海底熱水系周辺堆積物の有機成分に関する地球化学 的研究

九州大学学術情報リポジトリ

Kyushu University Institutional Repository

海底熱水系周辺堆積物の有機成分に関する地球化学 的研究

山中, 寿朗

九州大学理学研究科地球惑星科学専攻

https://doi.org/10.11501/3150700

出版情報：Kyushu University, 1998, 博士（理学）, 課程博士 バージョン：

権利関係：

Geochemical studies of organic components in sediments around seafloor hydrothermal systems

by

Toshiro YAMANAKA

Abstract

Sedimentary fatty acids associated with hydrothermal activity were studied to obtaine clues of community structure and biomass. Composition of sedimentary fatty acids are varied according to community structure and sedimentary environment. Concentration of total fatty acids in sediment related with in situ biomass and sometime with input of organic matter.

In this study, surface sediments were obtained from four hydrothermal sites, Manus Basin, Okinawa Trough, Kagoshima Bay and Izu-Ogasawara Ridge by RSV Shinkai 2000 _and seven sediment cores were obtained from Sagami Bay that expect to show conventional fatty acid compositions of marine sediment. Below discussion of fatty acid compositions of the hydrothermal sites was based on comparison with results of the investigation of Sagami Bay.

The fatty acid compositions in the sediments show high proportions of monounsaturated and methyl-branched acids with an exception, Okinawa Trough.

Monounsaturated and methyl-branched acids are typical biomarkers for aerobic and anaerobic bacteria, respectively, and the high proportions indicate large biomass of bacteria in sediment surface. Furthermore, concentrations of total sedimentary fatty acids obtained from Manus Basin and Kagoshima Bay are one order higher than that of normal coastal basin. The fatty acid compositions and concentrations are one evidence for the argument that seafloor hydrothermal areas develop large biomass of vent communities which are supported by chemosynthetic bacterial production. On the other hand, the sediments obtained from Okinawa Trough were enriched in normal (i.e. saturated straight chain) fatty acids and long chain acids. The long chain acids are typical terrigenous origin. Okinawa Trough is characterized by high sedimentary rate. The sedimentary fatty acid compositions of the hydrothermal sites in Okinawa Trough are predominated by allochthonous input and autochthonous input are diluted by the sediment. Sagami Bay sediments were provided mainly from Tokyo Bay and proportion of bacterial biomarkers are relatively low. The acid compositions were characterized by balance between allochthnous and autochthonous input.

Autochthnous input is mainly photosynthetic production in water column. While a sediment

sample was obtained from a cold seep zone in the bay, the sedimentary fatty acid composition, preference of branched acids to monounsaturated acids, referected presence of a cold seep community.

The second purpose of this investigation is clarified the extent of hydrothermal alteration of organic matters and generation of hydrothermal petroleum. This study first identified of hydrothermal petroleum generation in a submarine caldera of the Kagoshima Bay.

This is the first example of hydrothermal petroleum in arc-backarc system. The small amounts of similar petroleum were also found in submarine calderas of the Izu-Ogasawara Ridge and Manus Basin. In the caldera of Kagoshima Bay fumarolic and hydrothermal activities associated with volcanism was reported. The petroleum in the caldera of Kagoshima Bay is enriched in aromatic hydrocarbons and contained significant amounts of pyrolitic origin poly-aromatic hydrocarbons, indicating occurence of pyrolisation of orgaic matter over 300°C.

The petroleum contained high molecular weight hydrocarbons and similar petroleum was not found at hydrothermal site outside of the caldera. It suggests that the petroleum is not migrated and is generated from present sediment as a source rock within the caldera.

Furthermore, together with the occurrence of Kuroko type sulfide deposits in the same caldera are shown simultaneously generation of petroleum and sulfide ore deposits involving with hydrothermal activity. This may be an analogue of pair distributions of petroleum and Kuroko mines in the Green Tuff region of the northeast Japan.

From this study, organic geochemical significance of seafloor hydrothermal systems are summarized. The sedimentary fatty acid compositions at hydrothermal vent sites are represented by high proportion of bacterial biomarker acids such as monounsaturated and branched acids. The high proportions of monounsaturated acids indicate predominance of aerobic bacteria at the hydrothermal areas. Chemosynthetic bacteria such as sulfur-oxidizing bacteria are aerobes. It is indicating that large biomass of bacteria present at hydrothermal areas and aerobic bacteria including chemosynthetic bacteria predominant in the community.

Petroleum like hydrocarbons are identified in three submarine calderas of Kagoshima Bay, Izu-Ogasawara Ridge and Manus Basin. The petroleums are currently forming associated

11

with hydrothermal activities. In these calderas Kuroko type sulfide ore generations are also occerred. It suggests that simultaneously generation both of petroleum and the sulfide ore are occured by seafloor hydrothermal activities associated with island arc systems. Furthermore, hydrothermal vent community is possibly linked with carbon recycle involving with hydrothermal petroleum generation. The petroleum is considered an energy source of vent community. Large biomass at hydrothermal system is supported two system such as chemosynthesis and carbon recycle.

ill

Contents Abstract List of figures List of tables Chapter ^1.

VI Vlll

Introduction ¹

1-1. Seafloor hydrothermal system and related biological community ¹ 1-2. Study of bacterial community using biomarker fatty acids in sediments ³

1-3. Previous biomarker studies of hydrothermal systems ⁵

1-4. Hydrothermal alteration of organic matter and hydrothermal petroleum formation ⁶

1-5. Main purpose of this study ⁸

Chapter ^2.

Geological, biologica� and hydrogeological background 2-1. Background of hydrothermal areas

2-1-1. Geological setting

2-1-2. Significance of biological communities

2-1-3. Geochemistry of hydrothermal fluids and gases at the study areas

13 13

2-2. Geological, biological and geochemical background of the Sagami Bay ¹⁸ 2-2-1. Geological background

2-2-2. Special bottom communities associated with cold seepage 2-2-3. Chemistry of cold seeping fluids

Chapter ^3.

Sampling and analytical methods 3-1. Sampling sites

3-2. Analytical methods

3-2-1. Handling of sediment before analysis

3-2-2. Extraction and fractionation of lipids and estimation of microbial biomass 3-2-3. GC/MS analyses

3-2-4. Lipid identification and nomenclature

3-2-5. Isotope analyses: Carbon and Sulfur isotope ratios

Chapter ^4.

