Dear Charlotte,
> This paper reports preliminary findings based on a small sample of trees
> from Porsuk and is clear about this from the outset. Although
> potentially exciting and sufficient to build a case for future work,
> data from such a small sample of trees would never have been sufficient
> to unequivocally prove a direct link between the growth anomaly and a
> volcanic eruption, nor to specify the Minoan eruption as an exact source
Yes, my comments did not dispute that.
> The data were however sufficient to provide a firm basis to
> offer alternate hypotheses as to the origin of the Porsuk growth anomaly
> and worthy of publication as they demonstrate the potential of
> dendrochemistry to contribute to evidence for or against specific years
> put forward by various scholars for the date of the Thera eruption. I
> stand by our conclusion based on these data - exactly as stated in the
> paper- that a direct volcanic association is implied and that this
> hypotheses now requires rigorous testing.
I dispute that (see further 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.
>
> The amount of S actually present in the wood cannot be used to measure
> the amount of S deposited by a particular pollution event. If a whole
> range of growth environment variables (soil chemistry in particular)
> were known, along with key physiological requirements of the species of
> tree to be investigated, then it might be possible to model uptake of S
> and other elements and thereby retrospectively attempt to quantify the
> actual amount of acidic deposition required for the type of response
> observed. The fact that for one of the trees there was an observable
> increase of S and that for others there were increases of other
> essential elements fits with a hypothesis of element mobilization
> relating to increased environmental acidity. (Based on the findings of
> 50 or more studies on the impact of anthropologically induced acid rain
> on terrestrial ecosystems).
If only one of the four trees showed any increase in S, then your paper
should have stated that.
Additionally, S is vastly more abundant than any of the trace elements;
so the low abundance increase in S is evidence that none of the
trace-element increases are due to assimilation from tephra.
> This hypothesis is put forward in addition to evidence for relatively
> major S deposition during the Bronze Age from at least three independent
> paleoenvironmental records (e.g. Frisia et al. 2008, Siklosy et al 2009,
> Eastwood et al. 2002) - which all conclude the Thera eruption to be the
> source.
Frisia et al. do not "conclude" that the S is from Thera. Rather, they
assume that, on the basis of their date and the radiocarbon dates for
Thera. If Thera's eruption was dated to a time other than the late
17th century, then they would have matched Thera to a different S
spike; there are several such spikes during 1700-1450. Moreover, the
S spike that they attribute to Thera is not the largest spike during
that time period. Please see their graph of S--a copy is attached.
Siklósy et al. do not conclude that Thera is the source. Again, the
authors assume such, on the basis of the radiocarbon dates and their
apparent--false--belief that Thera (only) erupted at that time.
Eastwood et al. study a site near the line of maximum tephra dispersion,
about 385 km from Thera. Certainly, their site received much direct
deposition from Thera. Porsuk, however, would have received very
little direct deposition from Thera. So the S deposition at the site of
Eastwood et al. may well be unrelated to the S deposition at Porsuk.
We know that Aniakchak had a very large eruption (perhaps twice as
massive as Thera's) within several years of 1629 BC. We know that
there was a hemispheric climatic event beginning in 1629 BC. And
we know that the details of that event, though few are available, are
consistent with the event being volcanically induced. Thus Aniakchak
was the very probable, though not certain, cause of the event.
The Porsuk event was originally dated via tree-ring matching. That
matching is very untrustworthy: see my review. Some support for
my review's general arguments is given by Griggs & Manning [2009].
Unfortunately, G&M also demonstrate that the Cornell lab still does
not have competence at tree-ring matching in Anatolia (I have
described to Sturt an approach to addressing this). So the Porsuk
event is really only dated via a few 14C dates. As I understand
things, then, the Porsuk event might be part of the 1629 BC event--
if so, it is very probably due to Aniakchak.
> > 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.
>
> With regard your argument for the quantity of S evidenced from the tree
> rings not being enough to be from Thera I hope I have answered this
> above — with the limited dataset presented in the paper we cannot begin
> to estimate 'how much' additional S was present at the onset of the
> growth anomaly — just that there was a change in environmental acidity.
> As to Cl — this element is particularly mobile in the terrestrial
> environment and sparsely studied in terms of wood physiology, so has not
> so far been a key focus. It is problematic to detect with ICP-MS due to
> interferences from other elements, and as yet we have not been able to
> gain sufficiently good detection via SXFM to find it. If we are able to
> move forward with some further analyses this would be a particular
> priority — as you rightly suggest — to establish or not the likelihood
> of a 'local' eruption source.
Cl is much less abundant, in the eruption, than S. Since you found very
little S, and since Cl tends to be more mobile than S, it is extremely
unlikely that you would find any Cl, even with more sensitive tests.
Even assuming that the Porsuk event was due to Thera.
> On the basis of evidence at the time of writing, the paper suggests (not
> claims) that Thera would be the most likely candidate as a source of S
> deposition based on the following reasoning: According to field
> observations of tephra deposition and associated isochrone maps, the
> Porsuk trees were growing downwind of Thera, and so are likely to have
> received some fallout when the eruption occurred. The fact that they are
> only 400km away — the same distance (though not quite the same
> direction) as a speleothem record in which a large increase in S
> (concluded to be the result of the Thera eruption) has been
> independently identified during the Bronze Age (Frisia et al. 2008)
> makes this a logical assertion.
