E-mail to C.L. Perason [2009-10-23]

Dear Dr. Pearson,

Malcolm Wiener (Cc'd) sent me a copy of your recent paper in JArchSci and asked me for my comments on it. It seemed fair to write to you directly; my comments are below.

A central claim of your paper is that there is an increase in S at the time of the growth spike. Data for two trees is presented (fig.4): one tree shows a definite, but not huge, increase; the other tree shows almost nothing. Hence the S deposit at Porsuk does not seem to have been extremely large.

Your paper claims that "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". The contrary seems indicated: Thera seems an unlikely candidate because it would be expected to deposit much more S, as well as substantial Cl--and no Cl is mentioned. (The issue with Cl is That HCl does not survive stratospheric transport: the presence of HCl is a near-conclusive sign of proximity to an eruption.) Too, HF is similar to HCl in this.

Given the approximate date of the anomaly, Aniakchak is a highly plausible candidate for a volcanic source. Yet your paper makes no mention of Aniakchak. Here is a scenario: Aniakchak erupted, injected lots of S into the stratosphere, some of which was later deposited at Porsuk; the volcanogenic aerosol (in the stratosphere) greatly increased precipitation and cloud cover in parts of the eastern Mediterranean (such increases are attested by other eruptions), and that, together with the acidic deposition, induced the tree-growth spike at Porsuk.

Your paper makes little mention of volcano-climatic effects. The closest it comes to doing so seems to be this: "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." Your paper presents no good reason to accept this. It presents data that seems to counter-indicate this. And it ignores that volcanoes have been known to induce precipitation increases in parts of the eastern Mediterranean region.

Your paper additionally says that "Hf has been measured in the Minoan eruption deposits by numerous research teams": yes, but at less than one part per 100000; and tephra deposits at Porsuk were likely extremely tiny, as your paper states. Also, why would Hf become any more abundant than any of the other numerous elements? There was some Hf present at Porsuk prior to the eruption (fig.5), and biological processes that led to a rapid mobilization (as your paper states) of Hf seems to be a plausible a cause for the Hf abundance increase. Your paper appears to treat such mobilization cavalierly:

Whilst it could be argued that the observed increase reflects the impact of increased environmental acidity on REE availability in the soil/regolith, fresh deposition of acidic volcanic ash seems more probable. For example, Hall et al. (1990) found increased REE indicative of known deposition from an eruption of Mt St Helens.

How does the example of Hall et al. make fresh deposition of volcanic ash more probable?

A related quote is this.

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 quote purports to argue against a distant eruption, but provides no actual reasoning.

Regarding the date of Aniakchak, first consider this quote from Denton & Pearce (cited by your paper):

Keenan [2003, p. 2] states that Hammer et al. [2003] present "[data] for each of ten major chemical constituents of particles from Thera (38 particles) and Greenland (174 particles). For the most abundant constituent, SiO2, ... the means are 73.2 ± 0.26 [Thera] and 69.6 ± 0.14 [Greenland]; these do not even overlap at eight standard deviations."

Since the SiO2 abundances do not overlap at eight standard deviations, the Theran and Greenlandic tephras are different. The Copenhagen guys do not accept this point, but they have never tried to rebut it either: they ignore the point when I publish it (my 2003 paper was based on it), and they ignore it when a leading tephrochronologist repeats it. The underlying problem seems to be that they do not know how to calculate the standard deviations. There are statisticians at Cornell who would, I'm sure, be glad to review the calculation with you, if you are unsure of it.

Nick Pearce has identified the Greenlandic tephra as being from Aniakchak. There are problems with Pearce's analyses, but they can be fixed: the identification is really strong (considering both individual elements and the bimodality). Hence we can have confidence about when Aniakchak erupted, to within the error of the ice-core date.

Southon [QuaternRes, 2002] indicated that the GRIP ages should be "shifted by -20 yr (i.e., to younger ages)", on the basis of isotopic records. I have additional analysis to support this. The shift makes the ice-core date for Aniakchak correspond to that of the Porsuk growth event.

So the remaining question is about Aniakchak's stratospheric aerosol: would the aerosol mass have been big enough to leave significant deposits in Anatolia and to cause substantial climatic after-effects? Unfortunately, there are currently no precise estimates of Aniakchak's aerosol mass. The total mass erupted by Aniakchak was roughly twice that of Thera though (but erupted mass only indicates aerosol mass to within a factor of about 25). Thus, it is surely plausible that Aniakchak's eruption had some climatic after-effects--including in the eastern Mediterranean.

Regarding the radiocarbon dates for Thera's eruption, see recent and forthcoming papers by Malcolm, as well as

http://www.informath.org/apprise/a4106.htm

To summarize, it is really interesting to see your data, especially with the "direct evidence in support of a volcanogenic cause for the growth-ring anomaly". The finer analysis of the data, though, requires more assiduity and objectivity.

I also have a request: please publish all your raw data, e.g. on the web.

Good wishes,

Doug Keenan

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Douglas J. Keenan
http://www.informath.org
phone + 44 20 7537 4122
The Limehouse Cut, London E14 6N, UK