Characteristics of carbonate deposition in the environment are ubiquitous distribution, extensive production by organic activities and long-term stabilization of reactive organic carbon. Since enormous amount of carbon dioxide has been fixed in carbonate deposits during the evolution of life, the study of fixation-fluxes in carbonates and their mechanisms acquires considerable importance of clarifing the present globalclimate changes. From the view point of carbon dioxide sinks in the enironment, we propose three major objectives of carbonate study: the estimates of cardon dioxide fluxes fixed by carbonates, the detection of the global climate changes recorded in the past carbonates with fine time resolution, and the modeling of global carbon cycles. The following is a list of important subjects for each objectives. (1) Flux measurements: photosynthesis of calcium carbonate in coral reef, calcification by calcareous algae, carbonate desolution in deep-sea sediments, rate and mechanisms of carbonate preicipitation. (2) Detection of past climate changes: paleoclimate changes with fine time resolution, time determination with fine time resolution, microanalysis of trace elements and isotopes, quantitative measurements of diagenetic processes, and reading sedimental records of climate related to short time scales. (3) Simulation: chemical reaction of carbonates in aqueous systems and global circulation of carbon fluxes.

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Lake sediments have been attracting the growing interest from the various fields of geosciences for the last decades. This development of lacustrine sedimentology greatly owes to the fact that hydrocarbon source rocks have been found in the freshwater lacustrine deposits. Lacustrine study has also been stimulated by the recent growing interests and concern on the global climatic change and environmental crisis; nonglacial annual layers (varves) of lake sediments reflect the changing climatic conditions, providing high-resolution time series data. Lake sediments are also important as a sink of atmospheric CO₂. Profundal facies of northern temperate lakes are mostly composed of fine-grained calcite, forming nonglacial varve laminations. Lacustrine carbonates ranges in composition from low-Mg calcite to magnesite through high-Mg calcite, dolomite, hydromagnesite, nesquehonite, and even aragonite, reflecting the wide variations in salinity and Mg/Ca ratio of lake waters. Accumulation rate of lacustrine carbonates is estimated to be, at least, 1000mm per 1000years. This figure is similar to that of rapidly growing coral reef limestones. Precipitation of inorganic, lacustrine carbonates has been explained to be caused by a drastic increase of pH, which in turn, has been caused by a decrease in dissolved CO₂ and/or HCO₃^- through photosynthesis of blue green algea and diatoms. Thus the precipitation of inorganic carbonates in lake environments is strongly linked with bioactivities and is called as bio-induced carbonates.

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The geochemical cycle of CO₂ in a carbonate rock area was discussed by using for an example obtained for Akiyoshi-dai Plateau (Yamaguchi Prefecture), one of the big gest karst plateaus in Japan. The calcium concentration of the baseflow of the groundwater in the area showed seasonal fluctuations, and followed changes in CO₂ concentration in the soil. Soil CO₂ measured in the meadows which cover most of the area varied from a minimum of 0.08% at a soil temperature of 3.8℃, and a maximum of 1.2% at 20.8℃. The calcium concentration in the groundwater is controlled by waterlimestone dissolution equilibrium, under open system conditions depending on the meadow's soil CO₂ concentration. At the runoff peak of groundwater issuing from Akiyoshi-do Cave, which has the biggest drainage basin in Akiyoshi-dai Plateau, 18.5 km², the calcium concentration increases due to the flushing out of water with a long residence time in the deeper phreatic zone. During 1983-1986, a yeary average of 2,100 tons of limestone was dissolved in 2.1×10⁷m³ of groundwater issuing from Akiyoshi-do Cave, its catchment basin including 16.5km² of a limestone area: the mean solutional denudation rate is 51mm/ka. The amounts of CO₂ untilized on chemical weathering in carbonate rock areas in the world, corresponding to the same amounts of chemically weathered carbonate rocks in mol, were estimated by using the limestone denudation rate of 50mm/ka and found to be 8x10¹¹kg/y. The role of chemical weathering of carbonate rocks cannot be ignored in the geochemical cycle of CO₂.

