Graphitization of carbonaceous matter during metamorphism with references to carbonate and pelitic rocks of contact and regional metamorphisms,Japan

This study is an attempt to correlate the graphitization process of carbonaceous matter during metamorphism with metamorphic grade. Graphitization can be parameterized using crystal structure and chemical and isotopic compositions. The extent of graphitization could be characterized mainly by temperature, duration of metamorphism and rock composition. We compared the graphitization trends for two metamorphic terrains, a contact aureole of the Kasuga area and a regional metamorphic terrain of high-temperature/low pressure type of the Ryoke metamorphic terrain in Northern Kiso area, Central Japan, and for two different lithologies (carbonate and pelite), using X-ray diffractogram, DTA-TG analysis, and chemical and stable isotope analyses. During contact metamorphism, graphitization and carbon isotopic exchange reactions proceeded simultaneously in pelitic and carbonate rocks. The decreases in basal spacing d(002) of the carbonaceous matter in carbonate rocks is greatly accelerated at temperatures higher than about 400° C. Furthermore, carbon isotopic ratios of graphite in carbonate rocks also change to ¹³C-enriched values implying exchange with carbonates. The beginning of this enrichment of ¹³C in the carbonaceous matter coincides with an abrupt increase of the graphitization processes. Carbon isotopic shifting up to 5 in pelites could be observed as metamorphic temperature increased probably by about 400° C. Carbonaceous matter in pelitic rocks is sometimes a mixture of poorly crystallized organic matter and well-crystallized graphite detritus. DTA-TG analysis is an effective tool for the distinction of detrital graphitic material. Two sources for the original carbon isotopic composition of carbonaceous matter in pelites in the Kasuga contact aureole can be distinguished, about-28 and-24 regardless of the presence of detrital graphite, and were mainly controlled by depositional environment of the sediments. Graphitization in limestones and pelitic rocks in regional metamorphism proceeds further than in a contact aureole. In the low-temperature range, the differences in extent of graphitization between the two metamorphic regions is large. However, at temperatures higher than 600° C, the extent of graphitization in both regions is indistinguishable. The degree of graphitization is different in limestones and pelitic rocks from the Ryoke metamorphic terrain. We demonstrate that the graphitization involves a progressive re-construction process of the crystal structure. The sequence of the first appearance of crystal inter planar spacing correlates with the metamorphic grade and indicates the crystal growth of three-dimensional structured graphite.     

Effects of stand characteristics on tree species richness in and around a conservation area of northeast Bangladesh

We investigated the effect of tree cover, forest patch and disturbances on tree species richness in a highly diverse conservation area of northeast Bangladesh. A systematic sampling protocol was adopted and 80 sub-plots from twenty five 1 ha plots were used for the vegetation survey. Linear regression analysis was performed to understand the effect of patch area, disturbances and tree cover on tree species richness. Ordination using Redundancy analysis (RDA) and Non-metric Multi Dimensional Scaling (NMDS) were also performed to explore the tree species compositional similarities along the stand characteristics gradient and locations of the sample plots. Our study revealed that, forest patch size has greater influence on species richness. Areas with medium level of disturbances have shown greater species richness. In constrained ordination the selected explanatory variables regulated the richness of common species. Our findings can be useful for better forest management and restoration of landscapes of conservation needs using ecologically important species.     

Cryptotephra detection using high-resolution trace-element analysis of Holocene marine sediments, southwest Japan

