River runoff control on inshore salinity variability recorded by coral‐derived δ18Osw in the Seribu Islands, Indonesia

The skeletal geochemistry of Porites corals is commonly used to monitor and reconstruct sea surface temperature and seawater δ18O (δ18Osw), which are further used to reconstruct sea surface salinity (SSS). We reconstructed ∼30 years of δ18Osw from paired Sr/Ca and δ18O measurements in Porites corals from the Seribu Islands, spanning inshore (Bidadari Island) to offshore (Jukung and Air Islands) surface waters north of Jakarta, Indonesia. We find that coral-δ18Osw for inshore and offshore reef sites reveals significant differences due to localized δ18Osw (or SSS) variations, preventing the reconstruction of a regional composite δ18Osw (or SSS) record. Focusing on the inshore site, coral-δ18Osw shows clear variability at interannual (2–6 years) and decadal (9–10 years) timescales. However, season-specific and lag-adjusted analyses reveal that coral-δ18Osw does not correlate significantly with large-scale climate indices, including the Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO). Instead, coral-δ18Osw exhibits a strong and significant relationship with river runoff, indicating that freshwater input is the major factor controlling inshore SSS variability. These results suggest that coral-δ18Osw in the Seribu Islands primarily records the integrated hydrological response of the coastal system rather than direct large-scale climate forcing. Climate modes such as the IOD and ENSO likely influence SSS indirectly by modulating precipitation and runoff over the adjacent mainland. This highlights the importance of land-ocean coupling in controlling coastal salinity variability and its imprint on coral-based proxies. Longer coral records are needed to further resolve low-frequency climate variability and its regional impacts.