Unusual aliphatic hydrocarbon profiles at hydrothermal vent fields of the Central and Southeast Indian Ridges and Mid-Indian Basin

Abstract

The hydrocarbon (n-alkane and n-alkene) distribution in deep seawaters of the Mid-Indian Basin (MIB) and hydrothermal vent sites of the Central (CIR) and Southeast Indian Ridges (SEIR) were investigated to understand the impact of hydrothermal activities in deep seawater organic matter. The distinguishing feature of the detected hydrocarbons was in a bimodal distribution with a strong predominance of even-numbered n-alkenes (C16:1-C34:1) maximizing at n-C16 or n-C18, and a series of n-alkane hydrocarbons ranging from C16–C35 with slightly high content in long chain par but without an even/odd carbon number preference. The mono-unsaturated aliphatics were generally 1–4 times higher than the saturated counterpart and exhibited more complex vertical variabilities. The higher content of aliphatics in deeper water, i.e., the signals at 2500 m from the CIR and the increasing tendency towards the bottom from the SEIR in the near vent field sites, indicate a potential hydrothermal contribution. Sulfate-reducing microorganisms of bacteria and archaeon in seawater and the hydrothermal system may be responsible for the distribution feature of the saturated or unsaturated hydrocarbons, but the possibility of abiogenic sources cannot be excluded. In addition, the hydrocarbon profiles recorded the organic debris mineralization in the subsurface coinciding with dramatically hydrographic variability. The present aliphatic profiles feature reflected the complex bio-geochemical process, a minor impact of hydrothermal activity on the hydrocarbon distributions and deep water circulations in the Indian Ocean.