The 2003 annual meeting of the Sedimentological Society of Japan was held as its first meeting after the renewal of this society at Yayoi Theater, University of Tokyo in April, 2003. The symposium titled “New Horizons and Prospects in Sedimentology” on the first day of the meeting consisted of nine speakers, including two foreign ones, from various specialized fields in sedimentology. We invited Dr. Seung-Soo Chun from Korea and Dr. Li Sitian from China for constructing collaboration of study of sedimentology in East Asia. We were sorry that Dr. Li Sitian was absent for social disturbance in Beijing, China caused by SARS. Dr. Chun gave a talk, “Sedimentation and Holocene Evolution of Macrotidal-flat Depositional System in the Southwestern Coasts of Korean Peninsula”.
My talk as the president of the Sedimentological Society of Japan focused on the future of sedimentology in Japan as well as of the Sedimentological Society of Japan. I have done researches on modern fluvial and tidal deposits and depositional environments for more than ten years. Sedimentology on modern sediments and depositional environments are connected with social sedimentology. For example, the role of dams in rivers for environmental sciences is questionable, that is, the role must be discussed in social sedimentology. Because dams in the river break the transportation system of sediments to the sea, beach sands are hardly supplied through the river, and most of the beaches in the Japanese coasts are in erosional condition.
As one of the fields in which sedimentologists are able to do social contribution vigorously, I propose the system of “doctors for natural disaster”. Doctors of natural disaster observe geology, geography and ecology in the area. Based on the results of observations and the occasional characteristics, they point out hazardous areas and estimate type, size and frequency of natural disasters; for example, landslide caused by huge earthquake and debris flow after massive rain fall. And the most important role of doctors of natural disaster is to educate citizen how to act appropriately before and during disasters. This position of doctors of natural disaster in each municipality will be contribute reduction of damage by natural disasters.

View full abstract

Sedimentology is a basic discipline in geological sciences established only in the midst of the 20th century. It had, however, a history of pre-sedimentology as long as that of geology itself. Reviewing the history of sedimentology, the establishment and development of sedimentology in Japan are discussed on the following topics: (1) stratigraphy and lithology, (2) proposal of the term “Sedimentology”, and (3) the process of establishment of sedimentology and its development in the four stages, 1950-1960, 1960-1980, 1980-2002, and since 2002. A special mention is made of the first introduction of the term “Taiseki-gaku” (sedimentology in English) in 1929 by Tsugio Yagi, a professor of Tohoku University, which was much earlier than the proposal of the term “Sedimentology” by Hakon Wadell in 1932. In this respect, a tribute of praise should be paid to Tsugio Yagi not only in Japan but also internationally.

View full abstract

This paper attempts to figure out new perspectives for the future sedimentary basin studies on the basis of the historical overview and recent study trends on this field. Genetic viewpoint on sediments has developed prominently during the last thirty years, being affected by the sedimentary facies and sequence stratigraphic concepts. Understanding the origin of sediments and sedimentation controlling factors in basin filling is an important point in sedimentary basin analysis because it makes possible for us to conduct precise sedimentological studies including the past environment analysis and hydrocarbon exploration. Recent scientific research and petroleum exploration projects tend to require multidisciplinary approaches because of its usefulness for the research collaboration. The genetic viewpoint on sediments plays an important role to integrate the wide-ranging results derived from multiple study fields as a core of multidisciplinary approaches. Not only multiple “study fields” but also “analytical and modeling methods” for sedimentary basins are subject to the multidisciplinary approaches. In addition to the conventional backward and deterministic modeling methods of sediments such as facies and sequence stratigraphic models, forward, quantitative and stochastic modeling methods are recently developing. Depositional experiments and process simulations determined by multiple parameters provide important information on the relationships between sediments and sedimentation controlling factors. Stochastic simulations are also being introduced with the geostatistical concepts for sediment distribution analysis. Recent developments on the 3-D seismic and well-log technologies provide innovative solutions on the distributions and properties of sediments through seismic attribute, seismic facies and petrophysical data sets.
It is expected that the developments of the genetic viewpoint on sediments and the multidisciplinary approaches for study fields and analytical methods will bring a new synergy in the future integrated sedimentary basin analysis.

View full abstract

Sedimentology has been evolving into a new field in the Earth System Science from a part of classic geological science. A transcending time-scale study on active sedimentary processes including modeling by STRATAFORM and Source-to-Sink projects since year 1996 has been leading us to new stage after sequence stratigraphy. Such scientific trend and new research project on Deltas in the Monsoon Asia-Pacific region (DeltaMAP) are introduced.

