Stratigraphic separation of Laacher See Tephra and a discrete Younger Dryas Boundary fallout layer in southernmost Sweden

The temporal relationship between the Laacher See eruption (LSE), the Younger Dryas Boundary (YDB) layer, and the abrupt onset of Younger Dryas (YD) climate change has long been debated. The Körslättamossen fen in southernmost Sweden provides the first robustly dated Scandinavian record demonstrating that these events are clearly separated stratigraphically and temporally. The Laacher See Tephra (LST) layer at 288.5 cm (~13,006 cal BP) lies 6–10 cm below a discrete PGE-rich horizon (282–279 cm) marking YDB deposition, interpreted here as a 3-cm-thick impact fallout layer. This interval has an onset Bayesian-modeled age of 12,861 ± 54 cal BP, ~150 years after the eruption, indicating that the LST and YDB deposition represent separate events. Within this fallout layer, Fe–Si microspherules with dendritic quench textures peak at 282–281 cm, while maximum PGE-bearing nanoparticle mass occurs at 280–279 cm, consistent with delayed settling of submicron particles. LA-ICP-MS reveals Si-rich microspherules containing PGE-bearing microdomains but negligible Ni, Co, and Cr. These compositions indicate that the microspherules are dominated by terrestrial target material rather than meteoritic metal, while the associated PGE-rich microdomains and nanoparticle populations record a minor but detectable extraterrestrial component. Single-particle ICP-TOF-MS detects a sharp Pt anomaly with Pt-dominant elemental ratios (e.g., high Pt relative to Fe and Ir), consistent with the platinum enrichment reported in the YDB in the GISP2 ice core. Immediately above the fallout layer, a dark, organic-rich horizon records the abrupt hydroclimatic shift to cooler, wetter YD conditions. These observations document three stratigraphically distinct horizons: the LSE, a separate YDB fallout layer, and the near-synchronous onset of YD climate change, providing a well-constrained European stratigraphic framework for evaluating mechanisms proposed for the YD onset.