When was the Minoan eruption of Thera?

When was the Minoan eruption of Thera?, draft, 2015‐09‐17, of 10

When was the Minoan eruption of Thera?

Petra Ossowski Larsson* and Lars-Åke Larsson, Sweden * Corresponding author: [email protected]

Abstract

The Minoan eruption of the Thera (Santorini) volcano provides an archaeological key marker for the Bronze Age chronology of the Eastern Mediterranean civilizations. However, the exact date for this large eruption is still unknown. Based on published tree ring and ice core chronologies, we investigate the candidates for major volcano eruptions in the middle of the second millennium BC. Ice core analysis provides indication for the volcanic nature of prominent events which resulted in climatic downturns and which are therefore visible in the tree ring chronologies. Our conclusion is that there are only two candidates for a "supervolcano" eruption in the time range -1675 to -1450. Only one of them has so far been scientifically considered as a candidate for the Thera eruption. Recent investigations seem to indicate it to be less likely that this candidate is Thera. But there is one unexplored candidate left! The evaluation of which of the two eruption candidates is the most probable, is backed up by a re-investigation of the so called "Ugarit Eclipse". We also suggest a new synchronization of tree ring and ice core time lines for the time range mentioned.

Introduction

The question about the exact date of the Minoan eruption of the Thera (Santorini) volcano, which ended the Minoan culture and left impressions on the archaeology of the whole eastern Mediterranean, is still enigmatic. While archaeologists seem to favor an eruption date around -1500 (the so called "low" chronology), other scientists want to place the eruption around -1620 (the "high" chronology) i.e. about 120 years earlier, because of radiocarbon dates and tree ring and ice core evidence. There is still a hot debate running about this topic (refs.1,2) and a recent article by Felix Höflmayer may illustrate the case (ref.3).

However, concerning one point the archaeologists seem to have reached consensus: the Minoan eruption took place around the start of the Egyptian New Kingdom, i.e. the start of the 18th dynasty. Therefore a reliable date of the Minoan eruption would pin down the Egyptian New Kingdom on the absolute time line.

Radiocarbon dating

Many articles have been written about the radiocarbon dating of samples from the Thera eruption and the Egyptian 18th dynasty. We do not want to discuss the


radiocarbon results here, as the resolution of this dating method appears to be so low that it was not capable to detect dendrochronological errors as large as 200 years, if we are right (ref.4). A peculiarity related to the Minoan eruption/start of the 18th dynasty question is that the Thera samples have radiocarbon dates around -1620 (where the proponents of the "high" chronology want it), while the early 18th dynasty samples have radiocarbon dates around -1550 (where the archaeologists want it). 70 years radiocarbon offset is quite a lot for two events which are archaeologically regarded as contemporary. In a recent article by Ritner and Moeller (ref.5), Ahmose's "Tempest stela" is interpreted as a contemporary record of the cataclysmic Thera event. Ahmose was the first ruler of the Egyptian 18th dynasty. He erected the Tempest stela in Thebes to commemorate a very heavy storm which had devastated Egypt, and to describe the restoration work ordered by him. However, other scholars see the Thera eruption later in the 18th dynasty around -1500 (which would increase the radiocarbon offset even more).

Tree ring and ice core chronologies

Volcanism affects climate which affects tree growth. Therefore a lot of work has been directed at finding "volcano markers" in long tree ring width chronologies. LaMarche and Hirschboeck (ref.6) detected frost damage to the cells of high elevation bristlecone pines which already had started growing for the spring/summer season. They were able to connect some of these severe frost throw-backs to volcanic cooling events in historical times. It appeared to be only one frost damage event during the second millennium BC, at -1626. Therefore this date was tentatively connected with the Thera eruption as being the "supervolcano" of that period.

Later, more growth depressions and "frost rings" were detected around the same time in various tree ring chronologies. We can indeed see growth anomalies in pines from Northern Fennoscandia at -1648 and -1633. But were these growth depressions caused by volcanic eruptions? And is one of them really Thera? Salzer and Hughes (ref.7) carefully allow a wide possible range (-1675 to -1450) for the Thera eruption, but seem to prefer a date around -1626.

A recent article by Helama et al. (ref.8) identifies sudden climatic downturns in the more than 7000 years long pine chronology from Northern Finland (which we have access to and which we have verified as being absolute and continuous). For the second millennium BC, two cold summer events are suggested as being caused by volcanic eruptions: at -1463 and -1583. For the period -1626 to -1622 no anomalous conditions could be found, and the events at -1648 and -1633 are apparently regarded as not severe enough. However, a second article (ref.9) considers also -1633 among the 20 most negative years for pine growth in Northern Fennoscandia, and excludes the downturn at -1583.

