AMS Carbon-14 Dating of Microbial Carbonates of Shallow-Water Holocene Coral Reefs in the Philippines

Abstract:

Microbial carbonates (microbialites) occurred in the Holocene coral reefs at Paraoir and Currimao of western Luzon, the Philippines, as revealed in drill cores. These microbialites appeal grayish to buff in color with a laminated to thrombolitic structures, and encrust on corals directly or on a thin layer of calcareous red algae or encrusting foraminifers that covered the corals first. At both study sites, the microbial carbonates tend to occur in the lower (earlier) parts of the Holocene reef sequence where Heliopora coerulea dominated. No microbial carbonates were observed in the upper (younger) part of the Holocene reefs that is interpreted to grow in water depth shallower than 6 m on the basis of coral community. Selected samples of microbial carbonate and associated coral were dated by AMS ¹⁴C. The results show that the microbialites formed about 9 to 8 thousands years ago, coeval to or a few hundred years younger than the corals on which the microbialites encrusted. Some corals above or below the dated microbialites were ²³⁰Th dated in previous study. The AMS ¹⁴C ages of samples recovered at reef margins are consistent to ²³⁰Th ages, but those recovered in the back reef are much older that the ²³⁰Th ages and believed to be reworked. The current consensus is that microbial carbonates result from carbonate precipitation induced by bacteria (cyanobacteria or others) activity, or sedimentation trapped by microbial mat. Considering the ages and occurrences of the microbialites, it is proposed that the microbialites in coral reefs of western Luzon formed from 9 to 8 ky ago in dysphotic condition at greater water depth when the deglacial sea level was rising rapidly. Toward 7 ky ago when the sea-level rise slowed down and coral reef caught up, the condition became favorable to corals prospered in euphotic shallow water, and no longer suitable for microbial carbonate formation.