Koji Minoura

Tsunami deposits in a lacustrine sequence of the Sanriku coast, northeast Japan

Abstract Lacustrine deposits of the Pacific coast of northeast Japan, consisting mostly of black organic mud, contain intercalated thin beds of well-sorted medium sand. Examination of the deposits from a coastal site of Sanriku has revealed that sand grains are of marine origin and are fractions of deposits in marshy ponds, transported from the littoral environment by a great flooding of seawater. The mode of sedimentation shown in the thin beds of sand implies that they were deposited by tsunamis, each with a maximum rise of 1 m or more above sea level.

Discovery of Minoan tsunami deposits

The Hellenic arc is a terrane of extensive Quaternary volcanism. One of the main centers of explosive eruptions is located on Thera (Santorini), and the eruption of the Thera volcano in late Minoan time (1600–1300 B.C.) is considered to have been the most significant Aegean explosive volcanism during the late Holocene. The last eruptive phase of Thera resulted in an enormous submarine caldera, which is believed to have produced tsunamis on a large scale. Evidence suggesting seawater inundation was found previously at some archaeological sites on the coast of Crete; however, the cause of the tsunami and its effects on the area have not been well understood. On the Aegean Sea coast of western Turkey (Didim and Fethye) and Crete (Gouves), we have found traces of tsunami deposits related to the Thera eruption. The sedimentological consequences and the hydraulics of a Thera-caused tsunami indicate that the eruption of Thera volcano was earlier than the previous estimates and the tsunami did not have disruptive influence on Minoan civilization.

Sequence of sedimentation processes caused by the 1992 Flores tsunami: Evidence from Babi Island

Sedimentation processes caused by a modern tsunami have been discussed from the point of view of geologic and numerical investigations of the 1992 Flores tsunami in Indonesia. Geologic evidence on Babi Island shows an invasion of two waves of different direction and magnitude, which resulted in widespread deposition of marine sand on the north and south-southwest shores. On the latter, coarse and well-sorted carbonate sand containing molluscan shells suggests that much more destructive waves swept across the southern coast, as compared with the northern coast, where the deposit included medium carbonate sand with a silty component. A physical explanation for such destructive waves on the southern coast of Babi is provided by a numerical simulation of the tsunami generation and propagation. The geologic and numerical results indicate that an isolated island surrounded by a circular shoreline or reef edge will be subject to the most destructive waves on the coast on the back side of the island relative to the tsunami source.

CHAPTER THREE – TSUNAMIS AND TSUNAMI SEDIMENTOLOGY

Abstract Tsunamis are one of the most catastrophic wave motions, which cover a large parts of the sea and behave intricately especially in coastal zones. Studies on tsunami sedimentology have revealed that tsunamis induce various types of sedimentation in marine, lacustrine and onshore environments. Tsunami deposits are sedimentological evidence of tsunami events. This contribution describes the nature of tsunamis and of tsunami sedimentation as an introduction to this volume. Hydrodynamic aspects are introduced to explain the propagation of tsunamis. The diversity of tsunami deposits is illustrated on the basis of literature data. Onshore tsunami sedimentation is discussed in particular. Tsunami sedimentation appears to depend on the hydrodynamic and hydraulic character of the tsunami. The distribution pattern, grain-size variation and many other sedimentological structures reflect the characters of the tsunami such as the height, current velocity and period. Therefore, tsunami sedimentation should be interpreted based on careful consideration of the characteristics of tsunamis. This may result in a reliable reconstruction of ancient tsunami events.

Last-Glacial to Post-Glacial 10Be fluctuations in a sediment core from the Academician Ridge, Lake Baikal

10Be concentrations, sediment composition and sediment accumulation rates were determined in a sediment core from the Academician Ridge of Lake Baikal. The 10Be concentrations vary from 5.07 × 108 atoms/g in the last glacial section to 11.25 × 108 atoms/g in the section representing the post-glacial climate stage. 10Be fluxes, on the other hand, increase toward the last-glacial stage coinciding with an increase of the sediment accumulation rate. These facts imply that climate-related sedimentation is related to both flux and concentration of the nuclide in the sediment core. During the last-glacial stage, arid conditions in the lakes watershed caused a marked increase in flux of clay particles with low 10Be content to the lake, resulting in an increased flux but lowered concentration of 10Be in the sediments. It is suggested that the 10Be record of the bottom sediments of Lake Baikal can be used as a proxy indicator of climate change in central Asia.