In ref.8 the identified severe downturns are compared with volcano markers (sulfate peaks) from various ice cores. This is not really successful, but as the authors put it: "errors in the ice chronology may accommodate it". After our own analysis of available ice core data (ref.10), the reason for these errors becomes evident: not all seasonal rings were visible and this had to be compensated for already at about 500


BP in the GISP2 ice core. At 1340 BP about 60 consecutive rings seem to have been invisible. Vinther et al. (ref.11) identified the problem and tried to rely on interpolation. For this they had to identify an exactly dated old enough historical event which could also be identified within the ice cores. The well documented Plinian eruption of mount Vesuvius in RomAD 79 seemed to be a good candidate as there was a large volcanic marker visible in the ice cores at about the place where they had expected it to be. The only problem is that, according to our hypothesis, this eruption took place in 311 and not in 79 ....

Since then, this fatal calibration of the Greenland ice core time line has been abandoned and methods for unbiased counting of seasonal rings have been improved (ref.12). The new approach uses worldwide observed precise time markers generated by cosmic abrupt enrichment events (14C in tree-rings and 10Be in ice cores). After correction, the "79" volcanic marker former attributed to Vesuvius is now dated 88 and attributed to a volcanic source from the high latitudes. However, the ice core record for the second millennium BC is still awaiting refinement. Therefore the old ice core time line (Meese/Sowers timescale) is used in Figure 1 which shows volcanic sulfate and particle concentration (dust) measured in GISP2 for the period -1665 to -1340 (ref.13).

Figure 1: Volcanic sulfate (ppb) and Particle concentration (#.10³/ml) for the period -1665 to -1340. GISP2 Greenland summit ice core.

There are only two prominent sulfate peaks in this range, at -1623 and at -1453, with 170 years with only minor recorded volcanic acidity in between. The peak at -1623 is considered as marking the Thera eruption, as no other candidates are available. Though in a recent study by Coulter et al. (ref.14) several dust layers during the 17th century BC, not only for -1623, have failed to comply with the chemical composition of the Thera fallout (instead an Alaskan volcano is suggested). As the authors laconically state:

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A cache of over 300 cuneiform tablets recording diplomatic correspondence of the Pharaoh (known as the "Amarna Letters") were found in the ruins of Amarna. Among these letters was a message to Akhenaten shortly after he became Pharaoh, from Abimilku ruler of Tyre, with news concerning another vassal:

... Fire destroyed the palace at Ugarit; (rather) it destroyed half of it and so half of it has disappeared. ... (EA 151)

In the destroyed palace at Ugarit, another - charred - clay tablet was found with a report on a fairly recent event which possibly was a total solar eclipse. If the event reported by Abimilku would mark the final destruction of the palace at Ugarit and not a previous fire incident, we would have a chance to determine an absolute date for the eclipse and the Amarna period. This is because the archives at Ugarit were kept up to date and only contained documents of the recent half century, except for legal texts and treaties which were kept for longer time (ref.15). However, if Abimilku's report was about an earlier fire incident, we would have to use historical considerations to estimate the time distance between the eclipse and the Amarna period.

Several scientists have tried to interpret the short text on the Ugarit clay tablet (KTU-1.78). All agree that it is about something that happened to the sun and her "attendant" (probably a star or planet) at new moon in a certain month, and that this was a bad omen. Most scientists (but not all) suggest a solar eclipse, because this would occur only at new moon. And as the "attendant of the sun" was visible, this hints at a dark sky and a total eclipse. The name of the attendant is given as "Rashap" and interpreted as the planet Mars. A crucial point is the name of the month "hiyaru", which is interpreted either as February/March, or April/May. There is also a dispute whether the eclipse occurred at daytime or at sunset. A modern reading of the clay tablet with interesting explanations is available in ref.16.


Maybe it was not a solar eclipse at all, maybe the clay tablet just contains an astrological prophecy? Therefore it is misleading that the NASA Eclipse Web Site (ref.17) reports about the "Ugarit Eclipse" without reservation and gives a definitive date (3 May -1374), which is at risk to be wrong as we shall see below.

But let's assume that the Ugarit clay tablet describes a real total solar eclipse. Sawyer and Stephenson (ref.18) identify 14 July -1405, 3 May -1374, 8 January -1339 and 5 March -1222 as possible candidates and finally they point out 3 May -1374 (the date reported on the NASA Eclipse Web Site). De Jong and van Soldt (ref.15) principally agree but prefer 5 March -1222 because they identify "hiyaru" as February/March, and because this is the only eclipse in the range during which the planet Mars was above the horizon (!). Not only that, but Mars was at this occasion only 3.5º from the center of the eclipsed sun at Ugarit. David Rohl (ref.19) reads the clay tablet as describing a sunset eclipse and therefore chooses 9 May -1011. Wayne Mitchell (ref.20, p.18-20) reviews the case and publishes sky views for these three suggestions (and two other candidates). Here we see the absence of Mars in -1374, Mars in the vicinity of the sun in -1222, and Mars between Jupiter and Mercury (triple conjunction) in -1011.

