Stable Carbon And Oxygen Isotonic Compositions Of Biogenic Shells From The Southwest India Ridge And Santa Monica Basin

Biogenic shells are good candidates for investigating the deep sea environment. The barnacle shell samples discussed in this paper were recovered from the southwest India ridge (SWIR). The bivalve shell samples studied here were obtained from the cold seep area of Santa Monica Basin, off shore of California. In order to investigate the circumstantial information recorded by biogenic shells a polarizing microscope, X ray powder diffraction and stable isotopes gas mass spectrometer were used for the mineralogy and stable isotopic analyses,which provide as a basis for studying deep-sea environments.The experiment results of barnacle shell from the hydrothermal area samples show that the barnacle belongs to slab barnacle Pachylasmatidea, Pachylasma sp. The X-ray Diffraction (XRD) results indicate that barnacle shell is predominantly of calcite (only two samples contain a little quartz which less than1%). The513C values of30samples are scattered between0.54%and1.80%with a mean of1.15%, within the range of normal biogenic shell δ13C values. The δ18O values of the30samples are scattered between4.02%o and5.67%o with a mean of5.21%. which derives that the salinity of seawater changed when shells were formed. The analysis of δ13C and δ18O values indicate that the barnacle lived near a hydrothermal vent, and the carbon gas concentration in ambient seawater is close to the normal marine environment level.The results of bivalve shells from the cold seep area samples illustrate that the bivalves belong to Calyptogenia elongata Dall and Lucinoma sp. However, the X-ray Diffraction (XRD) results indicate that bivalve shells are predominantly aragonite (more than99%). The δ13C values of30samples are from-23.05%to2.11%with a mean of-6.62%, lower than δ13C values of normal biogenic shell, and the carbon isotopic compositions of a shell were largely changed during its growth. Meanwhile, several shells δ13C value is depleted with minimum value of-23.05%. which is the most13C-depleted shells of fresh marine bivalves so far has been discovered. The δ18O values of the30samples are within the range of2.19%o to3.66%o with a mean of 2.91%o, and some samples are heavier than biogenic carbonate shell which precipitated in equilibrium with seawater. The analysis of δ3C and δ18O values has verified that the methane concentration in ambient seawater was increased.