Organic geochemistry of hydrothermal petroleum
generated in the submarine Wakamiko caldera,
southern Kyushu, Japan
Toshiro Yamanaka
a,*, Junichiro Ishibashi
b, Jun Hashimoto
c aVenture Business Laboratory, Kyushu University, Hakozaki, Fukuoka 812-8581, JapanbDepartment of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan cMarine Ecosystems Research Department, Japan Marine Science and Technology Center, Natsushima, Yokosuka 237-0061, Japan
Received 28 January 2000; accepted 17 August 2000 (returned to author for revision 27 March 2000)
Abstract
Four sediments impregnated with hydrothermal petroleum were dredged from the shallow sea¯oor (200 m) of the submarine Wakamiko caldera in northern Kagoshima Bay, southern Kyushu, Japan. Their organic geochemical parameters were studied. The hydrothermal petroleums were characterized by higher Pr/n-C17and Ph/n-C18ratios and
lower phenanthrene/methylphenanthrene ratios than hydrothermal oils known from comparable sea¯oor hydro-thermal systems around the world. The former characteristic is interpreted to re¯ect the large contribution of unaltered terrigenous organic matter because of the shallow water depth and land proximity of the caldera. The latter char-acteristic indicates high maturity rather than simple pyrolysis of the source organic matter. Furthermore, the input of components from thermally-unaltered sediment and mild biodegradation are evident in the oil compositions. Among the collected samples, dierences in hydrocarbon compositions between lithi®ed sediments and normal shallow sea muds were notable. The lithi®ed sediments had higher maturity levels for the sterane and triterpane distributions of the petroleums. Higher BeP/BaP [benzo(e)pyrene/benzene(a)pyrene] ratios suggest that petroleums in the lithi®ed sedi-ments are more altered, because the ratio re¯ects the extent of secondary oxidation and/or thermal loss of BaP once formed. These organic geochemical parameters suggest that the lithi®cation sediments are derived from a signi®cant depth below the sea¯oor and had erupted recently onto the sea¯oor. The frequent eruptions would enhance organic maturations and lithi®cation of sediment and transport of subsurface sediment to the sea¯oor.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Hydrothermal petroleum; Wakamiko caldera; Sakurajima volcano; Biomarkers; Pristane; Phytane; Polycyclic aromatic hydrocarbons; Phenanthrene to methylphenanthrene ratio; Eruption
1. Introduction
Petroleum generation associated with sea¯oor hydro-thermal systems was ®rst identi®ed at the Guaymas Basin, Gulf of California in 1978 (Simoneit et al., 1979). Since the ®rst discovery, hydrothermal petroleums have
been discovered at three other sea¯oor hydrothermal ®elds, Escanaba Trough on the southern Gorda Ridge (Kvenvolden et al., 1986), Middle Valley of the Juan de Fuca Ridge (Simoneit, 1994), and the Red Sea (Simoneit et al., 1987; Michaelis et al., 1990). These marine hydrothermal petroleums were described only from sea-¯oor depths greater than 1400 m, although petroleum seeps associated with continental hydrothermal systems have also been identi®ed. Prior to the discovery of leum generation in sea¯oor hydrothermal systems, petro-leum seeps associated with active continental hydrothermal
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systems in Yellowstone National Park were reported by Love and Good (1970), and thoroughly characterized by Clifton et al. (1990). Other examples of oil seeps from continental hydrothermal systems have been reported from the Waiotapu geothermal region in New Zealand (Czochanska et al., 1986), Lake Tanganyika in the East African Rift (Tiercelin et al., 1989; Simoneit et al., 2000) and the Uzon caldera in the East Kamchatka region (Bazhenova et al., 1998). Simoneit (1990) suggested that hydrothermal petroleums can be encountered in both continental and marine domains where they are asso-ciated with magmatically driven hydrothermal systems. However, no hydrothermal petroleums from sea¯oor depths between 0 and1000 m have been found to date. A currently forming hydrothermal petroleum has recently been documented from the active submarine Wakamiko caldera, northern Kagoshima Bay, southern Kyushu, Japan (Yamanaka et al., 1999). This petroleum is generated at the shallowest sea¯oor depth (200 m) of all known hydrothermal petroleums. The situation of petroleum formation in the caldera could be comparable to continental systems rather than marine systems because this area is in¯uenced by frequently active volcanism and the oil generating area is as small as in continental systems. The tectonic setting of the caldera, located on the volcanic belt of an island arc, is analogous to the case of the Uzon caldera and the Waiotapu geothermal area.
