Response of Philippine reefs to an overall rise of sea level during the Late Pleistocene-Holocene based on high-resolution bathymetric data

Coral reefs provide an archive for the reconstruction of past sea levels because reef-forming corals grow at a limited depth range and have a good preservation in the geologic record. High-resolution multibeam bathymetry to a depth of 70 m in Apo Reef, Occidental Mindoro; Masinloc, Zambales; Patnanungan, Quezon and Danajon Bank, Leyte show a series of pairs of terraces and steep scarps. Diver observations and drop camera images indicate the proliferation of corals in the mesophotic depths especially along the outer edges of the terraces. The terraces are interpreted to represent reef flat to reef crest environments and the scarps as reef fronts. The series of pairs of terraces and scarps are possibly the back-stepping reefs that were drowned during the overall rise of sea level starting in the late Pleistocene.  The number and depths of the terraces vary within a reef indicating a possible local tectonic influence on the sea level history. Comparison of the distribution of the depths of the terraces across reefs shows clustering in three depth ranges - 14-20m, 28-38m, 56-60m which may correspond to the three sea level still stands in the past 13 ky. In Apo Reef, the outer edges of the terraces are lined with mounds that are 2 to 10 m high.  These mounds may represent catch-up growth which is continuing to the present.  This response may have been made possible by clear waters resulting to deeper photic limit, biodiversity in the region which can provide an assemblage with greater depth range, and relative protection from wave action.