Tree growth downturns in the Iron Age

Pines growing in northern Scandinavia are regarded as notably sensitive to climatic effects of e.g. volcanic eruptions. A chronology of climatic downturns through the mid- and late-Holocene has been established for Finnish pine (the same pine master as we used to date European oak) in a recent article by Samuli Helama et al. (ref.1). Sudden negative change in growth conditions were registered for the years Astr 1601, 536, -329, -1463, -2563, -2849 and -4865. (About year count and notation read explanation here, opens in new tab)

For Iron Age and Roman time, the downturns at Astr -329 and Astr 536 might be relevant to look at. If we place Hollstein's Iron Age/Roman complex (HollsteinRoman, RomBC 546 to RomAD 315) in its new position according to the proposed 218-years-offset against Scandinavian pine, it exactly fits the gap between these two extraordinary events visible in the Finnish tree-ring record.

Fig.1: HollsteinRoman (red normalised curve and green ring width curve) matched against Finnish pine (black normalised curve and blue ring width curve) using the Hollstein normalisation, offset 218 years, corr.coeff. 0.14, t=4.3 at 860 years overlap.

The downturn at Astr -329
In the Finnish master, the event at Astr -329 caused a severe timber depletion which made it difficult to bridge this period (Eronen 2002, ref.2). The following is a citation from ref.2:

There was a sudden drop in the pine growth in 330 BC. A very thin ring was formed in that year and the recovery from the blow took about 20 years. Based on the ring widths, this was the most unfavorable year for the growth of tree-line pines in Lapland during the entire 7500-year period (Lindholm et al., unpublished data).

A deeper analysis of the causes for the severe growth depression is made in the article, with the conclusion that it "is likely to have been caused by increased wetness" as no volcanic fall-out or other anomalies could be noticed.

Update 2015-08-02: In a recent study by Sigl et al. (ref.11) a new refined ice core chronology with global volcanic markers is presented. No volcanic activity around  Astr -329 is registered.

The Astr -329 event is clearly evident in the Torne träsk master as well (ref.3):

Trees from around this period show extremely suppressed growth, often associated with distorted ring patterns and the occurrence of reaction wood.

Here is a zoom of this period in the tree-ring record:

Fig.2: HollsteinRoman (red normalised curve and green ring width curve) matched against Finnish pine (black normalised curve and blue ring width curve), offset 218 years, Astr -329 event.

The oldest tree-ring of HollsteinRoman (conventionally RomBC 546) directly succeeds a severe timber depletion in the German oak record called "Hallstatt gap", which with our 218-years-offset coincides with the Scandinavian pine depletion at Astr -329. The Hallstatt gap marks the transition between North European Bronze Age and North European Iron Age.

Bernd Becker (ref.4) characterises this gap as follows:

The gap in the Hohenheim river oak chronology during the later part of the Hallstatt period exists even after 20 years of intensive field sampling, and is still unexplained. On the one hand, continuous overlapping older oak sequences (and in the oldest part, pines) show a unique continuity of flooding events over the entire Holocene. The exception to this is the lack of evidence for tree accumulation in river gravels in south-central Europe between 540 BC (the latest accumulation date from the Lahn River) and 382 BC (the earliest accumulation date of a series of subfossil trees at the Danube). Because of this lack of evidence, a 10-yr gap in the subfossil river oak series occurs from 540 to 530 BC. Unfortunately, this small gap coincides with a period when finds of archaeological oak samples in southern Germany are also very rare. The early Hallstatt oak samples from archaeological sites end at 684 BC, but the tree-ring curves of the hitherto earliest dendrodated La Tene oak constructions do not start before 546 BC (included in HollsteinRoman, our comment).

Friedrich 2004 (ref.5) comment:

Even after 30 yr of continuous fieldwork and more than 3000 samples from the river Main alone, a phase of very low replication still exists in the “Hallstatt” period (2600–2300 BP) of that chronology.

What ever it was that struck northern Scandinavia (and also Siberia, Helama, personal communication) so that most trees died, it was not limited to that region if we are right. It had influence on whole continental north-western Europe, but not on the British Isles. QUB has no difficulties to bridge RomBC -548/Astr -329 (with SwanCarr and GarryBog) and the samples do not show anomalies in ring growth.

So, what type of catastrophe could have killed almost all trees in continental north-western Europe and Scandinavia, and could have made it difficult for the survivors to regain normal growth? Wildfire, storm, landslide, earthquake: yes, but the affected area is very large and the event was very distinct in time. Mega-tsunami: less probable because the British Isles were not affected. Some kind of disease: would not have killed different species at the same time, and not immediately. Would not have caused reaction wood. Mega-volcano eruption: would not have killed that many trees immediately, and would have had influence over the whole northern hemisphere. Moreover it would not have caused reaction wood. Remains some cosmic influence.

Our association goes directly to the Tunguska event in 1908, when a small asteroid or comet exploded over a remote location in Siberia. Due to the remoteness no fatalities were reported, but it is estimated that some 80 million trees over an area of 2,150 square kilometres were felled, and that the shock wave from the blast would have measured 5 on the Richter scale. Read eye witness reports and see pictures here and here.

