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Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (W) of seawater at 25∞C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing W aragonite values (1.08-7.77) of seawater. The increasing trend of calcification rate (c) for W is expressed as an equation, c = aW + b (a, b: constants). When W was larger than ~4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on W of ambient seawater. A decrease in saturation state of seawater due to increased pCO 2 may decrease reef-building capacity of corals through reducing calcification rate of corals.Keywords: coral, calcification, aragonite, saturation state, cultured experiment As a result of coral calcification (Eq. (2)), the W aragonite of seawater is altered through releasing CO 2 in surrounding environments. The elevated CO 2 in marine environments lowers pH through decreasing CO 3 2-concentration and leads to a decrease in W of seawater.The relationship between coral-reef calcification and W of seawater has been reported through field observations. Broecker and Takahashi (1966) observed that precipitation rate of CaCO 3 on the Bahama Bank was proportional to the degree of supersaturation, as a secondorder reaction. Smith and Pesret (1974) pointed out that coral calcification in the lagoon of Fanning Island was affected by the carbonate mineral saturation state. On a coral reef flat of Okinawa, Ohde and van Woesik (1999) also observed that calcification rate increased with the increase in W aragonite of seawater during daytime. From a global viewpoint associated with atmospheric CO 2 budget, Kleypas et al. (1999) strongly pointed out that coral calcification responds to W of seawater.On the other hand, the relationship between coral calcification and W aragonite of seawater was also studied through laboratory experiments under controlled conditions. Gattuso et al. (1998) observed a nonlinear increase in calcification rate of Stylophora pistillata with increasing W of seawater with a plateau in the rate when saturation state was larger than 3.90. Marubini and Atkinson (1999) also observed that calcification rate of Porites compressa decreased with decreasing W aragonite of seawater. However, only a few studies have provided clear relationship between coral calcification and seawater W.In order to elucidate the relationship between coral

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The drill core of Funafuti Atoll, collected by The Royal Society Coral Reef Expedition in 1896-1898, has been dated using high-precision 14 C and Sr isotope measurements determined by mass spectrometry. The core top is a Holocene reef deposit, reflecting 26.4 m of reef growth since 8 ka, and is underlaid by Pleistocene reef-lagoonal deposits separated by a series of inferred hiatuses at 26.4 m, ca. 30 m, ca. 65 m and ca. 80 m (ca. 0.1 Ma, ca. 0.2 Ma, ca. 0.3 Ma and ca. 0.6 Ma, respectively), reflecting atoll carbonate deposits associated with late Quaternary sea-level changes. In turn, the underlying sequence below ca. 80 m comprises an upper section of additional Pleistocene reef-lagoonal deposits, having a high accumulation rate greater than 100 m Ma −1 , and a heavily dolomitized lower section. Sr isotope data in the lower section provide maximum (oldest) ages for dolomite formation and, depending on the extent of the isotopic memory from precursor CaCO 3 , reflect ongoing diagenetic alteration from ca. 2.3 Ma (i.e. the oldest dolomite) or in one or more discrete diagenetic events at ca. 1 Ma, and possibly at ca. 2 Ma. This sequence reflects interaction between atoll carbonate sedimentation, tectonic subsidence and eustatic sea-level change.

This report describes protodolomites found in Daito jima to the east of the Okinawa Islands, Japan. To investigate the conditions for sedimentary protodolomite formation, the distribution and chemical composition of protodolomites were studied through field observations and laboratory analyses. From the thin section studies of carbonate rocks, it was observed that porous parts of coral were cemented by proto dolomite crystals and that aragonite in hard tissue of coral was transformed into protodolomite. These observed facts may show that protodolomite was formed through the transformation from aragonite and magnesian calcite in magnesium rich solution. The distribution and chemical composition of protodolomite in Minami-daito jima suggest that protodolomites may be formed in concentrated seawater.