造石灰岩生物による環境解析

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Two different approaches were taken to analyze chemically and quantitatively a non-biological environment where carbonate-precipitating organisms occurred; 1) analysis of the mineralogical and geochemical properties in the carbonate (hard tissue) of the organisms, or 2) stratigraphical and sedimentological analysis of the sediments enclosing the organisms. The first approach follows to the principle that the physiological requirements for living species can be extrapolated to fossil counterparts, like several workers have reccntly claimed in the field of functional morphology. From the view point of comparative physiology, the biogenic calcium carbonates of calcareous algae, hermatypic corals and molluscan and brachiopod shells were examined for their potentialities as quantitative thermometer or salinometer at three different levels, 1) mineral species, 2) elements and 3) isotopes both radioactive and stable. 1) Aragonite content in Mytilus edulis shells at Tsukumo Bay, Noto Peninsula appears to show a negative correlation with the salinity. 2) Mg content in magnesian calcite of Lithophyllum okamuraii, a coralline alga, show a negative correlation to the latitudes from Okinawa to Hokkaido along the coast of the Japanese islands, hence most probably, positively correlated with temperature. 3) ^<18>O/^<16>O paleotemperatures obtained from the Pectinid and Tridacna bivalves and brachiopod shells in the Younger Member (60,000〜80,000B.P.) of the Riukiu Limestone at Kikai-jima, Kume-jima and Okinawa-jima indicate that the first locality of about 2° further north than the others was 3.5〜4.7℃ colder than the other two localities. Differences in the paleotemperatures between the Younger and Older Members of the Riukiu Limestone and the Younger Member and the Raised Coral Reefs (4000〜6000 years B.P.) are about 1℃ and 10℃, respectively, which substantiates the interpretation resulted from biofacies analysis. Eight annual increments successively sampled from the banded nacreous layer of a Late Pleistocene Tridacna shell at Kume-jima showed about 8℃ for a probable seasonal variation in water temperature under which the gigantic clam lived. This value appears to be compatible to that of the surface water at Okinawa at present. 4) It was shown that the apparent ^<230>Th/^<234>U ages of some Late Pleistocene and Holocene corals from Taiwan, Ryukyus and Japan may be corrected under the assumption that corals dwelling in the continental marginal water contain ^<230>Th of 1.5±0.2% equil. uranium unit initially in their aragonitic corallites; hence apparently 1100 years older than the true ages. It is suggested that these correction values may change depending on the habitat, food chain and taxonomic position of the analyzed corals. Two examples were selected for the second approach which attempted to clarify the ecology of ancient organisms through the stratigraphic and sedimentological analysis of the associated sediments. The Permian carbonate rocks in the Nyugawa area, Gifu, are classified by Ca/Mg ratios and other lithological characteristics into two facies, reefy and backreef embayment, where the biota such as calcareous algae and fusulinid foraminifers clearly indicate habitat segregation. The second example was taken from the Permian beds of Neo-mura, Ibi-gun, Gifu, where the limestones represent a reefy complex strongly similar to those of the Nyugawa area, both in Ca/Mg ratios and fossil assemblages. A conclusion was drawn from these and other geochemical data such as the contents of Al_2O_3+Fe_2O_3, P_2O_5 and others, that the geochemical parameters mentioned above can be useful and informative to analyze a sedimentary environment. Finally, some recent works in prospect were briefly outlined and appended; 1) Necessity of phosphate thermometry of biogenic phosphate associated with carbonates in order to solve the ambiguity of isotopic abundancy in the ancient ocean, 2) possible correlation betwee
日本地質学会の論文
1968-12-28

造石灰岩生物による環境解析

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