Maximum grain size record from sediments of a Czech headwater lake reveals precipitation patterns during the Allerød to Younger Dryas transition

This study examines environmental changes recorded in the sediments of Plešné Lake (48°46.6’N, 13°51.9’E, Bohemian Forest, Czech Republic) between 13,200 and 12,200 cal yr BP, encompassing the Allerød-Younger Dryas (AL-YD) transition (∼12,900 years ago). Using sedimentological, geochemical, and chronological analyses, we reassess the prevailing interpretation of consistently arid conditions at the onset of the Younger Dryas (YD) and instead propose a more complex hydroclimatic pattern. We introduce a methodological innovation based on microscopic measurements of maximum grain sizes as a high-resolution proxy for precipitation intensity. The chronological framework integrates radiocarbon dating with the identification of the Laacher See cryptotephra, extending the known distribution of this important volcanic marker within the Bohemian Forest region. 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. An organic-rich layer spanning the AL-YD transition shows that organic accumulation began prior to both the tephra deposition and the YD onset. Geochemical analyses reveal a phosphorus anomaly and associated elemental patterns, likely reflecting a combination of volcanic input, acid rain-induced mobilization, and localized lacustrine processes. This suggests gradual environmental changes were already underway before these events. 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. Overall, these results highlight the complex nature of climate and environmental changes during the AL-YD transition, and provide local constraints that can be integrated into broader discussions of volcanic influences and hydroclimatic variability around the YD onset.