IMPACT-RELATED MICROSPHERULES IN LATE PLEISTOCENE ALASKAN AND YUKON “MUCK” DEPOSITS SIGNIFY RECURRENT EPISODES OF CATASTROPHIC EMPLACEMENT

Large quantities of impact-related microspherules have been found in fine-grained sediments retained within seven out of nine, radiocarbon-dated, Late Pleistocene mammoth (Mammuthus primigenius) and bison (Bison priscus) skull fragments.The well-preserved fossils were recovered from frozen "muck" deposits (organic-rich silt) exposed within the Fairbanks and Klondike mining districts of Alaska, USA, and the Yukon Territory, Canada.In addition, elevated platinum abundances were found in sediment analysed from three out of four fossil skulls.In view of this new evidence, the mucks and their wellpreserved but highly disrupted and damaged vertebrate and botanical remains are reinterpreted in part as blast deposits that resulted from several episodes of airbursts and ground/ice impacts within the northern hemisphere during Late Pleistocene time (~46-11 ka B.P.).Such a scenario might be explained by encounters with cometary debris in Earth-crossing orbits (Taurid Complex) that was generated by fragmentation of a large short-period comet within the inner Solar System.During the time period leading up to the Last Glacial Maximum (~23-19 ka B.P.), when eustatic sea level was substantially lower, Alaska and the Yukon Territory were part of the largest circumarctic area to remain unglaciated, called Beringia (Fig. 1), which extended from eastern Siberia (Chukotka) across the exposed Bering Strait region into Alaska and western Canada 1,2 .The glacial steppe environment of Beringia 3 was a refugium for Plio-Pleistocene tundra-grassland plant communities 4 as well as for the now-extinct mammalian megafauna 5 .Relict permafrost within the region 6-9 has preserved an extraordinary frozen record of plant, pollen, insect, and vertebrate fossil remains, as well as their ancient DNA 1,10,11 .Early expeditions to Alaska 12-14 and the Yukon Territory 14,15 found large quantities of megafaunal bones along beaches, riverbanks, and in minor-stream valleys 8 .Even greater collections of these fossils were made after industrial-scale placer-gold mining operations began in the Fairbanks and Klondike districts in the early 20 th century 16 .Otto W. Geist undertook extensive fossil collecting in Alaska on behalf of the American Museum of Natural History (AMNH), and in a typical year (1938) shipped more than eight thousand select specimens, weighing nearly eight tons (~7257 kg), to New York City 8 .The fossil bones collected included those of bison, mammoth, horse, musk ox, moose, lynx, lion, camel, mastodon, bear, and caribou, with many of these animals also appearing as frozen partial carcasses or mummies 1,8,17 .The three most common genera found were bison, mammoth, and horse, which represent more than 90% of Beringia's large mammalian biomass 18 .Many tens of thousands of specimens were collected in the 20 th century from Alaska and the Yukon Territory 5,8,19 , and hundreds to thousands more are still being recovered every year from mines in the Klondike district alone 1 .

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