Overview and Assessment of the Evidence for the Date of the Eruption of Thera

I should add that I have been extremely skeptical of 'big-bang' approaches to history, but I now believe that in certain circumstances, especially when there are multiple, converging lines of evidence, one is sometimes justified in looking for 'big-bangs' and their effects.

ABSTRACT

1. Marinatos's 1939 evidence for a 1500 BC eruption
2. Additional archaeological evidence since 1939
3. The first University of Pennsylvania radiocarbon dates: 1625 BC ±
4. The second Pennsylvania radiocarbon dates: all over the map
5. The new radiocarbon dates from Simon Fraser University: high or low?

6. The 1390 BC Danish ice-core date (now withdrawn)

7. LaMarche and Hirschboeck's bristlecone pine frost rings in 1626 BC

8. M.G.L. Baillie's 1626 Northern Irish bog oak rings

9. The revised Danish ice-core dates: 1645 ± 20 BC

10. D. Sullivan's Theran ash near Sardis and his ¹⁴C dates for it

11. Other reported Thera ash halls in the islands and in Anatolia

12. Ralph Rowlett's thermoluminescence dates

13. Dendrochronological evidence as of 1988 from the Turkish mainland

14. Dentrochronological non-evidence from Thera

15. The Hyksos and the argument from silence

1.   MARINATO'S 1939 EVIDENCE FOR A 1500 BC ERUPTION

It is fitting that we meet on the 50th anniversary for Professor Marinatos's paper (1939) in which he proposed a date of 1500 BC for the eruption. Upon re-reading his paper, I submit that the key pieces of his evidence were the synchronisms with Egyptian material such as had been found just a few years earlier by Xanthoudides (1924) in reused tombs in the Mesara. While this may not seem like 'hard' evidence, it was the best Marinatos had at his disposal fifty years ago. It does presume that Egyptian chronology was and is in good order and that the tombs' use and reuse did not continue for too long a time. If the diagnostic pottery was deposited with earlier burials, and the scarabs were deposited with later burials, for example, decades or more later, then the alleged synchronisms lose much of their value.

2.   ADDITIONAL ARCHAEOLOGICAL EVIDENCE SINE 1939

It is frustrating that half a century's excavation throughout the Aegean and eastern Mediterranean has shed so little additional light on the date of the Theran eruption. Indeed, Marinatos's declaring for 1500 BC so early in the archaeological game has often meant that everybody else has adjusted his or her dates to fit the, by now, canonical 1500 BC. In the 1970s, following the publication of the first Thera radiocarbon dates (see below), there were a few questions, notably those voiced at the 1977 Archaeometry Meeting in Philadelphia (Hood 1978), but not until the last five years, in which there has appeared a spate of papers discussing and re-discussing the subject, has there been a serious question of this conveniently round figure, even though in off-the-record discussion some excavators will admit that firm evidence is lacking.

3.   THE FIRST UNIVERSITY OF PENNSYLVANIA RADIOCARBON DATES: 1625 BC ±

The first serious effort at applying the then relatively new radiocarbon dating method to 16 samples of carbonized material from Thera (Michael 1976; Betancourt and Weinstein 1976; Fishman et al. 1977) yielded excellent uncalibrated results (approximately 1500 BC) and disturbing calibrated results (approximately 1625 BC or one or two years earlier, depending on whose calibration table one uses). The group of samples was remarkably homogeneous, and the one outlier (about a millennium too old) was explained away as possibly an inner ring of a very old tree. People who did not believe in calibration said the bristlecone pine calibration technique was thereby disproved. People who believed in calibration, on the other hand, were forced to conclude that something was possibly amiss with Aegean Bronze Age chronology. A third group tried to rationalize the discrepancy by saying that ¹⁴C dates from volcanic areas are contaminated by gases and are therefore corrupt, or that there had been possibly some kind of laboratory error. The gas theory I have never felt competent to judge, but I do not understand how 15 dates could have clustered together so neatly purely on the basis of a laboratory error. A recent study (Betancourt and Michael 1987) in which undersized samples are excluded from the calculations makes the cluster look even better.

4.   THE SECOND PENNSYLVANIA RADIOCARBON DATES: ALL OVER THE MAP

A second batch of Thera samples was analyzed at Pennsylvania in 1979 ff., presumably as a control upon the first batch (I suppose we could ask Professor Doumas what his motives were for submitting it), with horrible results for all parties concerned (Betancourt et al. 1978; Weinstein and Michael 1978). The dates ranged all the way from the Early Bronze Age to the end of the Late Bronze Age. The very few reasonable dates fell into both early and late camps. Indeed, the authors themselves pointed out that 'the Series II radiocarbon dates are of little or no value for Minoan chronology'. Here a claim of laboratory or some other kind of error is, allowable. Moreover, no statistical analysis is capable of sorting out the good and bad dates, although there have been some valiant recent efforts (Manning 1988; Aitken 1988). The wonder is that Professor Doumas was willing to agree to any kind of ¹⁴C analysis at Akrotiri again after such an unpromising beginning.

