An absarokite from a phlogopite lherzolite source

An absarokite (SiO₂ 47.72 wt %, K₂O 3.41 wt %) occurs in the Katamata volcano, SW Japan. The rock carries phenocrysts of olivine, phlogopite, clinopyroxene, and hornblende. Chemical compositions of bulk rock (FeO^*/ MgO 0.73) and minerals (Mg-rich olivine and phlogopite, Cr-rich chromite) suggest that the absarokite is not differentiated. Melting experiments at high pressures on the Katamata absarokite have been conducted. The completely anhydrous absarokite melt coexists with olivine, orthopyroxene, and clinopyroxene at 1310° C and 1.0 GPa. The melt with 3.29 wt % of H₂O also coexists with the above three phases at 1230° C and 1.4 GPa; phlogopite appears at temperatures more than 80° C below the liquidus. On the other hand, the melt is not saturated with lherzolite minerals in the presence of 5.13 wt % of H₂O and crystallizes olivine and phlogopite as liquidus phases; the stability limit of phlogopite is little affected at least by the present variation of H₂O content in the absarokite melt. It is suggested that the absarokite magma was segregated from the upper mantle at 1170° C and 1.7 GPa leaving a phlogopite lherzolite as a residual material on the basis of the above experimental results and the petrographical observation that olivine and phlogopite crystallize at an earlier stage of crystallization sequence than clinopyroxene. The contribution of phlogopite at the stage of melting processes is also suggested by the geochemical characteristics that the absarokite is more enriched in Rb, K, and Ba and depleted in Ca and Na than a typical alkali olivine basalt from the same volcanic field.     

Thermal decomposition of serpentine during coseismic faulting: Nanostructures and mineral reactions

This paper reports a detailed characterization of an antigorite-bearing serpentinite, deformed at seismic slip-rate (1.1 m/s) in a high-velocity friction apparatus. Micro/nanostructural investigation of the slip zone (200 μm thick) revealed a zonal arrangement, with a close juxtaposition of horizons with significantly different strength, respectively consisting of amorphous to poorly-crystalline phases (with bulk anhydrous composition close to starting antigorite) and of highly-crystalline assemblages of forsterite and disordered enstatite (200 nm in size and in polygonal-like nanotextures). The slip zone also hosts micro/nanometre sized Cr-magnetite grains, aligned at low angle with respect to the slipping surface and inherited from the host serpentinite.Overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820–1200 °C (in agreement with flash temperature theory), responsible for serpentine complete dehydration and amorphization, followed by crystallization of forsterite and enstatite (under post-deformation, static conditions). The results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite-hosted faults through the seismic cycle.     

Origin of the giant eruption cloud of Pinatubo, June 15, 1991

The series of eruptions of June 15, 1991 at Mt. Pinatubo, Philippines were observed hourly by satellite. A giant discshaped cloud covering an area of 60,000 km² appeared in the satellite images at 14:40, Philippine time. The cloud expanded radially against wind of 20 m/s and spread to an area of more than 120,000 km² within an hour. According to eyewitness accounts there was heavy fine-ash fall after 14:00, intermittent lapilli fall started at about 14:20, and heavy and continuous lapilli fall widely started at about 15:00. The occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall.The air-fall deposits of the major eruption are widely distributed, including upwind from the vent. They are composed of 3 units; a silt-size fine-ash layer (Layer B), a lapilli layer commonly including pumice grains of > 1 cm in diameter (Layer C), a lapilli bearing volcanic sand layer (Layer D). Judging from its wide distribution and depletion of coarse, grains, most of the fine ash of Layer B is not distal deposits of a small eruption, but is originated from a large co-ignimbrite cloud. It is suggested that the major eruption started with the generation of a pyroclastic flow, which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall.The results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 10⁹ kg/s.     

Grain-size variation of tephra derived from volcanic umbrella clouds

The relationships among the thickness and grain-size of tephra-fall deposits and the volumetric flow rate of their source umbrella clouds are analytically obtained. The logarithm of the ratio of the probability distribution function based on grain size (ln R _f) in fall deposits at two localities from the vent (r ₁ and r ₂, respectively) has a linear relationship with the particle-settling velocity, v, as: where Q is the volumetric flow rate of the umbrella cloud and A is a constant for a given pair of localities. The volumetric flow rate of the umbrella cloud can be estimated from granulometric data using this formula. Generally, the thickness–distance relationship of tephra-fall deposits depends on the initial grain-size distribution and the volumetric flow rate of the umbrella cloud. The empirical relationship of the exponential thinning behaviour can be extrapolated towards infinite distance only for a specific initial grain size which is similar to a log-normal distribution with σ _φ=2.5, otherwise it holds only in a limited range of distances. In applying these results to the 1991 eruption of Mt. Pinatubo, it is shown that the volumetric flow rate of the umbrella cloud during the climactic phase of 15 June was approximately 5×10¹⁰ m³/s, which is fairly consistent with the expansion rate of the umbrella cloud observed in the satellite images. Received March 20, 1993/Accepted September 11, 1993     

