Hydrothermal petroleum in the sediments of the Andaman Backarc Basin, Indian Ocean

Applied Geochemistry 18 (2003) 845–861 www.elsevier.com/locate/apgeochem

Hydrothermal petroleum in the sediments of the Andaman Backarc Basin, Indian Ocean§ M.I. Venkatesana,*, E. Ruthb, P.S. Raoc, B.N. Nathc, B.R. Raoc a

Institute of Geophysics and Planetary Physics and NASA Astrobiology Institute, University of California, Los Angeles, CA 90095-1567, USA b Dept. of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095-1593, USA c National Institute of Oceanography, Dona Paula, Goa 403 004, India Received 1 March 2002; accepted 13 August 2002 Editorial handling by B.R.T. Simoneit

Abstract Recent sediments of the Andaman Backarc Basin, Indian Ocean, between the Andaman Nicobar islands and the Malay Peninsula have been analyzed for biomarker lipids. Three cores were selected: one each from the fault zone in a deep basin (a graben between two fault systems), another from a location adjacent to the fault, and the third from the topographic high in the rift valley. The molecular composition of the lipid classes (n-alkanes, isoprenoids, alkylbenzenes, alkylcyclohexanes, hopanoids, polycyclic aromatic hydrocarbons, steranes, alcohols, sterols and fatty acids) was examined by gas chromatography (GC) and GC/mass spectrometry to understand the nature and source of the hydrocarbons present and the processes of maturation of organic matter. The data show that the hydrocarbons are of hydrothermal origin, derived from thermal alteration of sedimentary organic matter, consisting of a mixture predominantly of marine-derived components with some terrestrial inputs. Normal alcohols and fatty acids also corroborate the distribution of n-alkanes. The distribution profiles and various parameters computed from the concentration of the target compounds suggest that oxidative reactions and microbial degradation in this environment are insignificant. Triterpane and PAH compositions indicate that the thermal maturity of the bitumen in the samples is comparable to or lower than that found at other hydrothermal regions such as the Northern Juan de Fuca Ridge, Guaymas Basin and Escanaba Trough. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The Andaman Basin is a typical backarc basin with strike slip faults and a spreading ridge. The intermittent volcanic eruptions in the early 1990’s in the Barren Island (part of the Andaman Arc system) reflect the tectonic activity of the general region. After a detailed geological and geophysical investigation of the Andaman backarc Basin, Rao et al. (1996) reported the presence § Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA publication no: 5738. National Institute of Oceanography, India, Publication no: 3780. * Corresponding author. E-mail address: [email protected] (M.I. Venkatesan).

of chimney fragments and rocks and, cratered seamounts containing disseminated and vein-type pyrite embedded in a silicate matrix. Pyritic rods were also recovered in the sediments adjacent to the active segment of the rift valley. These features document the hydrothermal activity in the region. Subsequently, Chernova et al. (2001), from their study of hydrocarbons in the sediments, concluded that the sediment organic matter has been altered by hydrothermal/volcanic events. Most of the sites where hydrothermal activity and associated mineralization occur are located near divergent boundaries (Rona, 1984; Simoneit, 1993, 2000). Several elegant organic geochemical studies from hydrothermal environments such as Guaymas Basin and Juan de Fuca Ridge have been reported which exhibit unique hydrocarbon distributions different from normal,

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tectonically passive regions receiving organic detritus from primary production in the upper ocean (i.e. Simoneit and Lonsdale, 1982; Simoneit, 1984; Brault and Simoneit, 1989; Kvenvolden and Simoneit, 1990). A few other studies also have reported related data on hydrothermal petroleum (Spiro, 1984; Michaelis et al., 1990). Hydrothermal plume signals have recently been recorded along the SW Indian ridge (German et al., 1998). However, Andaman Basin is along the convergent margin, and it was of interest to carry out detailed molecular characterization of the organic matter in the sediments from this Basin in light of the results from the above mentioned two studies (Rao et al.,1996; Chernova et al., 2001) from the same region. The molecular composition of the lipid classes (i.e. n-alkanes, isoprenoids, alkylbenzenes, alkylcyclohexanes, hopanoids, steranes, polycyclic aromatic hydrocarbons, alcohols, fatty acids and sterols) was, therefore, examined in selected core sections from the Andaman Basin to understand the nature and source of the hydrocarbons present and the processes of maturation of organic matter. 1.1. Geological setting The Andaman Sea is a marginal basin extending from Myanmar in the north to Sumatra in the south and, from the Malay Peninsula in the east to the Andaman and Nicobar islands in the west (Fig. 1). The Andaman basin is a region of geologic and tectonic importance between the southern extension of the Himalayas and Indonesia (Rudolfo, 1969). The central part of the basin represents a complex topography depicting a volcanic arc system, backarc spreading ridge, and several seamounts and faults. The central Andaman trough extends from Martaban Canyon to Nicobar Ridge. The complex system of short spreading rifts and faults in the central Andaman basin was suggested by Dasgupta (1992) to be the consequence of backarc spreading. The spreading center is probably connected to the ShanSagaing fault system in the eastern Burma highlands in the north and to the Sumatra fault system in the south. The recent volcanic eruptions at the Barren Island, which is a part of the Andaman Arc system, indicate that the region is tectonically active. 1.2. Oceanographic setting The Andaman Sea is a partially isolated water body and is influenced by the large quantity of fresh water run off from the Irrawaddy and Salween rivers. The Irrawaddy River is the major source of sediment to the Andaman basin. The sediment deposition is controlled largely by the bathymetry of the basin (Rao et al., 1996). The Andaman Sea, similar to the adjoining Bay of Bengal, experiences the semi-annually reversing SW monsoon (June–September) and NE monsoon

(December–February) resulting in seasonal changes in surface circulation and productivity. In spite of large freshwater discharge, the nutrient concentration in the Andaman Sea is only modest. The average primary productivity of the Andaman Sea is rather low and is estimated to be approximately 6107 tons of C/a (Qasim, 1977). Shallow sills ( 1300 m) limit deepwater exchange between the Andaman Sea and the Bay of Bengal.