The purpose of this paper is the further characterization of hydrothermal petroleum from the Wakamiko caldera, located at a coastal and shallow water locale, with an emphasis on gas chromatography (GC) and GC/mass-spectrometry (MS) techniques. We also compare the Wakamiko oil with several well-known hydrothermal oils. Unfortunately, the four samples analyzed in this study are not enough for evaluating the process of hydrothermal petroleum formation in this area.
2. Geological setting and previous studies
The submarine Wakamiko caldera is located on the northeastern foot of the Sakurajima volcano, one of the most active volcanoes in Japan (Fig. 1). The volcano is located in the northern part of the Kagoshima Graben, which has a tectonic origin from extentional stress (Tsuyuki, 1969). The caldera ¯oor is approximately 200 m in depth and is covered by thick layers of recent sediments (up to80 m thickness). Numerous instances of high temperature gas bubbling (>200C) from the
sea¯oor, related to the activity of the Sakurajima vol-cano, have been reported (Ossaka et al., 1977). The gas is composed mainly of carbon dioxide (7792 vol.%), methane (520 vol.%), nitrogen (27 vol.%) and a small amount of hydrogen sul®de (Ossaka et al., 1992). The main magma chamber of the Sakurajima volcano is located just beneath the Wakamiko caldera (Takahashi,
1997), and the gas is expected to be derived from this magma chamber.
Strati®ed bottom water in the caldera develops during the summer, and the pH and dissolved oxygen level of the water drops (pH=6.5, DO<0.5 ml/l) due to the input of acidic and anoxic gas and hydrothermal ¯uids (Kamata et al., 1978). The muddy bottom sediments are composed mainly of pyroclastics and a small amount of plant remains. The C27 to C29 sterane composition in
the caldera sediment indicates that sedimentary organic matter is composed of material of both terrigenous and marine origins (Yamanaka et al., 1999).
This area also exhibits Kuroko-type mineralization, mainly gypsum, carbonate, native sulfur, barite and kaolinite, and a minor amount of pyrrhotite, stibnite, realgar, orpiment and As±S amorphous particles, which are mainly occurred in lithi®ed sediment observed on the caldera ¯oor (Nedachi et al., 1991). Base metals, mercury, arsenic, and antimony are also concentrated in the sediment around the caldera (Sakamoto, 1985; Sakamoto et al., 1997).
3. Sample description and methods
Samples were dredged from a depth of 200 m on the caldera ¯oor (Fig. 1, area A) by the ROV Dolphin 3K of the Japan Marine Science and Technology Center (JAMSTEC). The samples analyzed include two lithi®ed sediments and two common seabed muds, which had a strong petroleum odor (Table 1). The lithi®ed sediments are often observed studded around the hydrothermal seepages and fumaroles on the caldera ¯oor. The mud samples were collected with a rake or push core sampler and the hard sediments were grabbed with the manip-ulator of the Dolphin 3K. These samples were stored in plastic bags and frozen immediately after sample recovery pending analysis.
The organic components for chemical analysis were extracted by the following methods. The sediment samples were freeze-dried prior to extraction. The bitumen was extracted from dried samples with benzene:methanol (3:1 by volume) by ultra sonication. After extraction, the bitumen was separated into three fractions, aliphatics (F1), aromatics (F2), and asphaltic (NSO) compounds (F3) by silica-gel column chromatography, following the procedure of Simoneit and Lonsdale (1982). The ali-phatic and aromatic fractions were analyzed using a gas chromatograph/mass spectrometer (GC/MS) (Shimadzu model QP-5000 or model QP-2000) with capillary col-umn (Neutrabond-5, 30 m length, i.d. 0.25 mm, ®lm thickness 0.25 mm) and helium as the carrier gas. The GC oven temperature was programmed from 50 to 300C at 8C/min, with a ®nal isothermal hold for 30
min at 300C. Normal alkanes and PAHs are identi®ed
authentic standards (Supelco Inc). Concentrations were determined using three compounds (C12 fatty acid
methyl ester and 5a-cholestane for F1 and duteride naphthalene for F2) as internal standards added to sample solutions (either to total extracts or to each fraction).
