Thera and the Aegean World III: Volume Three: Chronology (excerpt)

Significantly the dates of these extreme "narrowest ring" events coincide with the estimated dates for major volcanic eruptions as recorded in the Greenland ice-cap. One of these events occurs in the decade of the 1620s bce and coincides very precisely with a previously suggested volcanic event at 1626 bce (now 1627 bce) put forward by LaMarche.

One is led to the inevitable conclusion that some major hemispheric event took place in the decade of the 1620s bce, and a strong circumstantial case can be constructed that the event was volcanic in origin. Since the dating is based on precisely dated tree rings no further refinement of the date of the event — probably 1628 bce — is required. There remain several points to be addressed.

Is the 1628 bce event, identified by the dendrochronologists, really Santorini, or did Santorini erupt at some other time, as suggested by some archaeologists? Do other tree-ring records assist in our understanding of the problem? Are there other ways of refining the evidence — i.e., are there isotopic alterations within the tree rings which might allow additional fingerprinting of the event? This paper will review progress in at least some of these areas.

Three lines of evidence — ice cores, bristlecone frost rings and an Irish oak minimum-growth event — combine to make it virtually certain that there was a large volcanic dust-veil event in 1628 bce. None of these lines of evidence is capable of specifying the volcano involved. They cannot therefore prove that Santorini erupted in 1628 bce. However, none of these continuous records, which between them appear capable of detecting most large eruptions, indicate any other consistent event between 1628 bce and 1159 bce. How therefore are we to date Santorini?

There seems to be a consensus opinion that the eruption of Santorini was violent. Watkins et al. (1978) went so far as to say that it was one of the largest explosive eruptions in post-glacial times and indeed that "eruptions of this magnitude are yet to be chronicled." To the outside observer it appears that the main event must have happened in a particular calendar year. It seems unlikely that either archaeology or radiocarbon dating can ever specify that year precisely. Yet, in the overall scheme of things, the date "exists" and, sooner or later, it ought to be possible to pin it down. This article aims to define the problem from a chronological point of view and detail the evidence for the event, seen in the Irish tree rings and elsewhere, in the 1620s bce.

The starting place has to be recognition that the widespread occurrence of narrowest growth rings in Irish bog oaks — minimum-growth events — coincides with the suggested dates of major dust-veil events derived from the Greenland ice cores (Baillie and Munro 1988; Hammer et al. 1980, 1987). Table 1 lists the two sets of dates which have already been tabulated by Hughes (1988). In addition, two of the dates, 1628 bce and 207 bce, coincide remarkably with two bristlecone frost-ring events in 1627 bce and 206 bce respectively. (For the former see LaMarche and Hirschboeck 1984; Hughes 1988; for the latter LaMarche pers. comm.)

Let us accept for the purposes of argument that the coincidence between the ice-core evidence and that from the bristlecones and Irish oaks is the result of a major eruption. The ice-core acidity peak at 1644 bce ± 20 tells us there was a volcano; the brisdecone pine and Irish oak effects tell us that something "hemispheric" happened within the limits of the ice-core date range — specifically at the beginning of the decade of the 1620s bce, most probably in 1628 bce. Let us look at what is seen in the tree-ring record. LaMarche and Hirschboeck reported a notable frost event in the growth ring for 1626 bce (now 1627 bce) in high altitude bristlecone pines in the White Mountains of California. At the time the article was written this was the only notable frost event between 42 bce and 2035 bce. Since then LaMarche has uncovered another frost event at 206 bce. There is no question of the accuracy of the bristlecone tree-ring record, so we can take it that there was a rare and extreme freezing event, in at least 50% of the trees studied, sometime in the growth season of the year 1627 bce.

When we look at the Irish record we see another rare event occurring in the 1620s bce. Six trees on four different bogs produced their narrowest growth rings in 1624 bce. Such extreme single-year events occur only nine times in 6000 years (Baillie and Munro 1988). However, it is clear from examination of the Irish record that the "event" at 1624 bce is not restricted to a single year. In reality 1624 bce is the low point of a reduction in growth which in general appears to start around 1628 bce and lasts for a number of years.

The first observation which drew attention to the minimum growth events in the Irish oaks centred on a small group of trees from Garry Bog, Co. Antrim. Several of these trees (see Fig. 1) showed a band of very narrow rings in the 1620s bce. Another tree, Q1863, had originally been assessed as "impossible" from 1630 bce onwards (from a chronology-building point of view there is little point in attempting to measure such rings) and indeed re-examination showed that the ring structure broke down completely at 1626 bce. Other Garry Bog trees, Q1933 and Q2032, showed a distinctly narrow band of rings beginning in 1627 bce. In trees from other northern bogs a similar picture emerged. For example, Q5392 from Sentry Hill showed a colour change associated with a narrow band of rings starting in 1628 bce. These findings were the first indication that LaMarche's event was not something localized to the White Mountains of California, but was potentially part of a hemispheric phenomenon. Overall there is a body of northern Irish oaks which show a dramatic effect starting a few years before the rare narrowest-ring event of 1624 bce. Having isolated the event, it seems reasonable to look at tree-ring patterns from that immediate period in order to obtain the clearest picture of what happened to the trees in question.