2. Sampling and analysis Ten sediment gravity cores of varying lengths from 75 to 465 cm were collected during March–April, 1995, onboard Research Vessel A.V. Sedorenko, in the Andaman Basin. Based on lithological variations from the core logs and the results of Chernova et al. (2001), 8 subsamples from surface and subsurface sections of 3 cores were selected for a detailed study (Fig. 1). They were one each from the fault zone at the deep basin, West Andaman Fault, and the topographic high. GC-2 was raised from the deep basin from a water depth of 3492 m, and the sediments are mainly greyish olive clays. Subsurface structure appears to be a graben between two fault systems. GC-5 was collected from a location west of West the Andaman Fault and part of the Andaman Nicobar Ridge system at a water depth of 1539 m. This fault offsets the Andaman spreading center. Since GC-5 is from the slope of the Andaman Nicobar Ridge, the depth is much shallower than the topographic high, which is a small isolated feature. The core consists of olive grey compact clays. GC-10 was collected from an isolated topographic high, which runs parallel to the spreading centre. Water depth on this topographic high is 2947 m while the surrounding area is 3100 m deep and the sediments are greyish brown soft clays at the top and olive grey soft clays in the deeper strata. Total organic C (%TOC) was computed as the difference between total C and total inorganic C. Total C content was determined with an Elemental Analyzer (NCS 2500, CE Instruments). Sediment samples were introduced in small aliquots of 5–10 mg in Sn cups through an auto sampler into the combustion tube of the analyzer. The calibration was done using Sulfanilamide standard as the reference material. The measurement precision for total C was 4% estimated by repeat analysis. Total inorganic C was calculated from the CaCO3 content of the sediments. A digital gasometer, calibrated with standard reagent grade CaCO3 powder, was used for determining CaCO3 content. Precisely weighed sediment samples were taken into the reaction chamber of the gasometer and treated with a known quantity of 1N HCl. The pressure generated in the chamber (proportional to the volume of CO2 released from the reaction)

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Fig. 1. Location map for core samples collected from the Andaman Basin. Water depth is given in parentheses.

was measured with a digital pressure gauge. CaCO3 percent in the sample was then calculated with the K factor established using standard reagent grade CaCO3 powder as follows: Pressure developed for varying amounts of standard reagent grade CaCO3 was measured and each time, pressure per one unit CaCO3 was calculated, averaged and identified as K factor for that set of analysis. The measurement precision for CaCO3 was

estimated to be  5% by repeat analysis. Total inorganic C content of the sample was obtained by multiplying the CaCO3 content by 0.12. About 15–25 g of the air-dried samples were powdered in a mortar and pestle and weighed into a centrifuge jar. After spiking with alkane and PAH surrogate standards (deuterated alkanes and hexamethyl benzene, dodecylbenzene, and terphenyl, respectively) and sonication for

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10 min, they were extracted 3 times by homogenization with 90–100 ml of dichloromethane. The solvent extract was concentrated, weighed and treated with activated Cu for elemental S removal. The S-free concentrate was subjected to silica gel column chromatography, and aliphatic, aromatic and polar fractions were isolated according to the methods of Venkatesan et al. (1987). Only the polar fractions were weighed, the other two were not because their yield was small from the limited amount of sample processed. Such low yields required concentration to a very small volume of the entire fraction to enable accurate weighing which would have resulted in a considerable loss of low molecular weight components. The alkanes and isoprenoids, pristane and phytane, were quantified by a Varian Model 3400 gas chromatograph (GC) equipped with a fused silica capillary column (DB-5, 30 m long with 0.25 mm diameter and 0.25 mm film thickness) using a Flame Ionization Detector. The PAHs were quantified from the extracted ion current profiles obtained from a Finnigan 4000 GC/ MS equipped with a GC and column the same as above (Venkatesan et al., 1987). A Galaxy 2000 for Windows data system was used to acquire data. Confirmation of alkanes and identification of other compounds (di- and triterpanes, steranes, branched and cyclic alkanes, alkenes etc.) in the aliphatic fraction were also obtained by the above GC/MS system. The alcohols, sterols and fatty acids in the polar fraction were derivatized to their silylethers and quantified with a Varian GC with fused silica column as above following the method of Venkatesan et al. (1986). Compound identification of the derivatized fraction was confirmed by GC/MS under conditions as described above.

3. Results and discussion 3.1. Total organic carbon, elemental sulfur and total organic extracts Core tops down to 10 cm were disturbed while retrieving because the samples were collected with a gravity corer. Any section within the top 10 cm generally should be considered as the surface section. All the samples had a petroliferous odor. The TOC of most of the samples is close to or lower than 1% and ranges from 0.5 to 1.84 (Table 1), similar to other samples from the same general region reported by Chernova et al. (1999, 2001). The low values are consistent with the low productivity in the region (Qasim, 1977). The GC-5 samples have uniformly lower %TOC levels than the other two cores. The total organic extracts from all the samples are of similar magnitude but include elemental S (Table 1). Elemental S was detected in all the samples,

with sections GC-2 (288–290) and GC-5 (38–40) and (71–73) containing more than the others, and the GC-10 samples containing only trace amounts. In general, judging from the amount of activated Cu required for its complete removal, elemental S increases with depth in a given core. This observation suggests that the sediments are more reducing at the bottom than in the upper strata. The presence of elemental S is generally associated with the occurrence of sulfides in the proximity of active vents (i.e. Simoneit, 1994). All the extracts exhibited blue fluorescence indicating the presence of polynuclear aromatic hydrocarbons. The polar fraction constitutes from 20 to 30% of the total organic extract. This result is comparable to the relative abundance of the polar fraction in total extract from other hydrothermal environments (Kvenvolden and Simoneit, 1990). 3.2. Aliphatic hydrocarbon compounds The resolved n-alkanes from the sediments are given in absolute concentrations (ng/g) in Table 1. The concentrations are all of the same order of magnitude, but vary about 4-fold among the samples, in an order by stations GC-5 > GC-2 > GC-10. When expressed in terms of organic C, resolved n-alkane content in GC-5 samples is 5–10 times more abundant than the other two cores. This confirms the above trend that especially GC-5 and-2 are generally more enriched in n-alkanes than GC-10 samples. Fig. 2 shows representative chromatograms containing distribution diagrams of the resolved n-alkanes dominated by shorter chain homologs with consistent bimodal patterns (Table 1). The first mode between n-C10 and n-C25, suggests possibly a microbial aquatic origin of these compounds (i.e. Cranwell, 1976). The second modal range from n-C26 to n-C35, with n-C31 as maximum, and a significant odd C number predominance in the C range from 27 to 36 (OEP31 53) is typical of higher plant inputs (Eglinton and Hamilton, 1963). A stratigraphic relationship is evident in the cores in that the OEP31 (odd–even-predominance at C31 (Table 1) increases with subbottom depth. The presence of terrestrial alkanes in the sediments is consistent with the sedimentology of the region which receives continental river runoff as noted earlier. However, since the plant wax alkanes are more resistant to biodegradation, their proportion may represent to a certain extent an overestimate of the terrigenous influx in the region. It is possible that in the previous study by Chernova et al. (2001), high molecular weight wax alkanes couldn’t be resolved well under their gas chromatographic conditions due to an excessive bleed of the column at higher temperatures (i.e. Chernova et al., 2001, Fig. 2b, GC-5, 51–53 cm). Although, the overall distribution profiles of alkanes appear comparable in both the studies the authors are unable to make further comparisons of the data because

