Open-system degassing of sulfur from Krakatau 1883 magma

We present the first sulfur and oxygen isotopic data for tephra from the catastrophic 1883 eruption of Krakatau. Sulfur isotopic ratios in unaltered Krakatau tephra erupted August 26–27, 1883 are markedly enriched in ³⁴S relative to mantle sulfur. High δ³⁴S values of +6.3 to +16.4‰ can best be explained by open-system or multi-stage degassing of SO₂ from the oxidized rhyodacitic and gray dacitic magmas with ³⁴S enrichment of SO²⁻₄ remaining in the melt. Lower whole-rock δ³⁴S values of +2.6‰ and +4.0‰ in two oxidized gray dacitic samples indicate more primitive subarc mantle sulfur in the 1883 magma chamber. Initial δ³⁴S of the rhyodacitic magma was probably in the +1.5‰ to +4.0‰ range and similar to δ³⁴S values measured in arc volcanic rocks from the Mariana Arc.     

Bio-Optical Properties of Seawater in the Western Subarctic Gyre and Alaskan Gyre in the Subarctic North Pacific and the Southern Bering Sea during the Summer of 1997

Chlorophyll a concentrations (chla) and the absorption coefficients of total particulate matter [a _p()], phytoplankton [a _ph()], detritus [a _d()], and colored dissolved organic matter: CDOM [a _CDOM()] were measured in seawater samples collected in the subarctic North Pacific and the southern Bering Sea during the summer of 1997. We examined the specific spectral properties of absorption for each material, and compared the light fields in the Western subarctic Gyre (area WSG) with those in the Alaskan Gyre (area AG), and the southern Bering Sea (area SB). In the area WSG, the irradiance in the surface layer decreased markedly, indicating high absorption. In the area AG, the radiant energy penetrated deeply, and the chl a and absorption values were low throughout the water column. In the area SB, light absorption was high in the surface layer on the shelf edge and decreased with increasing depth; on the other hand, light absorption was low in the surface layer in the shelf area and increased with increasing depth.     

Vertical coupling and dynamical source for the intraseasonal variability in the deep Kuroshio Extension

Ocean Dynamics - In the power spectrum, the upper and deep parts of the Kuroshio Extension have distinctly different peaks. The former peaks around 200 days, while the latter is mainly at the...     

Petrological variation of large-volume felsic magmas from Hakkoda-Towada caldera cluster: Implications for the origin of high-K felsic magmas in the Northeast Japan Arc

Abstract The Hakkoda‐Towada caldera cluster (HTCC) is a typical Late Cenozoic caldera cluster located in the northern part of the Northeast Japan Arc. The HTCC consists of five caldera volcanoes, active between 3.5 Ma and present time. The felsic magmas can be classified into high‐K (HK‐) type and medium‐ to low‐K (MLK‐) type based on their whole‐rock chemistry. The HK‐type magmas are characterized by higher K₂O and Rb contents and higher ⁸⁷Sr/⁸⁶Sr ratios than MLK‐type magmas. Both magmas cannot be derived from fractional crystallization of any basaltic magma in the HTCC. Assimilation‐fractional crystallization model calculations show that crustal assimilation is necessary for producing the felsic magmas, and HK‐type magmas are produced by higher degree of crustal assimilation with fractional crystallization than MLK‐type magmas. Although MLK‐type magmas were erupted throughout HTCC activity, HK‐type magmas were erupted only during the initial stage. The temporal variations of magma types suggest the large contribution of crustal components in the initial stage. A major volcanic hiatus of 3 my before the HTCC activity suggests a relatively cold crust in the initial stage. The cold crust probably promoted crustal assimilation and fractional crystallization, and caused the initial generation of HK‐type magmas. Subsequently, the repeated supply of mantle‐derived magmas raised temperature in the crust and formed relatively stable magma pathways. Such a later system produced MLK‐type magmas with lesser crustal components. The MLK‐type magmas are common and HK‐type magmas are exceptional during the Pliocene–Quaternary volcanism in the Northeast Japan Arc. This fact suggests that exceptional conditions are necessary for the production of HK‐type magmas. A relatively cold crust caused by a long volcanic hiatus (several million years) is considered as one of the probable conditions. Intensive crustal assimilation and fractional crystallization promoted by the cold crust may be necessary for the generation of highly evolved HK‐type felsic magmas.     

Determination of Stress State from Focal Mechanisms of Microseismic Events Induced During Hydraulic Injection at the Hijiori Hot Dry Rock Site

—?The stress state at the Hijiori hot dry rock site was estimated based on the inversion from focal mechanisms of microseismic events induced during hydraulic injection experiments. The best fit stress model obtained by inverting 58 focal mechanisms of seismic events simultaneously indicates that the maximum principal stress σ₁ is vertical, while the minimum principal stress σ₃ is horizontal and trends north-south. The average misfit between the stress model and all the data is 6.8°. The inversion results show that the average misfit is small enough to satisfy the assumption of homogeneity in the focal mechanism data and that the 95% confidence regions of σ₁ and σ₃ are well constrained, i.e., they do not overlap, suggesting that the inversion results are acceptable. The stress estimates obtained by the focal mechanism inversion essentially agree with other stress estimates previously obtained. It is therefore concluded that the focal mechanism inversion method provides a useful tool for estimating the stress state. The hypocentral distributions of microseismic events associated with the hydraulic fracturing experiments are distributed around the plane that spreads to almost east–west from the injection wells and declines to the north at a high angle. The vertical orientation and east–west strike of the seismic events are essentially coplanar with the caldera ring-fault structure in the southern portion of the Hijiori Caldera. This indicates that tensile fractures of intact rock were not being created, but pre-existing fractures were being re-opened and developed in the direction of the maximum horizontal principal stress, although microseismic events were caused by shear failures.     

