The most marked tree-ring growth anomaly in the Aegean dendrochronological record over the last 9000 years occurs in the mid 17th century BC, and has been speculatively correlated with the impact of the Late Bronze Age eruption of Thera (Santorini). If such a connection could be proved it would be of major interdisciplinary significance. It would open up the possibility of a precise date for a key archaeological, geological and environmental marker horizon, and offer a direct tie between tree-ring and ice-core records some 3600 years ago. A volcanic explanation for the anomaly is highly plausible, yet, in the absence of a scientifically proven causal connection, the value of the proposed correlation is limited. In order to test the hypothesis, dendrochemical analysis via Synchrotron Radiation Scanning X-ray Fluorescence Microscopy (SXFM), Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) was carried out on growth-ring series from four trees displaying the anomaly. Increases of sulfur, calcium, and rare earth elements following the onset of altered growth, plus concentration spikes of zinc and hafnium in the first affected growth-ring provide promising new evidence in support of a volcanic causal factor. Although a volcanic association is implied, the new data are not sufficient to prove a link to the exact eruption source. […]
Radiocarbon evidence puts the eruption between 1660 and 1600 BC (Manning et al., 2006; Friedrich et al., 2006). Whilst this evidence, produced by two independent research teams, is compelling, an absolute date, the actual year in which the eruption took place, would provide the most useful and appropriate starting point for any analysis of subsequent impact on society and climate. The dendrochronological record offers the potential for such absolute (precise and accurate) dating to a specific annual growth season. […]
In 1627/1628 BC narrow growth or frost events are found in North European (Grudd et al., 2000; Baillie and Munro, 1988) and North American (Salzer and Hughes, 2007; LaMarche and Hirschboeck, 1984) dendrochronological records. These have been correlated with acidity horizons in the GISP2 (Zielinski et al., 1994) and Dye 3 (Clausen et al., 1997) ice cores, and are supported by the 1627–1600 BC date range (Friedrich et al., 2006) derived from an olive tree buried by eruption debris. The accuracy of this narrow date range has been questioned, however, due to major difficulties inherent in identifying annual growth increments in olive wood (Vinther et al., 2008).
Around 1645 BC, (within the broader radiocarbon range of Friedrich et al., 2006 and Manning et al., 2006), a second cluster of anomalous growth events includes North American, Siberian and Finnish ring width minima in 1652 BC and 1648/1649 BC, and a frost damaged bristlecone pine ring in 1653 BC (Salzer and Hughes, 2007; Hantemirov and Shiyatov, 2002; Eronen et al., 2002). Again, possible volcanic forcing is evidenced by correlation with a significant acidity horizon around 1642 5 BC in the Greenland ice core record (Vinther et al., 2006, 2008). While the relevance of any of these specific dates in relation to Thera will continue to be argued (e.g. Denton and Pearce, 2008), what can be agreed upon is that there is evidence to place both volcanic eruptions and climate perturbations at these two points in the early Late Bronze Age.
Within the 16th or early 15th century BC time range favored for the eruption by proponents of conventional archaeological dating (Wiener, 2003, 2007; Warren, 1999), such evidence is more sparsely distributed. Whilst a bristlecone pine ring width minimum in 1597 BC may link with a minor ice-core signal (Salzer and Hughes, 2007), this is not well substantiated. Minima in 1544 BC, 1524 BC and 1418 BC (with frost damage in 1419 BC) have no other proxy correlations, and the latter in particular is far outside the date range suggested by all radiocarbon evidence. […]
Unfortunately, as yet, there is no Aegean equivalent to the long, continuous, absolutely dated tree-ring series for the second millennium BC derived from German and Irish oaks, Swedish pines, or the bristlecone pines of the American southwest. Samples from the Aegean for this critical time period are limited to rare finds preserved at archaeological sites, cross-dated to build floating chronologies and then anchored in time by wiggle-matched radiocarbon dates, rather than by more conventional dendrochronological procedures. Nevertheless, The Malcolmand Carolyn Wiener Laboratory for Aegean and Near EasternDendrochronology at Cornell University has such unique samples fromthe early Late Bronze Age in its collection.Of these, 61 of at least 64 individual trees, represented in over 200 samples from Porsuk, SE Turkey (Fig.1), display an exceptional short-lived (c. 7 year) growthspikewhichisby far themost significantgrowthanomaly tobe found in over 9000 years of Aegean tree-rings. Although the exact provenance of the trees from Porsuk is unknown, it is almost certain that they grew c. 5 miles to the south of the site, in the Taurus Mountains, approximately 840 km downwind of Thera (Kuniholm et al., 1992), during the time frame in which, according to the radiocarbon evidence, the Minoan eruption of Thera took place. […]