Table 1 Extractable organic matter yield and molecular ratios of hydrocarbons in sediments of the Andaman Basin GC-2 (7–10)

GC-2 (131–133)

GC-2 (288–290)

GC-5 (7–8)

GC-5 (38–40)

GC-5 (71–73)

GC-10 (8–9)

GC-10 (178–180)

Percent total organic carbon Total organic extract + S, mg/ga Polar fraction, ug/ga Total resolved n-Alkanes, mg/ga,b P C –C (L), ng/ga P 12 24 C25–C35(H), ng/ga n-alkanes carbon range, (Cmax)c L/He Pr/Ph Pr/nC17 Ph/nC18 CPIf CPIg OEP31h 30H/29H Tm/Ts S/(S+R) for 31H

1.1 1328 310 2346 (213) 2093 253 13–35 (16,31)d 8.27 4.15 3.44 2.09 0.87 0.77 3.23 1.63 1.14 0.44

1.08 2962 826 2623 (243) 1683 936 11–36 (14,31) 1.81 2.96 2.14 1.81 1.35 0.80 4.75 2.48 1.50 0.29

1.11 3488 1060 4457 (402) 3262 1193 10–36 (14,31) 2.73 3.51 1.52 1.19 1.17 0.77 5.93 2.71 1.67 0.23

0.5 5796 1380 5152 (1030) 4549 599 12–36 (14,31) 7.59 1.84 1.19 0.48 0.62 0.51 3.79 2.27 1.64 0.24

0.68 7740 2078 5896 (867) 5275 619 11–36 (14,31) 8.52 2.26 1.39 0.62 0.64 0.52 4.54 3.13 1.14 0.33

0.7 6123 1716 5300 (757) 4482 815 10–36 (14,31) 5.50 2.31 1.72 0.72 0.60 0.41 5.32 2.5 1.71 0.32

1.84 3119 648 1388 (75) 1030 354 10–36 (14,31) 2.91 1.96 1.52 1.13 0.93 0.68 2.89 1.50 1.08 0.46

1.1 2848 570 2790 (254) 1373 1413 11–36 (14,31) 0.97 2.22 2.43 2.77 1.79 0.83 5.58 2.21 1.55 0.47

a b c d e f g h

Based on dry sediment weight. P ( C12–C36); mg/g organic carbon in parentheses. Bimodal maxima in parentheses. Dominant homolog is underscored. Pristane is in higher concentration than C16. (L)/(H). CPI is odd/even n-alkanes in the carbon range from C13 to C35. CPI is odd/even n-alkanes in the carbon range from C13 to C25 (Tissot and Welte, 1984). OEP31 is odd/even n-alkanes in the carbon range from C27 to C36.

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Core (section in cm)

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Fig. 2. Representative gas chromatograms of the saturated (aliphatic) fraction. Arabic numerals refer to carbon chain length of n-alkanes. D14, D24 and D36 are deuterated surrogates. IS=Internal standard, phenyldodecane. Pr=pristane, Ph=phytane. 16i, 18i=isoprenoids. 16:1, =alkene. br=branched and/or cyclic alkanes. See Fig. 3 for identification of ‘br’. Not all identified compounds are marked in the chromatograms.

Chernova et al. (2001) didn’t report the absolute concentrations of the individual n-alkanes. Seven of the 8 samples exhibit L/H ratios (cf: Table 1) much higher than one, except for GC-10, 178–180 cm, where it is close to unity (Table 1; Fig. 2). The occurrence of uniformly higher relative concentrations of the more volatile resolved components with an even C predominance, a maximum at C14 or C16, and a CPI 41 (cf: Table 1) represents hydrothermal alteration of the marine biogenic sedimentary organic matter (Elias et al., 1997; Brault et al., 1988). The alkanes in the C range from 14 to 20 could have been derived from thermal and catalytic reaction processes typical of the catagenetic stage of petroleum formation (Tissot and Welte, 1984). The abundance of even C numbered low molecular weight alkanes also skews the CPI13 35 towards unity despite the presence of terrigenous wax alkanes as discussed above. An unresolved complex mixture (UCM, hump) of branched and cyclic compounds is present in all the samples in the C number region of 16–22 with a maximum between C18 and C19. This UCM is typical of light petroleum or biodegradation. Thermal alteration of microbial organic matter could have resulted in such a distribution as also previously observed for samples from the East Pacific Rise (Brault et al., 1988). Although these samples have been thermally altered, lipid signatures of the two endmember precursors, namely marine and terrestrial inputs, can still be identified. Pristane and phytane (Pr/Ph > 1) are dominant hydrocarbons in the C15–C20 region in all the samples. Other isoprenoids such as C16 and C18 (C17 absent) are found in all the samples while C15 isoprenoid is detected in a few. Relatively high abundance of pristane and phytane have also been observed in the petroleum residues associated with hydrothermal minerals from the Mid-Atlantic Ridge and in petroleum from active mounds of the southern rift from the Guaymas Basin (Brault and Simoneit, 1989; Simoneit, 1984). Pristane is always more abundant than n-C17 and phytane is more abundant than n-C18 in most of the present samples. Although pristane and phytane distributions have generally been exploited for paleoenvironmental implications (Didyk et al., 1978) these ratios in the present samples may be related to the thermal processes of the organic matter as well as their sources. Furthermore, Pr/Ph ratios are believed to increase with maturity while phytane/n-C18 will decrease (ten Haven et al., 1987). However, there is no clear trend in these ratios with depth in a given core in the samples. Considering that this environment is rather unique, the values in general may be related to the original source being altered by hydrothermal process rather than reflective of microbial degradation. The GC/MS analysis also confirmed the presence of some alkenes, branched alkanes, alkylbenzenes and

