Shusaku Yoshikawa

Depositional processes of primary to reworked volcaniclastics on an alluvial plain; an example from the Lower Pliocene Ohta tephra bed of the Tokai Group, central Japan

Abstract A change from primary to reworked volcaniclastic deposits in the Lower Pliocene Ohta tephra bed has been identified, using facies analysis. The Ohta tephra bed is a widespread marker-tephra, distributed over a distance of more than 130 km. The source volcano of this tephra is inferred to lie 50 to 100 km north of the most proximal Ohta tephra bed. The Ohta tephra bed shows a change in volcaniclastic facies in distal areas. Eight sedimentary facies are recognized. These are interpreted as pyroclastic fall, pyroclastic flow, and reworked (mudflow, flood flow, channel fill, swamp or backswamp, lacustrine and ephemeral pond) deposits. In underlying and overlying non-volcaniclastic beds, four sedimentary facies are recognized, indicating a fluvio (-lacustrine) depositional system on an alluvial plain. From the successions of volcaniclastic sedimentary facies, deposition of the Ohta tephra bed can be divided into stages I to VI. Each depositional stage is intimately related to volcanic activity. Stage I is the first eruption, during which only the northern part of the study area was covered with fallout ash. The second eruption stage of stage II involved fallout ash distributed over all the study area. These two fallout ash beds are derived from precursor eruptions. Stage III reflects a period of volcanic quiescence and development of widespread lacustrine conditions. The fallout ash may have destroyed vegetation, and prompted greater discharge to form the large lacustrine deposits of stage III. A pyroclastic flow stage in stage IV is associated with an explosive eruption, and resulted in pyroclastic flow deposits reaching the northern part of the study area. Stage V is characterized by mudflow deposits. Widespread mudflow deposits also originated from high water and clastics discharge on the unvegetated surface. Mudflows extend more than 180 km from the source volcano. Abundant supply of volcaniclastic material and high water and clastic discharge enhanced a rapid progradation of alluvial plains. Final reworking occurred during stage VI, when volcaniclastic sediments were deposited from a fluvio-lacustrine system. The reworking system comprises initial mudflows, followed by a normal fluvial system (normal stream flow). The mudflow deposits of stage V are traceable with roughly constant thickness, but the volcaniclastic content of the deposits decreases in distal areas. The fluvial system deposits of stage VI are thicker and have less abundant volcaniclastics in distal areas. Fluvial systems were braided streams promoted by volcanic eruption, while they were meandering streams during the pre-eruption period. Some eruption-induced braided streams remained for a while following the cessation of volcanic activity.

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.

Benthic foraminiferal assemblages in Osaka Bay, southwestern Japan: faunal changes over the last 50 years

ABSTRACT Live benthic foraminiferal assemblages from surface sediment in Osaka Bay collected in 1999 were analyzed to characterize the distribution of the modern foraminiferal assemblages. Foraminiferal assemblages were compared with those of previous studies to document environmental changes in Osaka Bay over the past 50 years. Sixty-one species of foraminifera belonging to 37 genera were recognized from the 1999 surface sediment samples. An agglutinated assemblage containing Trochammina hadai and Eggerella advena is dominant in the inner part of the bay and is related to eutrophication. The foraminiferal assemblage in areas deeper than about 20 m is composed of Eggerella advena, Ammonia beccarii forma A, and Pseudorotalia gaimardii. This assemblage may be influenced by the large clockwise Okinose Circulation Current which extends throughout the western bay. Foraminiferal assemblages in Osaka Bay have changed dramatically during the last 50 years. The Trochammina hadai-Eggerella advena assemblage became established in the inner part of the bay, reflecting eutrophication that progressed from the 1960s through the 1970s. This assemblage became dominant in 1983, and typically dominated the inner part of the bay. From 1983 to 1999, however, the abundance of taxa belonging to this assemblage decreased greatly following implementation of 1973 Osaka City bylaws that restricted wastewater discharge. Changes in benthic assemblages such as the decrease of Ammonia beccarii forma A and increase of Eggerella advena have occurred in response to decreased incidence of red tides, and floral change in the species that cause these tides. The results of this study demonstrate that the abundance and distribution of benthic foraminifers in Osaka Bay are intimately related to environmental changes related to the urbanization of coastal areas.

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.

Depositional records of plutonium and 137Cs released from Nagasaki atomic bomb in sediment of Nishiyama reservoir at Nagasaki

In a sediment core of Nishiyama reservoir at Nagasaki city, depth profiles of (240)Pu/(239)Pu isotopic ratio, (239+240)Pu and (137)Cs activities were determined. Sediments containing plutonium and (137)Cs, which were deposited immediately after a detonation of Nagasaki atomic bomb, were identified in the core. Observed below the sediments were macroscopic charcoals, providing evidence for initial deposit of the fallout of the Nagasaki atomic bomb. This is the first entire depositional records of plutonium and (137)Cs released from the Nagasaki atomic bomb together with those from atmospheric nuclear tests.

Magnetostratigraphy of the Late Cenozoic Tokai Group in central Japan and its sedimentologic implications

Abstract The Late Cenozoic Tokai Group consists of fluvial sediments with many volcanic ash layers. Based on paleomagnetic measurements and tephrostratigraphy of this group, we have established the Tokai Polarity Superzone, which is divided into the TS-E, TO-D, TO-C, TO-B, and TO-A Polarity Zones in ascending order. These magnetopolarity zones are correlated with Chron 5 to 1. According to the magnetostratigraphy of the Tokai Group, sedimentation rates are at a maximum in the northwestern part of the basin and a minimum in the northeast. The depocenter collectively migrated clockwise and tilted westward.

Characteristic Differences in the Chemical Composition of Spheroidal Carbonaceous Particles in Japanese and Chinese Cities

Spheroidal carbonaceous particles (SCPs) are produced by the high-temperature combustion of fossil fuels and are emitted to the atmosphere. We examined the surface morphology and the chemical composition of SCPs in samples of surface sediment from five industrial cities each of Japan and China, using scanning electron microscopy with energy dispersive spectroscopy (EDS). There exists a relationship between surface morphology and the chemical composition of SCPs in China, but not for SCPs in Japan. The chemical compositions of SCPs differ between Japan and China: those in Japan are S-rich, whereas those in China are Ti-rich. The results suggest that EDS can be used to identify China-derived SCPs in East Asian countries such as Japan, South Korea, and Taiwan.

Nishiyama Reservoir: Lead Sources, Inventory, and the Influence of the Nagasaki Atomic Bomb

Sediment core samples were obtained from Nishiyama Reservoir, 3 km east of the hypocenter of the Nagasaki atomic bomb, to evaluate both sources and inventories of lead delivery to the sediments. Using microwave digestions and ICP-MS analytical techniques, elevated concentrations of isotopically unique lead were identified in the 1945 sediment horizon. These results illustrate that the Nagasaki bomb attack can be linked with unique lead loading to the sediments in the region of the hypocenter. There is a possibility that the roll-up of in-situ lead accounted for a majority of the increase in lead flux delivered to reservoir sediments during the atomic bomb attack.