Geochemical Journal, Vol. 46, pp. 31 to 43, 2012 Characteristics and source of polycyclic aromatic hydrocarbons in the surface hydrothermal sediments from two hydrothermal fields of the Central Indian and Mid-Atlantic Ridges JIWEI LI,1,2 XIAOTONG PENG,3* HUAIYANG ZHOU,3 JIANGTAO LI,3 SHUN CHEN,3 ZIJUN WU3 and HUIQIANG YAO2 1Southwest Jiaotong University, Chengdu 610031, China 2Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China 3State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China (Received March 24, 2011; Accepted November 2, 2011) The analysis of major elements and polycyclic aromatic hydrocarbons (PAHs) were carried out on surface hydrother- mal sediments collected from the Kairei hydrothermal field on the Central Indian Ridge (CIR) and the Logatchev hydro- thermal 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 character- ized 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. Keywords: polycyclic aromatic hydrocarbons, inorganic geochemical characteristic, hydrothermal sediments, Kairei hydrothermal field, Logatchev hydrothermal field carbon, and subsequently migrate to surficial sediments INTRODUCTION (Pikovskii et al., 1996; Rudenko and Kulakova, 1986; PAHs are believed to be formed under high-tempera- Chernova et al., 1999). For example, Konn and coworkers ture synthesis, incomplete burning of organic fuel or as a detected the aromatic compounds in the hydrothermal flu- result of thermal impact on organic matter (Simoneit, ids from the Rainbow and Lost city hydrothermal fields 1993, 1995; Rudenko and Kulakova, 1986; Gennadiyev in the MAR region (Konn et al., 2009), and after that, et al., 1996). They are not only the indicator of industrial through the simulation experiment, they proved that PAHs and municipal contamination (Budzinski et al., 1997), but could be formed by biomass degradation in the extremely also one of the major components of hydrotherma1 bitu- hydrothermal environment (Konn et al., 2011). Since the men (Simoneit, 1984; Kawka and Simoneit, 1990). Both first discovery of hydrothermal petroleum at the Guaymas field study and laboratory simulation indicated that these Basin in the Gulf of California in 1978 (Simoneit et al., thermogenic hydrocarbons were likely to form from the 1979), numerous studies devoted to investigation of PAHs deep in the high temperature zone within the earth crust in the hydrothermal field have been carried out. Espe- as a result of synthesis and polycondensation of simple cially, in the Escanaba Trough on the southern Gorda Ridge (Kvenvolden et al., 1986), the Middle Valley of the Juan de Fuca Ridge (Simoneit, 1994), the Red Sea *Corresponding author (e-mail: [email protected]) (Simoneit et al., 1987; Michaelis et al., 1990), the Copyright © 2012 by The Geochemical Society of Japan. Andaman Basin (Chernova et al., 2001; Venkatesan et al., 31 Logatche v Edmond Meters Meters –3000 –2800 TVG13 –3100 –3400 Legend MAR6 –3400 –3800 MAR1 Hydrothermal field Sample site CIR7 Fig. 1. Locations of sampling sites at the Kairei and Edmond hydrothermal fields in CIR and the Logatchev hydrothermal field in MAR. 2003), the Rainbow vent field (Simoneit et al., 2004; Konn By contrast, the PAHs profiles in the hydrothermal solid et al., 2009) and the Lost City hydrothermal field structures, such as sulfide deposits (Simoneit et al., 2004) (Delacour et al., 2008; Konn et al., 2009). and chimneys (Simoneit and Fetzer, 1996), are charac- These previous investigations have made a great ad- terized by high concentrations of heavy PAHs (e.g., vancement in the knowledge of modern marine hydro- coronene) and only traces of LMW PAHs. thermal organic matter and provided a large amount of Based on the analysis of sulfide deposit at the rain- useful information to compare with the ancient counter- bow hydrothermal field, Simoneit et al. (2004) suggested parts found on land. However, the spatial distribution of that the solubility of the LMW PAHs was high enough PAHs in the hydrothermal geological occurrence at mod- for them to dissolve in the fluids. This factor resulted in ern marine hydrothermal fields is still a hot topic. the HMW PAHs analogs were abundant and LMW PAHs Chernova et al. (1999) reported that sediments in the