4. Results and interpretation
The results of the extractions, chromatographic separations, and GC and GC/MS analyses of the acyclic hydrocarbons are listed in Table 2. The mud samples (D341R and D411C) are relatively enriched in aliphatic
Fig. 1. Submarine topography and sampling locations in the Wakamiko caldera, northern Kogoshima Bay, Japan; (A) sampling area on the caldera ¯oor; (B) another active bubbling area on a knoll.
Table 1
Sample locations and types collected from the sea¯oor of the Wakamiko caldera
Sample Date Latitude Longitude Depth (m) Methods Sediment types
D341R 2 September 1997 3139.5240N 13046.4360E 208 Rake Mud
D374H 16 June 1998 3139.5380N 13046.4320E 203 Grab Lithi®ed sediment
D411C 27 April 1999 3139.5290N 13046.4350E 201 Core Mud
and aromatic hydrocarbons, and the lithi®ed sediment samples are enriched in asphaltic (NSO) compounds. The normalized percentage compositions of these three fractions of each sample are plotted in a ternary dia-gram (Fig. 2). In the same ®gure, the gross compositions of other hydrothermal petroleum-bearing samples from Guaymas Basin and Escanaba Trough are also plotted (Kvenvolden and Simoneit, 1990). The hachured area in the ®gure represent typical conventional petroleum compositions (Tissot and Welte, 1984). The samples in this study are scattered in the ternary plot, as is the case for the Guaymas and Escanaba hydrothermal petroleums. The wide variation in gross chemical composition of hydrothermal petroleums is mainly due to the great variability inherent in the hydrothermal processes and post-generation eects (Simoneit, 1985). Lower abun-dances of aliphatic and aromatic hydrocarbons in the lithi®ed sediments are attributed to removal of these compounds by a combination of post-generation pro-cesses such as selective solubilization due to enhanced solubilities in hot water, and possibly biodegradation. A similar depletion was reported for samples from deep sections of DSDP holes in Guaymas Basin (Kawka and Simoneit, 1994).
4.1. Normal alkanes and isoprenoids
TIC chromatograms of the aliphatic fractions are shown in Fig. 3. Most of the aliphatic hydrocarbons, except for sample D411C, are present as an unresolved complex mixture (UCM) with only minor amounts of resolved individual components. In samples D341R and D411C n-alkanes were clearly resolved, as labeled in
Figs. 3 and 4. All the samples containedn-alkanes ran-ging from C13to C3336with a maximum at C19. Sample
D411C had another maximum at C27. Carbon number
preferences (CPI; Simoneit, 1978) ranged from 0.97 (D341R) to 1.24 (D374H). The CPI values close to 1 indicate complete maturation (Simoneit, 1978) and the
n-alkane distribution extended >C24indicate contribution
of terrigenous organic input (Tissot and Welte, 1984). Pristane (Pr) and phytane (Ph) were major compo-nents and the pristane/phytane ratio was greater than 1 in all samples; this ratio showed slightly ¯uctuating values ranging from 1.11 to 1.43. These ratios are com-parable to those of known marine hydrothermal petro-leums (e.g. Kvenvolden and Simoneit, 1990; Simoneit, 1994). Pr/Ph ratios of those petroleums were interpreted to indicate maturation of organic matter rather than a separate source. In the case of our samples, the Pr/Ph ratios are the likely result of processes rather than source, because organic source signatures are expected to be uniform in such a small caldera (about 6 km in diameter). Pr to normal C17-alkane (Pr/n-C17) ratios are
quite variable ranging from 1.22 (D374H) to 12.31 (D411H). Didyk et al. (1978) suggested that high Pr/n-C17
ratio (>1) of a petroleum was evidence that terrigenous plants played a major role in the origin of the petroleum. The values of >1 for our samples are consistent with this expectation about the source. Ph to normal C18
-alkane (Ph/n-C18) ratios are also variable, ranging from
0.95 (D411C) to 4.86 (D411H). The high values for our samples are again comparable with the Waiotapu oil and indicate a high contribution of terrigenous organic matter. These ratios are also consistent with hydrothermal oils that have undergone minor to moderate biodegradation.
