in the Journal of Archealogical Science in 2009, conducted elemental analysis on a wide growth-ring anomaly from one of the tree-ring site chronologies overlapping with the Gordion record (Porsuk in southern central Turkey). In that study, they found significant changes in elemental chemistry associated with a wide growth-ring anomaly, which was then dated to about 1650 BC; at the time, this was within the possible radiocarbon range suggested for the Thera eruption, at odds with certain lines of archaeological evidence. The elemental response was consistent with what might be expected from a volcanic event but as noted at the time, also consistent with what might be expected following a forest fire. The date for this elemental change and growth response is now outside the possible range for the Thera eruption, although it may originate from some other unidentified eruption; such as the Yali-Nisyros volcano, at the eastern edge of the Aegean volcanic arc. The revised radiocarbon ranges for Thera-relevant materials suggested by Pearson et al. in the earlier study indicate that the majority of the 16th century BC should now be searched for evidence of the eruption.
Left: The 854 anomaly in sample C-TU-POR-3, from Porsuk in southeast Turkey; right: a similar (though extended) growth-ring anomaly from a tree which grew about 30 km from Katmai Volcano, Alaska. The tree in question was inundated with a few feet of pumice following the 1912 AD eruption of Novarupta, attributed to Katmai. The pumice killed or suppressed low vegetation cover, enhancing conditions for established trees. Inset: a short growth anomaly from a single application of fertilizer to a tree in an experimental forest. Pearson et al. (2009).
In the new study, a combination of two approaches was used for improving and securing the date range for the floating tree-ring series from Gordion. First, Pearson et al. compared a sequence of annual Carbon¹⁴ measurements from single rings of the Gordion series with a contemporary time series of annual Carbon¹⁴ from absolute, calendardated Bristlecone Pine, Pinis spp., and Irish Oak, Quercus spp., across the period 1700 to 1500 BC.
Similar applications have relied on detecting the presence of significant rapid excursions in the annual tree-ring Carbon¹⁴, in particular the largest of these discovered so far an approximate 1.2% change between the years 774 and 775 AD. This event has also been used to provide an independent verification of the calendar dating for established multiregional tree-ring records and to synchronize tree-ring Carbon¹⁴ with Berylium¹⁰ (which forms by spallation of nitrogen and oxygen in the atmosphere and precipitates onto and into surface layers) in the ice cores. In the case of the 774/775 AD marker event, the potential is clear, but for time periods where no such dramatic markers are present, like 1700 to 1500 BC, a different strategy has to be applied. Pearson et al. make use of less pronounced and consequently, less secure Carbon¹⁴ time markers for a proposed annual Carbon¹⁴ pattern-matching approach..
Second, this is tested using an anticorrelation between tree growth response to the same volcanic forcing events in both the Mediterranean Juniper, Juniperus spp., trees and calendar-dated North American Bristlecone Pine. This test uses a well-established temporal association between high-elevation Bristlecone Pine frost rings and large-scale volcanic eruptions. It has been clearly demonstrated that latewood frost rings in Bristlecone Pine occur the year of or the year following a volcanic event, and this causal connection has been strongly confirmed across the last 2500 years. Beyond this period, Bristlecone tree-ring chronologies are accurately dated to the calendar year for over 5000 years, and therefore, the record of precisely dated Bristlecone response to volcanism covers the period across which the Juniper sequence lies according to both conventional radiocarbon wiggle matching and the annual Carbon¹⁴ pattern-matching approach used by Pearson et al.
In western Turkey, the years of or following many of the same major volcanic eruptions that affected Bristlecone growth in the more recent period are marked by wide growth rings in Austrian Pine, Pinus nigra. This indicates that an increase in May–June precipitation caused more favorable growth in this region as part of a chain of climatic disturbances associated with Northern Hemisphere cooling following major mid- or northern latitude volcanic eruptions. Assuming that similar climatic forcing prevailed during the Bronze Age and knowing that Pine and Juniper tree-ring chronologies from this region show strong interspecies correlation, Pearson et ai. hypothesised that wide rings in the floating Juniper sequence should correlate with calendar-dated frost events in Bristlecone Pine and that, if so, this could provide a means to test the annual Carbon¹⁴-matching approach and to refine to a fixed tree-ring date based on synchronization with the calendar-dated Bristlecone record (in a similar approach to previous studies that used Bristlecone Pine frost rings as fixed date volcanic markers to refine dating for volcanic acidity layers in ice cores).
Finally, Pearson et al. report the chemical study of this newly secured tree-ring sequence with the objective of seeing if any chemical indicator could be found that might help to further constrain the dating possibilities for the Thera eruption.
