Laser Raman detection of authigenic carbonates from cold seeps at the Formosa Ridge and east of the Pear River Mouth Basin in the South China Sea

Abstract

Laser Raman spectroscopy has been successfully applied for the in situ detection of gases and liquids, but has not yet been extensively applied to rocks in the deep sea. Authigenic carbonate rocks are widely distributed at the Formosa Ridge (Site F) and the eastern region of the Pear River Mouth basin (Site P) in the South China Sea (SCS). Considerable Raman spectral information on authigenic carbonates was obtained from laboratory measurements of slices of authigenic carbonate, direct measurements in a simulation environment, and in situ measurements in the deep sea. The dominant band of authigenic carbonate from Site F ranges from 1084 to 1087 cm−1 while the dominant band from Site P ranges from 1088 to 1091 cm−1, suggesting that compositional differences exist between the samples from the two cold seep areas. The results of the point-counting method indicates that the aragonite content in Site F is higher than that at Site P. The wider peak width of the aragonites at Site P shows the aragonites at Site P have lower degree of crystallinity, which suggests that aragonite is likely to be destroyed when the cold seep weakens or dies. The above information obtained via the Raman spectroscopy can guide sample collection in the deep sea and can be used in conjunction with other analysis methods to better understand environmental changes or the geological processes.