Dendroecologically speaking, the most logical cause for an anomaly such as this would be a sudden, short-term improvement in growth conditions for these trees, specifically to a cooler, moister environment, possibly with increased cloud cover. Although the degree of climatic impact of the Thera eruption has been much argued in the literature (Pyle, 1997; Eastwood et al., 2002), even conservative estimates of sulfur dioxide emissions, the key volcanogenic factor influencing surface temperatures (Rampino and Self, 1982), are predicted to have resulted in 2 or 3 years of slightly warmer winters and cooler summers in high latitudes (Pyle, 1997). Such conditions could have extended the growth season, resulting in the observed short-term improvement in growth. This makes plausible the suggestion of Kuniholm et al. (1996) that this may be the Aegean equivalent to other European growth-ring anomalies attributed to the Thera eruption, especially in light of recent evidence suggesting the eruption may have been larger than previously estimated (Sigurdsson et al., 2006). […]
The onset of the anomaly, starting with relatively dated ring 854, is currently radiocarbon dated to 1650 BC þ4/ 7 (Manning et al., 2003, 2001). Given the proximity to Thera, the nature of the growth pattern, and the coincidence of the date, a Theran causal hypothesis appears highly possible. Yet, in the absence of a provable causal connection, such conjecture is of somewhat limited use.
In this paper, high-resolution multi-elemental mapping via Synchrotron Radiation Scanning X-Ray Fluorescence Microscopy (SXFM) in combination with Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Mass Spectrometry (ICP-MS) is used for a dendrochemical investigation of the tree-ring anomaly from Porsuk. The volcanic causal hypothesis is tested via an investigation to establish prospects for isolating an elemental marker signature which could be used to positively link the eruption of Thera, or some other event, with the change in growth pattern at this point in time. […]
The combination of the three analytical methods resulted in detection of a range of over 30 elements, with some detectable by only one method (e.g. sulfur (S) via ICP-AES or the rare earth elements (REE) via ICP-MS) and others (e.g. calcium (Ca)) replicated by all methods allowing for intercomparison of data series for the same element. […]
The elemental changes observed in the four trees from Porsuk from the onset of the growth-ring anomaly are not entirely consistent. Whilst the majority of elements appear to increase at or following ring 854, that change is not equally strong in all trees, nor does it affect all elements in the sameway at exactly the same time. In the absence of any means to prove the exact growth provenance of the material it is not possible to address these inconsistencies further as they are likely to reflect localized, micro-site specific variables.
The key point is, whilst the exact type of the elemental change is not replicated in each tree, some degree of change occurs for most elements, in all the trees, from the onset of the growth anomaly. This was replicated via three separate analytical methods. The fact that the change is slightly different in individual trees indicates a response to external elemental variability, impacted by a wide range of micro-site/tree specific factors (e.g. soil depth, chemistry, or the age of the tree), rather than some type of physiological effect common to Juniperus sp. in response to more favorable growth conditions. This is supported by the fact that where longer sequences were analyzed (e.g. Fig. 4), the change can be observed to extend beyond the physical anomaly into the resumed regular growth. The implication of all this is that, when considering the probable cause of the ring 854 growth anomaly, we are looking not only for a change in environmental conditions which caused a short-term improvement in growth, but also an event which caused a significant change in environmental chemistry.[…]
So increases and decreases in elements observed in the Porsuk trees can be explained by a sudden influx of acid and its effect on micro-site specific variables for each of the trees. The distinct increase in S concentrations shown in Fig. 4 indicates a large influx of S to the forest system, while increased Ca, Mn, and Sr (e.g. Fig. 5) may indicate mobilization of these elements in response to that increased acidity. Whilst a forest fire might result in similar changes in environmental chemistry (Bondietti et al.,1989), the growth-ring pattern is not consistent with the impact of such an event, nor were any fire scars found in the Porsuk samples. Given the approximate date of the anomaly, the most logical source of sulfuric deposition would be volcanic, and given its proximity, Thera would seem the most likely candidate. Yet S alone does not prove a connection with the Minoan eruption. Likewise, the spikes of Zn (see Fig. 4) and Zr, whilst indicating an influx of new chemistry to the growth environment (or rapid mobilization of these elements in the soil), cannot be directly linked to a particular source. The only potential indicators detected for which a specific volcanic origin might be more directly hypothesized are the Hf spike (e.g. see Fig. 5), which occurs in ring 