Stable Carbon and Oxygen isotopic analysis of sub-microgram quantities of CaCO3 and the application to the analysis of single small foraminiferal shells.

~ We developed an analytical system to determine stable isotopic compositions (δ ¹³C and δ ¹⁸O) of sub-microgram quantities of CaCO₃ for the purpose of analyzing individual foraminiferal shells, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The system consists of a micro volume CaCO₃ decomposition tube, stainless steel CO₂ purification vacuum line with an introduction quantity regulating unit, helium-purged CO₂ purification line, gas chromatograph, and CF-IRMS. By using this system, we can determine stable carbon and oxygen isotopic compositions as low as 0.2 μ g of CaCO₃, with standard deviations of ±0.10 ‰ for δ ¹³C and ±0.18 ‰ for δ ¹⁸O within a 4-hour reaction time and 30-minute analysis. Our method has the advantage that we do not have to regulate the weight of CaCO₃ to react with H₃PO₄ because we can regulate the introduced quantity of CO₂ within the stainless steel vacuum line. ~ By using the system, we can determine the δ ¹³C and δ ¹⁸O values for individual small foraminifera whose weight is below 10 μ g (e.g. Globocassidulina sp.). That is to say, we can determine the δ ¹³C and δ ¹⁸O values for almost all kinds of calcareous foraminifera. Thus, we can apply the isotopic studies of foraminifera to seafloor sediments in which limited numbers of small shells are available (e.g. at high latitudes, in the deep sea around CCD, or in the seafloor fluid seepage area). In addition, in the case of large-sized calcareous foraminifera, we can determine the δ ¹³C and δ ¹⁸O differences between the fragments within a single shell. Partial analysis of their shells will be useful not only for paleoceanography but also for ecological research. ~ Using the system, we determined δ ¹³C and δ ¹⁸O values of a small-sized hyaline calcareous foraminifera Globocassidulina sp., about 210 μ m in diameter and 3 μ g of weight, collected from the top part (0-1cm) of a sediment core. The analytical result shows the isotopic compositions and dispersions of Globocassidulina sp. as -1.66±0.17 ‰ _PDB for δ ¹³C and +2.88±0.31 ‰ _PDB for δ ¹⁸O (1σ , n=10). Each measured value of single Globocassidulina sp. coincided well with those of multiple shells. No systematic difference was observed for the isotopic values between the living and the dead.