Biomarker study of sedimentary fatty acids at the hydrothermal areas and lipid

22 22 31

compositions of the Sagarni Bay ³⁶

4-1. Fatty acid compositions of the sediments at the hydrothermal areas ³⁶ 4-2. Characterization of the fatty acid compositions at the hydrothermal areas ⁵⁰

lV

4-3. Distributions of sedimentary lipids in Sag ami Bay 4-3-1. Fatty acids

4-3-2. Hydrocarbons

4-3-3. Fatty alcohols, phytol and cholesterol

4-4. Lipid compositions as source indicator of sediments deposited at a coastal

57

system in Sagami Bay 67

4-4-1. Distributions of the low CPI hydrocarbons in Sagami Bay

4-4-2. Depositional processes of terrigenous organic matter in Sagami Bay 4-4-3. Characterixzation of cold seepaze zone at Site 5

4-5. Summary 73

4-5-1. Fatty acid as indicator of biological community structure around hydrothermal vents

4-5-2. Lipid compositions as indicator of the sediment source and biological community structure in the Sagami Bay

4-5-3. A comparison of fatty acid compositions between the hydrothermal areas and the Sagami Bay

Chapter 5.

Hydrothermal petroleum generation at hydrothermal areas associated with

arc-backarc system 7 5

5-1. Hydrothermal petroleum identified from Wakamiko Caldera, Kagoshima Bay 75 5-1-1. Hydrocarbons and biomarker maturity indies in Wakamiko Caldera,

Kagoshima Bay

5-1-2. Carbon isotope ratios of light hydrocarbons in fumarolic gases and sulfur isotope ratios of hydrothermal minerals in W akamiko Caldera 5-1-3. Organic geochemical features of petroleum-like hydrocarbons yielded

from Wakamiko Caldera

5-2. Hydrocarbon distributions of the other hydrothermal sites 88

5-3. Discussion 99

5-3-1. Distributions of petroleum-like hydrocarbons at the hydrothermal areas 5-3-2. Possible fossil model of the simultaneously generation of petroleum and

sulfide ore deposits associated with arc-backarc tectonics

5-4. Summary 108

Chapter 6.

Conclusion

Acknowledgements Reference

v

109

111 112

List of figures

Fig. 1 Schematic circulation model for a typical sediment-starved ridges and heavily sedimented ridges using data from 21 ON East Pacific Rise and Guaymas Basin as example.

Fig. 2 Global distribution of modern seafloor hydrothermal vents.

p.10

p.ll Fig. 3 Ternary diagram of saturated hydrocarbons, aromatic hydrocarbons and NSO plus asphaltic components.

p.12 Fig. 4 Location map of sampling site in tills study.

Fig. 5 Geologic structure of the Sagami Bay.

Fig. 6 Distribution of Calyptogena and chemosynthetic communities in the Sagami Bay and geologic structure.

Fig. 7 Submarine topography and locations of sampling sites in the DESMOS Caldera, Manus Basin.

Fig. 8 Submarine topography of the North Knoll, Iheya Ridge.

Fig. 9 Locations of sampling sites and submarine topography of the Wakamiko Caldera, Kagoshima Bay.

Fig. 10 Locations of sampling points and submarine topography of the Myojin Caldera, Izu-Ogasawara Arc.

p.17 p.20

p.21 p.26 p.27

p.28

p.29 Fig. 1 1 Submarine topography of Sagarni Bay with the locations of the sampling sites. The thick solid lines

represent active faults.

Fig. 12 Flow chart of extraction and frctionation of lipids^. Fig. 13 Fractionation of total extract for petroleum analysis.

Fig. 14 TIC chromatograms of the fatty acid methyl-esters in Samples D916 and D921 obtained from the DESMOS Caldera, Manus ^Basin.

Fig. 15 Fatty acid compositions of Samples D91 6 and D9 21 .

p.30 p.34 p.35

p.3 9

p.40 Fig. 16 TIC chromatograms of the fatty acid methyl-esters in Samples D857R D857B and Dl030 obtained

from the North Knoll of Iheya Ridge^, Okinawa Trough.

Fig. 17 Fatty acid compositions of Samples D857B, D857R and D1030.

Fig. 18 TIC chromatograms of the fatty acid methyl-esters in the D3 41 and D345 sediments of the Wakamiko Caldera.

Fig. 19 Fatty acid composition of Sample D345 from the Kagoshima Bay.

VI

p.42 p.43

p.45 p.46

Fig. 20 TIC chromatograms of the fatty acid methyl-esters in Samples D1008 and D1011 obtained from the Myojin Caldera, Izu-Ogasawara Arc.

Fig. 21 Fatty acid compositions of Samples D 1008 and D 1011.

Fig. 22 Comparison of the fatty acid compositions by classification using alkyl chain types such as normal-saturated, monounsaturated, methyl-branched and PUF As.

Fig. 23 Fatty acid compositions of the Sagarni Bay sediments. Open columns; saturated, hatched columns; monounsaturated, filled colunms; methyl-branched fatty acids.

Fig. 24 n-Alkane compositions of the Sagarni Bay sediments.

Fig. 25 n-Alkanol compositions of the Sagarni Bay sediments.

Fig. 26 Proportions of the long chain (>C2 o) fatty acids and alkanols derived from terrigenous input into the Sagarni Bay.

Fig. 27 Discrimination diagram using major element compositions of sediment samples from the Sag ami Bay.

Fig. 28 MORE-normalized spider diagram of Pearce (1983). a) Sample l AC, 2AC, 6AC, and 7 AC,

b) Sites 3AC, SAC, and 8AC. Elemental compositions were determined by XRF method.

Fig. 29 TIC chromatograms of the aliphatic fractions in Samples D341 and D344 obtained from the northern part ofKagoshirna Bay.

Fig. 30 Ternary diagram of saturated hydrocarbons, aromatic hydrocarbons and NSO plus asphaltic components.

Fig. 31 TIC chromatogram of the aromatic fraction in Sample D341 from the Wakamiko Caldera.

Fig. 32 Mass fragmentgram ofbiomarker steranes and triterpanes in Sample D341 from the Wakamiko Caldera.

Fig. 33 Ternary diagram of Biomarker sterane compositions (C27-29) as source indicator.

Fig. 34 Mass fragmentgrams (M/Z=57 and 99) of the aliphatic fractions in Samples D916 and D921 obtained from DESMOS Caldera, Manus Basin.

Fig. 35 TIC chromatogram of aliphatic fraction of Sample Dl030 obtained from the North Knoll of Theya Ridge, Okinawa Trough.

Fig. 36 TIC chromatograms and mass fragmentgrams of aliphatic fractions in Samples D 1008 and DlOll .

Fig. 37 Paired distribution of the metallic mineralization belt (Kuroko belt) and the oil-producing belt

p.48 p.49

p.56

p.60 p.63 p.65

p.66

p.71

p.72

p.80

p.81 p.83

p.85 p.86

p.91

p.94

p.97

in the Neogene system of the northeast Japan. p.l 04

Vll

Fig. 38 b34S distributions of the sulfur compounds in the sediment and the fumarolic gases of Wakamiko Caldera comparison with Kuroko ore sulfides and organic sulfur in the petroleum yeilded in the northeast Japan.

Fig. 39 Plot of bD ^vs. 613C of the methane in the fumarolic gases and natural gases of the northeast Japan.