Annual Carbon¹⁴ measurements were made on 186 consecutive years (relative years 834 to 1019) of the 1028-year Gordion Juniper sequence (which starts with relative year 737). These measurements into the IntCal13 Radiocarbon Age Calibration Curve using the OxCal 4.3 radiocarbon calibration program to provide an end date for the entire calibrated tree-ring chronology, within an 8-year range: 758 to 751 BC at 95.4% confidence level. This was in good agreement with previous wiggle matching of 128 decadal or 11-year blocks spaced over 987 years of the same Juniper sequence, which placed the end of the tree-ring sequence at 751 BC +6/–8 at a 95.4% confidence level. A chi-squared (χ²) test (used to determine whether there is a statistically significant difference that is unlikely to be due to chance alone between expected frequencies and observed frequencies) for the Mediterranean Carbon¹⁴ time series vs. the weighted mean of the annually resolved combined Oak and Pine data placed the last ring of the Mediterranean sequence at a more precise date of 745 ± 4 BC (95.4% confidence level); this is statistically slightly younger (10 ± 6 years) than when the same data are wiggle matched to IntCal13. Pearson et al. considered the position using the annual Carbon¹⁴ data as significantly more reliable as it is a result of comparing fine structure that is not available in IntCal13, which is primarily based on decadal data. Using the fine structure yields dating results free from the regional or laboratory offsets that may be combined in the coarser-resolution calibration data. The reasonably close agreement of the results via the different methods does, however, demonstrate that, for wiggle matches spanning multiple decades, the improved curve shape offered by the annual Carbon¹⁴ data may have a relatively small effect on the final calibrated date range.
Positioned relative to an end date of 745 ± 4 BC the visual correlation of the annual data around the increased production event of circa 1528 BC is clearly evident. The Gordion data more closely agree with the annual Oak and Pine data than with IntCal13 and show the same offset from the curve as shown by the other annual data between 1650 and 1540 BC. They are also valuable in providing an annually based record of Carbon¹⁴ fluctuation from the Mediterranean region in this time period relevant for the Thera eruption. While no large-scale localized offsets in Carbon¹⁴ are evident, for the years where contemporary Oak, Pine, and Juniper measurements from the same laboratory can be directly compared (1680 to 1580 BC), the Mediterranean Juniper is offset from the Irish oak by +9.0 ± 3.5 Carbon¹⁴ years, whereas they are only +3.4 ± 2 Carbon¹⁴ years different from the North American Pine. While this slight difference is within the stated measurement errors, it is possible that the closer agreement between the Pine and Juniper may reflect a shared, more southerly latitude than the Irish Oak. These data agree with previous findings that there is no major regional offset in the period. Pearson et al. also note that the data indicate that, around the period of lower solar activity (around 1600 BC) and during the period of more rapid Carbon¹⁴ production (roughly 1540 to 1528 BC), there is no significant difference between the multiregional annual Carbon¹⁴ data, which might be related to growth season. Pearson et al. do, however, note the possibility of a localized excursion in Carbon¹⁴ around 1548 BC. This requires further investigation as, if it is not an analytical outlier, it could represent an influx of 'old carbon' into the environment, potentially consistent with a volcanic eruption such as Thera.
The validity of the dated position produced by chi squared analysis (745 ± 4 BC at a 95.4% confidence level) and supported by annual Carbon¹⁴ pattern matching around the 1528 BC Carbon¹⁴ excursion was then independently tested using the previously described correlation between years of known eruptions, calendar-dated Bristlecone Pine frost-ring years, and wide tree rings in Mediterranean sequences. Pearson et al. hypothesised that, if their temporal placement of the Juniper chronology was correct at 745 ± 4 BC, then it should show wide rings in the year of or following a Bristlecone Pine frost ring. On this basis, superposed epoch analysis (a statistical tool used in data analysis either to detect periodicities within a time sequence or to reveal a correlation, usually in time, between two data sequences) was used to test the significance of the effects of a mean tree-ring response to the proxy record of volcanic forcing across the full Bronze–Iron Age Juniper chronology in the adjusted position suggested by this study. In this position, the superposed epoch analysis analysis showed significantly wider rings than would be expected by chance in the Mediterranean chronology in the year following a Bristlecone frost ring. This nonrandom association provides strong corroborative evidence for the annual Carbon¹⁴ position to, in fact, be correct to the year. Within the 4 years on either side of the 745 BC dating placement, no other positions provide this strong association. This provided additional support that the position of the Gordion chronology determined by the chi-squared analysis is indeed correct to within 1 year and allowed Perason et al. to derive an exact calendar-dated position for the tree-ring series.