Table 2
Analytical results of bulk and molecular parameters for samples in the Wakamiko caldera
Total bitumen extract Alipahatic hydrocarbons Aromatic/naphnethnic Asphaltic (NSO)
Sample Total yield (F1) hydrocarbons (F2) compounds (F3)
(mg/g dry weight) (mg/g) (% of three fractions)
(mg/g) (% of three fractions)
(mg/g) (% of three fractions)
D341R 5.22 1.08 20.7 3.02 57.8 1.12 21.4
D411C 47.94 8.84 19.4 15.65 34.3 21.09 46.3
D374H 35.18 0.02 0.1 2.97 10.8 24.47 89.1
D411H 13.14 0.11 0.9 2.99 26.9 8.03 72.2
Sample n-Alkanes Isoprenoids
Range Cmax CPI(C14ÿ32)a Pristane/phytane Pr/n-C17 Ph/n-C18
D341R C13ÿC33 C19(monomodal) 0.97 1.23 8.14 3.05
D411C C13ÿC36 C19,C27(bimodal) 1.08 1.13 1.22 0.95
D374H C13ÿC35 C19(monomodal) 1.24 1.11 1.94 2.23
D411H C13ÿC34 C19(monomodal) 1.09 1.43 12.31 4.86
a CPI (C
4.2. Cyclo-alkane biomarkers
The dominant terpanes in all the samples were hopanes [17a(H),21b(H)-hopane series] and moretanes [17b(H),21a(H)-hopane series] (Fig. 5). Theab-hopanes ranged from C27 to C33(C28 absent) as a homologous
series with a maximum at C30. Theba-moretanes were
also present as a homologous series beginning at C29
and extending to C33. In addition to these compounds,
all the samples contained hopenes, which are immature biomarkers.
The ratios amongab-hopanes andba-moretanes are shown in Table 3. Careful comparison between the ratios shows a slight dierence between the mud sam-ples and the lithi®ed sediment samsam-ples. The C31
hopa-nes(22R+22S)/C30hopane ratios for the mud sediment
samples are 0.62 and 0.52, whereas those for the lithi®ed sediment samples, are 0.38 and 0.46. The C33hopanes are
absent in the lithi®ed sediments and the smaller ratios of C31and C32hopanes to C30hopane suggest a relatively
higher maturity of the lithi®ed samples than the mud samples because of their stability under thermal stress. The mud samples show slightly higher ratios in para-meters other than the lithi®ed sediment ratios in Table 3.
The presence of C31and C32ab-hopanes that occur as
a mixture of two epimers, 22Sand 22R, provides rela-tive maturity levels of the associated oils. With increas-ing maturity the 22Rcon®guration, which is a biological precursor, is converted to a mixture of 22S and 22R
epimers, and the ratio increases from zero to an equili-brium ratio [22S/(22S+22R)] of about 0.61 (Ensminger et al., 1974). The epimer ratios of our samples are close to the equilibrium ratio with the exception of C32hopanes
(ratio=0.48) for sample D411H, although this low value may be a re¯ection of error introduced as a result of calculating this epimer ratio with components at very low levels. The ratios of 17a(H)-22,29,30-trisnorhopane (Tm) to 18a(H)-22,29,30-trisnorneohopane (Ts), which
re¯ect maturity and/or source parameters for hydro-carbons (Seifert and Moldowan, 1978), are variable, ranging from 4.68 to 7.5.
The predominant sterane in all the samples is 5a(H),14a(H),17a(H)-cholestane (20R) (Fig. 6). Ratios of 27R/29R[aaa-cholestane (20R)/a-24-ethyl-cholestane (20R)] are variable in the four samples, ranging from 0.53 to 1.48 (Table 3). Steranes are present as complex mixtures with signi®cant peak overlap, making the identi®cation of individual compounds dicult. The
ratios [20S/(20S+20R)] for C29 steranes are the only
epimer ratios that could be measured with con®dence (Table 3). The equilibrium ratio of C29 steranes 20S/
(20S+20R) is about 0.54 (Mackenzie et al., 1980). They are variable in all four samples ranging from 0.29 to
0.45. Diasteranes (C27) are also present with the
13b(H),17a(H)-diacholestane 20S and 20R compounds
most obvious (Fig. 6). The epimer ratio of these isomers are 0.5 and 0.56 in the mud samples and 0.61 and 0.59 in the lithi®ed samples.
Low molecular weight steranes, i.e. pregnane (C21
-sterane) and methylpregnane (C22-sterane), typically
appear in highly mature condensates (Suzuki et al., 1987). In laboratory studies, these steranes have been generated by heating at 300C for extended periods
(Wingert and Pomerantz, 1986). Therefore, a high ratio of 5a-steranes (C21+C22)/(C21+C22+C27+C28+C29)
[C21ÿ29sterane L/(L+H)] indicates extended maturation
and thermal cracking of steranes (Suzuki et al., 1987). In the lithi®ed sediment samples the C21 and C22steranes
are signi®cantly more abundant relative to 5C27
-ster-anes, and the L/(L+H) ratios of 0.68 (D374H and D411H) are much higher than the values in the mud samples (0.27 and 0.14) in Table 3. This relationship is clearly obvious in Fig. 6a±d and supports the higher maturity and/or alteration at higher temperature of the lithi®ed sediments compared to the muds.
