Characteristics and source of polycyclic aromatic hydrocarbons in the surface hydrothermal sediments from two hydrothermal fields of the Central Indian and Mid-Atlantic Ridges

Abstract

The analysis of major elements and polycyclic aromatic hydrocarbons (PAHs) were carried out on surface hydrothermal sediments collected from the Kairei hydrothermal field on the Central Indian Ridge (CIR) and the Logatchev hydrothermal field on the Mid Atlantic Ridge (MAR). The most characteristic PAHs in the samples were the low molecular weight (LMW) analogs present in a significantly high proportion. Values of the fluoranthene/pyrene (F/P) ratio indicated that PAHs originated from a pyrolytic process. The relative abundance of phenanthrene and its alkyl homologs characterized by the following order C0 > C1 > C2 > C3 > C4, and this result suggested that less biodegradation had occurred in the hydrothermal organic matter. However, a severe secondary oxidation or thermal loss of Benzo[a]pyrene (BaP) would have happened due to the presence of Benzo[e]pyrene (BeP) and absence of BaP in the PAHs. The hierarchical cluster analysis (HCA) showed that these samples could be grouped into four clusters, and all members in the same cluster had a similar inorganic geochemical characteristic. On the basis of canonical correspondence analysis (CCA), the PAH compositions were correlated well with those of inorganic elements. Among them, LMW PAHs such as biphenyl, naphthalene and fluorene were more related to the Talc formation environment, where the Si rich fluids meet the Mg rich seawater. These intermediate weight (IMW) PAHs such as phenanthrene, dibenzothiophene and retene showed a positively relation with metal elements (Fe, Cu and Zn), which may represent a plume fall-out environment. Lastly, the high molecular weights (HMW) PAHs including chrysene, fluoranthene and BeP showed a major positive correlation with environment of the chimney wall. These results suggest that the different molecular weight PAHs might be prone to distribute in different hydrothermal occurrence among the hydrothermal system.