Assuming that the δ 18 O-Sr/Ca vital effect relationship remained constant between mid-Holocene and modern day corals, δ 18 O sw **data** provide a robust method to assess hydrological differences between different epochs. The seasonal patterns observed in δ 18 O sw show that during the summer monsoon rainfall season, the oxygen isotopic composition of seawater at King Reef reached negative values, as SSS decreased. This indicates that the seasonal variation in δ 18 O sw at King Reef is influenced by the influx of freshwater from river runoff and the depleted oxygen values from direct rainfall.