4.3. Aromatic hydrocarbons
The aromatic hydrocarbons are present as complex mixtures also with a signi®cant UCM (Fig. 7). Unsub-stituted polycyclic aromatic hydrocarbons (PAH) are distinguished from their alkyl-substituted analogs except sample D341R (Figs. 7 and 8). Phenanthrene, ¯uoranthene, pyrene, chrysene, benzopyrenes, benzo-¯uoranthenes, and benzoperylene are common to all the samples. Coronene is also identi®ed in three of the four samples, the exception being D341R. In sample D341R pyrene is a single major PAH and other PAHs are relatively minor. The compounds that were resolvable are listed in Table 4 and their concentrations were determined by GC/MS analyses. The concentrations of the PAHs are similar in range as for the Guaymas oils with the exception of pyrene which is much higher (Kawka and
Simoneit, 1990). Although the concentration of anthra-cene is signi®cantly lower than that of phenanthrene in the Guaymas oils, the concentrations of anthracene in the Wakamiko oils are similar with those of phenan-threne. The mechanism of higher occurrence of anthra-cene is not well understood yet.
Various ratios of PAHs are given in Table 3. The phenanthrene to methylphenanthrene (P/MP) ratio is higher for the muds than for the lithi®ed samples. All these ratios, ranging from 0.12 to 0.31, are slightly lower than those reported for Guaymas oils (Kvenvolden and Simoneit, 1990) (Table 3). Miocene mudstone samples
obtained from northeastern Japan showed decreasing P/ MP ratios with increasing maturation of organic matter and the ratio reached a minimum value of around 0.3 at the highest maturation stage, indicated by the vitrinite re¯ectance (Rm> 1.0%) (Sampei et al., 1994). Similar
ratios for the Wakamiko oils support a high maturity of the bitumens.
The methylphenanthrene indices (MPI1 and 2: Radke and Welte, 1983: MPI3, Angelin et al., 1983) of the mud samples ranged from 0.71 to 0.89, and were distin-guishable from those of the lithi®ed sediments, which ranged from 0.43 to 0.67. This dierence could be
interpreted to indicate that the oils in the lithi®ed sedi-ments are immature at the onset of the oil window and the oils in the muds are more mature. This seems to be in contrast with the evaluation based on the sterane signatures. However, this interpretation must be quali-®ed as being not necessarily comparable with normal geological conditions, due to the unknown eects of water solubilization processes and alterations of organic compounds at high temperature (Simoneit, 1984).
Pyrene (Py) is more concentrated than ¯uoranthene (Fa) for the samples. Fa/Py ratios ranged from 0.18 to 0.32 for three of the four samples, which is at the lower limit reported for conventional crude oils (Ne, 1979) with the exception of D341R (Fa/Py=0.09). The ratios of benzo(e)pyrene to benzo(a)pyrene (BeP/BaP) were 1.23 to 1.14 for the mud samples and 1.44 and 2.11 for the lithi®ed sediments. This ratio has been used as an indicator for the extent of secondary oxidation of the PAH once formed, because benzo(a)pyrene is less stable than benzo(e)pyrene (Lane, 1989; Nielsen et al., 1984). The slightly higher BeP/BaP ratios in the lithi®ed sedi-ments implys that they have had an oxidative or thermal loss of BaP (Boni et al., 1994). The oils in the lithi®ed
sediments would be aected at higher temperatures than those in the mud samples.
All samples except D341R contained Diels' hydrocarbon (30-methyl-1,2-cyclopenteno-phenanthrene) (Simoneit et
al., 1992) as a major compound (Table 4 and Figs. 7 and 8). This hydrocarbon occurs in hydrothermal petroleum derived from marine, algal-rich organic matter in Guaymas Basin, however, not in those from Escanaba Trough and Middle Valley in Juan de Fuca Ridge, where terrigenous organic matter is redominant (Kven-volden and Simoneit, 1990; Simoneit, 1994). It suggests that autochthonous input is also an important organic matter source of the Wakamiko oils.