It is immediately clear that not all trees show a dramatic effect. This is almost certainly due to the fact that some trees were growing on mineral soils while others were growing rooted on peat. Trees growing on mineral soils should be much less susceptible to down-turns in growing conditions than the marginal trees rooted on deep peat. Looking at a selection of trees from the north of Ireland it is fairly clear that the real downturn is around 1628 bce. Fig. 1 shows some of the original Garry Bog trees together with a mean chronology for nine of the trees which show the most dramatic effects. There is a very definite drop in mean ring width after 1628 bce.

From 1985-88 efforts were being made to outline a prehistoric oak chronology in England. Material was drawn from two main areas, East Anglia and Lancashire. In both areas the oaks were preserved in peat and in the case of the Lancashire material it is fairly clear that these are bog oaks in the Irish sense, i.e. they were rooted on peat. The situation with the oaks from East Anglia is not so clear and at least some of them may have grown on mineral soils with subsequent burial in peat.

The approach to building the English chronology was to construct floating chronologies and to date these against the pre-existing Irish and German chronologies. This proved highly successful and a continuous chronology is now available back to ~3800 bce (Baillie and Brown 1988). One interesting finding from this work was the observation that all of the East Anglian oaks, which could be cross-dated, fell into one continuous chronology spanning 3196 bce to 1681 bce. This exactly paralleled an independent chronology from the main Lancashire site, Croston Moss, which spanned 3198 bce to 1682 bce. In this case an extension of the Croston Moss chronology ran on from 1584 bce to 970 bce.

Here then are two independent chronologies which are neatly bracketed by two of the narrowest-ring events in the Irish oak chronology (the dust-veil events in the 3190s bce and 1620s bce). In the case of Croston Moss the only break in an otherwise continuous chronology, over 22 centuries long, falls across the 1600s bce. Clearly something happened in the 1600s bce which adversely affected bog oaks in England.

At the time of writing there are five oak chronologies from Germany relevant to this period. Of these, four show definite evidence for reduced growth at exactly the same time as the Irish and Hasholme trees (Fig. 2, (b)-(e)). These chronologies were constructed at Köln (Schmidt pers. comm.) and Göttingen (Delorme and Leuschner pers. comm.).

The fifth chronology, which derives from the south of Germany (Becker pers. comm.) shows no dramatic effect in the 1620s, though it does, on average, show narrower rings in the 1620s than in the preceding decade. As the trees included in this southern chronology are known to have grown on mineral soils, this may explain their lack of sensitivity. Referring to Fig. 2, it is clear that the event begins at essentially the same time as the event recorded in the Irish trees. However comparison of Fig. 1 and 2 show that while the Irish ring widths tend to drop after 1628 bce, the English and German trees show 1628 as already a narrow ring. By summing the year-to-year trends in each area and converting the excess trend to a percentage, it is possible to mean two equally weighted data sets. The "Trend Mean" curve in Fig. 2 is therefore an "average" picture of the trend in oak growth over the relevant period. Again it is clear that the drop in oak is dramatic from 1628 bce on.

Most of the bog oak material used in the construction of the Belfast 7200-year chronology was derived from sources in the north of Ireland. However, in the course of dating oak timbers for interested archaeologists and others, several timbers have come to light which span the 1600s bce. Fig. 3 shows three of these timbers.

Timber (a) is a single, definite bog oak from Derrycashel, Co. Longford. While its mean ring width is around 1.0 mm until 1628 bce, thereafter its ring width decreases steadily. By 1620 bce its rings are less than 0.5 mm in width and this continues until 1593 bce. Here we are seeing a random sample from a new area going into decline after 1628 bce.

Timber (b) is another random sample, this time from a bog trackway at Corlona, Co. Longford. Since in this case the sample is archaeological we might expect that it would originally have grown on mineral soil. Again its mean ring width drops to less than 0.5 mm by 1624 bce and does not recover for some twelve years.

As has been demonstrated, the 1620s bce saw a traumatic episode for oaks not just in Ireland but in England and Germany. Given that the oaks studied by dendrochronologists represent only a tiny random sample of the oaks which were growing at the time, we can conclude that there must have been a very general reduction in oak productivity. We can also see that the onset of this reduction is around 1628 bce. If LaMarche's 1627 bce date is superimposed on Fig. 1-3 it is clear that he specified a year not just of trauma for bristlecone pines but for European oaks as well.