Fig. 40 3He/4He ratios of fumarolic gases and comparisons with the northeast Japan naturel gases and other hydrothermal systems.

List of tables

p.l0 5

p.l06

p.l07

Table 1 Sampling sites of the sediments from hydrothermal areas. p.24 Table 2 Site locations and the features of sample sediments that were collected from Sagami Bay. p.2 5 Table 3 Fatty acid compositions in the sediments ofDESMOS Caldera, Manus Basin. p.38 Table 4 Fatty acid compositions in the sediments of the North Knoll, Iheya Ridge. p.41

Table 5 Fatty acid compositions in the sediment of the east rim of the Wakamiko Caldera, Kagoshima

Bay. p.44

Table 6 Fatty acid compositions in the seiments of Myojin Caldera, Izu-Ogasawara Arc. p. 4 7

Table 7 The relationship of the fatty acid compositions in the sediment samples obtained from the

hydrothermal areas. p. 55

Table 8 Fatty acid compositions in the sediments obtained from Sagami Bay. p. 59 Table 9 The relationship of the fatty acid compositions in the sediment samples of the Sagami Bay. p. 61 Table 10 Alkane compositions in the sediments obtained from Sagami Bay. p.62

Table 11 The concentrations of individual n-alkanols, phytol and sterol present in the sediments of

Sagami Bay. p.64

Table 12 Elemental compositions of top 3 em thick ( 5cm at Site 1) of sediment cores obtained from the

Sagami Bay. p.70

Table 13 Alkene compositions of the sediments in Wakamiko Caldera and the on the knoll, Kagoshima

Bay. p.79

Table 14 Major PARs in the botto sediment ofWakamiko Caldera. p.82

Table 15 Comparison of the molecular mature indices of biomarkers and PAHs between Wakamiko

Caldera and other hydrothermal areas bearing hydrothermal petroleum. p.84

Vlll

Table 16 Fumarolic gas compositions and isotope compositions of carbon and sulfur in the gases. p.87 Table 17 Alkane compositions in the sediments of DESMOS Caldera, Manus Basin. p. 90 Table 18 Major P AHs in the sediments of DESMOS Caldera, Manus Basin. p. 92 Table 19 Alkane compositions in the sediment of the North Knoll, Iheya Ridge. p. 93 Table 20 Major P AHs in the sediments of the North Knoll, Iheya Ridge. p. 95 Table 21 Alkane compositions in the sediments of Myojin Caldera, Izu-Ogasawara Arc. p. 96 Table 22 Major P AHs in the sediments of Myojin Caldera, Izu-Ogasawara Arc. p. 98

IX

Chapter 1.

Introduction

1-1. Seafloor hydrothermal system and related biological community

Since high-temperature fluid venting extruded from the seafloor was first discovered in 1977 at spreading axes of the Garapagos Ridge and the East Pacific Rise (Corliss et al., 1979�

RISE Project Group, 1980), seafloor hydrothermal system has been considered as one of the most interesting and important target in hydrogeology. Seawater, which penetrates into the oceanic crust through the fracture system, is heated up by the heat source magma and ascends back to the seafloor due to its volume expansion. This flow of seawater is termed hydrothermal circulation. Hydrothermal circulation of seawater through the oceanic crust plays a role in many geologic and hydrogeologic processes, including heat transfer from the earth's interior, alteration of the oceanic crust, geochemical cycling of the elements, biogeochemistry of deep ocean waters, and possibly general seawater circulation (Fig. 1;

Kadko eta/., 1995� Scott, 1997).

Fig. 2 shows the global map for active hydrothermal areas at seafloor. It suggests that the many seafloor hydrothermal vents are located around ocean spreading zones, such as mid­

oceanic ridge (MOR) systems. During the last decade, a number of hydrothermal activities have been newly discovered from active convergence margin in the western Pacific. These hydrothermal activities exclusively occur in association with submarine volcanism found either on backarc spreading center, backarc rifts, or volcanic fronts in arc-backarc systems (Ishibashi and Urabe, 1995), and they involve hydrothermal mineralization as occurence in MOR hydrothermal systems. Hydrothermal minaralization occurence in the western Pacific are formed volcanic massive sulfide deposits (VMSDs) which are quantitatively and qualitatively more important than sulfide deposits occurence associated with MOR hydrothermal systems (Mosier eta/., 1983; Lowell and Rone, 1985).

The most important discovery was new biological communities, which develops below

1

euphotic zone, from the Garapagos Ridge in 1977 (Lonsdale, 1977� Ballard, 1977� Corliss et a/., 1979) where copious populations of animals, composed many new species, clustered around the vents. Especially, animal amounts developed around hydrothermal vents reach up to 15kg wet weight/m2 (Laubier and Desbruyeres, 1985; Ohta and Laubier, 1987), it values 4 orders higher than that of ocean floor (approx. 1g/m2� Ohta, 1983). When those communities were observed, the foremost question concerned how those unexpectedly high quantities of biomass are supported. Lonsdale ( 1977) and Jannasch and Wirsen ( 1979) were considered that the microbial chemosynthesis is primarily caused by an aerobic oxidation of hydrogen sulfide. Over the last two decades, the bacterial production due to chemosynthesis has been well documented by Jannasch (1983� 1989� 1995). Bacterial chemosynthesis is a carbon fixation process utilized chemical reaction as energy source in stead of photosynthesis.

Jannasch demonstrated major energy source for bacterial chemosynthesis was provided mainly by oxidation of hydrogen sulfide, hydrogen, ammonia, and methane, which are remarkable chemical components dissolved in hydrothermal fluid. Amount of carbon fixation involving with sulfide oxidation at hydrothermal systems are estimated to reach 1 0% of total primary production at the whole of bottom seawater (Jannasch, 1989). Many animals around the vents have been thought to be maintained by symbiotic and/or feeding chemosynthetic bacteria (Grassle, 1985).

The bacterial community in shallow water sediment above euphotic zone is generally dominated by heterotrophic bacteria which depend on primry production by photosynthesis.

In contrast, the hydrothermal community at deep seafloor is controled primary production by bacterial chemosynthesis. Therefore, chemosynthetic bacteria must be predominant member in sediment around hydrothermal vents. Although bacterial communities between shallow water sediment and hydrothermal vents are expected to be obviously dissimilar, little is known about organic geochamical defference between them.

Similar chemosynthetic communities have been found other areas where high-temperature hydrothermal venting and seeping can not be found (e.g. Suess eta!., 1985; Kulm eta!., 1986;

2

Okutai and Egawa, 1985). Subduction zones is an example, where cold interstitial water are squeezed from the accretion prisum due to tectonic compaction. Therefore, those communities was called "cold seep communities" (Ohta, 1988). The seeping water include hydrogen sulfide and methane as the case hydrothermal systems, which are energy source for chemosynthetic microflora.

