Gas hydrate is an ice-like solid compound composed of methane and water molecules. Since the gas hydrate, often called "methane hydrate", was found in deep sea sediments in late 1980's during the Deep Sea Drilling Project, marine gas hydrate has been attracting growing interests from the viewpoint of future natural gas resources as well as environmental mediators as enormous carbon sink. Exploration of marine gas hydrate has rapidly increased our knowledge as to the occurrence, origin and significance both in resources and environmental changes, but, we know little of the mechanism of accumulation and major controlling factors of the emplacement and evolution of gas hydrate system in marine sediments. As a matter of fact, gas hydrate has been often identified by means of anomalous reflectors called BSR's in seismic profiles, and believed to develop as strata-bound type deposits at around few hundred meters below seafloor. However, recent development of ocean floor observation and marine geochemical studies have revealed various occurrence of gas hydrate in shallow subsurface. Massive gas hydrates sometime expose on the seafloor are concentrate in shallow sediments < 5 meters. Seismic surveys have also identified chimney structures below gas hydrate bearing unit. These findings strongly suggest a formation of gas hydrate deposits in shallow sediments, totally different from strata-bound type deposits of gas hydrate associated with BSRs.

Integrated geological, geochemical, and geophysical exploration since 2004 has identified massive accumulation of gas hydrate associated with active methane seeps on two ridges called Umitaka Spur and Joetsu Knoll in Joetsu Basin, eastern margin of Japan Sea. The ridge structures are asymmetric anticlines formed along an incipient subduction that extends throughout the western side of the Japanese island-arc system, suggesting that the accumulation of gas and gas hydrate in Joetsu Basin is closely, and perhaps genetically related with the tectonics of Japan Sea Basin.

This study has two approaches: one is a tectono-stratigraphy to investigate the geologic background of the formation of shallow gas hydrates. The other is a geochemical aspect to constrain the mechanism of gas migration and accumulation of gas to form gas hydrate in shallow horizons. Discussion on the tectono-stratigraphic control on the gas hydrates of Umitaka Spur is based on 2D single channel seismic profiles. The study recognized chimney structures which seem to be strongly controlled by a complex anticlinal axial fault system. Seismic profiles exhibit high amplitude events with pull-up structures, probably due to massive and dense accumulation of gas hydrate above. BSR's are widely developed, in particular, within gas chimneys and in the gently dipping eastern flank of the anticline of the Umitaka Spur. The spur sediments are largely composed of clayey hemipelagic, while SCS identified several sharp horizons to indicate debris-flow deposits. BSR's are often strengthened within debris-flow units as well as in gas chimneys. Debris flow deposits are expected to contain high concentration of gas hydrate due to high permo-porosity characteristics, and are considered to be a possible target of shallow gas reservoirs.

Methane of gas hydrate has been observed to be of thermogenic with relatively heavy carbon isotopic composition. For comparison, deep subsurface marine gas hydrates as those of Nankai Trough and Blake Ridge, are entirely microbial, no indication of thermogenic. However, the shallow subsurface and even ocean-floor gas hydrate of the Joetsu Basin is mostly derived from deep-seated thermogenic methane. The unique feature of the Joetsu Basin gas hydrate is related with the evolution and tectonics of the eastern margin of Japan Sea.

The axial fault system, the anticline shape, and the permeable horizons as conduits induce gas migration to the top of the Umitaka Spur, providing strong seepages and giant plumes in the sea water column. Well developed conduit system carries large amount of thermogenic methane and forms massive gas hydrate buildup within the shallow part of gas chimney structures. Gas hydrates are also observed associated to debris within the gas hydrate stability zone in the eastern flank of Umitaka Spur, and may represent a potential natural gas resource in the future.

Migration of deep-seated methane has been well documented by organic and inorganic geochemistry of the Quaternary sediments of the Umitaka Spur, which was strongly suffered from sea-level change during the glacio-interglacial cycles.

Geochemical analyses and lithological description were used to understand the late Quaternary depositional history of Joetsu Basin. Geochemistry of sediment samples enabled the characterization of the background signatures and the origin of the organic matter of the Holocene and late Pleistocene sediments, on the basis of δ13Corg, TOC/TN and TS/TOC ratios coupled with palynofacies analysis. Based on this study the paleoenvironmental settings of the study area were reconstructed.

The Holocene sediments are characterized by high TOC and TN contents, low TOC/TN and TS/TOC ratios, and heavier δ13Corg values, which indicate a predominant marine organic matter origin, probably due to the warming and inflow of warm ocean and coastal currents along the East China Sea. These currents carried abundant phytoplankton from the Pacific Ocean as a result of the sea level rise. Occurrence of particulate organic matter shows abundant primary productivity during the Holocene under marine conditions. On the other hand, LGM sediments are characterized by low TOC and TN contents, high TOC/TN and TS/TOC ratios, and lighter δ13Corg signatures, which are characteristic of terrestrial organic matter, probably due to seaward migration of shorelines and strong input of freshwater carrying terrestrial materials. This terrestrial influence decreased gradually from the LGM to the Holocene because of the sea level rise and consequent increase in the marine organic matter productivity.

