Maximum Grain Size Record from Sediments of a Czech Headwater Lake Reveals Precipitation Patterns During the Allerød to Younger Dryas Transition

Gunther Kletetschka^1*Daniel Vondrák²Verena Meier³Tomáš Hubáček⁴Petr Porcal⁴Hendrik Küpper⁵Jiří Jan⁶Joanne P Ballard⁷Eva Löbelová-Švecová¹Marian Takáč¹Lucie SmrčinovṊárka Matoušková⁸Jan Rohovec⁹Evžen Stuchlík¹⁰

• ¹Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Faculty of Science, Albertov 6, CZ-128 43, Charles University, Prague, Czechia
• ²Institute of Environmental Studies, Faculty of Science, Benátská 2, CZ-12801, Charles University, Prague, Czechia
• ³Charles University, Prague, Czechia
• ⁴Institute of Hydrobiology, Na Sádkách 7, CZ-37005, Biology Centre Czech Academy of Sciences, České Budějovice, Czechia
• ⁵Faculty of Science, University of South Bohemia,, ceske budejovice, Czechia
• ⁶Biologicke centrum Akademie ved Ceske republiky Ustav pudni biologie, Ceske Budejovice, Czechia
• ⁷Institute of Plant Molecular Biology, Branišovská 1160/31, CZ-37005, Biologicke centrum Akademie ved Ceske republiky, Ceske Budejovice, Czechia
• ⁸Institute of Geology, Rozvojová 269, 165 00, Czech Academy of Sciences, Prague, Czechia
• ⁹Geologicky ustav Akademie ved Ceske republiky, Prague, Czechia
• ¹⁰Biologicke centrum Akademie ved Ceske republiky, Ceske Budejovice, Czechia

This study examines environmental changes recorded in the sediments of Plešné Lake (49°10'N, 13°52'E, Bohemian Forest, Czech Republic) from 13,200 to 12,200 cal yr BP, which includes the Allerød-Younger Dryas (AL-YD) transition (ca. 12,900 years ago). Using sedimentological, geochemical, and chronological analyses, we challenge the prevailing interpretation of a generally arid climate at the beginning of the YD and suggest a more complex pattern instead. We also introduce a methodological innovation using microscopic measurements of maximum grain sizes as a high-resolution proxy for precipitation intensity. Our findings indicate an initial drying at the beginning of the YD followed by episodes of intense precipitation during the early YD, suggesting a two-phase model rather than uniformly arid conditions. The chronological framework incorporates both radiocarbon dating and the identification of the Laacher See cryptotephra, extending the known distribution of this important volcanic marker in the Bohemian Forest region. We also document an organic-rich layer spanning the AL-YD transition, with organic accumulation beginning prior to both the tephra deposition and the YD onset. This suggests gradual environmental changes were already underway before these events. Geochemical analyses revealed a phosphorus anomaly and associated elemental patterns, likely resulting from a combination of volcanic input, acid rain-induced mobilization, and localized lacustrine processes. Several lines of evidence suggest that the Laacher See eruption (~13 cal kyr BP), which preceded the YD onset by approximately 100-200 years, contributed to environmental stress in the region. Our results highlight the complex nature of climate and environmental changes during the AL-YD transition, and they will contribute local constraints that can be integrated into broader discussions of volcanic influences and hydroclimatic variability around the YD onset.