1-2. Study of bacterial community using biomarker fatty acids in sediments

Accurate amount determination for microbial biomass in sediment have proven to be difficult (Bobbie and White, 1980). Culture techniques using selective media distort and underestimate in situ population because the culturing conditions required to provide sufficient numbers of bacteria for identification are unlikely to be identical to in situ conditions and will prove more favourable to some species than others. The microscopic analyses provides very little information on types of present microorganisms (Mita and Maeda, 1988).

To overcome these difficulties, in situ analysis of biochemical components were proposed to identify and quantify the viable biomass in sediments. All organisms metabolize organic compounds associated with their growth and reproduction. Some of metabolized organic compounds acumulate and are buried. The metabolized organic compounds are unique to each organism, especially lower microorganisms such as bacteria and planktons to which are metabolized each specific compound ( cf Killops and Killops, 1993 ). Hence, A specific organic compound to indicate source organism is termed biomarker.

Lipids, especially fatty acids, are an useful indicator (described below) since they are essential components of bacteria with the exception of archaebacteria. Fatty acids are major constituents of the membranes and are formed phospholipids. Only archaebacterial membrane is consist of fatty alcohols instesd of fatty acids (De Rose eta/., 1986). A part of lipids reach seafloor and some of them bound with residual organic compound. Then the bound lipids are accumulated and are preserved. The preserved fatty acids provide information of source organisms. Analyses of fatty acids have been of great value in

3

understanding bacterial phylogenie and taxonomic classifications (Lechevalier, 1977). Perry et al. (1979) proposed that bacteria make a major contribution to the solvent-extractable fatty acids found in sediment.

A number of investigations revealed that branched fatty acids, monounsaturated fatty acids, cyclopropyl fatty acids, and certain saturated fatty acids present in sediments are proxies for the in situ bacterial populations (Gillan and Hung, 1984; Perry et al., 1979;

Volkman eta!., 1980; White eta!., 1984). Branched fatty acids are a good proxy for anaerobic bacteria (Guckert et al., 1987; Kaneda, 1977), and sulfate-reducing bacteria (SRB) Desulfovibrio spp. (Boon eta!., 1977; 1978; Edlund eta!., 1985). For example, sediments in an eutrophic bay, where is anoxic condition and a number of SRB present, are enriched in branched fatty acids ranging from 29.9�36% of total fatty acids (Rajendran, et al., 1992).

During the last two decades, study for monounsaturated fatty acids in bacteria has developed (e.g. Parkes and Taylor, 1983; Perry eta!., 1979), and it has also been recognized that these acids metabolized by bacteria contribute to major fatty acids in marine sediments (Gillan et al., 1983; Parkes and Taylor, 1983; Perry eta!., 1979). Polyunsaturated fatty acids (here after PUFAs) are derived mainly from eukaryotes such as planktonic algae, diatom, phytoplankton and invertebrates (e.g. Harvey, 1994; Killops and Killops, 1993). They are labile materials. Most of them are decomposed in the water column and/or at the sediment surface (Harvey eta/., 1987; Harvey, 1994, Canuel and Martens, 1992). PUFAs are not identified in bacteria (Bobbie and White, 1980) with the exception of eubacterium, which provid 20:5 fatty acid to sediment (White et a/^., 1979b). On the otherhand, significant amounts of PUFAs are recognized: in marine and estuarine sediments (Perry et al., 1979;

Baird and White, 1985), intertidal sediments (Volkman et a/., 1980), and seafloor hydrothermal area (Hedrick et al., 1992). Fatty acid profiles in the sediments of the Antarctic lake system, show that PUF As are involved at a significant level (Mancuso et al., 1990). If the content of PUF As in the sediments is low, it is most likely that the material is rapidly altered during settling through the water column or just after accumulation at the sediment surface. For example, deep marine sediments containe large proportions of monounsaturated

4

fatty acids ranging from 20.6 to 41.4% of total fatty acids and small proportions of PUF As up to 11.7% in the northern Carolina continental slope (Harvey, 1994 )^. At the continental slope benthic fish and macrobenthic infaunal biomass appear about six times higher than comparable slope areas in other regions (Blake eta!., 1985). Harvey (1994) concluded that bacteria play an important role for alteration of organic matter. Hence, high content of monounsaturated fatty acids and low content of PUF As represent aerobic bacterial metabolism. Frothermore, chain length of fatty acid provide a good information in origin.

Generally the short chain fatty acids (<C2o) are autochthonous and the long chains (�C2o) are allochthonous (i.e. terrigenous origin) (Fukushima and Ishiwatari, 1984).

1-3. Previous biomarker studies of hydrothermal systems

Hedrick et a/. (1992) found evidences for a dense microbial community of archaebacteria and possible 1hiobacilli in the interior of the flange of a black smoker obtained from the Juan de Fuca Ridge and a red Beggiatoa-type colony in a sediment sample. In the same time

significant amounts of PUF As ( 18% of total fatty acid) of the type previously found in barophilic eubacteria. In the sediment sample high concentration of total fatty acid (24 7 J..lg I gdry sediment; Hedrick et a/., 1992) is detected, and the concentration is 1 ^� 2 orders higher than that of organic rich sediments in a continental slope ranging 1 0� 70J..l

g/

^g dry sediment (Harvey, 1994). It is considered that higher bacterial biomass is present at the seafloor around hydrothermal vents. The ridge has been covered by thick pelagic sediments with significant amount of terrigenous organic matters (Davis, Mottl, Fisher, et al., 1992). At the Middle Valley of the ridge the sedimentary organic matter are altered by hydrothermal activities and significant amount of petroleum-like hydrocarbons are generated and migrated to seafloor (Simoneit, 1994). The hydrocarbons can be utilized by bacteria in sediment surface (Simoneit, 1993), therefore, benthic biomass at hydrothermal area will be developed. Little is known about importance of the hydrocarbons as energy source for benthic biomass. About the hydrocarbon generation is described below.

Compositions of organic compounds have poorly compared among several hydrothermal 5

sites. Venting hydrothermal fluids are variable in chemical and phisical properties. The vent communities are also look different from each other (Hessler and Kaharl, 1995), and this difference caused by the different stages of hydrothermal activity (Von Damm, 1995). The relationship between the varieties of the animal compositions and the fluid chemistry is poorly understood.

1-4. Hydrothermal alteration of organic matter and hydrothermal petroleum formation

The Guaymas Basin in the Gulf of California is actively spreading, where sedimentation rate is high enough to cover the rift floor topography to a depth of �300-500m (Curray et al., 1982� Einsele, 1985� Lonsdale, 1985� Lonsdale and Becker, 1985). High concentration of petroleum-like hydrocarbons were found in the hydrothermally altered sediments obtained from gravity core samples (Simoneit et al., 1979) before the discovery of active hydrothermal venting. DSDP Leg 64 core samples and direct observation by submersible, DSV Alvin in 1982, tell us that hydrocarbons were generated instantaneously by hydrothermal alteration.

