Holocene Hydroclimatic Variability on Jeju Island, Korea: Reassessing the 4.2 ka Event via Diatom Records and Westerly Jet Dynamics

The 4.2 ka event is widely recognized as a major cooling and aridity episode across the Northern Hemisphere. However, its specific impact on the Korean Peninsula remains under-researched compared to neighboring China. In this study, we present a 9,000-year hydroclimatic reconstruction from the subalpine Sara-oreum wetland on Jeju Island, using diatom assemblages and monitoring data. Contrary to the typical "drought" narrative of the 4.2 ka event, our findings reveal prevailing humid conditions on Jeju, evidenced by an increase in summer-associated tychoplanktonic species. This moisture pattern aligns with records from Southern China, suggesting a southward shift of the westerly jet that anchored the monsoonal rain belt over the region. Furthermore, strong correlations between lake-level indicators (PC1 and sand content) and the δ¹⁸O  records from Xianglong Cave and the Westerlies Effect Index highlight the sensitivity of Jeju’s diatom records to large-scale atmospheric circulation. This study underscores the complex spatial heterogeneity of the 4.2 ka event and its linkages to westerly jet variability.