Three-year investigations into sperm whale-fall ecosystems in Japan

We report the first study of sperm whale‐fall ecosystems, based on mass sinking of whale carcasses at shelf depths in the northwest Pacific. We conducted three observations over a 2‐year period on replicate sperm‐whale carcasses implanted at depths of 219–254 m off the southern part of Japan from July 2003 to August 2005. The study was made possible by a mass stranding of sperm whales in January 2002, and the subsequent sinking of 12 carcasses in the waters off Cape Nomamisaki. Dense aggregations of unique chemosynthesis‐based fauna had formed around the whale carcasses after 18 months (July 2003). The mytilid mussel Adipicola pacifica was the most abundant macrofaunal species and covered most of the exposed bone surfaces. The general composition of the fauna was similar to that of deep‐water reducing habitats, but none of the species appearing in this study has been found at hydrothermal vents, cold seeps or deep‐water whale falls. A new species of lancelet, which was the first record of the subphylum Cephalochordata from reducing environments, a new species of Osedax; a rarely encountered benthic ctenophore, and a rare gastropod species were discovered at this sperm whale‐fall site. Benthic communities were similar across all the carcasses studied, although the body sizes of the whales were very different. The succession of epifaunal communities was relatively rapid and the sulphophilic stage was considerably shorter than that of other known whale falls.     

Primary productivity in the offshore Oyashio in the spring and summer 1990

Primary productivity was measured byin situ method using¹³C in the offshore Oyashio region in the spring (May) and summer (September) of 1990. Most of the values were within the range of 0.1 to 4 gC 1^–1 h^–1 although a very large value, 7.96 gC l^–1 h^–1, was observed in summer. Most daily primary production fell within the range of 372 to 633 mgC m^–2 d^–1 although a very large value, 2,109 mgC m^–2 d^–1, was observed around the frontal area in summer. Chlorophylla (Chl.a) exceeded 1 g l^–1 in many cases, and the maximum was 4.61 g l^–1 in spring and 7.53 g l^–1 in summer. Most primary productivity per unit Chl.a (photosynthetic assimilation ratio) was within the range of 0.1 to 3 gC gChl.a ^–1 h^–1 although higher values, 3–6 gC gChl.a ^–1 h^–1, were observed where small-size phytoplanktons (<2 m) were dominant. These results were compared with results obtained until now in the Oyashio region. The values beyond the range obtained so far in the offshore region were also observed in this study. Furthermore, it was pointed out that the size composition of phytoplankton community has significant influence on the results of Chl.a and photosynthetic assimilation ratio in the Oyashio region.     

Nitrogen circulation in the water column in the Oyashio region with reference to its high productivity

In order to determine quantitatively the reason for the high productivity in the Oyashio Region, which is the southwest part of the Pacific Subarctic Region, the annual-mean vertical circulation of nitrogen in the region was estimated from the vertical profiles of nitrate, dissolved oxygen and salinity, and sediment-trap data by adapting them to the balance equations. Estimates of the upwelling velocity (1.7×10⁻⁵cm sec⁻¹) and the vertical diffusivity (2.1 cm² sec⁻¹) in the abyssal zone and the primary and secondary productivities (44 and 4 mgN m⁻²day⁻¹, respectively) in the euphotic zone were close to those of previous works. The estimated vertical circulation of nitrogen strongly suggested that, since the divergence (5 mgN m⁻²day⁻¹) is caused by the abyssal convergence (6 mgN m⁻²day⁻¹) and the positive precipitation, the local new production (22 mgN m⁻²day⁻¹) necessarily exceeds not only the sinking flux (10 mgN m⁻²day⁻¹) itself but also the sum of the sinking flux and the downward diffusion of dissolved and particulate organic matter (7 mgN m⁻²day⁻¹) produced probably in the euphotic zone. The important roles of the abyssal circulation, the winter convection, and the metabolic activity in the bathyal zone to support the high productivity in the euphotic zone were clarified quantitatively.     

Rotational instability of a horizontal shear flow in a stratified rotating fluid

The rotational instability of a thermally stratified, viscous, conducting, rotating fluid is investigated by means of linearized perturbation equations. It is assumed that the basic horizontal flow is vertically uniform and that the horizontal shear is confined in a thin layer. By solving a simplified boundary value problem as a model of rotational instability in the sea, we have shown that the vertical wave length of the neutral disturbance is of order 10 times as large as the laminar Ekman layer thickness, and that this scale is proportional to (L/N)^1/3, whereL is the width of the shear layer andN is the Brunt-Vaisala frequency.     

Practical determination of mechanical design parameters of intact rock considering scale effect

As a rule, the change in mechanical properties of rock by specimen size is regarded as scale effect. A rock mass basically consists of two components: intact rock and discontinuities, each of which has different scale effects. In order to numerically analyze the behavior of discontinuous rock masses, sufficient mechanical properties must be determined not only for discontinuities but also for intact rock taking into consideration the different scale effects. This paper reviews the scale effect on strength and deformation characteristics of intact rock, and also discusses the possibility of using standard specimens to estimate strength and deformation characteristics of actual-scale rock. Finally, we propose an adequate method for the practical determination of mechanical design parameters based on the above discussions.