View full abstract

Study of sedimentary organic matter in the latter half of 20^th century had been greatly promoted by fossil fuel industry. As a result, various organic geochemical indicators to reconstruct thermal/burial history and depositional environment of the sedimentary rock were established. Computer modeling of basin evolution and oil/gas accumulation with organic geochemical indicators is now a matter of routine for petroleum exploration. The great leap forward in organic geochemistry relevant to petroleum exploration seems to have come to an end.
In addition to a problem of energy resource exhaustion, another our great concern is a rapid change of global climate. Organic-rich sediments are important not only as energy resources but also as a huge carbon reservoir in global climate system. The elucidation of temporal and spatial global climate change rests upon organic geochemical/biogeo-chemical study of sediments. Researches related to carbon-burial fluxes and to the formation and preservation of biologically controlled paleoenvironmental records in sea-floor sediments have been extensively conducted in the last few decades. Development of paleoenvironmental proxies, reconstruction of ecological system by molecular biomarkers, temporal and spatial variation of fluxes of biogenic materials and so on are also researches in area of organic geochemical and biogeochemical study of sediments. This type of research field is recently called “Biogeochemical Sedimentology”.
Problems of energy resource exhaustion and global climate change are still driving forces to promote development of the study of organic matter in sediments and sedimentary rocks. Organic and biogeochemical Sedimentology takes responsibility for development of non-conventional energy resources such as methane gas hydrates. Microbial activity in sediments plays an important role for carbon circulation and the formation of gas hydrate. The Earth system is based upon the Earth and Life's interaction, which always plays an important role to induce evolution of biosphere. Development of organic and biogeochemical sedimentology in this new century will increasingly depend on collaborations with biology. Organic and Biogeochemical sedimentology will contribute to understanding the Earth system and to promote neo-science of natural history for a new view of nature in the 21^st century.

View full abstract

GAS HYDRATES-A HUGE CARBON SINK OF SHALLOW GEOSPHERE:
Findings of gas hydrates in marine sediments in late 20^th century have made a strong impact on the study of carbon cycle and global changes as well as on the exploration for energy resources. Gas hydrates are crystalline materials composed of cages of water molecules that surround low-molecular-weight gas such as methane, ethane, propane, carbon dioxide, and etc. Naturally occurring gas hydrates are mostly composed of methane and water, then called as methane hydrate, but often referred as gas hydrates (e. g., Lancelot and Ewing, 1972; Shipley et al., 1978).
Recent geological and geophysical surveys for natural gas hydrates have revealed that the upper few hundred meters of the deep sea sediments of continental margins are often associated with laterally extensive gas hydrate deposits that underlain by free gas zone. Methane Hydrate Exploration Project of METI-JNOC (1995-) identified and recovered gas hydrate saturated sands from the Nankai Trough (e. g., Matsumoto, 2000). The pore saturation of the sands reaches up to 80%. The total carbon mass stored in modern gas hydrates is estimated to be 7, 500 to 15, 000 Gt (Gt=10¹⁵g), which is -25% of the total DIC in the ocean, -10⁴ times the amount of methane carbon in the atmosphere, and comparable to the conventional fossil fuel reserves (e. g., Kvenvolden, 1988, 1993). Methane of marine gas hydrates is mostly originated from microbial methanogenesis and is therefore extremely depleted in ¹³C (δ¹³C=-60 to -90%0 PDB).
Gas hydrates are stable at low temperature and high-pressure conditions with certain amounts of gas and water. Therefore, gas hydrates in marine sediments is sensitive to even a little change in the relative sea level, bottom water temperatures, and geothermal gradients. The modeling of carbon cycle and global change must take into account the presence of gas hydrates in marine sediments and the consequences of the dissociation and possible release of large amount of methane carbon into the ocean-atmosphere system.
δ¹³C OF MARINE CARBONATES AND GAS HYDRATE INDUCED EVENTS:
Stratigraphic boundary events with mass extinction are often associated with abrupt negative excursion in δ¹³C of marine carbonates. Extensive depletion in ¹³C in the shallow oceans had been explained in two ways; a mass extinction and reduced primary productivity-photosynthesis (Hsu et al., 1985; Hsu and McKenzie, 1990) and an oceanic overturning and consequent upwelling of anoxic bottom waters (e. g., Hoffman et al., 1991). Large releases of isotopically light carbon are also interpreted to have come from massive dissociation of gas hydrates (e. g., Dickens et al., 1995; Matsumoto, 1995; Kennett et al., 2000).
The “Latest Paleocene Thermal Maximum” (LPTM) ca. 55Ma is a typical example of “gas hydrate induced” boundary event. LPTM was characterized by a 4 to 6°C rise in deep ocean water temperature (Kennett and Scott, 1991), coincided with major extinction of benthic foraminifers (Kaiho, 1994) and negative carbon isotopic excursion (Δδ¹³C=-2 to -3‰). Early Paleogene was a period of general warmth, and futhermore a sharp temperature increase is recorded within a short interval of ca. 55Ma. High-resolution isotope records indicate that the main drop and gradual return in δ¹³C spanned less than 10, 000 years and 200, 000 years, respectively (e. g., Bains et al., 1999). The timing and magnitude of the δ¹³C excursion across LPTM are best explained by gas hydrate hypothesis. General increase of bottom waters, probably caused by recorded igneous activities, finally triggered thermal dissociation of marine gas hydrates and release of large amount of ¹³C-depleted methane to the ocean (e. g.,