The assumption that the final destruction of Ugarit was about contemporary with the Amarna period (case 1, refs.18, 19) has been disputed by other scholars (e.g. ref.21). The recent consensus places the first regnal year of Akhenaten about 150 years earlier than the fall of Ugarit (case 2). To calculate the start year of the Egyptian 18th dynasty, we estimate that the eclipse appeared within a fifty years period before the fall of Ugarit. Between Akhenaten's first year (-1350) and Ahmose's first year (-1548) there are 198 years according to recent consensus. With these conditions we arrive at alternative time ranges for the first regnal year of Ahmose, and the possible date for the Thera eruption should be within that range. The following table gives different eruption years for the three eclipse years suggested.

In this table we see that the suggested dates for the Thera eruption for the first two eclipse candidates fall into the total range (for both case 1 and 2), but not into the range for either case 1 or 2. This is because the time distance between suggested eclipse and eruption is about 250 years, while the time distance between the fall of Ugarit and the first regnal year of Ahmose is either max. 200 years (case 1) or max. 350 years (case 2). Historical considerations will have to settle this question once the year for the Thera eruption has been identified by tephra analysis. Maybe Thera


indeed exploded later during the Egyptian 18th dynasty, as suggested by Alexander MacGillivray (ref.22)?

Discussion and Conclusions

Our investigations of ice cores, tree ring chronologies and astronomy point out two probable dates for the Thera eruption within the range -1675 to -1450 (ref.7): -1633 and -1463. David Rohl's suggestion is apparently out of that range but could be considered later if the two other dates should prove negative.

Ice core analysis provides indication for the volcanic nature of the events resulting in climatic downturns visible in the tree ring chronologies. It also provides the exciting conclusion that there are only these two candidates for a "supervolcano" eruption in the time range. Dendrochronology provides the exact dates and an estimate of the severity (impact on climate) of the events. Astronomy helps to verify these dates in their archaeological/historical context.

The so far scientifically considered date for the Thera eruption is -1633, because this date is compatible with radiocarbon dates from archaeological context on Thera. However, this date is controversial as it implies a "high" chronology for ancient Egypt which is not accepted by all historians. Ice cores have been searched for volcanic ash in order to verify that its chemical composition is compatible with the Thera fallout in place, but no Thera fingerprint has been found yet. Moreover, the candidate for the Ugarit eclipse for this high eruption date does not match the eclipse report regarding an important detail: even though the "sun's attendant" has been identified as being Mars, that planet was not above the horizon during this occasion.

The other possible date for the Thera eruption, -1463, has not been considered scientifically because it is not compatible with the radiocarbon dates from Thera. Moreover, it maybe would lower the conventional Egyptian chronology with about hundred years, resulting in a "very low" chronology. But there are some plus points for this date. For this alternative the planet Mars is only 3.5º from the center of the eclipsed sun at Ugarit. The volcanic sulfate peak is larger than in -1633, and preceded by a prominent dust peak. This matches the eruption sequence as identified by geologists, which includes a massive ash fall a considerable amount of time before the lava flow commenced. Maybe also the acid peaks three and five years before -1463 are from the same eruption, as archaeological investigations have shown that people got an early warning and abandoned the settlements before the final collapse of the island. In that case the volcano signal around -1463 would mark one of the largest eruptions during the last five thousand years, in size challenging even the Samalas eruption of 1258.

For the time being we suggest to put aside the debatable and paralyzing radiocarbon dates connected to the Thera eruption, and we instead propose to focus on a tephra analysis in ice cores around -1463. There is a fair chance to find Thera here, especially as the possibilities already seem exhausted for the volcanic event around -1633. Except for the radiocarbon dates, the volcanic event around -1463 is the clearly better candidate for Thera because of better matching details. But the main


reason why (in our opinion) -1463 is the better alternative, is its increased probability in connection with our postulated re-dating of Roman and classic Greek time. As we slide Roman and classic Greek cultures more than two hundred years towards our time, some sort of "vacuum" is generated behind the classic complex which easily could accommodate the civilizations of the Bronze Age with a "very low" chronology.

Acknowledgement

We would like to thank Robert Porter for valuable discussions around the Ugarit eclipse tablet.

References

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When was the Minoan eruption of Thera?

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