5.   THE NEW RADIOCARBON DATES FROM SIMON FRASER UNIVERSITY: HIGH OR LOW?

The radiocarbon dating impasse was commented upon by Professor Doumas during the 1986 Archaeometry Meeting in Athens, and a third ¹⁴C study resulted. This will be reported on during this Congress, and I defer to Dr Nelson, hoping that he is at last in a position to resolve the radiocarbon dating problem for us.

6.   THE 1390 BC DANISH ICE-CORE DATE (NOW WITHDRAWN)

In 1980 an apparent confirmation of the conventional date for the Theran eruption appeared in the Greenland ice cores (Hammer et al. 1980). The date as published was 1390 ± 50 BC. This date has now been withdrawn by its authors although it lingers in the literature. As I understand it, the crucial core was broken just above the large acidity layer that was thought to represent the Theran eruption, and, since the eruption was thought to be more or less around 1500, the layer was therefore assigned its date.

7.   LAMARCHE AND HIRSCHBOECK'S BRISTLECONE PINE FROST RINGS IN 1626 BC

Shortly after he began work on bristlecone pines from the upper timber line, that is to say over 4000 m altitude, the late V. LaMarche knew he had a correspondence between frost rings and volcanic activity throughout the world. The governing principle seems to be that in years when the dust-veil index is high, solar radiation bounces back into outer space, and the mean temperature of the earth drops enough so that these trees are damaged by frost. In 1974 when I first visited LaMarche's laboratory in Tucson he showed me how he had looked at the 1500 BC tree ring to see what evidence there was for frost damage: nothing. He then looked a hundred years on either side of 1500 BC, and again: nothing. But in 1626 BC he had evidence of a volcanic explosion somewhere, several magnitudes that of Krakatoa. Of course this blast did not have to be from Thera. It could have been from anywhere in the world, but no one has yet proposed a reasonable alternative to Santorini. LaMarche asked whether this was going to get him in trouble with Aegean archaeologists, and I assured him that it would. He mentioned it casually in a National Geographic Magazine interview in 1977 and then published it formally in 1984 (LaMarche and Hirschboeck). Note that LaMarche's date has no plus-or-minus factor. It is correct to the year 1626 BC (LaMarche's computer was programmed to include a year 0 between 1 BC and AD 1. Thus his 1626 BC with a year zero = 1627 BC without a year zero). The event that caused this frost ring would have taken place a year earlier in 1628. Note that LaMarche's high-altitude bristlecone pine chronology replicates Ferguson's from lower altitudes, which, by the way, does not have these frost rings, but the cross-dating is secure.

8.   M.G.L. BAILLIE'S 1626 NORTHERN IRISH BOG OAK RINGS

As Dr Baillie will be doubt be saying in his presentation to this Congress, after LaMarche's paper appeared, he examined the 22 bog oak samples from this period in Northern Ireland which were by then part of an absolutely dated 7,272 year long chronology. He found that on a number of samples the years 1628 - 1626 BC were the narrowest rings in the entire lifetime of the tree (Baillie and Munro 1988). On two trees the rings after 1628 were so small that they were totally unmeasurable. Corroborative evidence for the volcano-narrow ring relationship included singularly narrow bands at AD 540, 207 BC, and 1153 BC, dates when it is believed significant volcanic activity took place. In the Irish bogs the mechanism for the abnormality is different from that of the frost rings in the American southwest: volcanic activity triggers both cold and possibly flooding, and the result is an extremely narrow band of rings which can continue for some years after the event. The Northern Irish oak chronology, replicated by several German laboratories, is also accurate to the year as is the bristlecone pine chronology. 1628 BC, then, and the years immediately following, were bad years in areas as widely separated as California, Northern Ireland, the Aegean (presuming Thera was the culprit), and even, perhaps, China.

9.   THE REVISED DANISH ICE-CORE DATES: 1645 ± 20 BC

The Danes have now published a new date based on new, intact core from the Dye 3 deep sounding in south Greenland (Hammer et al. 1987). They note a high signal at 1644 BC and suggest a 1645 BC eruption with an estimated standard deviation of ± 7 years and an estimated error limit of ± 20 years. Note that 1628 - 1626 is well within the limits of the ice-core evidence as revised. I am not in a position to judge the reliability of these error estimates, and I look forward to hearing from the authors as to their confidence in the date.

10.   D. SULLIVAN'S THERAN ASH NEAR SARDIS AND HIS ¹⁴C DATES FOR IT

Sullivan (1988) has found a 12 cm thick layer of Theran ash in Gölcük, a small lake above Sardis in western Asia Minor. He has a radiocarbon date of 7,400 ± 120 BP on peat from well below the tephra and another of 3,110 ± 160 BP from a peat layer 30 cm above the tephra. This is not immediately helpful to us, because the time range included everything from about 5000 BC down to about 1420 BC, and we have no idea how long it took for the 30 cm of material to accumulate above the tephra layer. But Sullivan is setting an example for the rest of us. The Gölcük work has two significant aspects:

a)      the Theran fall-out must have affected western Asia Minor in much the same manner as Denys Page (1970) posited for eastern Crete; and

b)      once these strata are identified and isolated, and if organic material can be recovered both from above and below the tephra and submitted for radiocarbon dating, we will have a time control far removed from any contaminating volcanic gases.