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Fig. 3. Mass fragmentograms of m/z 57 (n-alkanes), m/z 83 (n-alkylcyclohexanes), m/z 91 and m/z 105 (branched alkylbenzenes) from sample GC-5 (71–73 cm). Numbers refer to carbon skeleton size. Pr=pristane and Ph=phytane.

n-alkylcyclohexanes. Monoalkenes were detected in the C range from 16 to 19 in all samples and in some up to 22 with 16:1 as the predominant homolog. While it is likely that some of these olefins could have been derived from biogenic precursors, their presence under the huge hump along with several branched alkanes strongly suggests that a major proportion of them is from natural thermal cracking (Hoering, 1977). Further, the alkanes profiles in Fig. 2 are largely reminiscent of those from the oily crust and rock fragment from the southern rift from the Guaymas Basin (Simoneit, 1984) which are clearly of hydrothermal origin. Two types of branched alkanes were also identified in most of the samples in the C range from 15 to 25, one with an M-58 (butyl side chain) fragment and another with a characteristic fragment ion of m/z 127. Their complete structures could not be identified because of the lack of authentic standards. The mass fragmentograms at m/z 57, 83 and, 91 and 105 corresponding to the n-alkane, n-alkylcyclohexane, and branched alkylbenzene series (m/z 91 and 105), respectively, from a typical sample are presented in Fig. 3. The n-alkylcyclohexanes range from C14 to C22 and alkylbenzenes from C16 to C20. These compounds

have similar C number ranges and have previously been detected in association with hydrothermal minerals (Brault and Simoneit, 1989). However, unlike the resolved n-alkanes, the n-alkylcyclohexanes have the C number maximum centered at C18 and the alkylbenzenes at C19, coinciding with the maximum of the UCM (Fig. 2). The n-alkanes especially in the C range from 14 to 20 could have been generated from thermal and catalytic reaction processes as discussed above. Alkylcyclohexanes and alkylbenzenes could derive from the cyclization of straight chain fatty acid esters of both algal and bacterial origins that are present in the ambient sea water (Fowler et al., 1986). Kerogen cracking at elevated temperatures catalyzed by clay minerals could also generate these compounds (Spiro, 1984). Laboratory simulations by Rigby et al. (1986) of catalytic alkylation of benzene by alcohol at 200–270 C were found to form branched alkylbenzenes as a result of thermal processes. Fatty acid esters, also detected in these samples as will be described below, could have been one of the main sources of alkylating agents generating these compounds. Further, the presence of these alicyclic compounds (often in significant amounts in

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Fig. 4. Mass spectrum of the unknown bicyclic alkane in the aliphatic fraction. See text for details.

most of the samples) suggest that the conditions in this environment are not conducive for oxidative reactions and/or microbial degradation by chemosynthetic bacteria generally harbored in hydrothermal regions. This observation is consistent with the presence of elemental S in the samples. 3.3. Biomarkers In addition to several isoprenoids and branched alkanes, at least one bicyclic alkane with 15 C atoms was also detected. This compound has a Kovats index of 1464 (DB-5, 30 m column) which is within the expected general retention window, yet different from those of the two C15H28 bicyclic alkanes or 8b(H)-drimane reported from crude oils by Alexander et al. (1984). However, the mass spectrum of this compound with m/z 123> > 193>208> 137 (Fig. 4) is very similar to that of 8b(H)-drimane published by the above authors. In the absence of confirmation by coinjection with authentic standard, the authors tentatively identify this to be a C15H28 bicyclic alkane. Complex precursors in the microbial organic matrix, such as sesquiterpenoids or bacterial hopanoids, could have been degraded to simple bicyclic alkanes by thermal catalysis (Alexander et al., 1984; Simoneit, 1986). Bicyclic alkanes such as drimanes and rearranged drimanes have previously been found in hydrothermal environments at the East Pacific Rise (Brault et al., 1988). The bicyclic alkanes in the current samples could, therefore, have resulted from the thermal cracking and subsequent thermal alteration of the precursors.

Low levels of triterpanes which are similar in composition and maturity are detected in all the samples, and an example is illustrated in Fig. 5. The 17a(H),21b(H)hopane series ranges from C27 to C32 with a maximum at C30 (C28 absent). Ratios of the C30 to C29 hopane have been exploited for source rock correlations (Palacas et al., 1984). In the current study this ratio falls within a range from 1.5 to 3.13, indicating possibly a similar source for all the extracts (Table 1). Seifert and Moldowan (1978) suggested that the ratios of 17a(H)22,29,30-trisnorhopane (Tm) to 18a(H)-22,29,30-trisnorneohopane (Ts) can be used as a maturity parameter for hydrocarbons from a similar source or a source parameter if they had similar maturities. In the Andaman samples this ratio falls within a narrow window from 1.08 to 1.71 suggesting that they are of similar maturity and from a similar source. The extended hopanes (5C31) can be used to assess the maturity of the oils. The S(S+R) ratio for the C31 homolog spans over a range between 0.23 and 0.47 (Table 1). The C32 hopane was detected, but the S and R epimers were too small to measure accurately. With increasing maturity the 22R configuration is converted to a mixture of 22S and 22R epimers, finally reaching an equilibrium value with 22S/22S+22R close to 0.6 (Ensminger et al., 1974). The uniformly low epimer ratios in the Andaman samples confirm the immature character of their hydrocarbon assuming that the epimer ratios reflect the maturity of the whole oil. The natural product precursor compounds [17b (H), 21b(H)hopanes] which are thermally immature are present in significant amounts from C27 to C31. Only the C29

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Fig. 5. Representative mass fragmentogram (m/z 191) showing distribution of triterpanes in the aliphatic fraction from sample GC-5 (71–73 cm). Numbers indicate the carbon number of molecules. Suffixes designate configuration: ab=17a(H),21b(H)-; ba=17b(H),21a(H)-; bb=17b(H),21b(H)-hopane. Ts=18a(H)-22,29,30-trisnorhopane; Tm=17a(H)-22,29,30-trisnorhopane. S and R are epimers. E1=hop17(21)-ene ; E2=isohop-13(18)-ene; F1=Fern-9(11)-ene?; F2=cis-fernene? Exact identification of the two fernenes marked is not available due to their low concentration; D=diploptene.