The hydrothermal origin petroleums, "hydrothermal petroleum", are also found in the Escanaba Trough of the Gorda Ridge and the Middle Valley of the Juan de Fuca Ridge after the discovery of the Guaymas Basin (Simoneit, 1993� 1994). Thick organic rich deposits are source of hydrothermal petroleum because hydrothermal sites occurring hydrothermal petroleum generation are districted in sedimented spreading axes.

Conventional petroleum formation result from a geologically process tied to basin subsidence associate with organic matter maturation. Because generation of conventional petroleum is taken place usually up to 150°C ( cf. Killops and Killops, 1993), hence, burial depth and geothermal gradient are important factors for petroleum generation. Time is another important factor in petroleum generation. The higher the temperature experienced by a source kerogen the less time is required for oil generation. Necessary geologic time increases exponentially with decreasing temperature (Killops and Killops, 1993 ). In the case of hydrothermal systems, maturation of organic matter, petroleum generation, expulsion, and migration are compressed into an "instantaneous" geologic time. At seafloor spreading axes,

6

active hydrothermal systems with thick sedimentary cover (e.g. Guaymas Basin and Escanaba Trough) generate petroleum from generally immature organic matter in the sediments at seafloor spreading axes (Simoneit, 1993).

As showing in Fig. 3, most hydrothermal petrolums from the Guaymas Basin and Escanaba Trough are plotted in the outside the field of typical reservoir petroleums on the ternary composition diagram (Simoneit, 1993). This indicates that they are of diverse compositions and generally more polar than conventional petroleums. Typical oil from the Guaymas Basin (e.g. Didyk and Simoneit, 1990) have an intermediate content of n-alkanes

(

1

₈.4%) and a relatively normal content of iso, anteiso, isoprenoid, and naphthenic

hydrocarbons (81.6%), both percentages being comparable to those in the normal crude oil.

The n-alkane range for the stabilized oil

(Cw-c35)

was a monomodal and wide carbon number distribution with no carbon number preference as shown in

CPI10-32::::::1

(Cooper and Bray, 1963; Simoneit, 1978). The

CPI

is the carbon number preference index; for hydrocarbons, it is expressed as a summation of the odd-carbon-number homo logs over the range in this case from

Cw

_to

C35,

divided by a summation of the even-carbon-number homologs over the same range. Hydrocarbons are generated by cracking of alkyl-chain bounded to source organic matter, therefore, maturated oil dose not appear carbon number preference. This feature of n­

alkane is compared to the normal crude oil. The

CPI

_{value of}

-1

indicates complete maturation.

The aromatic hydrocarbons of hydrothermal petroleum are comprised of n-alkylbenzens and n-alkyltoluenes with significant amounts of polycyclic aromatic hydrocarbons (P AHs) and alkyl

P

_AHs. The hydrothermal petrolum showed remarkable abundance of

P

_AHs,

especially pyrolitic origin

P

AHs such as fluorene, methylenephenanthrene, fluoranthene, benzofluoranthene, and indenopyrene. These

P

AHs are ubiquitous in higher temperature pyrolysates (Geissman eta!., 1967; Blumer, 1975; Hunt, 1979), and are not easily reverted to periocondensed aromatic hydrocarbons after formation (Blumer, 1975, 1976; Scott, 1982).

Therefore, the

P

AHs found as major constituents in aromatic fractions of the Guaymas and Escanaba petroleums are evidence for that they have experienced very high temperature

7

condition.

The other feature of hydrothermal petroleum is coexistence of high-matured petroleum

and immatured organic compounds ( cf. Simoneit, 1993). Some biomarkers in hydrothermal petroleum indicate a variety of levels because the petroleum consists of mixing among bitumens of different maturity levels in maturity. Organic maturity in sediment is proceeding with incleasing of burial depth. High mature bitumen migrates with hydrothermal fluid and is mixing with immature organic matter in overlying sediment. While resevoir oil is sometime formed by accumulation of several kind of oils migrated from different source rocks, the maturity levels of the oil become usually uniform.

Simoneit ( 1993) proposes that the hydrothermal alteration of organic matter and hydrocarbon generation are probably a ubiquitous process along the rift systems where has been active over geologic time. Present Okinawa Trough and Miocene proto-Japan Sea are examplesd which consider to be sedimented spreading axes (Kobayashi and Nakamura, 1986).

These regions are potential fields for hydrothermal petroleum generation. In fact, major oil and natural gas fields of the Green Tuff region in the northeast Japan are also distributed along present shoreline of Japan Sea, although the origin of petroleum are poorly discussed from view point of hydrothermal alteration. Major sulfide ore deposits in Japan, namely "Kuroko"

ore deposits, have been considered to be formed by hydrothermal mineralization involved with bimodal volcanisms (e.g. Tanimura eta/., 1983; Sato, 1986). The spatial distribution of the Kuroko ore deposits are similar with the oil and natural gas fields in the Green Tuff region (Kajiwara and Sasaki, 1987). From sevral geochemical evidences, Kajiwara and Sasaki (1987) illustrate the genetic link between the Kuroko ore deposits and petroleum in the northeast Japan. The Green Tuff region is characterized by extensive submarine volcanisms (Horikoshi, 1981 ). The geological and geohistorical constraints of the Green Tuff petroleum and the Kuroko ore deposits are necessary to survey from view point of hydrothermal geochemistry.

1-5. Main purpose of this study

Main rational purpose of this study is to clarify distributions and character of organic 8

components deposited around seafloor hydrothermal areas. Especially, fatty acid composition is important to understand of bacterial community structure and to estimate biomass around hydrothermal areas. These geochemical data have not been accumulated sufficiently. Survey of generation and distribution of petroleum-like hydrocarbons are also important purpose of this study. Organic geochemistry for sedimentary organic matter around hydrothermal vents in the western Pacific are poorly understood and comparison enough among several independent hydrothermal areas. Accordingly, sample sediments were collected from four different hydrothermal sites and one cold-water seepage in Sagarni Bay, associate with arc-backarc systems in the western Pacific, and carried out comparison and characterization for the composition of organic compounds, especially fatty acids, in those sediments.

