Yoshio Inouchi

Turbidites as records of intense palaeoearthquakes in Lake Biwa, Japan

Abstract Some 20 turbidites were collected in the uppermost 3 m of surface sediments in Lake Biwa, southwest Japan. These turbidites correlate in age to historical earthquakes that caused severe loss of lives and property near Lake Biwa. The age of each turbidite was estimated precisely, based on sedimentation rate (by weight) and the age of tephra markers below the turbidites. The magnitudes and epicentres of major historical earthquakes during the last 1300 years are available from historical documents. Empirical equations relating intensity, distance to the epicentre and magnitude enable us to calculate the ground shaking at Lake Biwa from these historical earthquakes. Earthquakes having intensity stronger than grade IV L on the Japan Meteorological Agency (JMA) earthquake scale, that is 44 gal acceleration at Lake Biwa, all coincide with the turbidite ages. This intensity is apparently the lower threshold for turbidity-current initiation in Lake Biwa.

A diatom record for the past 400 ka from Lake Biwa in Japan correlates with global paleoclimatic trends

Abstract Detailed diatom records from a 140-m sediment core with a time resolution of approximately 1000–1500 yr were obtained from Lake Biwa to examine possible correlations between diatom concentrations and paleoclimatic changes during the last approximately 400 ka. In the sediments, the diatom assemblages are generally dominated by planktonic diatoms. The diatom stratigraphy is characterized by four zones with distinctly higher concentration values and 12 horizons with low concentration values. Diatom records from the 140-m core and another core indicate that these changes were not local phenomena, but occurred throughout the lake basin. Comparison of diatom concentration records with pollen records from Lake Biwa sediments indicates that higher diatom concentrations represent warmer and wetter climatic conditions, and that lower concentrations represent colder and drier climatic conditions. Comparison of diatom records from Lake Biwa with marine oxygen isotopic records shows that the number of peaks and their relative amplitudes are similar in both profiles; therefore, peaks and drops in diatom concentration profile might correlate with most oxygen isotopic events. This finding indicates that during approximately the last 400 ka, the changes in diatom concentration, represented as diatom productivity in Lake Biwa, correlate closely with global paleoclimatic changes at the scale of the Milankovitch cycle (precession cycles of 23 and 19 ka), and that diatom concentration was higher during warm events and lower during cold or cooling events.

Reconstruction of a climate record for the past 140 kyr based on diatom valve flux data from Lake Biwa, Japan

To reconstruct the pattern of past climate change in central Japan during the last 140 kyr, total planktonic diatom valve concentrations (valves g−1) and fluxes (valves cm−2 year−1) of total planktonic diatoms flux (PVF) and individual species were examined using a 140-m core taken from Lake Biwa, Shiga Prefecture. Most records had a sample resolution between approximately 150 and 300 yr. Based on characteristics of past and modern diatom responses to possible climate variables, we interpreted changes in Stephanodiscus suzukii flux (SVF) to reflect changes in phosphorus levels, which reflect, in turn, summer precipitation levels; changes in Aulacoseiva nipponica flux (AVF) reflect winter vertical lake-water mixing induced by winter temperatures and snowfall levels. Thus, changes in total planktonic diatom flux reflect a combination of summer precipitation, winter temperature, and snowfall values. During the 140–101 ka interval, changes in S. suzukii productivity at a millennial timescale may correspond to changes in summer rainfall in central Japan. The disappearance of A. nipponica during the same period could indicate weaker vertical mixing, possibly caused by increased temperatures and decreased snowfall levels in winter. During the 101–70 ka interval, the AVF record shows levels near or above those observed in present times, indicating that winter water temperatures fell within the optimal range for A. nipponica to prosper. Generally low AVF values during the 70–7 ka interval indicate weak winter vertical mixing and cold winters. The many intervals with low PVF values during the same period suggest decreased summer precipitation levels. Between 7 and 0 ka, PVF, SVF, and AVF records show levels near or above those of the present, suggesting winter temperatures favorable for A. nipponica growth, and snowfall and summer precipitation levels probably similar to or above those currently recorded.

