A geochemical model for coral reef formation

The conspicuous growth of a reef crest and the resulting differentiation of reef topography into a moat (shallow lagoon), crest and slope have long attracted the interest of scientists studying coral reefs. A geochemical model is here proposed for reef formation, taking into account diffusion-limited and light-enhanced calcification. First, to obtain data on net photosynthesis and calcification rates in the field, a typical coral community was cultured in situ on a reef flat. Using these data, equations including parameters for calcification were then developed and applied in computer simulations to model the development over time of reef profiles and the diffusion of carbon species. The reef topography simulated by the model was in general agreement with reef topography observed in nature. The process of reef growth as shown by the modeling was as follows. Increases in the shore-to-offshore gradients of the concentrations of carbonate species result from calcification by reef biota, giving a lower rate of growth on near-shore parts of the reef than on those further offshore. As a result, original topography is diversified into moat and reef crest for the first time. Reef growth on the reef crest is more rapid than in the inshore moat area, because more light is available at the crest. Reef growth on the nearshore side of the reef is further inhibited by damming of carbon-rich seawater on the seaward side of the reef by the reef crest. Over time, the topographic expression of the reef crest and moat becomes progressively more clearly defined by these geochemical processes.