11.   OTHER REPORTED THERAN ASH FALLS IN THE ISLANDS AND IN ANATOLIA

Sullivan's work may be coupled with reports of Theran ask on Kea, Rhodes (Doumas and Papazoglou 1980), and elsewhere in western Anatolia. W. Voightlander (pers. comm.) has now found Theran ash mixed with LM IA and LM IB pottery on the Anatolian coast at Teichioussa. Sullivan reports more Theran ash at Köyceğiz. More will no doubt be found as we learn what to look for.

12.   RALPH ROWLETT'S THERMOLUMINESCENCE DATES

Rowlett (pers. comm.) has been doing a large number of thermoluminescence dates on Theran ash, but I have not yet seen printed results. The error margin for TL are such that under normal circumstances the prospects of resolving this particular problem would be unlikely. As I understand it, he hopes that by the sheer number of TL determinations  (~1000) he will get a clustering that will be significant.

13.   DENDROCHRONOLOGICAL EVIDENCE AS OF 1988 FROM THE TURKISH MAINLAND

In our laboratory at Cornell we have a floating, almost continuous dendrochronological sequence for the eastern Aegean that runs from the 23rd to the 8th century BC with only one gap, about thirty years long, in the late 17th century BC. Before September 1989 we will have returned to Gordion and Porsuk, two sites which ought to give us the material to fill this gap, thereby providing us with a 1500 year continuous floating chronology wiggle-matched by 26 radiocarbon dates of specifically-numbered rings. Measurement of the newly-collected wood will start in October 1989 as soon as we return to Ithaca, N.Y. There is no obvious anomaly in the Aegean tree rings around 1500 BC, and as of June 1989 we do not yet have a ring for 1626 BC, so we are not in the position of either LaMarche or Baillie to make any claims about odd-looking rings, but give us several more months, and we will see. Our low-altitude (below 2000 m) Aegean wood does not exhibit the responses of either the high-altitude California wood or of the Irish bog oak, and, although we can point to some narrow rings following volcanic activity elsewhere in the world, I do not believe that these are statistically significant. Thera, being much closer, might have left a mark of some sort.

14.   DENDROCHRONOLOGICAL NON-EVIDENCE FROM THERA

In 1982 Professor Doumas kindly invited us to look at the charcoal on Thera, of which he has several kilograms. The notion was to see whether we could find a ring sequence which we could then try to cross-date with the long sequences from Anatolia. Interestingly, all that is preserved at Akrotiri is the charcoal from the hearths, and we discovered to our disappointment that the people of Thera did not burn their first-quality wood. Although we found several small flakes of conifer, the bulk of the charcoal is either olive or grapevine, neither of which are useful in dendrochronological analysis. The restored beams that one sees in all the Thera photographs are modern reproductions of unburned beams that have long since rotted away. We have told Professor Doumas that the day he discovers a burned building at Thera with well-preserved charcoal from constructional timbers all he has to do is send us a telegram and we will be on the next airplane.

15.   THE HYKSOS AND THE ARGUMENT FROM SILENCE

One oddity which proponents of the 1500 BC date have never been able to explain is why there is no mention of this catastrophic event in the Egyptian records. Thera ash has been excavated in the Nile Delta (Stanley and Sheng 1986), and surely the Egyptians should have left some king of comment. In the period of the Hyksos, however, internal Egyptian affairs were in sufficient disorder so that had the eruption occurred in 1628 - 1626 the lack of comment is explicable. It is of course illogical to say that because we hear nothing from the Egyptians, something, therefore, must have happened, but the earlier date does make more sense from an Egyptologist's point of view.

SUMMARY

Although at least one scholar (Betancourt 1987) has voiced pen support for a higher dating system for the Aegean Late Bronze Age, for many other colleagues the jury is still out. I, for one, find it difficult to believe that something as cataclysmic as the Theran eruption could have taken place without causing world-wide tremors or reactions and am therefore inclined to favor the earlier date because of what I believe to be the more than accidental clustering of the bristlecone pine frost rings, the extraordinarily narrow bands in Northern Irish oak, the acidity layer in the Greenland ice cores, and the bulk of the radiocarbon determinations, all around 1628 - 1627 BC, possibly corroborated by the silence from Egypt and more distant events commented on by Kevin Pang (1985) who believes that the 1628 event is the same one that ushered in the beginning of the Shang Dynasty in China, as reported in the Bamboo Annals for 1618 BC: 'yellow fog, a dim sun, then three suns, frost in July, famine, and the withering of all five cereals'. The Chinese evidence is not entirely secure as to its date, the Annals having been found, lost, and then recovered, and the question of the Shang Dynasty and its dates really depends on one line in the text (information from Professor Martin Bernal at Cornell), but these phenomena are symptomatic of a large volcanic eruption such as the one which now preoccupies us and which certainly must have had a global rather than a purely local impact.

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