LaMarche's frost rings are strongly correlated with dust veil events. Irish narrowest-ring events are also strongly correlated with dust-veil events by way of the Greenland ice-core acidity dates. Hammer et al. see the most prominent Dye 3 acidity peak for several centuries at 1644 bce ± 20. It is hard to imagine that there can be any other explanation for the accumulated tree-ring effects.

This brings us to the question of mechanism. What is the link between a large volcanic dust veil and reduced oak growth in northern Europe? Sear et al. (1987) have shown that there is noticeable cooling in the northern hemisphere for many months following major eruptions in recent times. Kelly and Sear (1985) have also shown significant low-pressure anomalies over northern Europe after major eruptions. Given that bog trees and some trees from mineral soils are badly affected by the conditions, it seems increasingly likely that the oaks were responding to raised water tables. This would account directly for the growth reduction in bog oaks and in those oaks on mineral soils susceptible to flooding.

So, we have to recognize the general problem of attempting to date volcanic events by dendrochronology. Since the likely mechanism is cooling, giving rise to climatic upset, giving rise to local difficulties for trees, there may be a time lag between the eruption and what is seen in the ring patterns. This possibility was addressed by LaMarche when he suggested that the eruption which caused the 1627 bce frost effect in bristlecone pines may have actually taken place one or two years before 1627 bce. European oaks might again be expected to suffer a time lag, especially if the mechanism did involve the raising of water tables. Additional attractions of this mechanism are that it would allow different areas to be affected at slightly different times — which we appear to see — with the overall effect lasting for a number of years. It might also explain the "damage" effects suggested by Fig. 3 if water table alterations actually killed the deeper root systems of some trees.

The way to answer this question is almost certainly the detailed analysis of stable isotope ratios (deuterium and carbon-13) in the annual rings of trees which grew across the later 1600s bce. Since stable isotopes might give some direct measure of cooling associated with particular growth rings, study of trees from a wide geographical area might give a clear picture of the actual conditions prevailing from the 1630s to 1627 bce and answer the question of the exact date of the cooling associated with the eruption. Preliminary results of analyses by Ramesh (pers. comm.) have shown that there is relevant information in Irish tree rings of the period.

If we review the "narrowest ring" story as it developed after 1986 it is possible to see the strength of the circumstantial case for a highly significant event at 1628 bce. Until 1986 only LaMarche and Hirschboeck's 1627 frost ring and Hammer et al.'s 210 ± 30, 260 ± 30, 1100 ± 50 and 1390 ± 50 dates were available to assess volcanic impact in the period 200 bce to 2000 bce.

In 1986, looking for support or otherwise for LaMarche and Hirschboeck's date, three narrowest-ring events were identified in Irish oaks with start dates at 1628 bce, 1159 bce and 207 bce. The coincidence of these and other events with Hammer et al.'s date ranges for acidity layers in Greenland suggested strongly that the Irish trees had been recording volcanic events.

In 1988 the Irish event dates were published. The relevance of two additional lines of evidence has since become apparent. (1) LaMarche added another frost-ring event at 206 bce. (2) Pang and Chou (1985 and pers. comm.) provided Chinese Dynastic-change dates. Their evidence included famines, 207~204 bce, obscured stars, 208 bce and Dynastic change, 203 bce.

This extremely tight clustering of observations in the decade 210~201 bce suggests a link between dust veils and the concept of the "Mandate of Heaven" associated with Chinese Dynastic change. When it is realized that suggested dates for the start and end of the Shang Dynasty can be found at variously ~1600 bce and ~1100 bce (Pang and Chou 1985) and 1618 bce (Kuniholm, this volume) and 1163 bce (Bernal pers. comm.) respectively, it is clear that the accumulated bristlecone, Irish oak, ice-core and Chinese evidence clusters dramatically.

Such is the case for a hemispheric — volcano-related — environmental event at 1628 bce. How does this relate to the Thera debate? It is clear from the proceedings of the Third Thera and the Aegean World Congress that Thera is the only eruption known to occur in the bracket 1670~1500 bce. None of the other suggested candidate eruptions have been proven to lie within this bracket. Thus, notwithstanding the volcanological assessment that Thera's sulphur output is incompatible with the 1644 ± 20 ice acidity, it must remain a possibility that Thera (or Thera in combination with some other eruption) caused the ice-core signal and contributed to the 1628 bce event. It is this strong circumstantial case, only available within the last few years, which prompts the suggestion that "1628 bce should become the working hypothesis for the date of the Theran eruption until proven otherwise."

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