Porsuk is 825 km away from Thera. And it is far from the line of
maximum tephra dispersal. There would have been very little direct
deposition at Porsuk.
The speleothem record of Frisia et al. is treated above.
> The fact that other researchers have
> linked acidic deposition directly with the Minoan tephra in lake
> sediments (e.g. Eastwood et al. 2002 — albeit in that case not to
> detrimental effect) can also be taken to support a logical argument for
> Thera being the most probable source for a change in environmental
> chemistry at Porsuk.
The lake that Eastwood et al. studied would have received vastly
more tephra than Porsuk, due to both distance (385 km) and,
especially, being near the line of maximum deposition. Additionally,
the Porsuk sequence ends in the mid 16th century. There might well
have been a large acidity spike at Porsuk in the late 16th century or
afterward, but the Porsuk sequence cannot tell.
Your paper does not state that the Porsuk sequence ends in the mid
16th century BC. Nor does it state that almost all archaeologists
believe that the date was around 1500 BC. Those two things
together will likely mislead readers who are unfamiliar with Thera
into thinking that the tree rings span all plausible eruption dates.
This is a serious issue.
> > 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.
>
> If we were able to undertake the further analysis I propose we
> test the climate change hypothesis against the Porsuk growth rings by
> looking at the oxygen and carbon isotope record for the same period — if
> there was an increase in precipitation it would be logical to expect a
> change in the isotopic record with the onset of the anomaly.
For carbon isotopes, a change due to precipitation would be
indistinguishable from a change due to direct deposition. Hence there
would seem to be little value in studying carbon isotopes.
For oxygen isotopes, a regional change in precipitation would indeed
be expected to be reflected in the ratios. But volcanoes tend to induce
a regional seasonal _decrease_ in precipitation. Your comment seems
to confuse regional seasonal precipitation with local storm precipitation:
the latter (only) is a very plausible cause of the Porsuk event.
As an example of a local event, in 1816 (after Tambora's eruption) there
was a storm in Egypt whose torrential rain continued for several days,
caused floods that nearly destroyed some villages, and seriously
threatened Cairo. (I suspect that the Ahmose storm text relates a similar
event.) See further Yair Goldreich's book "Climate of Israel" [2003].
> > 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?
>
> The reason we suggest that fresh deposition
> from volcanic ash might be more probable is the nature of the increase
> in Hf which is abrupt and tied to the onset of the anomaly. If it were
> the result of mobilization from pre-existing Hf in the growth
> environment one would expect the increase to be gradual as the
> acidity level increased and then lag as it slowly leached out over time.
> The singular nature of the Hf increase is rather more indicative of a
> sudden introduction of new material.
>
> The answer to settle all of this is really to undertake further analysis
> — I could find no references on the uptake and distribution of Hf in
> tree species, thus a model explaining how and why it might specifically
> become more abundant in tree rings as a result of acidification could be
> developed. The fact that Hall et al. found rare earth uptake as a direct
> result of known tephra deposition simply demonstrates that others have
> found that this can occur.
An abrupt increase of some elements is what would be expected if there
were extremely-severe storms. Your work does not seem to have
considered this point: a severe storm would provide much extra moisture
for tree growth (some temporarily retained in local lakes, perhaps) and
also increase local weathering--which adds elements to the ground water.
Also, your comment does not rebut my point that there was insufficient
Hf in the tephra at Porsuk to leave any trace in tree rings.
As for Hall et al.--they found no Hf at all. Moreover, they studied trees
where at least 10 cm of tephra was deposited; Porsuk, in contrast,
probably had at most 0.01 cm. A copy of their paper is attached.
> It is also agreed that it was a
> relatively big eruption which dispersed S and tephra in a broadly
> easterly direction.
The direction was broadly NE. (Tephra found on Crete appears to be
from the precursory phases [Keenan, G^3, 2003], which was weeks
earlier than the climactic phases and should not be counted here.)
McCoy & Heiken [Volcanic Hazards (editors--McCoy & Heiken), 2000]
suggest that the dispersal was due in part to the jet stream. I have
additional evidence that gives some support for that.
The total mass of the eruption was big. The best estimates that I know
of for Thera's S injection are given by Scaillet et al. [Volcanism and the
Earth's Atmosphere (editors--Robock & Oppenheimer), 2003]. Those
estimates are only claimed to be accurate to within a factor of about 25.
Your paper does not cite Scaillet et al.
> > I also have a request: please publish all your raw data, e.g. on the
> > web.
>
> I completely agree with the idea of publishing the raw data but this is
> not practicable. The complete raw datasets amount to many gigabytes of
> data and make little sense until the raw spectra are processed into
> elemental abundance.
Abundances of each element for each ring of each tree--that's less than
5000 numbers. Those are the numbers that people care about, and they
could easily be published.
> Thank you for raising some interesting issues — I hope I have gone
> some way towards addressing them.
Your paper presents some measurements and some preliminary
analyses of those measurements. I am sorry, but the analyses
are not logically coherent and they fail to understand the literature.
In particular, there is no reason to believe--and good reason to
disbelieve--that anything about a tephra can be determined from
current or future measurements at Porsuk.
Good wishes, Doug