SEAWATER

•COLD (tC)

A ALKALINE (pH-7.8)

•OXIDIZING

• S04 (27.9 mm)

A METAL-DEFICIENT (e.g. <0.001 �m Fe

<0.001 �m ^Mn

0.01 �m ^Zn 0^.007 �m Cu

• Mg (52. 7 mm)

• 315 oc

•pH^- 6

• VERY REDUCING

• H2S

(4.8 mm)

• METALS 1-20%

OF 21oN VALUES

• M -FREE

• HOT(350°C)

. . . . . . . . . + • ACID (pH ^-4.6)

. • . · . · . · . · . · . · . · + + + • REDUCING

· · · · · · · · +

+ + + • H2S (7.5 mm)

· _. · _. · _. · _. · _. · _. · ₊ + ₊ + ₊ • METAL-RICH

+ + + + +

. . . •

+ + + + + + + + + + + + + +

• . + + + + + +

. .^{' . + + + +}

MAGMA

. . + + + + + +

. . � � � � � �

(e.g. 1429 J.Jm Fe

885J.JmMn 85 J.JmZn

22 J.JmCu)

• Mg ^-FREE (0 J.Jm)

� � � � � � � � � � �

Fig. 1 Schematic circulation model for a typical sediment-starved ridges and heavily sedimented ridges using data from 21 ON East Pacific Rise and Guaymas Basin as example (Scott, 1997). A hydrothermal fluid is produced by reaction with hot rock and, in th� case of the sedimented ridges, this fluid is further modified by reaction with sediment before reaching the seafloor.

10

d

--- --

Kolbainsay I

0

•

(

^Roy�

Jb

Lucky Strike ^{T AGe}

^,;

Broken Spur ^{,,. )}

J c �

f

^SnakaP1t

·· . .

. ' , .,

Fig. 2

Global distribution of modern seafloor hydrothermal vents (after

Hannington

eta/.,

1995).

100°/o Aliphatic H.C.

Aromatic H.C.

100°/o

•

Escanaba Trough

•

Guaymas Basin

•

100°/o

Asphaltic (NSO)

Fig. 3 Ternary diagram of saturated hydrocarbons, aromatic hydrocarbons and NSO plus asphaltic components (after Simoneit, 1993). Typical crude oils fall within the hatched area (Tissot and Welte, 1984).

Chapter 2.

Geological, biological, and hydrogeological background

2-1. Background of hydrothermal areas 2-1-1. Geological setting

In this study, sediment samples were obtained from four hydrothermal sites associated with island arc-back arc systems of the west Pacific Ocean. Three samples were taken from submarine calderas, the DESMOS Caldera in the Manus Basin, the Wakamiko Caldera in the Kagoshima Bay, the Myojin Knoll Caldera of the Izu-Ogasawara Arc (Fig. 4). The other sample was obtained from a knoll of the Iheya Ridge in the middle Okinawa Trough. The Wakamiko and Myojin carderas were located on volcanic fronts of the Kyushu-Ryukyu and Izu-Ogasawara arc-trench systems, respectively (Hayasaka, 1987; Yuasa et al., 1991). The Kagoshima Bay is a part of the Kagoshima Graben which is tectonic origin (Tsuyuki, 1961 ).

The formation of the Wakamiko Caldera is considered onset in the late Pleistocene (22 .... 23ka), resulting from the eruption of the Ito pyroclastic flow (Aramaki, 1984; Hayasaka, 1987). The Myojin Caldera is also one of the Quaternary submarine volcano (Yuasa et al., 1991 ). The DESMOS caldera and Iheya Ridge were located on central rifting zones of back-arc basins (e.g. Tufa, 1990; Taylor et al., 1991; Japanese DELP Group on Back-Arc basins, 1991;

Kimura etal., 1986). Okinawa Trough is characterized as extensive sedimentation (Tsugaru et al., 1991) and is one of sedimented hydrothermal system comparable with Guaymas Basin, Gulf of California (Garno et al., 1991a). The Wakamiko Caldera were reported to be deposited up to 80 m thickness of recent sediments from seismic profiles (Hayasaka, 1987).

The other hydrothermal sites were covered only thin sediments from direct observation of submersible, RSV "Shinkai 2000 ".

Remarkable feature of these four hydrothermal areas are recognized occurrence ofKuroko type sulfides and/or native sulfur deposits (Nedachi etal., 1991; Iizasa etal., 1997; Aoki and Nakamura, 1989; Shitashima et al., 1997; Garno et al., 1996; 1997). Horikoshi (1977)

13

demonstrated hydrothermal activity occurrence of Kuroko type sulfides and native sulfur deposits in Miocene is related to arc-backarc systems. Sevral common characteristics of sulfide ores from Kuroko and present analogues are reported enrichment of Au, Ag, Cu, Pb and Zn in sulfide chimnys of Myojin Caldera (Iizasa et al., 1997), positive anomalous of As, Sb, Hg and base metal in hydrothermally altered sediment of Wakamiko Caldera (Sakamoto, 1985; Sakamoto et al., 1997), native sulfur deposits in DESMOS Caldera, Myojin Caldera and Wakamiko Caldera. At DESMOS Caldera and Wakamiko Caldera sulfide deposits are expected to form below seafloor (Shitashima et al., 1997; Nedaci et al., 1991). Any mineralized chimny has not been found at hydrothermal system of W akamiko Caldera yet.

2-1-2. Significance of biological communities

In many case, three kinds of animals symbiotic with chemosynthetic bacteria, such as bivalves, gastropods and tube-worms, are found at hydrotehrmal sites. Especially, two bivalves, Calyptogena and Bathymodiolus, and Vestimentiferan tube-worms are predominant constituents of hydrothermal community. It should be noted that one or two species among them are dominated in each hydrothermal area.

Calyptogena and three species of Vestimentiferan tube-worms (Arcovestia, Escarpia, Ridgeia) are thought to be the co-dominant members of the hydrothermal communities at the DESMOS Caldera (Hashimoto and Ohta, 1997). Two kind of shrimps ( Alvinocaris and Nematocarcinus), squat lobster Munidopsis, small gastropods, were the representative constituents of the thick community.

Biological community of the North Knoll of the Iheya ridge was also dominated by Calyptogena sp. nov. in biomass, primitive scalpellids (Neolepas, spp.) and slender vestimentiferan tube worm in number (Hashimoto et al., 1990; 1993a; Ohta, 1990). Two kind of shrimps (Alvinocaris and Lebbeus), mussels, squat lobster Munidopsis, a stone crab Paralomis sp., were the representative constituents of the thick community. A larger Vestimentijeran tube-worm Lamel/ibrachia sp., glass sponges Pheronema ijimai, Euplectella sp. and the fishes such as Aldrovandia, Chimaera and Synaphobranchus surrounded the field

14

of dense patches of organisms.

From the northern Kagoshima Bay, in 1993, during a series of surveys exploring the biological community accompanied by submarine fumaroles thousands of Vestimentiferan tube-worms were discovered to form clusters at a water depth of 82m at top of a knoll located east rim of the Wakamiko Caldera (Hashimoto eta!., 1993b ). This vestimentiferans are remarkable not only in the shallowest occurrence in the world but also in the first record from the euphotic zone. The energy source of the vestimentiferans are considered hydrogen sulfide provided from the fumarole, in fact, some dozen living worms has been maintained in the laboratory, Kagoshima City Aquarium, for more than one year by addition of sodium sulfide.