Detection techniques for invisible tephra, known as cryptotephra, have been exploited to construct precise and high-resolution correlations for a broad range of sedimentary sequences. We demonstrate that continuous trace-element profiles are an effective means for detecting probable positions of distal cryptotephra in Holocene hemipelagic sediments. Instrumental neutron activation analyses were performed on specimens of bulk sediments from five piston and gravity cores (water depths: 300-1500 m) taken from the southern Japan/East Sea. The down-core variations in the Ta/Sc ratio identify the positions of one to three alkaline cryptotephra in four of these cores. The Cr/Sc profiles show the position of one rhyolitic cryptotephra in three of the cores. The existence of tephra-derived components (glass ± crystals) was confirmed by microscopic observation, SEM-EPMA analysis and refractive index measurement on grains extracted from these layers. Based on microscopic observation and the stratigraphic correlations between cores, we identified eruption ages of the cryptotephras at 6.3, 7.5 and 9.3 ¹⁴C kyr BP, and two source volcanoes around 800 and 400 km from the study area.The tephra layers visible to the naked eye contained volcanic grains coarser than 200 μm, and the alkaline and rhyolitic tephra component comprised >20% and >33% of the sediment on weight basis, respectively. In contrast, the range of particle sizes of the cryptotephras detected in this study is finer than 125 μm, and almost all of the glass shards were finer than 40 μm. The alkaline and rhyolitic cryptotephras made up only 2-17% and 22-24%, respectively, of the sediment on weigh basis. The high sensitivity of this method stems from the significant difference in trace-element contents between the tephras and enclosing hemipelagic sediments in the core. Alkaline U-Oki tephra was enriched in Ta by one order of magnitude over that of the sediment, and depleted in Sc by one order. The rhyolitic tephra, K-Ah, was depleted by about one order in Cr relative to that of enclosing the sediment. The differences in chemical composition between within-plate alkaline tephras and hemipelagic sediments are usually so large that trace-element geochemical method is likely to be useful for alkaline cryptotephra detection in other areas with similar tectonic characteristics.     

Ocean anoxic events in the mid-Cretaceous simulated by a 3-D biogeochemical general circulation model

The mid-Cretaceous is well known for its ocean anoxic events. The causal mechanisms are controversial: stagnant deepwater, high biological productivity in the surface waters, and other possibilities have been suggested. Our study simulated the mid-Cretaceous ocean, using general circulation models combined with a marine biogeochemical cycle model to explore the relationship between thermohaline circulation and biogeochemical cycles and investigate the causes of ocean anoxic events. The simulated thermohaline circulation shows an unsteady inactive state. Oxygen concentrations in the deepwater decrease under the inactive state, but a horizontal gradient develops, with higher oxygen concentrations in the Tethys and lower concentrations in eastern Panthalassa. This is not due to the different ages of the deepwater but rather to the differences in biological productivity in the surface water, meaning that the relationship between thermohaline circulation and biogeochemical cycles under the inactive state is different from that in the present ocean. In the standard simulation, assuming the present level of the total amount of phosphate in the ocean, 29% of the bottom water is anoxic. The experiments increasing the amount of phosphate show its high sensitivity for extending the anoxic region with global-scale anoxia simulated under the doubled amount of phosphate. The high amount of phosphate would be reasonable because the inactive state would induce an imbalance of phosphate between riverine input and sediment output. Therefore, both the inactive thermohaline circulation and the increase in the total amount of phosphate in the ocean induce the global-scale anoxic condition in the deepwater.     

EPMA and PIXE dating of monazite in granulites from Stary Gierałtów, NE Bohemian Massif, Poland

Chemical Th–U–total Pb (CHIME) dating of monazite by electron probe microanalyzer (EPMA) and proton microprobe (PIXE) was carried out on felsic granulites from Stary Gierałtów, Poland, which represent part of the Orlica-Śnieżnik Dome in the NE Bohemian Massif. Analyzed monazite is characterized by mosaic zoning rather than simple core-to-rim growth, and strontium contents of up to 750ppm. An isochron age of 347 ± 13Ma represents timing of amphibolite-facies metamorphism, in agreement with previously published estimates.     

Large-scale landslides in Toyama Prefecture,central Japan,and their probable relationship with earthquakes

Large-scale landslides along the Kubusu and Besso rivers in Toyama Prefecture are developed in the Miocene Iwaine Formation, which is composed of andesitic lava, tuff, and tuff breccia. In the middle member of this formation, the tuff is easily altered to montmorillonite-bearing rock, and subsequently plays an important role in the development of landslides events, which tend to be large-scale events, as the massive lava of the upper member forms a cap rock over the tuff. The Kiritani and Koinami basins, which are flat intermontane basins located along the Kubusu and Besso rivers, respectively, are interpreted as landslide-dammed lakes, later filled with sediment. Accelerator mass spectrometry ¹⁴C ages show that the landslides forming each dam occurred simultaneously, at approximately 2500 BP. These ages were measured from wood fragments embedded in the landslide material of Kiritani, and from an in situ stump drowned during the impoundment of Koinami. If the trigger of these landslides was an earthquake, it is most likely to have been the penultimate event along the Atotsugawa fault zone.     

