Hit-or-Myth? Linking a 1259 AD Acid Spike with an Okataina Eruption

Article excerpt

Introduction: Bronze Age catastrophes and myth-making

In their recent provocative paper, Buckland et al. (1997) examined evidence for two Bronze Age 'catastrophes'. The first, the destruction of Bronze Age Thera (Santorini) by a cataclysmic volcanic eruption, was described as 'real and in need of a calendar date' (p. 581). The second, the apparent collapse of Middle Bronze Age settlement in upland Britain, was considered as a speculative event 'hypothesized on archaeological grounds and dated by a tenuous link through tree rings to an Icelandic volcano' (p. 581). The fundamental purpose of their critique was to demonstrate that great caution is required in the interpretation of interdisciplinary studies that attempt to link archaeological findings with those from other disciplines. This caution is essential because such age-based linkages may enter the literature as if proven fact because the limitations of the data are rarely communicated clearly, either unwittingly or otherwise. This is anathema to archaeology because the distinction between 'fact' and 'interpretation' may not always be obvious to its practitioners.

Whilst we agree in general with their conclusions, we think Buckland et al. have unintentionally violated one of their own tenets by constructing a linkage, based on the assumed correlation of radiometric and ice-core derived dates alone, between the 1259 AD acid spike in ice cores and an Okataina-derived volcanic eruption in New Zealand. Although only a minor part of their paper, the construction of this link by Buckland et al. nonetheless is viewed as the initial step in the mythicizing process they rightly wish to avoid. We demonstrate that such a link is untenable because the age data Buckland et al. applied to the Okataina eruption are flawed, and we suggest that the 'link' needs correcting before a new myth develops. This correction is particularly relevant to archaeological studies in the South Pacific because the Okataina-derived eruptive provides a valuable regional datum in dating New Zealand's exceptionally brief prehistory (Higham & Hogg 1997; Newnham et al. 1998).

The 1259 An acid spike and the Kaharoa eruption, Okataina

The 1259[+ or -]2 AD acid signal is one of the largest recorded in ice cores from Greenland and Antarctica for the past 2000 years (Hammer et el. 1980; Langway et al. 1988; Zielinski et al. 1994). Because the spike is common to ice-core records at both poles, Langway et al. (1988) suggested that the eruption must have been large and equatorial. El Chichon volcano (Mexico) is a possible source (Palais et al. 1992).

Buckland et el, however, attributed the 1259 AD acid spike to a mid-latitude eruption from Okataina volcano in North Island [ILLUSTRATION FOR FIGURE 1 OMITTED]. The 'Okataina' eruption is clearly the Kaharoa episode, the largest and most recent rhyolitic event in New Zealand, which resulted in extensive tephra fallout [ILLUSTRATION FOR FIGURE 1 OMITTED] (Lowe et al. in press a). The basis of Buckland et al.'s correlation with the 1259 AD ice-core acidity record is evidently the derivation of a date of 1259[+ or -]11 AD for the Kaharoa eruption via calibration of associated radiocarbon ages (Ramsey 1994). This calibrated date is based on the mean age of 770[+ or -]20 b.p. reported in Simkin & Siebert (1994) following Froggatt & Lowe (1990). We do not dispute the calibration process per se except to comment that the Southern Hemisphere offset correction (Vogel et al. 1993; McCormac et al. in press) does not seem to have been applied. based on Stuiver & Reimer (1993) and Stuiver & Becker (1993) and the intercepts method, 770[+ or -]20 b.p. corresponds to 1258-1283 AD without the offset, but with a -40-year offset correction the calibrated 1[Sigma] range is 1280-1291 AD, clearly incompatable with 1259[+ or -]11 AD at this level of significance. Of more importance to our discussion is the fact that the 770[+ or -]20 b. …