The biological study of the Myojin Caldera is now in progress. The first survey for biological study in the caldera was done in 1997. Deep-sea mussels, Bathymodiolus septemdierum, were co-dominant members of the community in the caldera. Bathograeid crabs were also representative constituents of the vent community. At the hydrothermal systems along Izu-Ogasawara arc, Calyptogena hav^e not been inhabited.

2-1-3. Geochemistry of hydrothermal fluids and gases at the study areas

Geochemistry of hydrothermal fluid compared with seawater are characterized by depletion of magnesium and sulfate ion, low pH ( <4. 6), reductive condition, and enrichment in calsium, potasium and heavy metal (Fig. 1

)

^. These features are attributed to chemical reactions taking place between seawater and reactant rocks, at high temperature of300�400QC and high pressure of 300�500 bars (Alt, 1995).

The DESMOS fluids are revealed as analogue of "high sulfidation epithermal system", which unique fluid geochemistry showed evidence for magmatic contribution, such as extremely low pH ( 2. 1) and high sulfate concentration (3 3

mM)

accompanied by native sulfur deposition around the vents (Garno et al., 1996; 1997). The maximum temperatures of hydrothermal fluids from the site was 120°C.

The hydrothermal fluids (43�238QC) collected from the North Knoll of the Iheya Ridge in the middle Okinawa Trough show low Cl concentrations ( 50�80% of the ambient

15

seawater), indicating that the hydrothermal fluids contain vapor phase formed by phase separation at depth (Chiba

et al.,

1996). The chemical compositions of the North Knoll fluids are similar to those of the fluids at Izena and Minarni-Ensei seafloor hydrothermal systems in the middle Okinawa Trough. The contents of volatile species such as C02 in the middle Okinawa Trough fluids are higher than that of MOR fluid (Ishibashi

et a!.,

1990). The relationship between temperature of the hydrothermal fluid and water depth indicates that the hydrothermal activity occurs at the condition very close to the boiling point of seawater. The shallow circulation of hydrothermal fluid is suggested by the silica concentration of the fluid (Chiba

et al.,

^1993). The other important geochemical features of hydrothermal fluids in middle Okinawa Trough are interpreted by reaction with sediment. The hydrothermal fluids obtained from the JADE and CLAM sites in the middle Okinawa Trough are characterized by high alkalinity and ammonium concentration (Garno

et al.,

1991a� 1991b� Ishibashi

et al., 1990;

^Sakai

et a/.,

^1990). These characteristics were evidence for organic matter decomposition during high-temperature fluid-sediment interaction.

Previous study of fumarolic gases sampled from the northern Kagoshima Bay reported that the gases were composed of mainly C02 (77�93% of volume), CH4 (5�20%), N2 (1. 7�2.4) and H2S (0. 02� 1.4%) (Ossaka

eta!.,

1992) and were derived from volcanic activity.

The gas temperature at a fumarole was reported up to 215°C. The stratified bottom water in the caldera floor is developed during the summer, and both pH and dissolved oxygen level of the water drops

(pH=6.5, DO<O.Sml/1)

due to acidic gas and hydrothermal fluid (Kamata

et

al.,

1978).

The bottom water has been detected small positive anomaly of mercury, arsenic and antimony (Sakamoto,

1985).

High temperature fluid from black smoker and white smokers in the Myojin Caldera, Izu­

Ogasawara Arc were collected by RSV "Shinkai 2000" in May

1998

^(Yuasa

eta!., 1998).

Preliminary result suggests small difference between the fluids and MOR fluid (Ishibashi

et

a!.,

1998).

The bottom and column water were obtained in the caldera showed that water is enrichment in methane and helium-3. The CH4f3He ratio of hydrothermal fluid was estimated to be about 106�107, which indicated sediment-poor environment of hydrothermal system

16

(Tsunogai

et

al., 1995).

100" 120" 140" 160" 180"

40"

2a· 2a·

�

a·

DESMOS Caldera

a·

, . Manus Basin

·,-..

-·

'

20" 20

40" 4a

100"

•

Reported hydrothermal areas

Fig. 4 Location map of sampling site in this study. Reported hydrothermal areas

are refered to Ishibashi and Urabe

(1995).

2-2. Geological, biological and geochemical background of the Sagami Bay 2-2-1. Geological background

The Sagami Bay, surrounded by the Miura and Izu Peninsulas with Oshima Island (Fig.

5), is a tectonically very active basin because the Izu Bonin Arc has collided onto the Honshu Arc along the Sagarni Bay (e.g. Taira, 1989). As a result of the deformation due to the arc-arc collision, many active faultings are developed within the Sagami Bay, which provides very complicated intra basin submarine topography (Soh et al., 1990; Fujioka eta!., 1989) with fault-controlled submarine banks, and deepwater channels, and submarine fan. On the basin floor, further, cold seepage enriched in methane feeds deepsea Calyptogena communities along the faults (Hattori et al., 1996; Kanie et al., 1992), together with exposure of authigenic carbonates associated with the oxidation by methane (Masuzawa et al., 1995).

In the Sagarni Bay, the continental shelf is poorly developed, then, the continental slope is very steep (grade: 3-5% ). The river-derived detritus directly flow down into the deep­

water basin floor of the Sagami Bay. Previous studies have described depositional processes of the detritus on the basis of the piston core, seismic profile and deep towing observation studies (Utashiro and Iwabuchi, 1971; Fujioka eta!., 1989, Ohkouchi, 1990; Soh etal., 1991).

As the results, it is concluded that the terrigeneous sediments were provided from the Sakawa River, Tokyo Bay, and Izu Peninsula into the basin floor of the Sagami Bay mainly by turbidity currents and flood induced hyperpycnal flows.

2-2-2. Special bottom communities associated with cold seepage

Dense deep-sea biological communities dominated by the giant clam Calyptogena soyoae were found off Hatsushima Island, western Sag ami Bay (the Hatsushima site) in 1984 by the submersible Shinkai 2000 (Okutani and Egawa, 1985). The composition of the communities, the occurrence of a positive temperature anomaly of about 0.5°C within the community beds and extremely high amount of methane in the bottom water, indicated the presence of cold seepage. The clam communities stretch for 7 km along the foot of a steep escarpment off Izu Peninsula at water depths of 900-1200m (Hashimoto et al., 1989). The area is located near

18

the convergence front of the northern tip of the Philippine sea Plate, which is subducting beneath the Eurasian and North American plates. These "cold seep" communities have been understood as being supported by microbial chemosynthesis that oxidizes reduced compounds in a manner similar to that occurring in deep-sea hydrothermal communities (Masuzawa et al., 1992). The giant clam communities have been found from several sites in Sag ami Bay (Fig. 6: Hashimoto and Hotta, 1994). Some vestimentiferan tube-worms (Lamellibrachia sp.) and bivalves (Bathymodi olus sp.), which are supported by chemosynthesis of symbiotic bacteria, have been also found around Calyptogena colonies (Hashimoto and Hotta, 1994).

