Abstract
In order to constrain relative sea level (RSL) and hence land-level change in northwestern Luzon, we have been working to extract slabs from fossil coral microatolls at a suite of sites in Ilocos Sur and Ilocos Norte provinces, Philippines. RSL in this region is driven by a combination of eustatic sea-level, glacial isostatic adjustment (GIA), and tectonics; however, the contribution of each of these components is poorly known. Coral microatolls robustly track not only RSL, but also rates of change in RSL. Our aim is to reconstruct RSL over the past 8,000 years at sites along the coast, using spatiotemporal differences in the positions and rates of past RSL to model its different drivers. Here we report observations and preliminary RSL reconstructions at one site in Ilocos Sur.
In Cabugao municipality, Ilocos Sur, all slabbed fossil coral microatolls are cup-shaped, defined by lower centers and higher outer rings - this morphology results from rising RSL as the corals grew. However, corals older than ~7200 yr BP record faster RSL rates (~ 2.9 ± 0.2mm/yr rate of increase) compared to younger corals that grew ~1000 yr BP. Our working hypothesis is that the differences in rates are primarily caused by eustatic sea-level rise (due to melting of continental ice sheets) and GIA prior to 7200 yr BP. In spite of this, corals estimated to be from ~7700 to ~7200 yr BP were systematically found to be between 0 and 1 m higher than their modern equivalents, indicating either that RSL in the absence of tectonics had already reached present levels much earlier than predicted by published GIA models, or that this site has experienced meters of tectonic uplift in the past ~7200 yr. Ongoing analysis of the coral microatolls in Cabugao, a better understanding of elevated inferred Pleistocene surfaces at the site, and comparisons to RSL reconstructions at sites elsewhere along the same coastline will help discriminate between these hypotheses.