DISTRIBUTION AND SIGNIFICANCE OF THE 2004 INDIAN OCEAN TSUNAMI DEPOSITS: INITIAL RESULTS FROM THAILAND AND SRI LANKA

Abstract The Indian Ocean tsunami of 26 December 2004 left tsunami deposits in the coastal areas of the surrounding countries. We conducted a preliminary investigation of the tsunami deposits on the coasts of Thailand and Sri Lanka 3 months after the event to clarify their distribution and significance. We observed abundant tsunami-transported reef blocks, megaripples and selective erosion and sedimentation in the backshore of Pakarang Cape in Thailand. Our data indicate eastward inundation of the west side of the cape by the tsunami. We also investigated sheet-like tsunami deposits at Bang Sak beach in Thailand and at Garanduwa in Sri Lanka, where the tsunami direction was recorded by bent grasses and trees. The thickness and grain size of the tsunami deposits are dependent on the local topography, but generally the deposits thin and fine landward. The deposits consist of silicate minerals, shell fragments and coral skeletal fragments transported from the beach or the offshore sea bottom by the tsunami waves. In the studied sites, the tsunami deposits are restricted to the lower half of the inundated zone, reflecting the low sediment transport energy of the waves.

Sediments and sedimentary processes in Mutsu Bay, Japan: Pelletization as the most important mode in depositing argillaceous sediments

Abstract Mutsu Bay is a large transgressive inland basin that originated in a set of structural depressions brought about by tectonic processes in early Quaternary time. The offshore beds of the bay consist of muds rich in organic material, while sands and gravels occur as a coastal sediment facies. Active sediment palletization of muds owing to feeding by polychaetes is recognized throughout the bay, and the bed surface is mostly covered with fecal pellets. Laboratory experiments in a seawater flume suggest that isolated fecal pellets are removed more easily than ambient muds. Continued pelletization dramatically reduces the critical entrainment velocity of the bed surface. Sedimentary processes in Mutsu Bay have been influenced not only by sea-level fluctuations but also by the oceanographic conditions of the Sea of Japan. Offshore mud deposition in the bay can be attributed to warm-water circulation which came into existence about 10,000 years ago. A wet and mild climate in consequence of temperatere seawater of post-glacial warming increased biochemical weathering of the land surface, leading to a voluminous discharge of fine-grained materials into the bay. Sediment pelletization by polychaetes facilitated easy transport of argillaceous grains in response to bottom currents, resulting in the development of offshore mud layers.

Paleoenvironmental Changes in the Eurasian Continental Interior during the Last 12 Million Years Derived from Organic Components in Sediment Cores (BDP-96 and BDP-98) from Lake Baikal

Organic components in combined sediment cores (BDP-96 and BDP-98) of BDP-96-1 and -2 (200 m) and BDP-98-1 and -2 (600 m) from Academician Ridge in Lake Baikal, in south-central Siberia, were studied to clarify the biological production, sources of organic matter, and paleoenvironmental changes during the last 12 million years (Myr) in the Eurasian continental interior as well as in the world. Total organic carbon (TOC) contents ranging from 0.13% to 2.8%, with an average of 0.93% (n = 7710), were fairly low, and were similar to those found in some oligotrophic lakes. TOC and total nitrogen (TN) contents had decreased by about half from 12 Myr ago to the present as a result of global climate cooling. Higher biological production periods are mainly attributed to the contribution of vascular plants.

Forest-desert alternation history revealed by the pollen-record in lake baikal over the past 5 million years

Publisher Summary The watershed of Lake Baikal constitutes the southern portion of the Siberian taiga and the northern part of the Mongolian steppe, and it is located at a high-latitude (51–56 N), far from the ocean. The paleoclimatic history of the Baikal region sensitively reflects past global changes, such as warm-cold and dry-moist oscillations of climate. Paleovegetational reconstruction of the region is indispensable to understanding how the Siberian taiga forest responded in timing and magnitude to past global changes. However, there are few palynological studies for paleovegetational reconstruction of the past million years in the region. This chapter presents a palynological study on the BDP96 Hole 1 core drilled by a Russian scientific drilling team in 1996. The results obtained in this study prove that the forest-desert alternations are closely related to global glacial–interglacial cycles during the Pleistocene. Considerable forest retreats are visible for relatively long periods during the late Pliocene, and a remarkable desert prevailing period during the early and middle Pliocene preceded the forest dominant period. The work in the Baikal area has also shown that these forest-desert alternations are more sensitive to global paleoclimate changes than those found in other continental region records.

Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan

ABSTRACTWe use temperature-dependent viscous remanent magnetization to estimate the emplace-ment age of tsunamigenic coral boulders along the shorelines of Ishigaki Island, Japan. The boulders consist of the hermatypic coral Porites , and the time of their deposition by tsuna-mis has been established using radiocarbon dating. Recently deceased corals at reef edges around Ishigaki Island record the Earth’s current magnetic field (present Earth field, PEF) as a remanence parallel to the field in the skeleton. Since the time when the coral skeletons were emplaced on the shorelines as boulders by destructive tsunami waves, a new viscous magne-tization was partially overprinted in the boulder parallel to the PEF. The results of thermal demagnetization indicated that the boulders were rotated at least once, and their emplace-ment ages determined from L. Neel’s relaxation theory for single-domain magnetite agree well with the radiocarbon ages, although there are traces of multidomain magnetites. New applica-tion of Neel’s theory to tsunamigenic coral boulders gives us an opportunity to ascertain the age and transportation mode of individual tsunamigenic coral boulders in this area.