homolog of the moretane [17b(H), 21a(H)] series, which are of intermediate stability, is found in small amounts. Based on the triterpane distribution, the thermal maturity of the samples is roughly comparable to hydrothermal petroleum from those of Northern Juan de Fuca Ridge (Simoneit et al., 1992) and are less mature than those from the Guaymas Basin and Escanaba Trough (Kvenvolden and Simoneit, 1990). The presence of hop-17(21)-ene, isohop-13(18)-ene, fernenes and diploptene further document the relatively immature character of the bitumen. The triterpanes are believed to derive from microbial organic matter (Ourisson et al., 1979; Venkatesan, 1988a). The triterpane profiles observed in the samples are the result of rapid maturation due to hydrothermal activity from contemporary organic matter in the region. Steranes are present only in GC-5 samples, but, at much lower levels than triterpanes. Because of the very low intensity of the ions and potential peak overlap, greater uncertainties could arise in the measurement of ratios of their epimers, and therefore, they are not

reported here. No mono or diaromatized steranes were detected. Traces of triaromatized steranes found in some samples at depth, could have been derived from the aromatization of monoaromatic steroid hydrocarbons during the hydrothermal alteration, especially considering that elemental S was present. This argument is based on prior laboratory thermal simulation results where elemental S was found to enhance the generation of triaromatized steranes from the monoaromatics (Abbott et al., 1984). Retene was detected in small amounts in all the samples. Simonellite (at a lower level than retene) was detected in all but GC-10 (178–180 cm). The presence of these diterpenoids indicate terrigenous input (Simoneit, 1977) consistent with the high molecular weight n-alkanes found. 3.4. Polynuclear aromatic hydrocarbons Polynuclear aromatic hydrocarbons (PAH) and their alkylated homologs are present in all the samples. A

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Fig. 6. Representative total ion current trace from GC/MS quantitation of PAH fraction from sample GC-2 (288–290 cm). 1, Naphthalene; 2,2-methylnaphthalene; 3, 1-methylnaphthalene; 4, biphenyl; 5, dimethylnaphthalenes; 6, hexamethylbenzenze (surrogate); 7, acenaphtheneD10 (IS); 8, methylbiphenyl*; 9, bibenzyl*; 10, trimethylnaphthalenes; 11, methylbenzofurans*; 12, phenanthrene D10 (IS); 13, phenanthrene; 14, anthracene; 15, phenyldodecane (surrogate); 16, methylphenanthrenes; 17, dimethylphenanthrenes; 18, fluoranthene; 19, pyrene; 20, terphenyl (surrogate); 21, chryseneD12 (IS); 22, chrysene/triphenylene; 23, benzo(b&k)fluoranthene; 24, peryleneD12 (IS); 25, perylene; 26, benzo(ghi)perylene. *Not quantitated.

representative total ion current plot from GC/MS is illustrated in Fig. 6. The total resolved PAHs (Table 2) range from 360 to 1400 ng/g and are 2- to 5-fold lower in concentrations than the resolved alkanes, but much higher (5–30 times) than those reported by Chernova et al. (2001, cf. Table 4) for sections close to those analyzed here from the same cores and from the central Indian basin sediments (Chernova et al., 1999). The two spreading zone samples from GC-10 contain the least amount of PAHs, which is also consistent when expressed in terms of the total organic C. Although the authors have identified and measured more PAHs than Chernova et al. (2001), the additional compounds in the analysis are found to be present only in small amounts. The major ones (as listed in Table 4 of Chernova et al. and the present Table 2) are common in both the studies. The difference in the quantitative data may, therefore, be due to different methodologies employed for PAH determination because Chernova et al. (2001) used

Shpolski spectroscopy while in the current work PAHs were measured by GC/MS which provides a better resolution of the compound peaks as well as higher sensitivity. However, the relative proportions of certain compounds in both studies are reasonably comparable since a wide range of values were reported by these authors for samples from the spreading zone and West Andaman fault and deep basin where different sub sections from the same cores have been analyzed by the present authors. The percentage of pyrene, benzo(a) pyrene, perylene and benzo(ghi)perylene in the total unsubstituted PAH from the present samples (Table 2), therefore, fall within the same range quoted by Chernova et al. (2001). Total PAH (t-PAH) comprises both parent and their alkylated homologs as listed under Table 2. Sulfur PAHs, such as dibenzothiophene and its homologs, were also found as expected from the presence of elemental S in the samples. Parent PAH (p-PAH in Table 2) ranges from 17

Table 2 Concentrations of various PAH and distribution parameters in sediments of the Andaman Basin Core (section in cm)

91.5 41.8 132.4 165.1e 83.3 17.5 1.7 4.1 11.5 41.9 25.2 13.1 50.9 2.5 54.9 29.1 6.9 0.0 0.0 4.0 8.1 3.4 0.0 12.5 10.1 14.2 0.0 1.7 6.8 3.2 0.0 8.4 2.3 3.8 1.4 66.5 2.1

GC-2 (131–133) 87.2 80.6 232.4 201.8 93.1 55.9 1.1 3.9 10.8 33.9 19.0 11.4 40.7 2.3 49.3 27.2 7.9 0.0 0.5 3.1 9.3 6.4 0.0 10.2 8.9 11.7 2.6 1.4 6.2 3.0 0.0 6.9 2.0 3.5 1.4 79.5 1.8

GC-2 (288–290) 75.6 123.5 363.9 305.5 106.8 77.4 0.9 2.3 9.4 26.8 13.5 8.9 31.0 1.7 45.7 29.6 10.1 0.0 0.6 2.0 10.4 8.1 0.0 7.2 7.2 9.0 3.7 1.2 5.1 2.8 1.1 5.4 1.3 3.3 1.2 91.4 1.4

GC-5 (7–8)

GC-5 (38–40)

7.4 23.2 115.4 137.7 60.0 11.0 0.4 1.6 6.0 27.4 12.0 0.0 38.7 0.8 55.1 35.7 19.6 0.0 0.4 2.2 2.0 1.2 0.0 5.2 3.8 3.1 1.8 0.9 3.4 1.7 0.0 3.1 1.0 2.3 0.3 27.6 0.0

51.4 89.3 339.5 341.0 115.2 33.7 0.3 3.1 11.0 32.8 9.8 8.2 36.6 1.0 39.6 28.4 18.5 8.9 1.4 11.4 28.6 32.1 15.3 6.3 5.0 8.9 6.3 1.1 4.6 3.5 2.1 4.7 1.2 3.5 0.8 59.9 0.0

GC-5 (71–73) 75.8 107.6 259.9 203.8 71.4 33.2 0.4 2.6 8.0 24.7 16.3 8.4 37.9 1.3 41.4 25.9 14.2 6.2 0.7 5.1 10.3 5.6 0.0 4.7 4.4 7.8 5.8 0.9 5.4 3.0 2.0 4.1 1.3 3.2 0.5 56.1 0.0

