The distribution and variation in the gas composition of macro-seeps on the near-shore Lingtou Promontory in the South China Sea

Abstract

Natural hydrocarbon seeps in a marine environment are one of the important contributors to greenhouse gases in the atmosphere, including methane, which is significant to the global carbon cycling and climate change. Four hydrocarbon seep areas, the Lingtou Promontory, the Yinggehai Rivulet mouth, the Yazhou Bay and the Nanshan Promontory, occurring in the Yinggehai Basin delineate a near-shore gas bubble zone. The gas composition and geochemistry of venting bubbles and the spatial distribution of hydrocarbon seeps are surveyed on the near-shore Lingtou Promontory. The gas composition of the venting bubbles is mainly composed of CO2, CH4, N2 and O2, with minor amounts of non-methane hydrocarbons. The difference in the bubbles’ composition is a possible consequence of gas exchange during bubble ascent. The seepage gases from the seafloor are characterized by a high CO2 content (67.35%) and relatively positive δ 13CV-PDB values (–0.49×10–3–0.86×10–3), indicating that the CO2 is of inorganic origin. The relatively low CH4 content (23%) and their negative δ 13CV-PDB values (–34.43×10–3—–37.53×10–3) and high ratios of C1 content to C1–5 one (0.98–0.99) as well point to thermogenic gases. The hydrocarbon seeps on the 3.5 Hz sub-bottom profile display a linear arrangement and are sub-parallel to the No. 1 fault, suggesting that the hydrocarbon seeps may be associated with fracture activity or weak zones and that the seepage gases migrate laterally from the central depression of the Yinggehai Basin.