Nutrient and Phytoplankton Dynamics in Harima-Nada, Eastern Seto Inland Sea, Japan During a 35-Year Period from 1973 to 2007

Long-term monitoring of water quality and phytoplankton was conducted at 19 sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan for 35 years from 1973 to 2007. There were two significant long-term changes, an increase in winter water temperatures of 0.042°C year^?1, and a decrease in dissolved inorganic nitrogen (DIN) from about 10 μM in the 1970s to ~5 μM in the late 1990s due to the reduction in nutrient inputs. DIN concentrations and total phytoplankton cell density were both higher during the 1970s to the early 1980s and then exhibited a significant decrease in the mid 1980s and remained relatively constant thereafter. Diatoms were the dominant phytoplankton group (>90%) over the 35-year period, and there was a dramatic shift from Skeletonema dominance (~70%) to Chaetoceros in the mid 1980s. This shift in diatom species may be attributed to differences in the life cycle of Skeletonema and Chaetoceros and the response to the decrease in DIN concentration.     

U–Pb zircon age from the radiolarian‐bearing Hitoegane Formation in the Hida Gaien Belt,Japan

The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian‐bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian‐bearing Hitoegane Formation, Japan, was undertaken. The Hitoegane Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones and felsic tuff. The felsic tuff and tuffaceous mudstone yield well‐preserved radiolarian fossils. Zircon grains showing a U–Pb laser ablation–inductively coupled plasma–mass spectrometry age of 426.6 ± 3.7 Ma were collected from four horizons of the Hitoegane Formation, which is the boundary between the Pseudospongoprunum tauversi to Futobari solidus–Zadrappolus tenuis radiolarian assemblage zones. This fact strongly suggests that the boundary of these assemblage zones is around the Ludlowian to Pridolian. The last occurrence of F. solidus is considered to be Pragian based on the reinterpretation of a U–Pb sensitive high mass‐resolution ion microprobe (SHRIMP) zircon age of 408.9 ± 7.6 Ma for a felsic tuff of the Kurosegawa belt, Southwest Japan. Thus the F. solidus–Z. tenuis assemblage can be assigned to the Ludlowian or Pridolian to Pragian. The present data also contribute to establishing overall stratigraphy of the Paleozoic rocks of the Fukuji–Hitoegane area. According to the Ordovician to Carboniferous stratigraphy in this area, Ordovician to Silurian volcanism was gradually reduced to change the sedimentary environment into a tropical lagoon in the early Devonian. And the quiet Carboniferous environment was subsequently interrupted, throwing it once more into the volcanic conditions in the Middle Permian.     

Source Scaling of Inland Crustal Earthquake Sequences in Japan Using the S-Wave Coda Spectral Ratio Method

We estimate corner frequencies and stress drops for 298 events ranging from M _w 3.2–7.0 in 17 inland crustal earthquake sequences in Japan to investigate the source scaling and variation in stress drops. We obtain the source spectral ratio from observed records by the S-wave coda spectral ratio method. The advantage of using the S-wave coda is in obtaining much more stable source spectral ratios than using direct S-waves. We carefully examine the common shape of the decay of coda envelopes between event pair records. The corner frequency and stress drop are estimated by modeling the observed source spectral ratio with the omega-square source spectral model. We investigate the dependences of stress drops on some tectonic effects such as regionality, focal mechanism, and source depth. The principal findings are as follows: (1) a break in self-similar source scaling is found in our dataset. Events larger than M _w 4.5 show larger stress drops than those of smaller events. (2) Stress drops of aftershocks are mostly smaller than those of mainshocks in each sequence. (3) There are no systematic differences between stress drops of events occurring inside and outside the Niigata-Kobe Tectonic Zone in Japan. (4) Clear dependence of the faulting type on stress drops cannot be seen. (5) Stress drops of aftershocks depend on their source depth. (6) The crack size obtained from the corner frequency corresponds to the total rupture area of heterogeneous slip models for large events.     

Early Paleozoic subduction initiation volcanism of the Iwatsubodani Formation,Hida Gaien belt,Southwest Japan

International Journal of Earth Sciences - In placing Japanese tectonics in an Asian context, variation in the Paleozoic geological environment is a significant issue. This paper investigates the...