We present results of a pollen analysis carried out on sediment cores collected from a smallSiberian lake (Lake Cheko) located close to the epicenter of the 1908 Siberian explosion, theso called Tunguska Event. The purpose of our study was finding evidence pro or contra thehypothesis that Lake Cheko might fill an impact crater left by a fragment of the cosmic bodythat survived the atmospheric explosion, and hit the ground ~10 km NWof the inferred epicenter.This hypothesis was suggested by a number of indirect evidence, including lake’sunusual funnel shape, stratigraphy of the lake’s deposits and the absence of documental evidencethat the lake existed prior to the 1908 (1, 2, 3).Material and MethodsPollen and micro-charcoal data were obtained from two sediment cores: TG99-21 and TG99-22. Analyses were carried out on 30 samples, following standard treatments and countingfrom 500 up to 1000 pollen grains/sample.We determined also the abundance of micro-charcoalsin order to estimate the frequency of forest fires in the region around the lake. Pollendiagrams are similar and they show a taiga vegetation, like the present one, mainly formedby conifers and birch with very few shrubs and herbs. Periodical fires are suggested by peaksin >250 μm micro-charcoals coinciding with decreases in tree pollen concentration.ResultsBoth diagrams show a similar major pollen Event (pEvent): the appearance, at about half ofthe cores, of hydrophytes, coinciding with a general fall in the concentration of taiga pollen.The pEvent divide both diagrams in two pollen zones: Zone I shows a taiga forest growingon wet ground and absence of hydrophytes; Zone II shows a lake surrounded by taiga foresttestified by the appearance and subsequent continuity of hydrophytes. Interestingly, age estimatecarried out on the TG-22 sediments using short-lived radioisotopes indicate that the pEventcould lay close to the beginning of the last century (Gasperini et al., in press).Moreover,while the sediments in the upper part of the TG99-22 core appear typical lacustrine, beingconstituted by faintly laminated, fine-grained deposits, below the pEvent we found massiveto chaotic coarser grained sediments and vegetation macro-remains (3).ConclusionsPollen diagrams suggest that no lake existed prior to the pEvent, but a taiga forest growingon wet ground. Furthermore, they suggest the presence of a lake starting from the pEvent. Theabrupt decrease in tree pollen concentration occurring during the pEvent and the absence ofmicro-charcoals are in agreement with the impact of a cosmic body which destroyed the localforest without causing a fire.
|Data di pubblicazione:||2009|
|Autori:||L. Forlani; C.A. Accorsi; L. Gasperini|
|Titolo:||Is Lake Cheko an impact crater created by the 1908 Tunguska Event? Some hintsfrom pollen and micro-charcoal analysis|
|Appare nelle tipologie:||Abstract in Atti di Convegno|
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