Phytoclasts in the form of plant debris in terrestrial sediments can be transported by water to distant areas because they are lighter than inorganic particles. The semi-enclosed East Sea, which is connected by narrow straits to other seas, is adjacent to continental shelves that are the source area of terrestrial sediment flowing into the East Sea. These shelves alternated repeatedly between terrestrial and marine environments as a result of eustatic sea-level changes during the Late Quaternary. Palynofacies analyses of the IODP Exp. 346 U1430 core, located in the Eastern South Korea Plateau (ESKP) of the East Sea, have revealed changes in the size and concentration of phytoclasts associated with glacial–interglacial cycles. These changes are generally negatively correlated with the global sea-level curve, and their anti-phase cycles with high amplitude are clearly evident during the last ca. 750 ka with the geotectonic stabilization period. In particular, several coarse-grained phytoclasts were observed during the glacial period, including the Last Glacial Maximum (LGM). These findings suggest that the concentration and size of phytoclasts flowing into the East Sea were influenced by changes in the distance of the source area, depending on the water depth of the strait and nearby shelves owing to sea-level changes in tandem with glacial–interglacial cycles and geotectonic events.

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East Asian summer monsoon (EASM)-driven rapid hydroclimatic variation is a crucial factor with major socioeconomic impacts. Nevertheless, decadal- to centennial-scale EASM variability over the last two millennia is still poorly understood. Pollen-based quantitative annual precipitation (PqPann) and annual precipitation reconstructed by artificial neural networks (ANNs) for the period 650–1940 CE were reconstructed from a paleo-reservoir in South Korea. ANNs reconstruction was performed to compensate for a hiatus section. On a decadal timescale, 10 high-precipitation periods were identified, and PqPann and ANNs reconstructions were comparable to local instrumental rainfall and historic drought records. Biotic lags to rapid climatic changes ranging from 25 to 100 years were recognized by asynchronous pollen and speleothem responses to precipitation. We suggest that PqPann-based decadal- to centennial-scale climatic change reconstruction should take biotic lags into account, although the lags can be ignored on the millennial scale. The position of the EASM rainband influenced rainfall magnitude.

The history of past hydroclimatic extreme events, essential information for predicting future changes, is preserved in fluvial sediments. Here, we reconstruct changes in decadal-scale extreme flooding events over the period 7700–1700 cal yr BP from floodplain sediments in the middle reach of the Nakdong River, Korea, based on lithogenic elemental ratios (e.g., Zr/Ti and Sr/Si). For example, Nakdong extreme flooding (NEF) events frequently occurred at 7700, 7200, 6000, 5000, 3800, 3200, 2900, 2600, 2300, 2000, and 1700 cal yr BP, and were associated with higher sea-surface temperatures and strong El Niño-Southern Oscillation (ENSO) activity. Notably, we found a significant change in the frequency of extreme events ca. 3700 cal yr BP over East Asia. The hydroclimate fluctuated with dominant periodicities of 950 and 540 years in 7700–3700 cal yr BP and shorter centennial to decadal cycles (320, 110–120, and 60–75 years) in 3700–1700 cal yr BP. This 3.7-ka climatic shift is consistent with a marked southward shift of the intertropical convergence zone, intensified ENSO activity, increased frequency of recurving typhoons, and deep-ocean circulation changes in both the northern and southern hemispheres, demonstrating the urgent need for investigating the critical role of past deep-water circulation in hydroclimate changes.

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