Coral Sr/Ca and Mg/Ca records in Con Dao Island off the Mekong Delta: Assessment of their potential for monitoring ENSO and East Asian monsoon

The climate of the South China Sea (SCS) is dominated by the East Asian monsoon (EAM) and can be related to the El Niño-Southern Oscillation (ENSO) owing to the interaction between ENSO and the EAM. An annually-banded coral (Porites sp.) collected from Con Dao Island in the southern SCS was measured for Sr/Ca and Mg/Ca ratios at near-monthly resolution through the annual bands of AD 1948–1999. This island is only ~ 90 km from the Mekong Delta coast and thus significantly influenced by riverine discharge, suggesting relatively severe environmental stress on corals. The Sr/Ca time series shows a clear annual cyclicity chiefly modulated by sea-surface temperature (SST), whereas the Mg/Ca time series exhibits an indistinct annual cyclicity, indicating that the previously-proposed coral Mg/Ca thermometry is greatly disturbed. An instrumental SST record in Con Dao Island (since 1980) has been compared with the Sr/Ca time series to calibrate a Sr/Ca thermometer. The Sr/Ca vs. SST comparison shows that the Sr/Ca thermometer is sometimes disturbed by some factor and that almost all of the disturbances occur around the annual-maximum SST in the warm/wet season. The Sr/Ca data around the annual-minimum SST in the cool/dry season is almost free from the disturbance and thus useful as a SST proxy. The disturbances of the Sr/Ca and Mg/Ca thermometers may be ascribed to the Mekong River discharge and its accompanying phenomena (i.e., large freshwater input, suspended-sediment loads, intense phytoplankton blooms, etc.), which are likely to disturb coral physiological processes. Applying the Sr/Ca thermometer to the whole Sr/Ca time series provides a SST reconstruction from 1948 through 1999. Reconstructed annual-minimum SSTs show a clear quasi-biennial oscillation significantly correlated with ENSO, indicating that the annual-minimum SST in the southern SCS tends to be higher (lower) in El Niño (La Niña) phases. This is compatible with previous observations that the East Asian winter monsoon is weakened (strengthened) in El Niño (La Niña) phases. The reconstructed SST record suggests a warming of 1.0 °C for the latter half of the 20th century. The Sr/Ca and Mg/Ca time series exhibit similar decadal-to-bidecadal variations, which do not seem to be primarily due to SST variability but rather due to some other factor possibly related to disturbance or fluctuation of coral physiological processes. Although both of our Sr/Ca and Mg/Ca data are affected, to a greater or lesser extent, by some non-temperature factor, a part of the Sr/Ca data provides a useful SST proxy and suggests that coral-based SST reconstruction in the southern SCS may be an effective means for monitoring the EAM and ENSO.     

Oysters and Tsunami: Iterative Learning and Nested Governance as Resilience in Post-Disaster Aquaculture in Hokkaido,Japan

This article examines how a community of oyster farmers in Hokkaido, Japan recovered from the 2011 Tōhoku Earthquake and Tsunami, which damaged their aquaculture in Lake Akkeshi. It focuses on how, over the years, nested governance structures facilitated iterative learning, which helped build resilience against unexpected external crises. Drawing on adaptive management and resilience studies, this study examines the historical development of the local fishery cooperative association (FCA) as a series of responses to socio-ecological disasters — responses that this article characterizes as iterative learning. During the recovery after the 2011 tsunami, the FCA functioned as an information hub that directed the flow of knowledge and resources through government and industrial hierarchies, to the benefit of the farmers. This research, conducted between 2015 and 2017, is based on interviews with farmers, cooperative administrators, and government officials, as well as an analysis of archived records, fishery regulations and post-disaster subsidy programs.     