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It was suggseted that a very delicate carbon balance between atmosphere, hydrosphere and geosphere on the present earth was attained by CO₂ fixation by photosynthesis and by biological calcification in the ocean. The "calcareous algae", which deposit massive calcium carbonate on the thallus, are mostly marine and are widely scattered among Cyanophyta, Rhodophyta, Chlorophyta, Chlomophyta and Haptophyta. The coral reef is especially important for standing crops of macro-calcareous algae and for their productive environment of calcium carbonate. Calcification in the macro-algae takes place extracellularly in the specially separated sites from the outer seawater, i. e., in intercellular space (Halimeda in Chlorophyta) or thickened cell walls (Corallinaceae in Rhodophyta). It seems to be coupled with an increase in pH in the same sites which is caused by CO₂ fixation by photosynthesis. Calcium carbonte is deposited as aragonite (Halimeda) or magnesian calcite (Corallinaceae). Unicellular algae, coccolithophorids in Haptophyta, produce disc-shaped scales of calcite called "coccoliths". The calcified scales are formed intracellularly and then extruded to the outer surface of the cell. The morphology of coccoliths is under genetic control, and an involvement of specific calcium-binding acid polysaccharides is strongly suggested in the formation of fine structure of coccoliths. The coccolith formation is intimately associated with photosynthesis. Emiliania hyxleyi, a species in coccolithophorids, is a cosmopolitan species and a main producer of pelagic carbonate. Annual production of calcium carbonate by calcareous algae are not known in detail, but it may be estimated as sevelal ten parcent of the total annual production of carbonate (2.0x10⁹ ton as CO₂) in the ocean. Calcium carbonate formation in the ocean accompanies CO₂ evolution as follows; 2HCO₃^-+Ca²⁺→CaCO₃+H₂O+CO₂, although CO₂ fixation by photosynthesis acts as a "driving force" for CaCO₃ precipitation. Therefore, for the evaluation of calcification in the effect on atmospheric CO₂, it must be estimated how photosynthesis overcomes the calcification in rate and what portion of organic matter synthesized in photosynthesis is remained without oxidation back to CO₂.

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Calcium carbonate deposition in the ocean results in a release of CO₂ to the atmosphere. Based on this process, a "coral reef hypothesis" was proposed to explain postglacial CO₂ increase as a result of reef growth. The model assumed that if twice the mass of atmospheric carbon (2 ACM) were deposited into coral reefs between 15,000 and 10,000 years B. P., the accompanying release of CO₂ would account for 40 ppm increase of the atmospheric CO₂. In this paper, calcium carbonate deposition rates into postglacial reefs with a time scale of 1000 years is calculated based on shallow core researches to test the hypothesis. The total reef mass is estimated to be 2 ACM. Calcium carbonate deposition rates of postglacial reefs attained their maximum between 5000 and 6000 years B. P. with a rate of 0.45 ACM for 1000 years. This major deposition phase does not match the period of observed CO₂ increase in the atmosphere. This discrepancy is not in agreement with the "coral reef hypothesis" .

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Carbonate sediments and/or rocks are subjected to various diagenetic processes and change their properties easily because of their chemistry. Consequently, their properties become different from that of primary carbonate sediments. Especially, the carbonate rock properties as porosity and permeability, change drastically by dissolution, precipitation and stabilization of carbonate minerals during early diagenesis. It is, therefore, very important and indispensable to understand the early diagenesis of carbonates. To elucidate the early diagensis, the following methods have been applied; 1) sedimentary petrological approach, 2) experimental approach, and 3) hydrogeochemical approach. A hydrogeochemical approach has been performed since 1970's and actively in the last several years. The approach is on the basis of chemistry of groundwater and it estimates diagenetic processes by mass balance and mass movement in the groundwater of each diagenetic environment. An advantage of the approach is that it is relatively easy to estimate quantitatively the proceeding diagenetic process, such as porosity change, transformation rate of aragonite to calcite, and so on, whereas the direct detection of the recent diagenetic process by other methods is difficult because of the very slow rate of reaction. A hydrogeochemical approach collectively evaluate a entire diagentic process in each diagenetic environment and deals macroscopically. Therefore, hydrogeochemical approach coupled with microscopic petrological analyses of rock samples will elucidate the diagenetic process synthetically.

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Global geochemical cycle has been described with boxes which indicate carbon reservoirs and fluxes between the boxes. Carbonate rock is one of the most important reservoirs because about 75% of carbon on the surface of the Earth is fixed in it. It is necessary to formulate processes related to carbonate precipitation and dissolution to reconstruct a dynamical system of an ancient climate. Carbon cycle models in the different three time scales (carbon productions in the Recent coral reef; carbonate deposition in the Quaternary; carbonate deposition in the Phanerozoic) were examined. Photosynthesis and calcification of communities are the primary processes in the Recent coral reefs, while the dissolution of the carbonate and weathering of silicate by CO₂ control the cycle in the geologic time.

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