Having arrived at a secure date range for the tree-ring series, Pearson et al. made multiple scans using a desktop ATLAS Micro-X-ray Fluorescence unit across the transverse surface of a subsample of GOR-76. The scans covered the period from circa 1630 to 1500 BC. These revealed a single major disturbance of the element Calcium around 1560 BC. The exact onset of the change may be as early as 1562/1 BC, and the effect appears to last until around 1557 BC. Other analytical techniques will be used to refine this temporal association. Calcium is an essential element in wood that is needed to support fundamental biological functions, including cell membrane stability and stress response. Declines in tree-ring Calcium have previously been associated with drought; however, in this case, the growth rings that feature the depletion are not unusually narrow (as would indicate drought). A forest fire response is also a possible explanation, and this can manifest as either an increase of Calcium as it becomes more available for uptake after burning or as a depletion where areas of the sample are scarred but again, the tree-ring growth pattern does not indicate a growth release or scar typical of fire impact.
A high-resolution X-ray fluorescence scan of the transverse section of GOR-76 featuring an unusual depletion of the element Calcium. The mapped area was identified as the only significant elemental anomaly in the 16th century BC growth rings from this sample. This scan shows that a calcium depletion occurs from around 1562 to 1558 BC and is centered on an unusually wide, slightly pale in colour growth ring at 1560 BC. A similar wide, pale ring occurs in 1550 BC but does not indicate the same degree of depletion. Pearson et al. (2020).
Alternatively, Calcium can be reduced in tree rings following foliar exposure to acid mist or other such precipitation. Therefore, the finding of a Calcium depletion is consistent with the impact of volcanically induced acid deposition [reported in lake sediments as a result of the Thera eruption. On its own, this Calcium response in a single tree might not be worth reporting; however, the date around which it occurs makes it worthy of further discussion because 1560 BC also coincides with evidence for volcanic impact indicated in two other records. Subfossil Pine trees from a calendar-dated record in Finnish Lapland indicate a possible eruption immediately preceding 1560 BC in the form of a negative departure in Carbon¹³ (drop in the proportion of Carbon¹³ relative to Carbon¹²), which has been shown to correlate with periods of reduced visibility due to volcanic acid fog. The high-altitude Bristlecone pine record also includes an indicator year at 1560 BC along with 4 other years in the 16th century BC when unusually narrow growth or frost-damaged cells are recorded. These dates (1597, 1560, 1546, 1544, and 1524 BC) are all indicative of major volcanic eruptions, the origins of which are not yet known. The coincidence of these two additional records around 1560 BC makes further investigation essential. The apparent increase in old carbon around 1558 BC also requires further exploration as, although the tree grew several 100 km from the eruption, this too could hypothetically connect with the Thera eruption, and all potential indicators should be explored. We note, however, that 1560 BC is more recent than indicated likely for the chemical change associated with the Thera eruption at Sofular cave and older than is indicated likely for the event via certain lines of archaeological evidence Nevertheless, these findings clearly merit further careful investigation to define better the onset and duration of the response and to see if it can be replicated in other trees and expanded via the detection of other more clearly volcanogenic (or otherwise) elemental markers.
Pearson et al.'s study shows that, even in the absence of a large-scale interannual Carbon¹⁴ excursion (such as at 774/775 AD), comparing the fine structure in annually derived Carbon¹⁴ time series via a range of approaches can offer a way to improve the dating precision and accuracy possible for floating tree-ring sequences previously dated by conventional radiocarbon wiggle matching to the IntCal calibration curve. First, critically, matching based on two annual Carbon¹⁴ time series (one of which is calendar dated via dendrochronology) offers a dated position for the floating sequence, which is fixed. This differs from modeled dates via conventional radiocarbon wiggle matching, which may change with new iterations of the calibration curve. Second, chi-squared testing of longer annually based time series can refine dating for floating tree-ring sequences to a precise year within a ± 4-year range, and this can be visually tested and confirmed across small-scale Carbon¹⁴ features (such as at 1528 BC). Third, as is the case in this study, additional proxy information can be used to refine the dating further. We found that other tree-ring associations strongly suggested that the dating indicated by the annual Carbon¹⁴-matching approach yielded a result that was in fact accurate to within 1 year. This combination of methods opens up opportunities to anchor floating tree-ring sequences in time outside the capacity of standard dendrochronological techniques, demonstrating potential to fill in a range of critical temporal and geographic gaps in the tree-ring record.
Anchoring the Gordion tree-ring series more securely in time is an important contribution to improving timelines in the ancient East Mediterranean and maximizing the potential of this record as a paleoenvironmental resource. The first step toward this is the identification of the calcium anomaly around 1560 BC, which while clearly requiring replication and much further substantiation, opens up potential that may now be pursued toward finding an exact date for Thera.
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