A geochemical model for coral reef formation

T. Nakamura 0 T. Nakamori 0 0 T. Nakamura (&) T. Nakamori Institute of Geology and Paleontology, Graduate School of Science, Tohoku University , Aobayama, Sendai 980-8578, Japan 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. - The upper parts of well-developed coral reefs have a unique topography that differs markedly from those of other marine coastal environments. Most of the fringing reefs of the Ryukyu Islands, for example, are characterized by both a convex reef crest and a concave moat (shallow lagoon). These structures are common, differing only in scale, not only on fringing reefs but also on barrier reefs and atolls. Understanding why such topography is regularly formed is one of the most important unresolved problems in coral reef geology. Growth histories of coral reefs in the Ryukyu Islands from the Holocene to the present have been reconstructed based on core samples from both raised coral reefs in uplifted areas, and on recent reef flats in more stable regions (Konishi et al. 1983; Takahashi et al. 1988; Kan and Hori 1993; Yonekura et al. 1994; Kan et al. 1995, 1997; Yamano et al. 2001b). Like coral reefs in the tropics, those in the Ryukyu Islands grew remarkably during the postglacial sea-level rise and the ensuing stillstand. Most Japanese Holocene reefs began to develop from around 10,000 years before present (BP), which is later than the time of initiation of typical reefs in the tropics. Catch-up reef growth dominated from 10,000 to 6,000 years BP because of rapid sea-level rise. The reef crest structures grew quickly after this stage and caught up with the rate of sea-level change during the period from 6,000 to 4,000 years BP (Takahashi et al. 1988; Yamano et al. 2001a). Construction of a shallow reef crest diversified the reef environment by the development of a wave-affected fore-reef, and a calm back-reef with a depressed topography in the form of a moat (Takahashi et al. 1988; Yamano et al. 2001a). This growth history suggests that reef crests grow rapidly during periods of sea-level rise. Montaggioni (2005) reported this peculiar pattern not only from the Ryukyu Islands but also from the other parts of coral reef areas, e.g., Pointe-auSable reef, Mauritius, Indian Ocean (Montaggioni and Faure 1997), Koror barrier reef, Palau, Western Pacific (Kayanne et al. 2002), One Tree Reef, Central Great Barrier Reef (Marshall and Davies 1982), Central area, Lord Howe Island, Tasman Sea, Western Pacific (Kennedy and Woodroffe 2000), and Punta Islotes fringing reef, Costa Rica, Eastern Pacific (Cortes et al. 1994). The growth rate of reef crests and the development of other morphological characteristics of the reef systems would have been affected by calcification of coral reef biota, because the reef framework is made of carbonate skeletons precipitated by them. Recent scientific work has shown that calcification can be defined by a simple function relating to environmental parameters, such as light intensity (Chalker 1981), and available chemical components (Gattuso et al. 1998a; Marubini and Thake 1999; Odhe and van Woesik 1999; Leclercq et al. 2000, 2002; Langdon et al. 2000). This study builds on that work, and proposes a model for coral reef growth that is based on calcification and other chemical and physical processes in the reef environment. It is important that the effect of light-enhanced calcification by hermatypic corals (Goreau and Goreau 1959; Barnes and Chalker 1990) is included in the model. This phenomenon has been reported on the basis of observations carried out at different spatial scales; for example, a coral colony (Goreau 1959), a coral community (Suzuki et al. 1995), and a complete reef (Barnes and Devereux 1984). Hermatypic corals are well known for their symbiosis with the dinoflagellate algae zooxanthellae. Incubation experiments suggest that photosynthesis by zooxanthellae enhances coral calcification (Goreau 1959; Gattuso et al. 1999), although the calcification does not have such effect on the photosynthesis under inhibited calcification (Yamashiro 1995; Gattuso et al. 2000). The photosynthesis of marine organisms is expressed by the following two equations: HCO3 ! CO2 OH CO2 H2O ! [CH2O] O2; and the calcification process can be expressed as follows: ! CaCO3: The production of OH during photosynthesis, as shown in Eq. (1), and of H+ during calcification, as shown in Eq. (3), is significant, as discussed subsequently. Gattuso et al. (1999) reviewed investigations into photosynthesis-enhanced calcification by hermatypic corals and pointed out that the endodermal cell layer secretes OH during photosynthesis and generates a pH gradient across the epithelial layer, with the endodermal side being alkaline. This OH can neutralize the H+ produced during coral calcification and accelerate the calcification. Although the interaction between calcification and endosymbiont photosynthesis remains unclear (Gattuso et al. 2000), it is generally accepted that photosynthesis and light availability are the main factors controlling the rate of calcification. Another factor controlling the calcification rate is the degree of aragonite saturation, Warag (= [Ca2+][CO32]/Karag, where Karag is the solubility product for aragonite), or [CO32], which is the dominant variable in the expression of Warag. The dependency of the calcification rate on Warag or [CO32] has been elucidated in laboratory experiments using t (...truncated)