GC-10 (8–9) 0.4 5.0 55.4 74.9 39.6 5.7 0.3 2.1 3.8 25.2 17.3 0 21.2 1.3 31.3 22.2 5.7 0 0 2.9 5.1 1.3 0 2.8 2.5 3.9 0 1.8 3.6 2 0 5.2 1.7 3.1 1.2 6.9 1.3

GC-10 (178–180) 0.3 5.6 75.5 95.3 59.7 6.4 0.1 0.0 4.0 29.4 20.1 0.0 23.5 1.3 42.7 28.8 6.8 0.0 0.0 3.1 6.0 1.5 0.0 3.3 3.2 5.5 0.0 1.7 3.9 2.0 0.0 6.1 1.9 3.6 1.4 7.7 1.4

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PAH (ng/g dry) Naphthalene C1-naphthalenes C2-naphthalenes C3-naphthalenes C4-naphthalenes Biphenyl Acenaphthylene Acenaphthene Fluorene C1-fluorenes C2-fluorenes C3-fluorenes Phenanthrene Anthracene C1-phenanthrenes/anthracenes C2-phenanthrenes/anthracenes C3-phenanthrenes/anthracenes C4-phenanthrenes/anthracenes 2,3-benzofluorene Dibenzothiophenea C1-dibenzothiophenesa C2-dibenzothiophenesa C3-dibenzothiophenesa Fluoranthene Pyrene C1-fluoranthenes/pyrenes C2-fluoranthenes/pyrenes Benz(a)anthracene Chrysene/triphenylene C1-chrysenes/triphenylenes C2-chrysenes/triphenylenes Benzo(k)fluoranthene Benzo(b)fluoranthene Benzo(e)pyrene Benzo(a)pyrene Perylene Indeno(1,2,3-cd)pyrene

GC-2 (7–10)

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Table 2 (continued) GC-2 (7–10)

GC-2 (131–133)

GC-2 (288–290)

GC-5 (7–8)

GC-5 (38–40)

GC-5 (71–73)

GC-10 (8–9)

GC-10 (178–180)

Benzo(ghi)perylene C1-C20H12 aromatics Simonellite Retene Total PAH(t-PAH) (ng/g)b Parent PAH(p-PAH) (ng/g)c %Naphthalenes in t-PAHd Naphthalene/methylnaphthalenes %Fluorenes in t-PAHd %Phenanthrenes/Anthracenes in t-PAHd Phenanthrene/methylphenanthrenes Fluoranthene/pyrene Fluoranthene+Pyrene/ Methylfluoranthenes+Methylpyrenes Benzo(e)pyrene/Benzo(a)pyrene %Perylene in t-PAH

1.5 0.0 Yes Yes 923 (84) 301 (27) 56 2.19 10 16 0.93 1.24

1.9 0.0 Yes Yes 1119 (104) 329 (30) 62 1.08 7 11 0.83 1.15

2.7 1.9 ndf Yes 1400 (126) 329 (30) 70 0.61 4 8 0.68 1.01

0.4 2.5 Yes Yes 615 (123) 117 (23) 56 0.32 7 24 0.70 1.37

1.5 3.9 Yes Yes 1370 (201) 238 (35) 68 0.58 5 10 0.93 1.26

0.8 0.0 ? Yes 1061(151) 246(35) 68 0.70 5 12 0.91 1.05

1.7 2 nd Yes 360 (20) 70 (4) 49 0.08 13 23 0.68 1.12

2.3 2.1 nd Yes 456 (41) 75 (7) 52 0.05 12 23 0.55 1.04

1.59 2.80 7

1.63 2.50 7

1.61 2.90 7

2.92 7.06 4

1.26 4.36 4

1.16 6.15 5

1.36 2.58 2

1.18 2.56 2

79.45 13 2 84 2

91.70 10 2 87 2

102.48 7 1 89 3

32.14 12 1 86 1

67.11 7 1 89 2

92.31 7 1 91 1

12.30 20 10 56 14

14.67 22 10 53 16

Unsubs PAH:Pyr+Bap+Per+GHIPer(ng/g) %Pyr in unsubs PAH %Bap in unsubs PAH %Per in unsubs PAH %GHIPer in unsubs PAH a

Low and inconsistent recovery. See text. t-PAH comprises of the following PAHs and their homologs: naphthalene, biphenyl, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, 2,3-benzofluorene, dibenzothiophene, flouranthene, pyrene, benz(a)anthracene, chrysene, triphenylene, benzo(b&k) flouranthenes, benzo(e&a)pyrenes, perylene, indeno(1,2,3,4)pyrene, benzo(ghi)perylene, C1–C4 homologs of naphthalene and phenanthrene/anthracene, C1–C3 homologs of fluorene and dibenzothiophene, C1–C2 homologs of fluoranthene/pyrene and, chrysene/ triphenylene and, C1 homolog of C20H12 aromatics; mg/gram organic carbon in parentheses. c p-PAH is the sum of all the parent PAHs listed under t-PAH; mg/gram organic carbon in parentheses. d Sum of the parent and all the alkyl homologs listed under t-PAH. e Most dominant methyl homologs underscored. f Not detected. b

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Core (section in cm)

