In order to reveal sedimentary processes in shallow lakes, we collected samples from 118 points in Lake Kitaura-a shallow coastal lagoon in the eastern margin of the Kanto region, central Japan, and analyzed grain size distribution, TOC, TN and C/N ratio. We propose a new depositional model for shallow lakes controlled by wave action owing to wind. The distribution of sandy sediments is restricted to areas near the perimeter of the lake, and correlates with areas of low contents of TOC and TN. Alternatively, clay-rich sediments are most widely distributed, and show high concentrations of TOC and TN. The TOC /TN ratio is low, about 7 to 8 throughout the basin, suggesting that major organic matter originated from planktons in the lake. High TOC/TN ratios (over 8) recognized in the northern end of the lake evidently suggests the influence of tributary rivers. The bottom sediments of Lake Kitaura are characterized by a predominance of clayey sediments which cover most of the sublacustrine plain and a narrow zone of sandy sediments along the southern coastal area shallower than 2.5 m. The former sediments are transported as suspended matter reworked from the shallow and shore areas by wave action. The latter are lag deposits sifted by surface drift generated by prevailing wind. Low topography around the lake and small drainage area of each river appears to be the cause of the very small amount or lack of coarse-grained sediments around the river mouths in the narrow inlets. The sandy sediments in the southern margin of the lake are interpreted as relict sediments deposited during the last high sea-level period.
Spheroidal carbonaceous particles (SCPs) are formed from incomplete high temperature combustion of fossil fuels. Charcoal is generated from biomass burning. SCPs and charcoal are well preserved in sediments. In this study, we examined the moat sediment cores around Osaka Castle by SCPs and charcoal analysis, and reconstructed fire history and combustion history of fossil fuel. The results show that macroscopic charcoal concentration reached its peak in 1940s. These charcoal fragments were probably generated from bombing of Osaka city in World War II. Also, SCPs began to increase in the early 1930s and drastically increased in 1960s. These trends show the start of fossil fuel use in 1930s and energy conversion from coal to oil in the early 1960s around Osaka Castle. The feature and grain size of SCPs after 1960s are different from those before then, indicating that SCPs before and after 1960s were derived from different fossil fuels. SCPs and charcoal analysis are effective techniques for paleoenvironmental study during the era of the ‘Anthropocene (Crutzen, 2002)’, particularly aftter the industrial revolution.
Tephric-loess sequences are distributed widely in Japan. We attempt to distinguish volcanic events from the tephric-loess sequences using a newly developed ’RIPL Method (Refractive Index Physical Lobas Method)’. The RIPL Method is a technique of sampling a few centimeter thick strata continuously from the vertical section of a tephric-loess sequence, measuring all the refractive indices of minerals (mainly volcanic glass, orthopyroxene, and hornblende) included in each sample, and identifying tephra horizones in the tephric-loess from the characteristic feature of changes in the refractive index and mineral compositions. We selected two outcrops around Yufudake Volcano, Kyusyu Island, Japan. Point A is on the southern foot of Yufudake Volcano, and point Y is between Yufudake and Kuju Volcanoes. Tephric-loess is well preserved at both points and the deposits include four widespread tephra layers, Aso-4 (86-90 ka), Kj-P1 (age unknown), AT (29 ka) and K-Ah (5330BC). Using the RIPL Method, we distinguished twelve eruptive event horizons (Event horizons 1 to 12) in the tephric loess sequences above the Aso-4, We indicated the possibility that Kuju and Yufudake Volcanoes are eruptive sources of these events as well as Aso Volcano southwest of the two points. Among the events, the Event horizon 9 recorded the Shirani pyroclastic flow deposites erupted from Kuju Volcano and ACP4 airfall pumice from Aso Volcano. We calculated ages of the twelve events as base levels of individual event horizons, with the assumption that depositional rate of the tephric-loess sequence of point A has been constant since 86-90 ka. The Event horizons 1 (74 ka), 2 (72 ka), 4 and 5a (67 ka) recorded the tephras erupted from Kuju Volcano and the Event horizons 3 (69 ka), 6 (64 ka), 7 (60 ka), 8 (58ka) and 10 (47 ka) recorded the tephras erupted from Yufudake or it’s near volcanoes because the similarity of each petrographic characteristics. The Event horizon 11 (21 ka) recorded the tephra from Onimoniyama Volcano and the Event horizon 12 (15 ka) recorded the tephra erupted Yufudake Volcano. When using the RIPL Method in the area located outside of isopaches of the tephras, the improvement of the grasping accuracy of the volcanic activity can be expected.
The Chinen Formation in southern Okinawa-jima, southwestern Japan, is composed of calcareous sandstone and sandy limestone correlative with the transitional lithofacies between massive siltstone of the upper Pliocene Shinzato Formation of the Shimajiri Group and detrital to reefal limestone of the Pleistocene Ryukyu Group. The calcareous nannofossil and planktonic forminiferal biostratigraphy of the Chinen Formation in the Urizun and Golf Links sections, both located near the type locality, were studied. Abundant specimens of the calcareous nannofossils, Gephyrocapsa spp. (small), whose base occured at 2.09 Ma, was recognized throughout the uppermost Shinzato to Chinen Formations in the Urizun section. Three latest Pliocene to early Pleistocene datum planes, the last occurrence of Discoaster spp. (1.97 Ma), the first occurrence of Gephyrocapsa caribbeanica (1.73 Ma) and the first occurrence of G. oceanica (1.65 Ma), were concentrated in the lower part of the Chinen Formation. The latest Pliocene planktonic foraminifera datum planes, the first occurrence of Globorotalia (Globorotalia) truncatulinoides and the coiling change of Pulleniatina spp. from dextral to sinistral, were also recognized in the same horizon in the lower part of the Chinen Formation. These indicate that the Chinen Formation correlates with the latest Pliocene to earliest Pleistocene between 2.09 Ma and 1.65 Ma. The Pliocene/Pleistocene boundary, indicated by the first occurrence of G. caribbeanica (1.73 Ma), is situated in the lower part of the Chinen Formation in which a chronostratigraphic gap of 0.3 m.y. is present. Conversely, the stratigraphic distribution of calcareous nannofossils and planktonic foraminifera in the Golf Links section, shows that the disconformity recognized in the lower part of the Chinen Formation at the Urizun section, is correlated with the boundary between the Shinzato and Chinen Formations. The lithostratigraphic boundary beween the Shinzato and Chinen Formations in these adjacent sections, does not correspond with the chronostratigraphy.
A revision of the Neogene lithostratigraphy of the Akan area of eastern Hokkaido is based on a reexamination of lithofacies and diatom biostratigraphy. The Neogene strata are divided into the Chichappugawa, Tonokita, Chichappu, and Kotan formations, in ascending order. The lower Miocene Chichappugawa Formation is newly defined and named in this study. The age of the Tonokita Formation is early middle Miocene, the Chichappu Formation is middle to late Miocene and the Kotan Formation is latest Miocene to Pliocene. The relationship between the Chichappugawa and Tonokita formations is inferred to be an unconformity, which suggests tectonic movement between the early and middle Miocene. The discovery of early Miocene strata in the Akan area indicates that the early Miocene sedimentary basin in eastern Hokkaido extended toward the northeast and east, and that it was connected to the Sea of Okhotsk.