Shallow to surface sediments at seep sites should be identical to the equivalent shallow sediments a bit far from seep sites, however, seep site sediments are greatly depleted in δ13Corg values, followed by high TOC/TN and TS/TOC ratios. These signatures are different from background surface sediments, but quite similar to the geochemical signatures of LGM sediments. Sulfate-methane transition promotes the anaerobic oxidation of methane and the consequent precipitation of carbonates and sulfides, which can explain the elevated TS/TOC ratio observed near seafloor sediments. Moreover it cannot explain the anomalous depleted values of δ13Corg and the high TOC/TN ratio signature, in an opposite trend of the neighboring Holocene sediments.

Anomalous features of seep sites sediments seem to imply migration of sediments as well as water and gas. Deep-seated sediments were migrated upward from deeper horizons to the top of mound-seep sites, showing appearance older and deeper LGM sediments on the seafloor. This mechanism is consistent with the findings of strong "pull-up" structures on seismic profiles. Migration of deeper sediments and gas must have occurred even in deeper and older sequences, which will be confirmed by finding of anomalous geochemical features in deeper horizons.

Keywords:δ13C(org;); gas chimneys; gas hydrates; Joetsu Basin; Japan Sea; methane seeps; organic matter; single channel seismic; tephrostratigraphy; TOC/TN; TS/TOC.

メタンハイドレ―トは、新しい天然エネルギー資源として、またグローバル気候変動の要因として、さらには海底地すべりやその発生に伴う津波災害の原因として、近年多方面から注目を集めている。日本近海においては、南海トラフと日本海東縁においてメタンハイドレートの存在が知られている。2004年以来、日本海東縁の上越海盆においてメタンハイドレ―トの分布や成因に関する調査が急速に進捗してきた。本研究は、同地域おけるメタンハイドレートの分布特性とそうした分布をもたらした要因とプロセスについて、地質学的、地球化学的、地球物理学的手法を駆使して、総合的に明らかにしたものである。

本論文は7章で構成されている。第1章では、メタンハイドレート研究の背景と意義、ならびに、調査研究地域の概要が述べられている。つづく第2章では、2Dシングルチャンネル探査にもとづく海底面下の活構造と層序の解釈を行い、これとメタンハイドレートの関係について議論している。とくにメタンハイドレートの分布が断層系の分布と大局的に一致していることから、ハイドレートの形成が断層活動と深く関わってきた可能性を指摘している。第3章では申請者が参加して広域にわたって掘削した多数の海底コア堆積物を対象として、火山灰(テフラ)編年学ならびに微化石層序学の手法等を用いて、第四紀後期の堆積層序を明らかにするとともに、同時期の堆積深度-時間モデルを構築している。とくに、火山灰は全て申請者が分析を行い、これまで記載されてこなかったローカルテフラの存在を明らかにしている。第4章では、コア堆積物の安定炭素同位体、有機炭素、無機炭素、全窒素、等の化学特性を分析し、それらの値とハイドレートの存否との関係を明らかにするとともに、第3章で明らかにした深度-年代モデルをもとに、これらの化学指標値の時空間変動を示している。その結果、最終氷期の海面低下期には陸源物質の割合が増し、後氷期には陸源物質は減少し、海生プランクトン起源物が増大すること、ヤンガードリアスの前後の急激な気候環境変動が記録されていることを示した。同時にメタンハイドレ―ト域では、こうした傾向が認められず、そもそも最終氷期極相期以降の地層が保存されていないことを明らかにした。第5章では、第4章までの成果を踏まえ、メタンハンドレートを構成する有機炭素の給源について論じるとともに、ハイドレ―トが海底表層環境へ与えた影響を評価している。第6章では5章までに論じた内容を統合した議論を展開している。とくに、現在メタンハイドレートが発達する場所は、地形的高まりをつくっており、活背斜軸を中心に分布すること、その表層に完新統がほとんど堆積していないこと、メタンハイドレートを構成する炭素同位体が海底の深部起源であることを示すことから、メタンハイドレートが活構造の弱線を通じて地下から上昇・集積している可能性が高いこと、ハイドレートを含む地層自体も造構運動によって隆起していて、新しい地層に被覆されにくいことを論じ、これらを説明するモデルを提示している。終章である第7章では、本研究全体の結論を述べている。

以上のように、本研究は上越海盆におけるメタンハイドレ―ド分布域に留まらず、日本海東縁地域を広く研究対象としてとらえ、近年進捗著しい日本海における最終氷期以降の古海洋環境の変遷研究の成果の妥当性を追試するとともに、そうしたバックグラウンドを持つ地域において、メタンハイドレートが分布する場所での海底表層環境とその変遷を復元することによって、ハイドレートの影響を総合的に評価した。とくに、表層物質の地化学分析、鉱物分析を多数実施し、火山灰層序学的研究を行い、さらには、微化石を用いた生層序学的研究成果を用い、ハイドレート活動域においては、完新統が概ね欠除していること、ハイドレ―トの給源が地下深部に想定されること、その上昇経路の分布が活構造に規定されていると考えられることを示した点にオリジナリテイ―を認めることができる。

これらの成果は、メタンハイドレート研究の推進に貢献するのみならず、海洋古環境や古気候研究上も重要な知見を与えうるものである。以上の理由により、博士(環境学)を授与できると認める。