Holocene land level and relative sea level are affected by a multitude of processes along the west coast of Luzon Island in the Philippines. Melting of higher-latitude continental ice sheets from last glacial maximum through the mid-Holocene led to eustatic sea-level rise, and the redistribution of ice and meltwater loads resulted in glacial isostatic adjustment (GIA) processes that are ongoing today, even in low latitudes. Off the west coast of Luzon, the South China Sea plate subducts under the Philippine Mobile Belt at the Manila Trench, inducing elastic deformation over the megathrust seismic cycle that has a maximum amplitude somewhere between the coastline and the trench but which may still be appreciable onshore. Yet some relative plate motion is taken up onshore, as evidenced by uplifted inferred Pleistocene terraces tens of meters above present mean sea level.
We have dated, slabbed, and analyzed a set of Holocene coral microatolls at the Pagsanahan site in Badoc municipality, Ilocos Norte province, on Luzon. Coral microatolls precisely track relative sea level (RSL), allowing the reconstruction of both the position of RSL and rates of RSL change. Preliminary analysis reveals the complicated interplay of multiple drivers of RSL: RSL rose rapidly until ~7500 cal yr BP, before dropping abruptly by ~1 m. We surmise these reconstructions capture rapid RSL rise toward a regional mid-Holocene highstand due to meltwater input and GIA, interrupted by ~1 m of coseismic uplift locally. We are working to elucidate details and identify potential causative faults.