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In modern Red Sea coral reef rock, pore spaces of corals are partly filled with fibrous aragonite precipitated subaqueously. By contrast, subaerially exposed reef rock about 115,000 years old, but with corals still composed of aragonite, lacks cement. Its porosity and permeability exceed those of modern reef rock. Emerged reef rock dating back 200,000-250,000 years may still consist of aragonite, but corals older than 250,000 years consist mostly of calcite. In these older corals dissolution removed the aragonite. Precipitation of a calcite mosaic preserved the outlines of the original corals, but the total skeletal framework preserved as calcite was less than that originally occupied by aragonite. Therefore porosity and permeability of the older reef rocks are markedly i creased compared with all younger reef rocks. The waters that passed through the older emerged reefs must have been barely saturated with respect to CaCO3.
As the emerged reef rocks lack interstitial fibrous cement, the corals must have been raised out of the sea before the onset of submarine cementation. An arid climate dating back 250,000 years prevented the dissolution of the aragonite of the corals. Although climatic changes more than 250,000 years ago were such that percolating fresh waters removed aragonite and precipitated calcite, the waters tended to remain undersaturated with respect to CaCO3. Hence the progressive sequence of emergence of reef rock before onset of submarine cementation, dissolution of aragonite, and minor calcite precipitation by fresh water led to increase in porosity and permeability.
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