View full abstract

Significance of the linkage between tectonics and climate is the long-lasting first order question in Geology that has never been answered clearly. The linkage between the uplift of Himalaya and Tibetan Plateau [HTP] and evolution of Asian-Indian monsoon has been regarded as the best example of tectonics-climate linkage because of its potential magnitude and importance with respect to global climatic evolution during the late Cenozoic. Although previous attempts failed to test this hypothesis because of the paucity and ambiguity of the tectonic and paleoclimatic data to constrain the timing and mode of both HTP uplift and monsoon evolution, the situation changes rapidly due to recent accumulation of tectonic data in HTP region and paleoclimatic data in inland Central and East Asia as well as continuous improvement in resolution and speed of climatic simulations.
In this synthesis, I will briefly summarize resent progress concerning our understanding on the timing and mode of the HTP uplift and the onset and evolution of the East Asian monsoon. I will then discuss potential significance of the linkage between the onset and evolution of millennial- to orbital-scale variability of Asian monsoon and the HTP uplift. Finally, I will demonstrate the significance of the impact of the HTP uplift and monsoon evolution on paleoceanography of the East Asian marginal seas.

View full abstract

Sedimentation in the open-coast tidal flats of southwestern Korea is controlled by the combination of wave and tide with seasonal variation. Environmental oscillation takes place between tide-dominated muddy deposition in summer and wave-dominated sandy deposition in winter. Winter storm is a major factor in sedimentation and preservation in the intertidal zone, producing extensive wave-generated parallel lamination and short-wavelength HCS/SCS. Winter storm waves dominate sedimentation over the long term in this setting, resulting in predominant preservation of amalgamated storm beds which are very similar to those associated with shoreface. It causes much confusion in differentiation between deposits in true shoreface and open-coast tidal flat, suggesting that some ancient shoreface deposits should be reinterpreted in terms of the concept of open-coast tidal-flat sedimentation.
The retrograding, coarsening-upward, late Holocene succession in this open coast has resulted from low sedimentation rates under low to moderate rates of sea-level rise. It shows a reverse pattern as compared with the models developed in embayed tidal flats with high sedimentation rate such as Jade- and Fundy-bay tidal flats which show a prograding, fining-upward succession. This result suggests that most southwestern open coasts in Korean Peninsula would have much higher potential to experience coastal erosion and inundation with the future sea-level rise than the coasts developed under high sediment supply.

View full abstract

To investigate the Recent lacustrine sedimentary environments, 16 bottom samples and 2 cores from Lake Amida and 7 bottom samples and a single core from Lake Kamegaike, both in Ehime Prefecture, Japan, were analyzed in terms of sedimentation rates and grain size distribution.
In Lake Amida, the average sedimentation rate calculated from a core taken at a shallower water depth site was 1.35cm/y, which is faster than the average rate of 0.92cm/y calculated from another core taken at a deeper site in the same lake. In the core from Lake Kamegaike, an increase of the sedimentation rate from 0.54cm/y to 1.36cm/y was detected at the core depth of 60cm. The surface of this increase is dated to be ca. 1954, which roughly corresponds the time when a water gate was constructed. This increase would be explained as an effect of the gate that should have prevented the suspended matter in the lake to flow out into the sea.
The sedimentation rates obtained in this study were generally larger than those in other Japanese lakes. This may be due to a probable higher rate of soil erosion and transportation, because the area around the lakes of this study has been exploited largely for agriculture such as orange fields. In addition, the variation of depth profiles of Pb-214 and K of the three cores is well correlated to artifact events such as shore protection and road works, which suggests that such events should have affected even on the distribution of nuclides.
These results have given some important clues to study sedimentation models and environmental changes of lakes.

View full abstract

Sediment transport on the riverbed of the Metoba River, flowing in the urbanized area of Matsumoto City, is influenced by vegetation. On the vegetated (Phragmites japonica) riverbed conveyance of coarser gravel was dominant during flooding in the spring to summer rainy season, and resulted sediments were found in the fall to winter dry season as gravel-floored longitudinal grooves in the vegetated area. A flume experiment indicates that the mobility of coarser fraction of bedload becomes higher on a vegetation-covered bed than on a non-vegetated gravel/sand bed. Once a larger grain flowed down on a vegetated riverbed, a continuous path for both coarse and fine material was established forming a gravel ribbon, then it changed into a longitudinal groove later.

View full abstract