2-2-3. Chemistry of cold seeping fluids

The bottom water and pore water chemistries have been documented well (Sakai eta/., 1987� Garno et al., 1988). The source of the seeping fluids, however, has been discussed several hypotheses, such as squeezing of the pore water by subducting plate compactions (Hashimoto et a/., 198 9� Yoshida and Tsukahara, 1991 ), seeping of quite low temperature hydrothermal fluid associated with magmatism below sediment (Fujioka eta/., 1989� Naka et a/., 1991) and seeping of deep groundwater in the land area through lateral migration (Tsunogai et al., 1994). The seeping fluid is enriched in methane which is demonstrated biological origin as a result of decomposition of organic matter (Tsunogai eta/., 1996). Helium isotope ratio also suggest not contribution of magmatism (Tsunogai eta/., 1996).

---

�

'

140 °

Subduction zone, thrust fault Fault

Hayama- Mineoka Tectonic Zone

140 °30'

Moving direction of the Phillipine Sea Plate

Fig. 5 Geologic structure ofthe Sagami Bay (Kanie eta/., 1996). OH: Oiso Spur, MnK: Manazuru Knoll, SKN and SKS: northern part and southern part of the Sagami Knoll, respectively, Muk: Miura Knoll, MsK: Misaki Knoll, OB: Okinoyama Bank.

20

e Chemosynthetic communities. 0 lOb

Fig. 6 Distribution of

Calyptogena

and chemosynthetic communities in the Sagami Bay and geologic structure (Hashimoto and Hotta, 1994).

21

Chapter 3.

Sampling and analytical methods

3-1. Sampling sites

In the DESMOS Caldera of the Manus Basin, two kinds of the sediment samples were collected. The Sample D916 was coarse and thin sandy sediment taken from near the Calyptogena community. The Sample D921 was sandy mud taken from foot of a Vestimentiferane tube-worm cluster. Both samples were collected by a push core sampler of RSV "Shinkai 2000" during BIOACCESS cruise in 1996 (Onboard scientists of the BIOACCESS cruise, 1997). The sampling sites are shown in Fig. 7. D916 site and D921 site were approx. 30 m and 250m away from a smoker, which venting fluids of moderate temperature (:S120°C).

In the North Knoll of the Iheya Ridge in the middle Okinawa Trough, two kinds of sediment samples were obtained by a push^-core sampler of RSV I(Shinkai 2000" during the dive program in April, 1996. The Sample D857B was collected from near Calyptogena colony and the Sample D857R was collected from the site far from the colony at a distance of about SOOm from active hydrothermal fields (Fig. 8). Both sediment samples were composed of sand to sandy mud with small amount of pumice. The Sample D1030 was also collected near the Calyptogena colony in July, 1998.

In the Wakamiko Caldera, Kagoshima Bay, two kinds of mud samples were collected: the

bottom of the caldera (D341, 200m depth, Fig. 9) and around the tube-worm communities (Hashimoto eta/., 1993b) at the top of the knoll located on the east rim of the caldera (D344 and D345, 90m). Around this area can be found some animals, which can be seen in normal coastal environment, however, at the bottom of the caldera any animals can not be found during anoxic condition in summer. The sediments were sampled using a rake and push core sampler, respectively, operated by the manipulator of the ROV "Dolphin 3k" in September, 1997. The fumarolic gas were· also sampled using the vacuum sampler operated by the ROV

"Dolphin 3k'' in June, 1998. The mud sample obtained from the bottom of the caldera had

22

strong gasoline-like smell and an oil slick was recognized. The other samples did not smell and have no oil slick.

In the Myojin Knoll Caldera, Izu-Ogasawara Arc, two kinds of sediment samples were obtained by the push-core sampler of RSV (IShinkai 2000" in May, 1998. Sampling point and seafloor topography was shown in Fig. 10. D 1008 mud sediments was obtained from the foot of a chimney which vents low temperature fluid (max 111 OC). Many deepsea mussels (Bathymodiolus septemdierum) inhabited around the sampling point. D 1011 sediment (sandy mud) was obtained from the mound of small chimneys of black smoker and near to the big chimney, called "Daimyojin Chimney", venting fluid of over 250°C high temperature.

The all sampling sites around hydrothermal areas was summarized in Tables 1. The detail feature of the sampling sites were described below.

From the Sagarni Bay, sediment samples were obtained during KF95-1 cruise , Ocean Research Institute of the University Tokyo in August 1995. Locations of sampling site (Fig.

11 ), the topographic features of the sites and the observations of the sediment cores are summarized in Table 2. The sediment core samples were obtained using a multiple corer, 6cm in diameter. Tokuyama et al., (1996) studied major element composition of the same samples by XRF analysis. They insisted two dominant source of sediment in the Sagami Bay. The south part of the Sagami Bay include Tokyo submarine fan (Samples lAC, 2AC, SAC, 6AC, and 7 AC) are dominated by material from Tokyo Bay, and the north part are dominated by river-derived material (Samples 3AC and 8AC).

�

Table I Sampling sites of the sediments from hydrothermal areas

-- ---- ---

Location Site Date Latitude Longitude Depth (m) Sediment type* Note

Manus Basin, ^{D916 96/ll/18} 3°41.542'S 151 °52.064'£ 1911 Sandy mud + pumice Near Calyptogena

Papua New Guinea +sulfur colony

D921 96/11/24 3°41.582'S 151 °51.996'£ 1905 Coarse sand + volcanic Near tube-worms glass fragment + sulfur colony

The North Knoll of ^{D857R 96/4/29} 2T47.178'N 126°54.203'£ 1035 Sandy mud Ambient sediment

Theya Ridge, _{D857B 96/4/29} 27°47.180'N 126°54.149£ 1049 Sandy mud White color patch

Okinawa Trough

D1030 98/7/9 2T47.176'N 126°53 .823'E 981 Mud Inside of Calyptogcna colony

Myojin Caldera, Izu- ^{D1008 98/5/6} 32°06.294'N 139°52.083'£ 1338 Sandy mud+ sulfur Ogasawara ^Arc _{D1011 98/5/10} 32°06.238'N 139°52.029'£ 1302 Sandy mud+ sulfide

mineral

W akamiko Caldera, D341 97/9/2 31°39 .524'N 130°46.436'£ 208 Mud+ sulfur Near fumarols

Northern Kagoshima _{D344 97/9/3} 3r39.59I'N 130°48.169'£ 96 Mud+ sulfur Foot of the tube-worm

Bay cluster

I D345 97/9/3 31 °39.540'N 130° 48.177'£ 92 Mud+ sulfur

* sulfur= small flagments of massive sulfur; sulfide minerals =automorphic sulfide minerals originate in hydrothermal mineralization.