854, and the increase in REE, Se and Y following the onset of the anomaly. Hf occurs with Zr in zircon crystals in silicate rich igneous rocks. The bedrock of the Taurus Mountains is calcareous, made up of sedimentary (limestone) and/or metamorphic (marble) facies. Given the rareness of Hf, its mode of formation, and the sudden, short-term increase, a volcanic origin seems a likely possibility. Hf has been measured in the Minoan eruption deposits by numerous research teams (Steinhauser et al., 2006; Eastwood et al., 1999; Huber and Bichler, 2003; Bichler et al., 2003), and zircons have been reported as part of the finer windblown fraction of the tephra (Vitaliano and Vitaliano, 1974). Equally it is difficult to offer an alternative explanation (other than the influx of new geological material) for an increase in the REE and unusual elements such as Se and Y. […]
It is with the REE that the main plausible potential exists to actually prove a link between a tree-ring and a specific volcanic eruption. These elements have unique ratios depending on source formation conditions and are widely used to provenance geological materials (e.g. Steinhauser et al., 2006; Eastwood et al., 1999). […]
Whilst further replication of the observed signatures is required for all elements, these data imply that the Porsuk growth anomaly was caused by an acid-producing environmental event which also resulted in the introduction of new geological material to the growth location. The presence of Hf and the REE in particular indicates a volcanic origin. This interpretation makes it possible to offer an alternative explanation for the Porsuk growth anomaly. Rather than the indirect, albeit well-established effect of sulfur dioxide on climate (Robock and Mao, 1995), it seems plausible that the growth anomaly was caused by more direct volcanogenic impact, a type of fertilization effect, either from tephra deposition, or due to the soil/chemical impact of volcanogenic acid. Alternatively it is possible that acidic/tephra deposition was sufficient to harm less established or lower lying vegetation, temporarily improving conditions for well-established trees until the ground cover recolonized. […]
The impact of acidic volcanic output on vegetation has been well recorded in the literature (e.g. Grattan and Pyatt,1994). It is possible that altered soil acidity made it difficult for certain ground cover to grow, lessening competition for mature trees, but it is the potential impact of nutrient release from increased acidity, in combination with a slight influx of newmineralogically enriched material, which fits best with the derived elemental evidence from the Porsuk trees. A recent study (Frisia et al., 2008) provides speleothem evidence for a major sulfate peak in the early Late Bronze Age (which the authors attribute to the Thera eruption), from a cave in northwestern Anatolia. The cave is located further to the north than Porsuk, but at approximately the same distance from Thera (see Fig. 1) and provides independent evidence of widespread S deposition across the region. Given this evidence, it seems reasonable to conclude that the root cause of the Porsuk growth-ring anomaly is not the indirect impact of an eruption on climate, but rather a direct impact from volcanogenic deposition. […]
This conclusion has parallels with that of Eastwood et al. (2002), who identified enhanced lake eocosystem productivity resulting from deposition from the Minoan eruption at a site 400 km east– northeast of Thera. The case is further strengthened by comparison with an early study of Pinus sp. growth-ring anomalies (including anomalously wide rings) resulting from eruptions of Paricutin volcano in Mexico (Eggler, 1967). In the study, where site-specific factors could be investigated and an eruption history was known, a combination of all the previously proposed impact factors was concluded to have resulted in similar anomalous growth. […]
For the first time we present direct evidence in support of a volcanogenic cause for the growth-ring anomaly beginning with relative ring 854 in trees from the Anatolian dendrochronologicalrecord.We do not use this to claim once and for all a positive causal connection between the Thera eruption and this sequence of treerings, but the presence of a replicable elemental change in all four trees, including significant increases in S and Hf at the onset of the growth-ring anomaly, provides convincing evidence that a volcanic eruption was the primary cause. Given the approximate growth location of the trees and the current date of the anomaly, the most logical source would be the Minoan eruption of Thera. These data can be cited as new proxy evidence to add to that of other paleoenvironmental archives which place the date for the Minoan eruption in the mid-late 17th century BC.[…]
For the data collected in this study, covering the years around both 1650 BC and 1628 BC, the only replicable indication of major elemental change was found at relative ring 854. If it could be demonstrated that this elemental change is unique over several hundred years (as is true of the actual growth anomaly), this would provide further evidence to improve the credibility of the proposed link with the largest volcanic eruption in the region at this time.