Climate change and variations of atmospheric CO2 during Cenozoic have been discussed by many studies. The model results for global carbon cycle and climate change are generally consistent with those of many analytical studies concerning climate events in early Cenozoic, e.g., early Eocene warming events. However, the climate events proposed by analytical studies in late Cenozoic are not wholly inconsistent with the results of the model studies, e.g., Miocene climatic optimum (warming), cooling events in Eocene/Oligocene boundary and middle Miocene (15 Ma). Many problems remain in relation to evaluating CO2 flux by hydrothermal solutions at mid-ocean ridges, island-arc, and back-arc basins in the models. Also, the discrepancy may be derived from errors in estimating weathering flux, organic carbon burial, and change in vegetation. Moreover, another greenhouse effect gas such as methane, land-sea distribution, albedo variations due to the formation of ice-sheets, and temperature distribution attributed to changes in the ocean circulation system should be considered. Recent analytical studies reveal that the CO2 level since Miocene has remained relatively low in spite of the suggested climate events in this period. The mechanism of oceanic environmental change, as well as atmospheric CO2, is especially important to elucidate climate change during Cenozoic.
The mean recession rate of Kegon Falls is examined, estimating original location and age. Two locations (LI and LII) at which the waterfall originated are estimated based on the distribution of Kegon lava, while the age of origination is considered to be the same as that of the Kegon lava eruption, i.e., 20, 000 years ago. Two values of the mean recession rate obtained are 0.018 m/y for the case of LI and 0.10 m/y for the case of LII. On the other hand, the recession rate can be evaluated, as an order estimation, using the empirical equation of the authors (Hayakawa and Matsukura, Earth Surf. Process. Landforms, 28, 2003), which shows relationships between the recession rates of waterfalls and the ratio of the erosive forces of rivers to bedrock resistance. The values of parameters used in the index of force/resistance ratio are obtained from existing data and field measurements for Kegon Falls. Substituting them into the equation, the recession rate is calculated to be in the order of 0.009-0.019 m/y. This result suggests that the recession rate for the case of LI is suitable for Kegon Falls.
The Median Tectonic Line active fault system (MTL), with an average slip rate as high as 5-10 mm/yr, is one of the most active inland faults in Japan. However, the long-term seismic risk of the MTL has been poorly known, because of insufficient paleoseismological data, especially timing and displacement associated with the most recent surface faulting. We carried out a trench excavation survey across the Hatano fault in Doi town, Ehime Prefecture, and were able to precisely determine the timing of the latest faulting event. The survey site is located between range-facing fault scarps level 0.8 m high on an alluvial fan. We first excavated two trenches across the fault to precisely locate fault traces. Faults cutting Holocene sediment were exposed on both walls of each trench. The sense of apparent displacement across the fault zone is down to the south, which is consistent with fault scarps around the trench site. Then we excavated two trenches parallel to the fault zone to expose stratigraphic evidence of horizontal displacement associated with past earthquakes. The sediment exposed on the trench walls contains evidence of two faulting events in the past 3500 years B.P. The most recent surface faulting along the Hatano fault occurred between 1520 cal A.D and 1660 cal A.D. This is the first paleoseismological data that precisely constrain the timing of the most recent faulting event of the MTL. We have estimated 2.5 ± 0.5 m right-lateral displacement and 0.3-0.5 m vertical displacement up to the north during the most recent faulting event based on an offset of paleo-channel deposit.
The Toyama Plain, central Japan, is a depression along the northward-flowing Zintsu River and Jogannji River. The western margin of the Toyama Plain is bounded by NNE- to NNW- trending Kurehayama fault. We estimated the formation age of fluvial terraces and the fault topography of the Kurehayama fault along the western part of Toyama Plain based on geomorphological and tephrochronological studies. Late Quaternary fluvial terraces in the western part of Toyama Plain are divided into ten levels : terrace I to X in descending order. Terrace III is overlain by the Kikai-Tozurahara tephra (75-95 ka) and the Tateyama D tephra (95-130 ka), Terrace V is overlain by the DaisenKurayoshi tephra (43-55 ka), and Terrace VI is overlain by the Aira-Tanzawa tephra (22-25 ka), respectively. The Kurehayama fault is a reverse fault extending for 20 km along the western margin of the Toyama Plain. The Kurehayama fault is characterized by fault scarps several meters high on late Quaternary fluvial terraces. Average vertical slip rates for the southern part of Kurehayama fault, Yatsuo area, are estimated at 0.08-0.41 mm/yr and vertical displacement at Terrace VI is more large northward. From the central to northern parts of Kurehayama fault, fault scarps 2 to 4 meters high on late Terrace IX and X, both terraces are Holocene terrace. Such a young offset implies that the last faulting events occurred since Terrace IX and X formed.