Perylene was identi®ed as a major peak in the aro-matic fraction only in mud sample D411C. Perylene is considered to be generated during diagenesis (Louda and Baker, 1984; Venkatesan, 1988) and its alkylation or degradation at elevated temperatures has been sug-gested by previous studies (Louda and Baker, 1984; Kawka and Simoneit, 1990). These results suggest that a fraction of the organic matter contained in these sediments has not experienced heating at catagenetic temperatures and likely re¯ects an input of thermally unaltered
Table 3
Degrees of maturation represented by indices of biomarkers and PAHs in the hydrothermal petroleums from the Wakamiko caldera and two other hydrothermal sites
Mud sample Lithi®ed sediment Guaymas Basina Escanaba Trougha
D341R D411C D374H D411H
Triterpane indices
C32hopane 22S/(22S+22R) 0.53 0.56 0.51 0.48 0.57 and 0.57 0.48 and 0.42 C31hopane 22S/22S+22R) 0.59 0.57 0.59 0.58 0.56 and 0.53 0.48 and 0.46
TM/TS 6.25 7.50 7.14 4.68 11 and 6.4 22 and 27
C30bamoretane/C30abhopane [17b(H),21a(H)/17a(H), 21b(H)]
0.21 0.25 0.20 0.20
C31hopane (22R+22S)/C30abhopane 0.62 0.52 0.38 0.46 C32hopane (22R+22S)/C30abhopane 0.33 0.31 0.20 0.30
Sterane indices
27R/29R 1.48 0.53 1.05 1.13
C27diasterane 20S/(20S+20R) 0.50 0.56 0.61 0.59 0.57 and 0.62 0.52 and 0.53 C29sterane 20S/(20S+20R) 0.43 0.29 0.36 0.45 0.12 and 0.28 0.20 and 0.13
C21-29sterane L/(L+H)b 0.27 0.14 0.68 0.68
PAH indices
Phenanthrene/methylphenanthrenes 0.25 0.31 0.22 0.12 0.36 and 0.57 2.69 and 3.33 Methylphenanthrene index
MPI1c 0.75 0.75 0.49 0.55 0.76 and 1 0.42 and 0.39
MPI2c 0.71 0.89 0.48 0.67 0.81 and 1.25 0.52 and 0.41
MP13d 0.72 0.77 0.43 0.43
Fluoranthene/pyrene 0.09 0.39 0.18 0.20 0.2 and 0.23 0.65 and 0.61
Benzo(e)pyrene/benzo(a)pyrene 1.23 1.14 1.44 2.11 4.17 and 1.28 1.49 and 5.56
a Data from Kvenvolden and Simoneit (1990). b 5a-sterane (C
21+C22)/C21+C22+C27+C28+C29) ratio. c Ratios, as de®ned Radke and Welte (1983).
Fig. 6. Mass fragmentograms of biomarker steranes in aliphatic fraction (F1) from bitumen in the samples. Numbers refer to the carbon skeleton: (a±d)m/z=217: key ion for steranes; (e±h)m/z=215: key ion for sterenes.a=5a(H), 14a(H), 17a(H)-steranes; D=diasteranes;RandSare epimer con®gurations at C-20.
T.
Yamanak
a
et
al.
/
Organic
Geochemistry
31
(2000)
organic matter to the hydrothermally generated petro-leum.
5. Discussion
Characteristics of the Wakamiko hydrothermal oil. The compositions of the marine and terrestrial hydrothermal petroleums are similar, but not identical (Simoneit, 1993). It is dicult to identify systematic dierences in the com-positions of marine and terrestrial petroleums. Speci®c molecular ratios and biomarker compositions have been used to classify oils accordingly to organic matter source and thermal maturity. The Waiotapu terrestrial oil is characterized by a high ratio of phytane/n-C18 alkane
(2.37) and a low ratio of pristane/phytane (0.64) (Czo-chanska et al., 1986), while the ratios of the marine
hydrothermal petroleums are51 and51, respectively, with a few exceptions. This high ratio of phytane/n-C18
is due to the high contribution of terrigenous organic matter, and the low ratio of pristane/phytane is due to low maturity of the organic matter (Czochanska et al., 1986). The Uzon terriginous oil is also characterized by a low ratio of pristane/phytane (40.52; Bazhenova et al., 1998). Another characteristic of the Uzon oil is signi®cantly lower concentrations of polynuclear aromatic hydro-carbons (PAH) than those of other known hydrothermal petroleums. The low concentration of PAHs indicate that the oil was generated at lower temperatures. Organic geochemical parameters of sterane and tri-terpane biomarkers in both oils from Waiotapu and Uzon indicate exceptional low maturity of the oil sources. Gross chemical composition and biomarker matura-tion parameters of the Wakamiko oils are similar to
those from other sea¯oor hydrothermal systems, such as Guaymas Basin and Escanaba Trough (Fig. 2 and Table 3). We noted the dierences in higher Pr/n-C17and Ph/
n-C18 ratios and lower P/MP ratios in the Wakamiko
oils. The range of Ph/n-C18in these oils is comparable to
that of the terrestrial oils from the Waiotapu geothermal area of New Zealand and the Uzon caldera of east Kamchatka, where the source organic matter is almost entirely plant remains, suggesting a high contribution of terrigenous input as source organic matter for the Wakamiko oil. The ratio is also consistent with a mildly biodegraded oil.
