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2 Acta Societatis Metheoriticae Polonorum

2 Walesiak T.

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Przegląd najważniejszych prac dotyczących próby ustalenia daty spadku meteorytów oraz wieku kraterów Morasko

100%

Walesiak T.

tom 12

129-148

One of criteria suggesting impact origin may be recognition of extraterrestrial matter inside or around suspected cavities. In case of Morasko some dating results throw doubt on link between craters and meteorites. Conclusion of some past research papers was that cavities were formed about 5 ka BP (e.g. palynological investigation, luminescence dating), while more or less facts testify fall in the Middle Ages (e.g. “young” charcoal pieces in crust of meteorites or shrapnel stuck in the roots of old tree). In this paper we perform comprehensive analysis of each result and check if there exists alternative explanation. During past palynological investigation, there were examined two craters. It was concluded that beginning of accumulation of sediments in smaller cavity started 5500–5000 BP. However, pollen spectrum for largest basin was different suggesting younger age. The problem is that according to bathymetric maps, samples could be collected from inner uplift (similar feature was observed inside Porzadzie and Jaszczulty, unconfirmed impact structures yet). If we compare profiles from craters with recent research (palynological study supported by radiocarbon dating) on sediments in Lake Strzeszynskie (6 km SW from Morasko) pollen spectrum seems to be similar more to results dated to <1000 BP. Especially percentage of non-arboreal pollen for both examined Morasko craters is much higher (5 times greater than it was for layer dated to 5 ka BP in Lake Strzeszynskie). Possibility of short disturbance (related to impact) cannot be excluded, but in such case any time of event should be taken into account. For small lakes (like those filling the craters) also local conditions could play important role. Analysis of luminescence dating may also discuss past conclusions. OSL method applied for samples taken from the thin layer of sand in the bottom of largest structure (under 3,9 m of organic sediments filling the crater) suggested age 5–10 ka BP for 24 aliquots. Same number of samples (24) revealed age 0–5 ka BP (13 samples with age <3 ka BP including several younger than 1 ka BP). Older dates can be explained by partial or even no zeroing, but last contact with light (zeroing signal) of sand grains (excavated from depth of almost 4 meters under organic matter) seems that might occur only during (or shortly after) the impact. It is difficult to find convincing arguments, which can undermine initial radiocarbon dating giving age <1 ka BP for 7 of 9 samples taken from the bottom layer of organic sediments from three Morasko craters. Same issue may exist with small charcoal pieces with age <2 ka BP (and several dated to ~700 BP) discovered deep in sinter-weathering crust of meteorites. Study of charcoal particles excluded their origin during post-sedimentary processes (like forest fire) unless meteorite fragments were laying directly on the surface for period between impact and wildfire (surviving whole time inhospitable climate conditions). There are two other possible explanations of charcoals. Either they were present at location as a result of past forest fire or they were produced during impact. However, similar small charcoal pieces were discovered around many craters (Kaali, Ilumetsa, Campo del Cielo, Whitecourt) and they were successfully used to estimate maximum age of those structures. During second stage of 14C dating the age obtained from three samples (taken few centimeters above the mineral bottom) was estimated ~5 ka BP. Looking for answer why these results are so different from previous once there may be mentioned at least two options. Lake sediments is difficult matter for radiocarbon dating and results may be hundreds or even thousands years older than real age. Second explanation may be that older matter (remnant of trees, paleosoil etc.), distributed around craters during impact, could be displaced by wind, rain, erosion and trapped in the bottom of cavities. The argument, which may be also against hypothesis of impact ~5 ka BP is meteorite shrapnel stuck in the roots of old tree. Result of expertise showed, that there exists mechanical damage in the wood and the only possible explanation is that meteorite has hit living tree. Maximum age, that this type of wood may preserve (inside building) is 1,8 ka while in natural environment not more than 500 years. Anyway some further examinations should be performed. The age obtained during AMS 14C dating of 2 samples from thin layer of paleosoil, discovered under overturned flap around largest Morasko crater, was ~5 ka BP. Observation that preserved layer of paleosoil is approximately 3 times thinner than thickness of modern soil may lead to conclusion that during impact top (younger) layer of organic sediments was removed and only older part “survived” in few locations close to the rim. The conclusion of the research was that dating provides maximum age of the impact (which does not exclude much younger impact <1 ka BP) so could be considered as right explanation of past discrepancies.

Jeszcze jedna dyskusja na temat daty spadku meteorytu Morasko

100%

Walesiak T.

tom 8

123-148

It’s over 100 years after discovery of the first iron meteorite in the area of Morasko, but still we are not sure about date of the fall. Recent researches suggest that it happened about 5000 years ago, however there are some other facts proving, that event took place in the last millenium. Verification o 14C radio carbon dating results revealed, that it can give us only the maximum date (meaning: not older than). The main problem was that probes were taken from mixed types of sediments – some created before and some after the impact. Few years earlier there was another 14C examination of sediments lying beyond the floor of 2 small cavities and their age was estimated to be approximatelly 700 years BP. Similar results were received after verification of organic matter under charcoal in the weathering crust of the meteorite, which was found in the Morasko Restricted Area. Another proof was discovery of shrapnel in the root of old tree, which could not be older than 800–1000 years. At least 2 times it was discussed, if finding of Morasko iron meteorites couldn’t be matched with described observation A.D. 1301, mentioned in annales, hovever nobody was able to prove it. Analysis of historical documents lead to information of appearance of 3 suns, after sunrise, what happened in 14 Feb. 1271. Further investigation of other sources gave us even more facts. We can read in annales that in the same year, castles and cities were burned and destructed to the ground. There is also information that Poznan, the largest city in the area (approx 7.5 km South from the Morasko), was burned in (or shortly before) the year of 1274. What is more Annales from Poznan and from its parent town Wielkopolska were finished in A.D. 1271, just in the middle of the sentence. Finally, confirmation is given by archeological researches, which tell us that city walls and castle of Poznan were ruined shortly after they were constructed, but scientists found difficulty to answer, when exactly it could happen. The main problem was unknown weapon, able to leave such a large destruction, even when considered next centuries. As we can find in annales that Poznan in A.D. 1253 had castle and fortifications so only iron meteorite impact in 1271, producing craters in the area of Morasko and Umultowo, can explain the scale of the devastation discovered by archeologists.