A model‐based test of accuracy of seawater oxygen isotope ratio record derived from a coral dual proxy method at southeastern Luzon Island, the Philippines

The oxygen isotope ratio in seawater (δ 18 O sw ) is an important indicator of past hydroclimatic conditions in the tropics. In this study, we apply a dual proxy approach using δ 18 O and Sr/Ca in coral skeletons to estimate δ 18 O sw . Due to a lack of long‐term continuous observational data for δ 18 O sw , a simplified one‐dimensional box model of the ocean surface associated with historical surface atmospheric data is used for comparison with coral‐derived δ 18 O sw record. The atmospheric data consist of the precipitation and evaporation values, together with their δ 18 O compositions from the global isotope reanalysis data. The model successfully reproduces a multidecadal time series for δ 18 O sw at southeastern Luzon Island, the Philippines, for the period 1979–2001 ( r  = 0.57, p  < 0.01). The result suggests that the coral records are an accurate indicator of hydrological cycle changes at the site. A discrepancy between the reproduction and the coral record in the wet season (December–February) may be caused by seasonal variation in the depth of the mixed layer and upwelling, since these factors are not accounted for in the model. The reproduction is in good agreement with the coral records in the dry season (April–June), implying that δ 18 O sw estimated using the coral dual proxy method is more robust at this time of year.