Did an unknown volcanic eruption create the conditions for the Black Death to enter medieval Europe?

Between 1347 and 1353, the Black Death spread across Europe, killing up to 60% of the population, leading to long-lasting demographic, economic, political, cultural, and religious changes which changed the continent, and eventually the world, beyond recognition. Recent palaeogenetic studies have now confirmed the long held belief that the pandemic was caused by the Plague Bacterium, Yersinia pestis, a zoonotic disease with wild reserves in a number of Rodent species. Despite the presence of a wild reserve, outbreaks of Yersinia pestis in Human and Domestic Animal populations are quite rare, with only three documented pandemics of the disease. The first of these, the Plague of Justinian, began around 541 AD, and persisted into the eighth century. The second, the Black Death, began around 1338 in Central Asia, and persisted in places into the early nineteenth century. The third began in China in the 1770s, and spread around the world; arguably this third Plague pandemic is still ongoing, with all wild reserves and outbreaks outside Asia apparently derived from this source.

Studies of the archaeological, historical and ancient genomic records have suggested that the Black Death was a genetically distinct strain of the Yersinia pestis Bacterium which probably originated in arid foothills of the Tien Shan mountains west of Lake Issyk-Kul in modern-day Kyrgyzstan. This spread along the trade routes of Central Asia, entering Europe via the northern Black Sea region in the 1340s. Notably, the Black Death abruptly entered Venice and other Mediterranean ports in 1347, presumably through the importation of infected Fleas from the Black Sea reason. 

From this time, there were repeated outbreaks of Plague across Europe until the early nineteenth century, although it is unclear if these represent a series of re-introductions, or the presence of a wild-reserve within Europe. Nor is it entirely clear why the Plague appeared in multiple Mediterranean ports at the same time, having been established in the Black Sea region for some time prior to this, without previously making the jump, though it has been suggested by several people that some socio-economic challenge led to a change in Human behaviour at this time, giving the disease an opportunity to spread.

In a paper published in the journal Communications Earth & Environment on 4 December 2025, Martin Bauch of the Department Humans and Environment at the Leibniz Institute for the History and Culture of Eastern Europe, and Ulf Büntgen of the Department of Geography at the  University of Cambridge, the Global Change Research Institute of the Czech Academy of Sciences, and the Department of Geography at Masaryk University, present evidence for a volcanic eruption at an unknown location in the years prior to the Black Death reaching the Mediterranean, which caused a regional famine, leading traders to seek new sources of grain, and thereby opening the region to the arrival of the Plague.

Bauche and Büntgen note that there has been extensive previous research into potential links between a volcanically induced climate crisis at the start of the  Late Antique Little Ice Age and the onset of the Plague of Justinian, but little previous investigation into such a link to the onset of the Black Death. They also note that ice cores from Greenland and Antarctica have revealed a spike in sulphur concentrations in 1345 considerably larger than the one caused by the Mount Pinatubo eruption of 1991, which probably represents an injection of about 14 megatons of sulphur into the atmosphere. Furthermore, there were also major spikes in 1329, 1336 and 1341, representing eruptions which would have injected roughly 3.7, 0.7, and 1.2 megatons of sulphur into the atmosphere, respectively.

Weather records from Japan, China, Germany, France, and Italy, all record the years from 1345 to 1349 as being exceptionally cloudy. A lunar eclipse in 1345 is recorded as having been exceptionally dark by witnesses in both Bohemia and China; something which can be another sign of a high volcanic dust level within the atmosphere. 

Studies of tree rings have shown that trees in the Spanish Pyrenees produced 'blue rings' in 1345 and 1346, which are interpreted as signs severe cold spells during the growing seasons affected growth. The production of blue rings in consecutive years is considered exceptionally rare. Studies of wood density across the Northern Hemisphere have suggested a progressive cooling from 1345 to 1347, with 1347 being the coolest year since 1257, when a cold spell was linked to an eruption on Mount Samalas on Lombok Island, Indonesia. 

May–September (MJJAS) temperature anomalies from 1119–2020 AD (uncertainties are expressed by grey shading), based on 534 maximum latewood density (MXD) measurement series from living and relict samples from Mountain Pine, Pinus uncinata, trees from undisturbed upper treeline ecotones in the Spanish central Pyrenees.  The pre-Black Death cold phase is indicated by the vertical blue shading. The right-side double-stained thin section shows two consecutive Blue Rings that were formed in 1345 and 1346 AD in a Mountain Pine, Pinus uncinata, from the upper treeline in the central Spanish Pyrenees. Bauche & Büntgen (2025).

While it is harder to assess rainfall in past than temperature, tree-ring data suggests that the cool period from 1345 to 1347 was accompanied by a prolonged west-east dipole in Europe, with wetter conditions around the eastern Mediterranean and dryer conditions around the western Mediterranean. Morocco, the British Isles, northern France, the Low Countries, Germany and southern Scandinavia, all appear to have suffered dry conditions, while the Iberian Peninsula, Italy, and the Balkans, had high spring and summer rainfall in those years.

Bauche and Büntgen also examined historical records from across Europe and beyond, which show a declining agricultural output from across Europe from 1345, and in particular a failure of (environmentally sensitive) Grape crops from northwestern Italy. Severe flooding was also recorded in Italy in the autumn of 1345 and the springs of 1346 and 1347, along with accompanying problems such as soil erosion. The winter of 1344/45 was exceptionally cold and snowy in the Middle East, with the winters of 1345/46 and 1347/48 being marked by drought and Locust invasions. 

