The composition and source of hydrocarbons in the hydrothermal products of Tangyin and Yonaguni Knoll IV hydrothermal fields from the Okinawa Trough

The hydrothermal sulfide and sediment of the Tangyin and Yonaguni Knoll Ⅳ hydrothermal fields in the southern Okinawa Trough were analyzed in gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS), by which the abundances of hydrocarbons and the individual C isotope compositions of n-alkane were determined. The n-alkanes in the hydrothermal products conformed to a bimodal distribution, and exhibited an odd-to-even predominance of high molecular weight n-alkanes with maxima at C₃₁ and an even-to-odd predominance of low molecular weight n-alkanes with maxima at C₁₈. The distribution and individual carbon isotopic compositions of n-alkanes suggest that the low molecular weight n-alkanes in hydrothermal products may be mainly the result of the metabolic activity of submarine hydrothermal microorganisms, and the high-molecular weight n-alkanes in hydrothermal products may be derived from mainly the terrigenous inputs. The content and proportion of low-molecular n-alkanes in hydrothermal sulfides are higher than those of hydrothermal sediment, indicating that hydrothermal microbial activity might be more flourishing in hydrothermal sulfide. The δ¹³C values of the n-alkanes in the hydrothermal sulfide samples tend to decrease as the number of carbon atoms increased, which suggest that the abiogenic contribution to the source of hydrocarbons in hydrothermal sulfides shall not be ignored.