analogs were depleted in the chimney walls at the rain- tectonically active zones of central part of the Andaman bow hydrothermal field (Simoneit et al., 2004). There- basin contained 10–100 times more hydrocarbons than fore, we hypothesized that the LMW PAHs would be apt pelagic sediments of the deep water basins in Indian to dissolve in the venting fluid and be transported out of Ocean, and concluded that the high levels of LMW ana- the hydrothermal chimney to the hydrothermal sediment logs PAHs, including naphthalene, phenanthrene and bi- around the hydrothermal deposit, whereas the HMW com- phenyl, in the deformation zone were associated with the ponents would be entrapped or redeposited from high tem- hydrothermal process. Venkatesan et al. (2003) also found perature fluid to the bitumen in the chimneys. This hy- that hydrothermal surface sediments of the Andaman pothesis sounds reasonable, however, it need more evi- backarc basin in Indian Ocean were predominated by alkyl dences to prove it. Here, we examined the relative abun- naphthalenes and alkyl phenanthrenes. Thus, this suggests dance of PAHs and concentration of major elements in that the low molecular weight PAHs would be commonly the hydrothermal surface sediments of two hydrothermal present in the surface sediments of hydrothermal systems. fields from two distantly geographic locations (CIR and 32 J. Li et al. Table 1. Sample location and description Sample Sample site Brief characteristics Main mineral Central Indian Ridge TVG 13 69.60°E, 23.88°S Fragment of sulfides chimney anhydrite Edmond hydrothermal field CIR7-1 70.04°E, 25.32°S Brown-yellow sediments with minor soft white sediment talc Kairei hydrothermal field CIR7-2 Gray-white soft sediment with some coarse grains talc, sphalerite CIR7-3 Gray-yellow sediments with some coarse grains talc CIR7-4 White and yellow sediment with dark-coffee coarse grains talc, chalcopyrite Mid Atlantic Ridge MAR1-1 44.98°W, 14.75°N Gray-white soft muddy sediments calcite Logatchev hydrothermal field MAR1-2 Red-brown metalliferous sediment Fe-oxyhydroxide MAR1-3 Dark-brown metalliferous sediment Fe-oxyhydroxide MAR6-1 44.98°W, 14.75°N White muddy sediment calcite MAR) to test the possibility of the occurrence of such ized by high contents of iron, silica, high H2S with high kind processes. chlorinity and low pH (Gallant and Von Damm, 2006). The Logatchev hydrothermal field GEOLOGICAL SETTING The hydrothermally active Logatchev field is located The Edmond and Kairei hydrothermal fields at 14°45′ N and 44°58′ W on a plateau immediately be- The Kairei vent field is located at 25°19′ S, 70°02′ E low a 350 m high cliff at a water depth of 3060–2900 m in segment S1 immediately north of the Rodriguez Triple (Fig. 1). Hydrothermal activity was first documented in Junction on the Central Indian Ridge (Gamo et al., 2001). 1994 along the eastern wall (Batuev et al., 1994). It ex- The hydrothermal activity was first reported by Herzig tends at least 800 m NW-SE and 400 m SW-NE and shows and Plüger (1988), and the hydrothermal vent site was a high diversity of vent sites and associated fauna. In this also first directly observed in 2000 (Hashimoto et al., area, tectonic accretion dominates the basaltic magma 2001). The main area of high temperature venting in this supply, resulting in a tectonic uplift of mantle and lower hydrothermal field occurs along a WNW trend, extends crustal rocks (peridotites and gabbros) to the sea-floor 80 m along the rift wall and is 30 m wide. The vent fluids (Cannat et al., 1997). Two main areas of high-tempera- of the Kairei hydrothermal field have a very high con- ture hydrothermal activity form the central part of the centration of hydrogen gas, a relatively high concentra- field, which is an area of at least three smoking craters, tion of silicon and a remarkably low CH4/H2 ratio. Large and a large mound with black smoker chimneys at its top. accumulations of weathered sulfides and peripheral rel- Black smoke is venting intensely at all three sites, either ict sulfide chimneys suggest the site has been active for a from the chimneys on the crater rim or from holes in the long time and there has been migration or focusing of sea-floor within the craters.