Are there any eye witness reports of the Astr -329 event? This date would correspond to RomBC 548, and written reports from that time are of course scarce. But maybe there is an oral tradition from that time, some kind of legend, which has been written down much later in the norse mythological sagas of the Edda. This is what Snorri Sturluson narrates about the "Weird of the Gods" (Ragnarök) in his "Gylfaginning":

Chapter LI. Then said Gangleri: "What tidings are to be told concerning the Weird of the Gods? Never before have I heard aught said of this." Hárr answered: "Great tidings are to be told of it, and much. The first is this, that there shall come that winter which is called the Awful Winter: in that time snow shall drive from all quarters; frosts shall be great then, and winds sharp; there shall be no virtue in the sun. Those winters shall proceed three in succession, and no summer between; but first shall come three other winters, such that over all the world there shall be mighty battles. In that time brothers shall slay each other for greed's sake, and none shall spare father or son in manslaughter and in incest; so it says in Völuspá:

Brothers shall strive | and slaughter each other;
Own sisters' children | shall sin together;
Ill days among men, | many a whoredom:
An axe-age, a sword-age, | shields shall be cloven;
A wind-age, a wolf-age, | ere the world totters.

Then shall happen what seems great tidings: the Wolf shall swallow the sun; and this shall seem to men a great harm. Then the other wolf shall seize the moon, and he also shall work great ruin; the stars shall vanish from the heavens. Then shall come to pass these tidings also: all the earth shall tremble so, and the crags, that trees shall be torn up from the earth, and the crags fall to ruin; and all fetters and bonds shall be broken and rent. Then shall Fenris-Wolf get loose; then the sea shall gush forth upon the land, because the Midgard Serpent stirs in giant wrath and advances up onto the land. Then that too shall happen, that Naglfar shall be loosened, the ship which is so named. (It is made of dead men's nails; wherefore a warning is desirable, that if a man die with unshorn nails, that man adds much material to the ship Naglfar, which gods and men were fain to have finished late.) Yet in this sea-flood Naglfar shall float. Hrymr is the name of the giant who steers Naglfar. Fenris-Wolf shall advance with gaping mouth, and his lower jaw shall be against the earth, but the upper against heaven,--he would gape yet more if there were room for it; fires blaze from his eyes and nostrils. The Midgard Serpent shall blow venom so that he shall sprinkle all the air and water; and he is very terrible, and shall be on one side of the Wolf. In this din shall the heaven be cloven, and the Sons of Múspell ride thence: Surtr shall ride first, and both before him and after him burning fire; his sword is exceeding good: from it radiance shines brighter than from the sun; when they ride over Bifröst, then the bridge shall break, as has been told before. The Sons of Múspell shall go forth to that field which is called Vígrídr, thither shall come Fenris-Wolf also and the Midgard Serpent; then Loki and Hrymr shall come there also, and with him all the Rime-Giants. All the champions of Hel follow Loki; and the Sons of Múspell shall have a company by themselves, and it shall be very bright. The field Vígrídr is a hundred leagues wide each way.

When these tidings come to pass, then shall Heimdallr rise up and blow mightily in the Gjallar-Horn, and awaken all the gods; and they shall hold council together. Then Odin shall ride to Mímir's Well and take counsel of Mímir for himself and his host. Then the Ash of Yggdrasill shall tremble, and nothing then shall be without fear in heaven or in earth. Then shall the Æsir put on their war-weeds, and all the Champions, and advance to the field: ....

This resembles the report of a larger cosmic explosion or impact, which sets the landscape on fire, fells trees and kills humans and animals within a large area. There are earthquakes, thunder and tsunami, and finally the poisoning of water and air which would cause chemical winter and slow down nature's recovery from the blast. This would in turn cause famine, disease, migration and warfare among the surviving people. Maybe we now know the exact year for a real such catastrophe: Astr -329.

Now, is there some direct evidence for an extraterrestrial impact around this time, except for the tree-ring anomaly and a saga? In 2007, Mike Baillie wrote a review on "The case for significant numbers of extraterrestrial impacts through the late Holocene" (ref.7) in which he mentions two possible candidates: the Kaali meteorite craters in Estonia, and the Chiemgau impact.

The Kaali craters on the island of Saaremaa, Estonia are definitely the result of an extraterrestrial impact. The date of the event is still controversial (ref.10), ranging from about 7600 to 2350 BP mainly based on radiocarbon measurement (which is difficult if the ground consists of carbonate minerals as in this case).
However, archaeological examinations place the impact in the late Bronze Age/early Iron Age, the site has probably been used for worship and perhaps animal sacrifice. The place has also been interpreted as being "Ultima Thule" which Pytheas from Massalia visited between RomBC 350 to 325 and where the barbarians showed him "the grave where the Sun fell dead".
There are signs of large wildfires and the pollen record suggests that farming ceased for a time after the impact. (Refs.8, 9).