Organic geochemical parameters such as high Ph/n -C18ratios and mature biomarker indices indicate that the
oils of the Wakamiko caldera have both characteristics of marine and terrigenous hydrothermal petroleums. The location of the caldera contributes to the accumulation of terrigenous organic matter. Furthermore, the in situ conditions on the sea¯oor (i.e. high water/rock ratio) play an important role in maturation of organic matter by hydrothermal eects, and post-depositional altera-tions such as water-washing and biodegradation. The presence of Diels' hydrocarbon supports that marine algal input is also an important organic matter source of the Wakamiko oils.
A low P/MP ratio of an oil has been considered to indicate high maturity as evidenced by that observation for Miocene mudstone samples (Sampei et al., 1994). Laboratory maturation studies of immature kerogen, in contrast, indicated that the P/MP ratio increases with thermal stress (Ishiwatari and Fukushima, 1979). How-ever, this laboratory study was carried out only under
dry conditions. It is not considered to re¯ect the exact conditions of hydrous pyrolysis such as would accom-pany hydrothermal petroleum generation. The low P/ MP ratio of the Wakamiko oils suggests high maturity rather than simple pyrolysis. This high maturity of the organic matter would be caused by the high thermal stresses associated with the frequent eruptions, characteristic of this submarine caldera. The Guaymas and Escanaba oils are generated in the thick sediments (5300 m thickness) covering the rift zones of the spreading axes. It is expected that a substantial portion of the sediments covering the Wakamiko caldera exhibit extensive thermal alteration because a relatively thin sediment (approx. 80 m thick) covering the caldera, which is the thermal source.
This suggests that there are two dierent sources for the extractable organic matter in these samples. One source is the hydrothermally generated petroleum exhibiting the characteristics of a mature oil, such as a smooth n -alkane distribution, mature biomarkers and emplaced with recently deposited sedimentary material. The other source of the extractable organic matter appears to be the fresh detritus which has not undergone any thermal maturation and is the source of the immature hopene biomarkers and diagenetically-derived perylene.
5.1. History of the lithi®ed sediments located on the sea¯oor
Organic geochemical signatures, especially for the distributions of steranes and hopanes, led to the con-clusion that the oils in the lithi®ed sediments are more
Table 4
Concentrations of polynuclear aromatic hydrocarbons in the hydrothermal petroleums from the Wakamiko caldera
ng/mg-bitumen
Compounds MW D341R D374H D411C D411H
Methylnaphthalenes 142 1269 24 4 18
Dimethylnaphthalenes 156 1461 n.d.a n.d. n.d.
Fluorene 166 73 n.d. n.d. n.d.
Phenanthrene 178 3935 38 131 42
Methylphenanthrenes 192 15,727 313 340 579
Anthracene 178 3013 30 16 43
Fluoranthene 202 676 441 76 88
Pyrene 202 7787 2491 236 434
Benzanthracene 228 1032 1093 606 1029
Chrysene(+triphenylene) 228 1290 1390 606 1387
Diels' hydrocarbon 232 n.d. 2156 173 1258
Benzo(b,k)¯uoranthenes 252 318 272 441 1019
Benzo(e)pryene 252 485 244 385 1235
Benzo(a)pyrene 252 395 169 338 585
Perylene 252 440 212 621 1053
Indeno(c,d)pyrene 276 37 16 14 101
Benzo(g,h,i)perylene 276 109 46 297 254
mature than those in the muds. The most signi®cant dierence between the mud and lithi®ed sediment oils is the sterane distribution ranging from C21 to C29. The
remarkably high C21ÿ29sterane L/(L+H) ratios of the
lithi®ed samples suggest that the hydrocarbons were formed at the catagenetic temperatures (> 300C). The
dierences between the oil compositions in the lithi®ed and mud sediments could also re¯ect dierences in the antecedents of the sediments.