Using foraminiferal Mg/Ca ratios to detect an ocean-warming trend in the twentieth century from coastal shelf sediments in the Bungo Channel, southwest Japan

To assess abnormal warming of temperature in southwest Japans coastal seas during the twentieth century, we developed a 200 yr interannually resolved record of planktonic and benthic foraminiferal Mg/Ca-based temperature using neritic coastal sediment cores. The winter to late spring bottom temperature (50 m) record, based on benthic foraminiferal (Nonion japonicum) Mg/Ca ratios in the Bungo Channel (BC), showed consistent variation with observed temperatures on a five-year average basis. The BC bottom temperature record showed a significant increasing trend of 1.5°C /100 yr during the twentieth century, which was never apparent in nineteenth century. That result suggests that our Mg/Ca-based thermometry approach using coastal benthic foraminifera can detect abnormally rising temperatures in neritic coastal seas in southwest Japan. The abnormal warming of winter to late spring bottom temperature in the BC contrasts with the lack of an increasing trend in the Globigerinoides ruber-based summer to autumn temperature for the upper 20 m in the slope region of southwest Japan and the lack of an increasing trend in the steric sea level in the region. These results indicate a warming trend of the neritic coastal ocean in southwest Japan, especially in winter to late spring. The timing of the onset of the prolonged abnormal high-temperature stage, which started in the early twentieth century, suggests a link of the neritic coastal ocean in Japan with human-induced global warming.

Large scale sand dredging and mode of slow recovery of sand dunes in the Seto Inland Sea, Japan

During the last 40 years, a huge amount of sandy sediments were dredged from the seafloor of the Seto Inland Sea and hence sea bottom environments have been suffering various kinds of damages. For example, bottom topography and sediments of Mihara-seto Sea have been greatly changed. In order to clarify the possibility and the speed of restoration of bottom topography and sediments, precise depth measuring survey using SeaBat in regular time intervals were carried out. Comparison of depth between the data of old navigation chart and our survey shows the disappearance of large sand dunes. At the central part of the surveyed area, where the water depth was less than 20 meters, water depth exceeds 50 meters at present. Sediment composition also changed in the dredged areas from sandy to gravelly bottom. Sand waves stretching in north-south direction are distributed at the northern part of the surveyed area. Result of recent depth comparison indicates shifting of sand waves from the channels to the area far away. Their shifting velocity is estimated to be a maximum 40 m per year. Their height, width and shifting velocity decreased toward the central part. However, other kinds of topographical changes were not observed in the surveyed area. In addition, based on bottom topography data obtained by SeaBat, it was found that the total volume of sediments in the area has not changed. Based on the data obtained during the last two years, it was also clear that the undulations made by sand dredging buried into flat bottom by the shifting of sandy sediments. Restoration of bottom topography in the area is less probable because of the little amount of sediments supplied by both channels but restoration of bottom sediments may be very much probable at areas of sand wave distribution.

Liquefaction of reclaimed land in Toyo City, Ehime Prefecture, by the 2001 Geiyo earthquake

2001年3月24日15時28分, 安芸灘を震源とするM6.7の地震(芸予地震)が発生した. この地震は愛媛県中予地方と広島県南部を中心として, 住宅などに多大な被害をおよぼした. ここでは, 愛媛県東予市の埋立地で発生した地盤の液状化について報告する.

Drilling from cover ice on Academician Ridge, Lake Baikal

1996年2月から3月にかけて, バイカル湖の中央部, アカデミー湖嶺において氷上ボーリングが実施された. 今回の据削では, 湖底から300メートルの試料採取が試みられ, 鮮新・更新世の環境変遷史解明が目標とされている(第1, 2図). 掘削現場には, 掘削のシステムを乗せた鉄製の台船と, 曳航船および宿舎としてのウランウデ号が氷 (厚さ50ないし100cm) 中に固定されていた (第3, 4図).なお, 固定のためのアンカー等は使用されていない.今年度および来年度冬には, 北部湖盆において1000mの掘削が予定されており, 掘削候補地点選定のため, 湖内各地でピストンコア試料が採取されている. 日・米・露・ECの共同研究として実施された本掘削の研究成果は, 国際地圏生物圏研究計画 (IGBP)－古環境解析コアプロジェクト(PAGES)や生物進化学への貢献が期待されている. 今回は, 第1報として, 現場の据削状況を中心に紹介する(第5～10図). また, 昨年秋に中部湖盆の湖底平原 (最深部付近) でベリシャーギン号により採取したビストンコア試料についても簡単に言及する(第11～13図). なお, 図を作成するにあたって, 地質調査所燃料資源部の棚橋 学氏と非常勤職員の久保紀子氏の協力を得た. ここに謝意を表します.