Coral-derived seasonal seawater δ18O records from the Northern South China Sea: Hydroclimatic insights into the Medieval Climate Anomaly and Little Ice Age

Seawater oxygen isotopes (δ¹⁸O_sw) are critical for reconstructing past climate and hydrological conditions, yet high-resolution δ¹⁸O_sw reconstructions during the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) remain scarce. Using paired monthly coral Sr/Ca-δ¹⁸O records from Hainan Island in the northern South China Sea (NSCS), we reconstructed wet and dry season δ¹⁸O_sw for these periods. During the MCA, δ¹⁸O_sw values were relatively positive, indicating drier conditions, while the LIA exhibited relatively negative δ¹⁸O_sw values, reflecting wetter conditions. Compared to 1984–2014 CE, the frequency of seasonal extreme δ¹⁸O_sw events increased by 25 %–74 %, likely driven by global warming caused by human activities. Long term trends reveal that dry season δ¹⁸O_sw closely mirrored wet season values, emphasizing the key role of the wet season δ¹⁸O_sw baseline in shaping seasonal hydrological conditions. When examined in the broader late Holocene context, East Asia exhibited a persistent meridional dry-wet contrast, with cold periods characterized by wet oceanic and dry inland conditions, and warm periods showing the opposite. These contrasting patterns were primarily driven by the combined influence of the East Asian Summer Monsoon and El Niño, with additional influences from the meridional shifts of the Intertropical Convergence Zone and zonal movements of the Pacific Walker Circulation. These findings underscore the dynamic interplay of regional climate drivers in shaping hydrological variability and highlight the NSCS's critical role in East Asia's climate. They provide valuable baselines for understanding natural climate variability and predicting future climate changes in this densely populated and economically significant region.