Formation of carbonate nodules, which contained a signi®cant amount of hydrothermal petoleum, are reported from Middle Valley, Juan de Fuca Ridge by Boni et al. (1994). Those nodules grew in the subsurface by precipitation of carbonate and entrapped hydro-thermal petroleum with compositions and maturity parameters essentially the same for each nodule and its surrounding sediment. Although similar carbonate nodules have not been found in the Wakamiko caldera, some hydrothermal mineral precipitates including carbonate (Nedachi et al., 1991) imply that the lithi®ed sediments are formed in the subsurface by precipitation of hydro-thermal minerals and alteration of in situ sediment.
The gross compositions of the oils indicate that aliphatic and aromatic hydrocarbons have been removed from the lithi®ed sediments. A similar loss of hydrocarbons was observed in deep subsurface sediments of the Guaymas Basin (Kawka and Simoneit, 1994). This loss may be caused by hydrothermal ¯uid circulation. High BeP/BaP ratios of the lithi®ed samples also suggest they have had an oxidative or thermal loss of BaP.
Although the number of samples was limited, these could be evidence that the lithi®ed sediments were located signi®cantly below the sea¯oor and had erupted to the sea¯oor. The observed distributions of lithi®ed sediments, studded around hydrothermal seepages and fumaroles, are in accordance with this hypothesis. Large-scale hydrothermal plumes would follow an eruption. In fact, temporary anomalies in surface seawater temperatures were observed in this caldera area by remote sensing on an airplane in September 1974 (Ozawa et al., 1976). The Sakurajima volcano continues to erupt frequently. The lithi®ed sediments located on the sea¯oor are evidence for intermittent eruptions at the caldera ¯oor. Further study, as for example sampling the deeper subsurface and inorganic chemistry of sediments and hydrothermal ¯uids, may provide detailed information about the mechanism of lithi®cation and processes of hydro-thermal petroleum generation in this area.
6. Conclusion
Hydrothermal maturation of sedimentary organic matter is observed on the sea¯oor of the Wakamiko caldera. The maturation levels indicated by the sterane and triterpane distributions of the hydrothermal oils are
similar to other known marine hydrothermal oils which are more mature than terrestrial oils of Uzon caldera and Waiotapu geothermal region. Nevertheless, the amounts of pristane and phytane in these oils are noticeably high and exceed the amounts ofn-C17andn
-C18 alkanes with one exception. This distribution is
comparable with terrestrial hydrothermal oil from New Zealand and suggests a high contribution of land plant detritus. This characteristic is due to the shallow water depth and land proximity of the caldera. The presence of Diels' hydrocarbon indicates that the Wakamiko oils are derived from not only land plant detritus but also marine algal input.
Furthermore, the oils of the caldera are characterized by noticeably low phenanthrene/methylphenanthrene ratios. The PAH distributions provide some additional information, suggesting high maturity rather than simple pyrolysis of organic compounds. The presence of immature hopene biomarkers and perylene, a PAH of primarily a diagenetic origin, indicates a portion of the organic matter is derived from a thermally-unaltered sediment. The low abundance of n-alkanes in some of the samples and the large UCM in the chromatograms of the aliphatic fractions are consistent with mild bio-degradation.
A comparison between the mud and the lithi®ed sediments suggests that the maturation is slightly higher for the lithi®ed samples than for the muds. Aliphatic and aromatic hydrocarbons concentrated in the muds, suggest migration of these compounds from the deeper part of the sediment blanket. In contrast, the lithi®ed sediments have lower amounts of aliphatic and aromatic hydrocarbons. High BeP/BaP ratios of the lithi®ed samples indicate oxidative and thermal loss of BaP. These characteristics imply that the lithi®ed sediments are derived from below the sea¯oor and were transported to the sea¯oor by eruption. The frequent eruptions of the Sakurajima volcano would enhance the maturation of sedimentary organic matter in the caldera and lithify the sediments in the subsurface.
Acknowledgements
Associate EditorÐB. Simoneit
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