The Chiemgau impact is still a disputed hypothesis about a vigorous extraterrestrial impact having formed a big crater strewn field in southern Bavaria. Abundant finds of characteristic microtectites, glas coated pebbles and other strange materials are used to prove the nature of the event, which is archaeologically dated to the late Bronze Age/early Iron Age. And once again there might be an echo of the catastrophe in the Celts saying  that they fear nothing unless the sky would fall on their heads (Strabon and Asterix).
Furthermore, a second site with the same type of glass coated pebbles etc. has been found 500 km away in the Nalbach/Prims area in Saarland. Here an impact crater has not been located yet, but the nature of the event that created the material has been investigated and confirmed (read the thesis here).

Maybe the three impact sites were caused in the same event, and maybe this event is related to the tree-ring anomaly in Astr -329. In case of the Kaali impact and the Chiemgau impact, the trajectory of the meteorite fall goes from north-east to south-west. If the impacts occurred simultaneously, we should find even more evidence on the path.

The downturn at Astr 536
The end year of HollsteinRoman (conventionally RomAD 315) appears directly before another downturn in tree-ring growth, if we apply the 218-years-offset. This is the well-known (multi-stage?) event of  Astr 536 to 542, described by Mike Baillie in his book "A Slice through Time" (ref.6), and clearly visible in e.g. the Whithorn collection from Scotland (in the QUB data).

Here is a zoom of this period in the tree-ring record:

Fig.3: HollsteinRoman (red normalised curve and green ring width curve) matched against Finnish pine (black normalised curve and blue ring width curve), offset 218 years, Astr 536 to 542 downturn.

The cause of this downturn is not fully understood, there might actually have been more than one elicitor e.g. one or more volcano eruptions together with a meteor or comet (ref.7) (a part of Halley's comet has been suggested).

Update 2015-08-02: In the recent study by Sigl et al. (ref.11), the authors pinpoint multiple volcanic eruptions as the elicitor of extraordinary prolonged northern hemisphere cooling. 

As this event happened in late antiquity, there are written reports which tell about sightings of "events in the skies" including falling boulders and hailstones, and the aftermath: "dry fog", dark clouds, famine all over the northern hemisphere and a pandemic called the "Justinian plague". And, if we are right, the final decline of the West-Roman Empire.

1. Helama, S., Holopainen, J., Macias-Fauria, M., Timonen, M. & Mielikäinen, K. 2013. A chronology of climatic downturns through the mid- and late- Holocene: tracing the distant effects of explosive eruptions from palaeoclimatic and historical evidence in northern Europe. Polar Research 32, 15866. read here
2. Eronen, M., Zetterberg, P., Briffa, K.R., Lindholm, M., Merilainen, J., Timonen, M., 2002. The supra-long Scots pine tree-ring record for Finnish Lapland: Part 1, chronology construction and initial inferences. The Holocene 12(6): 673-680.
3. Grudd, H., Briffa, K.R., Karlen, W., Bartholin, T.S., Jones, P.D., Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene 12(6): 657-665.
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5. Friedrich, M., Remmele, S., Kromer, B., Hofmann, J., Spurk, M., Kaiser, K.F., Orcel, C. and Küppers, M., (2004). The 12,460-year Hohenheim oak and pine tree-ring chronology from central Europe. Radiocarbon 46 (3), 1111-1122.
6. Baillie, M.G.L. 1995, A slice through time - dendrochronology and precision dating. ISBN 0713476540.
7. Baillie, M. 2007. The case for significant numbers of extraterrestrial impacts through the late Holocene. J. Quaternary Sci., Vol. 22 pp. 101–109. read here
8. Veski, S., Heinsalu, A., Kirsimäe, K., Poska, A. and Saarse, L. (2001), Ecological catastrophe in connection with the impact of the Kaali meteorite about 800–400 B.C. on the island of Saaremaa, Estonia. Meteoritics & Planetary Science, 36: 1367–1375. read here
9. Veski S, Heinsalu A, Lang V, Kestlane Ü, Possnert G. 2004. The age of the Kaali meteorite craters and the effect of the impact on the environment and man: evidence from inside the Kaali craters, island of Saaremaa, Estonia. Vegetation History and Archaeobotany 13: 197–206. read here
10. Raukas A., Stankowski W. 2011. On the age of the Kaali craters, Island of Saaremaa, Estonia. Baltica Vol. 24:1, 37-44. read here
11. M. Sigl, M. Winstrup, J. R. McConnell, K. C. Welten, G. Plunkett, F. Ludlow, U. Büntgen, M. Caffee, N. Chellman, D. Dahl-Jensen, H. Fischer, S. Kipfstuhl, C. Kostick, O. J. Maselli, F. Mekhaldi, R. Mulvaney, R. Muscheler, D. R. Pasteris, J. R. Pilcher, M. Salzer, S. Schüpbach, J. P. Steffensen, B. M. Vinther & T. E. Woodruff 2015. Timing and climate forcing of volcanic eruptions for the past 2,500 years. Nature 523, 543–549. doi:10.1038/nature14565. read here and here