Coral reef terrace age deduced from retreating knickpoints

• Novel knickpoint-based method dates coral terraces, bypassing diagenetic limitations. • Linking sea-level history with river knickpoint formation for terrace age inference. • Coastal uplift rate ∼1 m/kyr, aligning with tectonic studies south of the research area. The accurate dating of Quaternary coral terraces has long been challenging due to recrystallization of aragonite corals. Here, we present an approach to infer terrace age based on (1) the linkage between sea-level drop and river knickpoint formation and (2) the history of sea-level rise and fall recorded by the elevation–age relationship of knickpoints. Our study in northwestern Luzon, Philippines, combines field surveys, sedimentological analysis, and topographic and river-profile analysis, through which we map the coral reef terrace extent and correlate terrace outer edges to the initiation location of major knickpoints in adjacent river profiles. Through χ transformation, the knickpoint retreat distance is equivalent to knickpoint/terrace age, and therefore the elevation–retreat distance relationship of the knickpoints can be used to determine the age of the correlated terrace. Using this method, we deduce that the lower coral reef terrace sequences (T1-T4) correspond to Marine Isotope Stages (MIS) 5a, 5c, 5e, and 7a, respectively. Furthermore, we suggest the coastal area is gently tilting seaward with an average uplift rate of ∼1 m/kyr, and the highest terrace surface (420–440 m) may be more than 400 kyr old. We propose that this technique offers a promising solution for dating coral reef terraces in uplifting coastal regions where absolute age-dating techniques fail due to diagenetic alteration, and for terraces associated with bedrock rivers that developed before the penultimate glacial period.