Late Quaternary relative sea-level changes and reef development in the Coral Triangle recorded by coral reef terraces

Coral reef terraces (CRTs), usually observed along tropical coastal areas and are formed through repeated bioconstructional and erosional processes, are useful bio-geomorphological indicators of relative sea-level (RSL) changes in the past. Often considered as a process analog for predicted future changes for a warmer world is the Last Interglacial (LIG, also Marine Isotope Stage (MIS) 5e) and the analysis of sea-level indicators during this period can give us insights about potential drivers of local, regional, and global sea-level changes in the past. In this work, a standardized database of LIG sea-level indicators in Southeast Asia was produced. Overall, 43 unique RSL proxies (42 CRTs and one tidal notch), which were correlated to 134 dated samples, were compiled and data gaps in the region were recognized. Following this, a sequence of CRTs situated at the northern apex of the Coral Triangle was investigated in detail to understand RSL changes and reef development in the region during the late Quaternary. The Cape Bolinao CRTs in west Luzon, Philippines were examined using high-resolution morphological analysis and reef modelling coupled with new stratigraphic and geochronologic data. Distinct levels of CRTs (Terrace I to Terrace IX) were delineated with the lowest terrace (TI measured ~4 m above mean sea level (amsl)) dated mid-Holocene. Using best-fitting reef simulations that replicate the present-day morphology of the Cape Bolinao CRTs, the highest terrace (TIX at ~155 m amsl) was identified to form during MIS 5e. A morpho-chronological framework for the development of the Cape Bolinao CRTs is hereby proposed. In this framework, an extensive Late Pleistocene reef platform or table reef developed on top of a gently-dipping substrate and continuous uplift superimposed on a changing sea level led to the emergence of this table reef followed by the formation of fringing reefs (corresponding to lower reef terraces) along its slope. A parametric study was also conducted to examine the different parameters that control CRT architecture and morphogenesis in the region. Numerical reef simulations reveal that vertical land movement (uplift) is the principal driver of relative sea-level changes in western Luzon, being the main control of the architecture of the Cape Bolinao CRTs. The reef simulations suggest high uplift rates (1.17 mm/yr to 1.2 mm/yr) during the Late Pleistocene which is attributed to regional tectonics (i.e., subduction of the Scarborough Seamount Chain beneath Luzon Island). Other parameters that influence the CRT architecture in the study area include the slope of the basement, erosion rate, and reef growth rate. Accommodation space and antecedent topography also modulate the formation of CRTs especially in a rapidly uplifting region. Lastly, this study affirms that CRTs are a continuous record of sea-level oscillations superimposed on local forcings such as vertical land motion, bedrock topography, erosion rate, and reef growth rate.