M.I. Venkatesan et al. / Applied Geochemistry 18 (2003) 845–861

to 33% of the total PAH. The parent PAHs occur in the following general order: Naphthalene perylene phenanthrene > biphenyl > fluorene > fluoranthene > pyrene 5 chrysene/triphenylene5benzo(k)fluoranthene > dibenzothiophene  benzo(e)pyrene > benzo (ghi) perylene  benzo(a)pyrene  benzo(b)fluoranthene  indenopyrene. Coronene was not detected in the samples. Parent PAH are rather uncommon in crude oils, but are found to be ubiquitous in high temperature pyrolysates. Further, the presence of pericondensed PAHs such as pyrene, benzopyrenes, perylene, benzoperylene and PAH analogs with 5-membered alicyclic ring (e.g., acenaphthene, fluorene, fluoranthene, benzofluoranthene, indenopyrene) suggests a pyrolytic origin (Blumer, 1976; Youngblood and Blumer, 1975). Naphthalenes are the most dominant and phenanthrenes the next most abundant suite of PAH in all the samples. These two suites of PAH constitute 71–80% of the t-PAH (Table 2). Alkylated homologs uniformly dominate in all the samples. The most abundant homolog of all the PAHs measured are the C2 or C3 naphthalenes. The parent naphthalene and its alkyl homologs comprise from 49 to 70% of the total PAH (Table 2). Relatively high proportions of these compounds have also been reported by Chernova et al. (2001). Prior studies on other hydrothermal regions did not specifically report the dominance of these compounds and it is not clear if they naturally occur at low levels in those areas or if it is the result of an analytical artifact (i.e., Brault and Simoneit, 1989; Kvenvolden and Simoneit, 1990). However, the authors believe that the naphthalenes are of hydrothermal origin. It is possible many other PAHs detected here have also been resynthesized from indigenous organic matter under thermal stress (Simoneit, 1984). The tricyclic PAH (i.e. phenanthrene) and the alkyl homologs of phenanthrene/anthracene form a significant proportion of the total PAH ranging from 8 to 24%. The abundance of the phenanthrene and the alkyl homologs follows the order C1 > C0 > C2 > C3 > C4 and the distribution resembles that in the North Sea crude oil in which parent compounds are slightly more enriched relative to the alkyl homologs (5C2) unlike many other crude oils such as Jiang Han and Bradford (Kawka and Simoneit, 1990). Dibenzothiophenes, fluoranthenes and pyrenes also follow a similar trend in the proportion of the homologs. However, this homolog profile is very different from those of the surficial sediments from the urban coastal regions which generally are enriched in parent PAHs derived from fossil fuel/natural combustion sources (LaFlamme and Hites, 1978; Venkatesan et al., 1987). The PAH homolog distribution in the samples suggests that not much biodegradation has occurred as biodegradation would preferentially have removed the parent and monosubstituted triaromatics (Rowland et al., 1986). The result is consistent with

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the aliphatic hydrocarbon composition discussed earlier. However, some removal of the lower molecular weight homologs by water washing and aqueous partitioning during contact with sea water cannot be ruled out (May, 1980). In summary, the tricyclic PAH profile is characteristic of a hydrothermal petroleum which has undergone probably a limited post-depositional alteration. The ratios of phenanthrene to methylphenanthrenes vary from 0.55 to 0.93 (Table 2). Assuming negligible loss from sea water solubility, these ratios which probably reflect thermal alteration can be compared to laboratory simulation studies of kerogen from Tanner basin (off the coast of Southern California) heated at varying temperatures. For example, phenanthrene/ methylphenanthrene ratio varied from 0.59 to 0.83 at 310  C when the kerogen was heated for 18–100 h and from 0.9 to 5.0 at 410  C for a heating period of 5–32 h (Ishiwatari and Fukushima, 1979). These temperatures are well above the normal petroleum generation window (Tissot and Welte, 1984). Based on these findings, both the samples from GC-10, GC-2 (288–290) and GC-5 (7– 8) fall in the temperature window 4310  C whereas the remaining samples probably fit within the window between 310 and 350  C. It appears that the thermal maturity of the Andaman samples are roughly comparable to the oils associated with some of the hydrothermal minerals from the Guaymas Basin (Brault and Simoneit, 1989). However, the ratios of flouranthene/pyrene, fluoranthene+pyrene/methylfluoranthenes+methylpyrenes, benzo(e)pyrene/benzo(a)pyrene are all generally within the same range as was found in the Mid-Atlantic Ridge minerals and/or hydrothermal petroleums from the Guaymas Basin and Escanaba Trough (Brault and Simoneit, 1989; Kvenvolden and Simoneit, 1990) suggesting a similar thermal stress in the Andaman samples. Although the Andaman samples exhibit ratios of flouranthene/pyrene and benzo(e)pyrene/benzo(a)pyrene within the window reported for crude oils (0.6–1.4 and 0.2–3.3 respectively; Neff, 1979; Gschwend and Hites, 1981), the presence of a characteristic suite of PAH analogs and homologs and their overall composition in conjunction with the aliphatic hydrocarbon distribution point to the hydrothermally derived petroleum in the Andaman samples. It is not clear from the limited available data if and what type of PAH detected in these sediments could also have been contributed by the episodic volcanic activities in the general region. The levels of perylene vary from 2 to 7% of the total PAH with the two GC-10 samples having the lowest proportion. Perylene is believed to have been formed diagenetically from biogenic organic matter and could be degraded at higher temperatures (Louda and Baker, 1984; Venkatesan, 1988b). It is likely that the perylene measured here represents the residual amount after post-depositional loss under thermal stress.

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4320 10–32 (16,30) 0.92 5.3

g

f

e

d

c

b

a

Based on dry sediment weight. P ( C10-C26) fatty acids. No fatty acids generally 5C23 were detected except in one sample. Therefore, no Low/High molecular weight acid ratio is reported. CPI is even/odd n-fatty acids in the carbon range from C10 to C26 (Tissot and Welte, 1984). P ( C10-C32) alcohols. Bimodal maxima in parentheses. Dominant homolog is underscored. P P ( C10-C23)/( C24-C32). CPI is even/odd n-alcohols in the carbon range from C10 to C32 (Tissot and Welte, 1984).

4761 9–32 (18,30) 3.31 1.78 3571 10–32 (18,30) 1.79 3.83 8262 10–32 (16,30) 5.52 2.7 n-Alcohols Total resolved n-alcohols ng/ga,d n-Alcohols carbon range (Cmax)e L/Hf CPIg

4343 10–32 (16,30) 1.51 3.59

5071 10–32 (16,30) 1.8 3.71

2919 10–32 (18,30) 2.09 3.31

4654 10–32 (18,30) 1.62 2.85

3526 9–22 (16) 4.82 2681 8–20 (16) 2.46 1176 9–20 (16) 3.90 1333 9–22 (16) 3.80 6005 8–22 (16) 1.98 n-Fatty acids Total resolved n-fatty acids ng/ga,b n-Fatty acids carbon range (Cmax) CPIc

3124 9–22 (16) 2.67

2850 8–26 (16) 2.03

2001 8–22 (16) 2.78

GC10 (8–9) GC5 (7–8) GC2 (288–290) GC2 (131–133) GC2 (7–10) Core (section in cm)

Table 3 Yield and molecular ratios of n-fatty acids and n-alcohols in sediments of the Andaman Basin

GC5 (38–40)

GC5 (71–73)

GC10 (178–180)

