Kaali Meteorite impact induced Ecological catastrophe 800–400 BC, as revealed by pollen, XRD, LOI and 14C analyses of peat containing impact ejecta
by
Siim Veski
Tallinn Technical University, Institute of Geology, Estonia pst. 7, 10143 Tallinn, Estonia
Coauthors: Atko Heinsalu, Kalle Kirsimäe, Anneli Poska, Leili Saarse

A sequence of peat enriched with impact ejecta (iridium) from Piila bog, 6 km NW from the Kaali impact crater (Island of Saaremaa, Estonia) was examined using pollen, XRD, 14C and LOI analyses to date and assess the environmental effect of the impact. Significant changes occur in LOI, pollen accumulation, composition of pollen and mineral matter in the depth interval 170–178 cm, which contains over-background values of iridium. At this point of research the precision of dating the time of the Kaali impact crater lies between 800–400 BC. Rasmussen et al. (2000) showed a well-developed peak/plateau in the content of iridium (up to 0.5 ppb) in the peat of Piila bog at 172–177 cm below bog surface. This iridium rich layer is a marker horizon for the impact outside the crater. Associated with the Ir-enriched horizon are marked layers of charcoal, charred wood and wood stumps in the peat of Piila bog. The described layers are found all over the bog 20 km2 in size. The latter in connection with other evidence pointing to ecological catastrophe supports the idea of local impact-induced wildfires, which reached at least 6 km NW of the epicentre. LOI analysis of peat shows two levels of increased input of inorganic allochthonous material. Those coincide with the iridium and XRD data and visual observations of charcoal and mineral particles (quartz and feldspar grains were found in pollen samples at 169-cm level).

XRD analysis reveals that the main mineral phases in the peat ash are calcite, anhydrite and quartz with traces of K-feldspar and plagioclase. Quartz and feldspars are allochthonous phases and interpreted as direct impact ejecta or post-impact aeolian dust. The content of quartz reaches up to 20% of the mineral composition of ash in the middle of the section, at depth 173–180 cm. In the same interval small amounts of the K-feldspar and plagioclase were found. This abrupt change in the composition of peat ash at depth 173–180 cm is better expressed in quartz/calcite+anhydrite relative abundance ratio curve, which shows clear increase of quartz in this interval.

The meteorite explosion at Kaali (20 kt TNT) ranged between the effects of Hiroshima (15 kt TNT) and Tunguska (30 kt TNT). An intriguing topic in connection with fire is the burning of Asva fortified settlement 800–400 BC. Although this hypothesis is in need of further detailed investigation, we can assure that, indeed, the perishing of Asva and the impact of Kaali overlap in time and maybe in space.

The 3 zones of pollen evidence roughly correspond to pre-impact conditions, conditions directly after the impact and the recovery to “normal”. The impact explosion swept the surroundings clean of forest shown by the threefold decrease in pollen influx and the relative dominance of Pinus on the percentage diagram. Overrepresentation of Pinus percentages is a common feature for barren landscapes. The QM outside the bog on fertile soils was most affected, indicating that the disruptions in vegetation were not just local features around the sampling site in the bog.

Pollen evidence in zone 2 correlates with iridium, allochthonous minerals and pollen types indicating burning (Chamaenerion). Indicators of cultivated land, such as the cereals Triticum, Hordeum and Secale, which were present in pre-impact conditions, disappear in zone 2. Continuous signs of field cultivation on Saaremaa appear since 2300–2150 BC. The disappearance of cereals suggests that farming, cultivation and possibly human habitation in the region ceased for a period. The transition from zone 2 to 3 is interpreted as a gradual recovery of vegetation from the impact of the meteorite. The effect of the Kaali impact on landscape is hidden by new generations of vegetation.

There are currently two contradicting hypotheses about the age of the Kaali meteorite impact. Both rely on 14C dating of peat layers with extraterrestrial material and impact ejecta found in nearby bogs. The horizon with impact microspherules is dated back to about 6270–6500 BC (Raukas, 2000) and the one with elevated Ir to about 400 BC (Rasmussen et al., 2000). Our data speaks in favour of the latter, with some corrections, though.

References

RASSMUSSEN K. L., AABY B. AND GWOZDZ R. (2000) The age of the Kaalijärv meteorite craters. Meteorit. Planet. Sci. 35, 1067–1071.

RAUKAS A. (2000) Investigation of impact spherules – a new promising method for the correlation of Quaternary deposits. Quat. Intern. 68–71, 241–252.

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 BC on the island of Saaremaa, Estonia. Meteorit. Planet. Sci. 36, 1367–1376.

Date received: March 6, 2002