River Runoff Control on Inshore Salinity Variability Recorded by Coral‐Derived δ ¹⁸ O sw in the Seribu Islands, Indonesia

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The skeletal geochemistry of Porites corals is commonly used to monitor and reconstruct sea surface temperature and seawater δ 18 O (δ 18 O sw ), which are further used to reconstruct sea surface salinity (SSS). We reconstructed ∼30 years of δ 18 O sw from paired Sr/Ca and δ 18 O 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‐δ 18 O sw for inshore and offshore reef sites reveals significant differences due to localized δ 18 O sw (or SSS) variations, preventing the reconstruction of a regional composite δ 18 O sw (or SSS) record. Focusing on the inshore site, coral‐δ 18 O sw shows clear variability at interannual (2–6 years) and decadal (9–10 years) timescales. However, season‐specific and lag‐adjusted analyses reveal that coral‐δ 18 O sw does not correlate significantly with large‐scale climate indices, including the Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO). Instead, coral‐δ 18 O sw 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‐δ 18 O sw 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.