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In addition to the PAHs which were quantified as above, 1,1-biphenyl(3methyl), 1,1-biphenyl(4methyl), bibenzyl and methylated benzofurans were identified by GC/MS from comparison with the NBS library of spectra and they occur in significant amounts (Fig. 5) but were not quantified. These are most likely pyrolysates from organic matter. PAHs like coronene and heavier analogs/homologs which are indicative of hydrothermal fluids at temperatures 5350  C have been detected in other hydrothermal petroleums (Simoneit and Fetzer, 1996). This would suggest that the organic matter in those regions was subjected to high temperature alteration. However, coronene was not detected in any of the samples and the authors did not specifically isolate or search for PAHs heavier than coronene in the samples. 3.5. n-Alcohols and sterols Normal alcohols are found in the sediments generally from C number 10 to 32 (Table 3). The concentrations of individual n-alcohols range from not detected to 2018 ng/g. GC-2 (7–10) contains the maximum amount of alcohols. The abundance of total alcohols follows the order: GC-2 GC-10 > GC-5 and the low molecular weight alcohols predominate over the heavy ones in all but one sample. The bimodal distribution exhibits a maximum at C30 in 5 of the 8 samples where C16 or C18 is the secondary maximum. In 3 of the remaining samples the reverse trend is observed. The n-alcohol profile thus corroborates the presence of low molecular weight components of microbial/marine origin similar to that of n-alkanes. Even C predominance over odd C at the high molecular weight range is noted which again complements the n-alkane fingerprint, characteristic of land plants. Cholesterol and cholestanol are the only sterols detected and they occur in the range 100–300 and 40–80 ng/g respectively. This is much less than the levels normally encountered in marine sediments (Lee et al., 1979; Volkman, 1986; Venkatesan et al., 1987). These sterols must have originated from marine fauna (Lee et al., 1979). The low levels of sterols also explain the scarcity of steranes in the samples. However, C30 and C31 triterpenols are found in significant amounts, invariably more than cholesterol in all the samples. Triterpenols are believed to be derived from bacteria (Volkman et al., 1987; Venkatesan et al., 1990 among others). 3.6. n-Fatty acids Normal fatty acid esters are detected in the C range from 8 to 22 and in two samples it extends to C number 26 with the maximum at C16 (Table 3). The individual n-fatty acid content ranges from not detected to 2200 ng/g and the order of abundance of total acids in the

M.I. Venkatesan et al. / Applied Geochemistry 18 (2003) 845–861

samples is similar to alcohols, with GC-2 containing the maximum levels. Again the n-fatty acid distribution is consistent with that of n-alkanes at the low end and the even C dominance of heavy acids (detected in only one sample) complements that of high molecular weight n-alkanes typical of land plants. Low molecular weight alcohols and fatty acids could have originated from algae and bacteria whereas their high molecular weight homologs reflect inputs from terrestrial plants. Apparently, the immature organic matter in these sediments has been derived from the biogenic detritus and has undergone hydrothermal alteration even before the completion of normal early diagenetic changes. Normal alkanes, especially the even C homologs may have been generated from the reduction of n-fatty acid ester and alcohol precursors from autochthonous microbial/algal sources as well as from terrigenous plant wax components (Simoneit, 1978). The same fatty acid esters could have been the alkylating precursors for alkylcyclohexanes and alkylbenzenes as previously discussed.

4. Summary The hydrothermal petroleum in the Andaman Basin sediments are derived from thermal alteration of immature and recent organic matter. The sediments comprise mainly of marine organic matter mixed in with some terrestrial components. For example, the presence of significant levels of biogenic n-fatty acids, n-alcohols with even C number predominance and, hopanes and hopenes reflects the input of recent organic matter. Specifically, odd C dominated n-alkanes and even C dominated n-alcohols at the high molecular end represent land plant contribution. In contrast, the presence of i) gasoline range hydrocarbons, ii) a wide range of nalkanes with generally no odd C predominance, iii) high levels of pristane and phytane, iv) naphthenic hump of branched and cyclic alkanes characteristic of petroleums, v) significant concentrations of alkenes, vi) thermodynamically stable hopanes and vii) significant levels of parent and alkylated homologs of a suite of PAH document the hydrothermal origin of the hydrocarbons. Specifically, the PAH data suggest that the petroleum genesis is probably occurring in the region on a geologically instantaneous time scale (probably of the order of a few hundred years to millennia) at temperatures between 4310 and 4350  C and their thermal maturity is roughly comparable to those of the oils associated with hydrothermal minerals from the Mid-Atlantic Ridge and bitumen from the Guaymas Basin and Escanaba Trough. However, the triterpanes distribution would indicate a slightly lower thermal maturity for the present samples than those from the above regions. The distribution profiles and

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various parameters compiled indicate that the biodegradation of the bitumen is not significant because of the possible rapid removal of the products from the thermal zone. Samples from GC-2, from the deep basin (a graben between two fault systems), and GC-5, adjacent to fault, contain more hydrothermal petroleum than those from GC-10 from the topographic high. It is likely that hydrothermal fluids migrate from deeper strata to the recent sediments near surface and are discharged into the water column and/or retained by and accumulated in a reservoir under favorable conditions. However, it is premature to judge from the data gathered from a limited number of samples here as to how significant or widespread this hydrothermal generation/accumulation of hydrocarbons is in the region. Perhaps a similar molecular characterization of an extended number of core samples covering a wider area of this basin and the vicinity may be necessary to estimate the intensity of hydrothermal stress, in general, in the stratigraphic column and, correlate in particular, with paleotectonic/ volcanic activities. Since the extent of contribution from petroleum formed hydrothermally compared to the global crude oil reserves is unknown, the implications from studies similar to the above should also help assess if such areas could be considered as potential targets for unconventional oil exploration. Further, hydrothermal activity has been documented throughout the geological history of the earth and hydrothermally altered/synthesized organic matter could even be a significant source of C/nutrients for the chemosynthetic organisms in aquatic environments. An investigation of the chemistry of organic matter synthesis and alteration in such hydrothermal environments could, therefore, provide answers to some questions on the origin of life.

Acknowledgements We thank O. Merino for technical assistance, Dr. E. Desa, the Director (National Institute of Oceanography, Goa, India), for encouragement and Dr. Ch.M. Rao for his initiative to facilitate this work; and Drs. Bernie Simoneit and K. Kvenvolden and an anonymous reviewer for helpful comments and suggestions to improve this manuscript. Partial financial support to M.I.V. from NASA through the Astrobiology Institute, UCLA, is acknowledged.

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