Relationship between sinking organic matter and minerals in the shallow zone of the western Subarctic Pacific

We used time-series sediment trap data for four major components, organic matter and ballast minerals (CaCO₃, opal, and lithogenic matter) from 150, 540, and 1000 m in the western subarctic Pacific (WSAP), where opal is the predominant mineral in sinking particles, to develop four simple models for settling particles, including the “ballast model”. The ballast model is based on the concept that most of the organic matter “rain” in the deep sea is carried by the minerals. These four models are designed to simultaneously reproduce the flux of each major component of settling particles at 540 and 1000 m by using the data for each component at 150 m as initial values. Among the four models, the ballast model, which considers the sinking velocity increase with depth, was identified as the best using the Akaike information criterion as a measure of the model fit to data. This model successfully reproduced the flux of organic matter at 540 and 1000 m, indicating that the ballast model concept works well in the shallow zone of the WSAP on a seasonal timescale. This also suggests that ballast minerals not only physically protect the organic matter from degradation during the settling process but also enhance the sinking velocity and reduce the degree of decomposition.     

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb–Sr and fission-track ages of gneissose granitoid and numerical modeling

Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.     

Characterization of sulfate ion in travertine

The mode of incorporation of sulfate ion in travertine was discussed on the basis of chemical compositions, i.r. and laser Raman spectra. These data strongly suggest that most of the sulfate ions in the calcitic travertine replace carbonate ions. This conclusion is in good harmony with the facts that calcite incorporates more sulfate ions than aragonite does and that the sulfate content of manganoan calcite decreases with increasing manganese content (Takano et al. 1977). Based on this conclusion, retarding effect of sulfate ion on the precipitation of calcite from solution was discussed.     

BSRs and Associated Reflections as an Indicator of Gas Hydrate and Free Gas Accumulation: An Example of Accretionary Prism and Forearc Basin System along the Nankai Trough, off Central Japan

Abstract. Multi-channel seismic data obtained from the Nankai accretionary prism and forearc basin system has been studied to elucidate the migration and accumulation process of gas to the BGHS and examine the distribution pattern of BSRs and characteristic reflections associated with them.
BSRs are distributed widely in the Nankai accretionary prism and associated forearc basins (33,000 km²) and 90 % of them have migration and recycling origins. The widest distribution of the BSRs can be seen at the prism. A correlation between the BSR distributions and prism size shows that the BSRs tend to be more well-developed in a prism of large size. This suggests that a large prism may produce much amount of gas-bearing fluids that migrate to the BGHS and form the BSRs (tectonic control), hi the forearc basins, the BSRs are identified at topographic highs, anticlines and basin margins (structural control).
The upward migration of gas-bearing fluids is carried out through permeable sand layers and as a result, the distribution of BSRs is confined to alternating beds of sand and mud facies (sedimentary control). However, if there is enough time for upward migration and accumulation of gas to the BGHS, the BSRs can be generated widely in low-permeable mud facies (time control).
Those results imply that structural, tectonic, sedimentary and time controls are primary factors to decide the distribution of BSRs in the Nankai Trough area.     

Two recent flowslides in Yamashina area, Kanazawa City, Japan

Due to a continual rainfall, a flowslide occurred in Yamashina area, Kanazawa City, Japan on November 8, 2002 in the Tertiary mudstone area. The sliding mass was fully fluidized during the motion and moved downward the slope for a long distance. On December 31, 2003, slope failure was triggered by intensive rainfall and snowmelt water at the same site again, and resulted in the second occurrence of flowslide. The total displacement of the slope was recorded with an extensometer. Through field investigation, the difference of the sliding mechanism between the two flowslides was examined.     

Earthquake core inferred from near field observations

The source processes of large shallow earthquakes are investigated based on the various field phenomena and on the seismograms recorded at short focal distances. The results from coseismic and postseismic field surveys in some source regions strongly show that there must be a particular region characterized by a large dislocation, large acceleration and extremely low aftershock activity. This specific region seems to have a relatively small dimension compared with the length of the main fault.The predominant short-period waves on the strong-motion seismograms are concentrated within the short intervals at the initial parts of P and S waves. This fact also suggests that the rupture elements generating the predominant short-period waves are not distributed over the entire surface of a single main fault but are concentrated in a small region.We call this confined small region in the source area “earthquake core”. The earthquake core is formed a little later than the start of smoothing dislocation and it may be located at some distance from the starting point of rupture.     

Geochemistry of Late Permian to Early Triassic pelagic cherts from southwest Japan: implications for an oceanic redox change

Major, trace, and rare earth element abundances were determined for the southwestern Japanese pelagic chert sequence from the early Late Permian to early Early Triassic to investigate a redox change in deep-sea pelagic environments before and at the Permo-Triassic boundary (PTB) (251 Ma). The sequence was primarily deposited in the deep-sea of the superocean Panthalassa, and then was accreted to Japan in the Middle Jurassic. A remarkable lithostratigraphic change from red chert to siliceous∼carbonaceous claystone through gray chert is observed in this sequence. Constituent elements for these sedimentary rocks are essentially derived from two sources: (1) an ancient seawater via biogenic (mainly radiolarian), hydrothermal, and authigenic materials and (2) an average shale-like terrigenous material. The present measurement demonstrates significant stratigraphic changes of the Ce/Ce_non-ter* value (estimated Ce anomaly value of non-terrigenous component, recalculated by subtraction of terrigenous REEs from bulk REEs) and the (Mn/TE)_sample/(Mn/TE)_PAAS value (excess Mn component other than terrigenous one; TE=terrigenous elements including Ti, Al, Nb, Hf, Th; PAAS=Post-Archean Average Australian Shale) in the Permian chert. The Ce/Ce_non-ter* values increase from <0.2 to 1 and the (Mn/TE)_sample/(Mn/TE)_PAAS values decrease up-section, suggesting that the redox condition of deep-sea open-ocean changed from oxic to suboxic in an interval of approximately 10 Myr. The (∑Fe/TE)_sample/(∑Fe/TE)_PAAS and (Mn/TE)_sample/(Mn/TE)_PAAS values of carbonaceous claystone near or at the PTB are less than unity, suggesting that reductive dissolution of iron and manganese occurred under an anoxic condition. This supports the idea of the PTB oceanic anoxia in the superocean Panthalassa. The present data suggest that the anoxic condition prevailed in the deep-sea pelagic regions for an extremely long period, much more than 10 Myr, from the middle Late Permian to early Early Triassic. This long-term development of widespread oceanic anoxia may have been linked to the greatest mass extinction of the Phanerozoic.