Late medieval Italy had a highly urbanised population, with a complex grain supply system in place to support this population. Many city states, including major centres such as Bologna, Florence, Genoa, Siena, and Venice, had limited farmland and large urban populations, consequently importing grain over long distances to redress this imbalance. Only Rome and Milan were largely self-sufficient. Cities developed communal granaries run by officials with the power to manage these granaries, source supplies of grain from elsewhere, and prevent the export of grain from cities troubled by poor harvests or military conflicts. Typically, managing grain supplies was the second-largest source of expenditure for any late medieval Italian city-state, behind only military spending. Maritime powers such as Venice, Genoa, and Pisa, negotiated treaties with grain-producing areas such as Apulia, Sicily, Sardinia, North Africa,  the Aegean and the Black Sea region. 

In 1346/7 severe famines were recorded across parts of Spain, southern France, northern and central Italy, Egypt, and the Levant. This led to spikes in the price of grain in Spain, Italy, Egypt, and even the Arabian Peninsula. Strict grain regulations were implemented in many Italian cities from 1346, at least in part to grain-shortage induced civil unrest.

Northern Italy suffered a series of famines during this interval, which appear unrelated to any political crisis, supporting a climate-related problem as the cause. Initially, this shortfall was met by increased imports from southern Italy, but it quickly became clear that this would not be sufficient to alleviate the crisis, and that more imports would be needed from further afield. 

At this time, Venice and Genoa were in a state of conflict with the Mongols of the Golden Horde, who had been trying to eliminate Italian power in the Black Sea region. This had led to the Italian cities blockading the ports of the Black Sea, preventing the Mongols from trading with the Mediterranean. However, the onset of famine in Italy led to a re-appraisal of this situation, with a ceasefire and renegotiation of trading arrangements leading to a restoration of grain trading, saving Venice from starvation.

The Plague Bacterium, Yersinia pestis, had been present in the Black Sea region for some time, and it is likely that it would eventually have reached the Mediterranean at some point. However, Bauche and Büntgen contest that the lifting of the trade embargo against the Golden Horde in response to the volcanically-induced famine affecting northern Italy was the immediate cause of the Black Death reaching multiple Mediterranean ports in a short interval. 

The trade embargo was lifted in 1347, and shortly thereafter, Venetian and Genoan trading vessels began entering the trading ports of the northern Black Sea and the Sea of Azov, returning to Italy laden with grain. The Plague appeared in Venice less than two months after the first such trading vessel. In March 1348 Venice lifted an embargo on the export of grain to Padua, with the first Plague outbreak there coming shortly after. Records of the grain trade to Florence and Sienna are less clear, but again the first Plague outbreaks in these cities were associated with the abating of the famine. Notably, this first wave of Plague outbreaks did not affect cities such as Rome, Milan,Verona, Ferrara, Ravenna, and Bari, which controlled larger grain producing areas, and were not involved in grain imports from the Black Sea region. Elsewhere around the Mediterranean, Marseille, Palma de Mallorca, and many other important port cities had Plague outbreaks before the end of 1347, probably as a result of grain shipments from Genoa. Smaller cities such as Savona, Ventimiglia, and Tunis, began to suffer outbreaks in April 1348, again probably linked to Genoan grain shipments. The reached Trento along with grain shipments from Venice, and from their spread across the Alps into northern Europe. 

Grain trade and plague dispersal. Main aspects of the Venetian, Genoese, and Pisan grain trade network that prevented much of Italy from starvation in 1347 AD but also brought the Plague Bacterium, Yersinia pestis, to Venice and other Mediterranean harbours during the second half of 1347 AD, from where it spread rapidly. Location of the tree ring-based climate reconstructions is indicated (with two sites in Scandinavia not shown). Map is an equal-area, pseudo-cylindrical Mollweide projection with greyscale referring to elevations above sea level. Bauche & Büntgen (2025).

In 1344, a decade-long papal embargo on trade with the Mameluke Sultanate was lifted, enabling Italian merchants to resume trade with the Middle East. Following the resumption of trade with the Black Sea region, Venetian ships began carrying grain to Alexandria, and the ports of the Levant, while Mameluke traders began visiting the ports of the Crimea. While individual shipments are harder to trace for these ports than those of Italy, these ships are probably responsible for the arrival of the Black Death in North Africa and the Middle East.

Thus the resumption of trade with the Black Sea region appears to have resolved the immediate problem of a widespread famine, but also led to the simultaneous import of the Black Death to ports around the Mediterranean. The subsequent rapid spread and high fatality rate for the disease may also have been a result of a population weakened by famine. The sophisticated trade system developed by the Italian city states to ensure against famine appears to have made those states, and their allies around the Mediterranean and Europe, particularly vulnerable to the spread of the Plague. Bauche and Büntgen conclude that the Black was not just a singular event, but rather the culmination of a prolonged crisis which began with a climate crisis and then a famine.

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Cutting the Gordian Knot of tree-ring timelines in the East Mediterranean Bronze Age, and looking for a date for the Santorini/Thera eruption that destroyed the Minoan civilization.

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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