Ken Ikehara

Bio: Ken Ikehara is an academic researcher from National Institute of Advanced Industrial Science and Technology. The author has contributed to research in topics: Trench & Holocene. The author has an hindex of 30, co-authored 189 publications receiving 3158 citations.

Submarine Mass Movements and Their Consequences

Abstract: Submarine mass movements represent major offshore geohazards due to their destructive and tsunami-generation potential. This potential poses a threat to human life as well as to coastal, near shore and offshore engineering structures. Recent examples of catastrophic submarine landslide events that affected human populations (including tsunamis) are numerous; e.g., Nice airport in 1979 (Dan et al. 2007), Finneidfjord in 1996 (e.g., L’Heureux et al., this volume, Steiner et al., this volume), Papua-New Guinea in 1998 (Tappin et al. 2001), Stromboli in 2002 (Chiocci et al. 2008), and the 2006 and 2009 failures in the submarine cable network around Taiwan (Hsu et al. 2008). The Great East Japan Earthquake of March 2011 also generated submarine landslides that may have amplified effects of the devastating tsunami as shown in Fryer et al. (2004). Given that 30% of the World’s population lives within 60 km of the coast, the hazard posed by submarine landslides is expected to grow as global sea level rises. In addition, the deposits resulting from such processes provide-various types of constraints to offshore development (Shipp et al. 2004), and have significant implications for non-renewable energy resource exploration and production (Weimer and Shipp 2004; Beaubouef and Abreu 2010).

Rock-magnetic changes with reduction diagenesis in Japan Sea sediments and preservation of geomagnetic secular variation in inclination during the last 30,000 years

Abstract: A rock-magnetic and paleomagnetic study was conducted on a sediment core of about 4.4 m long taken from the northeastern part of the Japan Sea. The core covers the last about 30 kyrs, which was dated by nineteen radiocarbon (14C) ages. Remanent magnetization is carried dominantly by magnetite. Reductive dissolution of magnetic minerals occurs between 1.2 and 1.6 m in depth (about 5–8 ka in age). A rapid downcore decrease of anhysteretic remanent magnetization (ARM) begins at the shallowest depth. Saturation isothermal remanent magnetization (SIRM) follows, and a decrease of magnetic susceptibility (k) takes place at the deepest. Within this zone, coercivity of natural remanent magnetization (NRM) and the ratios of ARM to k and SIRM to k also decreases with depth. These observations indicate that finer magnetic grains were lost earlier than larger grains. A decrease of S ratios, wasp-waisted hysteresis curves, and a deviation from a mixing trend of single-domain and multi-domain grains in a Day plot occur as the dissolution proceeds, which suggests that high coercivity minerals like hematite are more resistive to dissolution than low coercivity minerals like magnetite. The start of the dissolution at 1.2 m in depth is synchronous with increases in organic-carbon and total-sulfur contents, but the horizon does not coincide with the present Fe-redox boundary at about 0.02 m below the sediment-water interface. From low-temperature magnetometry, it is estimated that magnetites with maghemite skin are reduced to pure magnetites prior to dissolution. There is no evidence for precipitation of secondary magnetic phases and acquisition of chemical remanent magnetization (CRM). Neither pyrrhotite nor greigite was detected. Information of paleomagnetic directions have survived the reductive dissolution. Inclination variations of this core resembles closely to the secular variation records available around Japan. Well-dated records older than 10 ka are still very rare, and hence our new record could be useful for establishing regional secular variations.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Abstract: The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.