Vendian-Cambrian Baratal limestone occurs as large allochthonous blocks in the Cambrian accretionary complex of the Gorny Altai Mountains, southern Russia. We analyzed the primary stratigraphy and depositional environments of Baratal limestone in the Kurai and Akkaya areas in the eastern part of the Gorny Altai Mountains. In the Kurai and the Akkaya areas, Baratal limestone conformably overlies basaltic greenstone. Geochemistry of this greenstone is similar to that found in modern oceanic plateau basalt or oceanic island basalt. The limestone lacks terrigeneous elastic influx. These suggest that the Baratal limestone was originally deposited on and around a plateau or seamount far from the continents in a mid-oceanic environment. Baratal limestone in the study area is lithologically divided into four types; 1) massive lime mudstone, 2) massive limestone conglomerate 3) bedded lime mudstone with slump structures, and 4) laminated lime mudstone. Massive lime mudstone contains stromatolites and ooids. This evidence suggests that the massive lime mudstone was formed in a shallowmarine environment. The massive limestone conglomerate contains angular clasts of lime mudstone, greenstone and chert. Its poorly graded and poorly sorted characteristics suggest that the limestone conglomerate was formed as debris flow deposits. Some parts of bedded lime mudstone have slump structures, and are interpreted as sliding deposits. In addition, laminated lime mudstone that shares an affinity with limestone turbidite, is associated with the massive limestone conglomerate. Sedimentary environments of these four types of limestone are inferred, respectively as follows ; massive lime mudstone may have been diposited on the top of a paleo-plateau/ -seamount, while massive limestone conglomerate, bedded lime mudstone with slump structures, and laminated lime mudstone on the slope of a paleo-plateau/-seamount.
Chemical and isotopic analyses (Sr isotopic ratio, major element, trace element, rare earth element, total carbon, nitrogen, and sulfur contents) of rock samples collected from middle Miocene to early Pliocene sedimentary rocks, Oga Peninsula, northern Japan were performed to elucidate the paleoceanographic environment of Japan Sea. The rocks studied include shale from Nishikurosawa, Onnagawa, and Funakawa formations in stratigraphically ascending order. The Onnagawa sedimentary rocks in the lower (ca. 12.6-11.4 Ma), middle (ca. 10.5-9.0 Ma), and upper (ca. 8.3-7.0 Ma) horizons are characterized by high Mo/Al, P/Al, and Ba/Al ratios and total organic carbon content. Positive Eu anomaly, K/Ti ratio and 87Sr/86Sr ratio are also high in the same horizons. These geochemical variations imply that high primary productivity, and reducing condition of deep paleoocean, and formation of petroleum source rocks were caused by an upwelling of deep seawater. The upwelling of deep seawater is considered to have been influenced by strong winds from the Asian continent, which was related to the uplift of Himalayan and Tibetan regions.
Dramatic changes of glaciers and vegetation are taking place in the glacier-covered high mountains of East Africa. The Tyndall Glacier on Mt. Kenya, which retreated at ca. 3 m yr-1 from 1958 to 1997, retreated more at ca. 10 m yr -1 from 1997 to 2002. Pioneer species such as Senecio keniophytum, Arabis alpina, mosses, lichen, and Agrostis trachyphylla have advanced over areas formerly covered by the glacier. The rate at which this vegetation was migrated up the former bed of the glacier (2.1-4.6 m yr-1 from 1958 to 1997) is similar to the rate of glacial retreat (2.9 m yr-1). In the interval from 1997 to 2002, when the glacier retreated at 9.8 m yr-1, pioneer species advanced at a rapid rate of 6.4-12.2 m yr-1. Larger woody plants such as Senecio keniodendron and Lobelia telekii, which showed no obvious advances before 1997, have advanced quickly since 1997. Rapid glacial retreat has been accompanied by rapid plant colonization. Pioneer species improve soil conditions and habitat for other plants. The distribution of large woody plants such as Senecio keniodendron and Lobelia telekii, which tend to grow in areas that were deglaciated 100 years previously, is not directly controlled by glacial retreat, but reflects different effects of debris flow and outwash, and spatial differences in the clast size of the sedimentary debris at the surface of any given area. For example, Senecio keniodendron grows most abundantly in areas covered by large debris, such as moraines.