Effects of relative motion between gas and liquid on 1-dimensional steady flow in silicic volcanic conduits: 1. An analytical method

We investigate the effects of vertical relative motion between gas and liquid on eruption styles by formulating a model for 1-dimensional steady flow in volcanic conduits. As magma ascends and decompresses, volatiles exsolve and volume fraction of gas increases. As a result, magma fragmentation occurs and the flow changes from bubbly flow to gas-pyroclast flow. In our model, a transitional region (‘permeable flow region’) is introduced between the bubbly flow region and the gas-pyroclast flow region. In this region, both the gas and the liquid are continuous phases, allowing the efficient vertical escape of gas through the permeable structure. We describe the features of conduit flow with relative motion of gas and liquid using non-dimensional numbers α, γ and ε. The parameter α represents the ratio of effects of wall friction to gravitational load, and is proportional to magma flow rate. The parameter γ represents the degree of decompression for the gas-pyroclast flow to reach the sound velocity at α = 1, and is proportional to r_c²/μ for given magma temperature and initial volatile content, where r_c is conduit radius and μ is liquid viscosity. The parameter ε is defined as the ratio of liquid–wall friction force to liquid–gas interaction force in the permeable flow region, and represents the efficiency of gas escape from magma. The values of γ and ε are determined only by magma properties and geological conditions such as liquid viscosity, magma permeability and conduit radius. We formulate a 1-dimensional steady-state conduit flow model to find non-dimensional magma flow rate α as a function of magma properties and geological conditions (e.g., γ and ε) under given boundary conditions. When the relative motion is taken into account with the assumption that magma fragmentation occurs when the gas volume fraction reaches some critical values, the pressure at the fragmentation level (P_f) decreases as the magma flow rate (α) decreases or the efficiency of gas escape (ε) increases, because gas escape suppresses the increase in the gas volume fraction accompanying magma ascent. When ε is so large that P_f is below the atmospheric pressure (P_a), the flow reaches the vent before fragmentation at low α. On the other hand, when ε is so small that P_f is greater than P_a, the flow reaches the vent after fragmentation at high α. These steady-state solutions of the flow at low and high α correspond to effusive and explosive eruptions, respectively. We present a graphical method to systematically find α. On the basis of the graphical method, a simple regime map showing the relationship between the assemblage of the solutions of conduit flow and the magma properties or the geological conditions is obtained.     

The Northwest Pacific—the UNEP action plan facing the sub-polar Asian seas

The countries surrounding the Northwest Pacific marginal seas have different levels of economic development and different political systems. With the rapid economic development in the region, the marine environment received high pressure, which requires high attention of the governments concerned, and urgent actions.As the Northwest Pacific marginal seas shared by several countries, regional co-operation and co-ordination are the effective means to address the marine environmental problems in the region. The Northwest Pacific Action Plan, the Northeast Asian Regional GOOS are providing the intergovernmental mechanism to facilitate the regional co-operation and co-ordination. Effective implementation of these projects is a critical task facing to all countries in the region.     

Estimation of Oxygen Consumption Rate Using T-DO Diagram in the Benthic Layer of Ohmura Bay, Kyushu, Japan

We have observed the temporal variation of oxygen deficient water with short time scale (less than a few days) in the central area of Ohmura Bay, Kyushu, Japan, in summer, 1995 and 1996. The vertical profiles of temperature were similar to those of dissolved oxygen. We noticed a linear relation between temperature and dissolved oxygen in the bottom layer, and applied the T-DO relation to estimate the net oxygen consumption rate, rather than direct evaluation of the advection and diffusion. Oxygen consumption rate just above the bottom was estimated to be about 0.21 g O₂ m^–3day^–1 in July 1995, and about 0.28 g O₂ m^–3day^–1 in August 1996. The net oxygen consumption rate estimated for the bottom layer below the second thermocline was about 0.61 g O₂ m^–3day^–1 with variability from 0.55 to 0.66 g O₂ m^–3day^–1 during July 25 to 29, 1995. This is was about 0.64 g O₂ m^–3day^–1 with variability from 0.18 to 1.4 g O₂ m^–3day^–1 during August 22 to 30, 1996. The net oxygen consumption rates are about half of those measured with a closed system in the Seto Inland Sea.     

Magma mixing in a squeezed conduit

The mixing of mafic and silicic magmas in a squeezed conduit is simulated by fluid dynamics experiments using glue and dilute glycerin in a squeezed vinyl tube. Lighter and more viscous glue initially stably overlies dilute glycerin in a tube. As the tube is squeezed downward by a roller, the glue and dilute glycerin axisymmetrically circulate in the tube. The stable density stratification overturns due to the circulative motion. Because of viscosity contrast between the two liquids, the convective motion becomes unsteady and chaotic, which leads to efficient mixing of two liquids, even when the tube is squeezed very slowly. It is suggested that magma mixing in a squeezed conduit may explain some occurrences of natural mixed lavas with nearly homogeneous groundmass.