Two extreme events near the Allerød-Younger Dryas transition: A story read from Bohemian Forest lake sediments (Central Europe)

Near 12,850 cal. yr. BP, the Younger Dryas cooling (YD) abruptly reversed the warming trend from the last glacial to the present interglacial at high northern latitudes. Subsequent YD-onset-related changes, including hydroclimate shifts, affected ecosystems and human societies worldwide. The main YD trigger – e.g., a massive meltwater input into the North Atlantic Ocean, volcanic gas aerosols from the cataclysmic Laacher See (LS) eruption in the Volcanic Eifel, Germany, or an extraterrestrial body impact or airburst – remains widely debated and unclear. We have obtained lake sediment cores from three sites located in the Bohemian Forest Mts., Czechia-Germany-Austria border area (distance of 450–470 km from the LS volcanic crater). The characteristic LS tephra glass shards were documented in all three cores using X-ray fluorescence scanning, magnetic susceptibility measurements, and direct observation by scanning electron microscopy, and their concentrations were quantified by a TESCAN Integrated Mineral Analyzer (TIMA). Our geochemical results show the closest match with the so-called MLST-B phreatomagmatic phase of the LS eruption. Moreover, a significant amount of LS-(crypto)tephra-related phosphorus (up to 0.15%), often the limiting nutrient in both terrestrial and freshwater ecosystems, was found in the sediments. The discovery of the LS volcanic ash in the Bohemian Forest points to a wider distribution of this (crypto)tephra than has been known so far (evident transport also in the eastern direction). It opens up new potential for tephrochronologically supported research of Late-glacial sediments in eastern Central Europe and exploring the role of the event in human prehistory. In addition to the LS cryptotephra, we observed magnetically extracted iron-rich microspherules with signs of high-temperature melting and quenching in all studied sediment cores. Their maxima (3–36 objects per 1 g of dry sediment) were situated 2.2–3.1 cm above peaks in the LS tephra shard concentrations. Such exotic objects were reported from numerous sites on several continents where more impact-related proxies were documented by proponents of the YD impact hypothesis. Based on this evidence, we hypothesize that the Allerød-Younger Dryas transition in Central Europe was likely affected by more than one extreme event. The LS eruption was followed by an event during which the iron-rich microspherules were formed. The ongoing study is supported by the Czech Grant Foundation (20-08294S – PROGRESS).