The Role of History-Dependent Rheology in Plate Boundary Lubrication for Generating One-Sided Subduction

We have developed a two-dimensional dynamical model of asymmetric subduction integrated into the mantle convection without imposed plate velocities. In this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at the plate boundary. We introduce a rheological layer that depends on the history of the past fracture to simulate the effect of the oceanic crust. The thickness of this layer is set to be as thin as the Earth's oceanic crust. To treat 1-kilometer scale structure at the plate boundary in the 1000-kilometer scale mantle convection calculation, we introduce a new numerical method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes. Using our developed models, we have systematically investigated effects of basic rheological parameters that determine the deformation strength of the lithosphere and the oceanic crust on the development of the subducted slab, with a focus on the plate motion controlling mechanism. In our model the plate subduction is produced when the friction coefficient (0.004–0.008) of the modeled oceanic crust and the maximum strength (400 MPa) of the lithosphere are in plausible range inferred from the observations on the plate driving forces and the plate deformation, and the rheology experiments. In this range of the plate strength, yielding induces the plate bending. In this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle than by the plate strength. To examine the setting of the overriding plate, we also consider the two end-member cases in which the overriding plate is fixed or freely-movable. In the case of the freely-movable overriding plate, the trench motion considerably changes the dip angle of the deep slab. Especially in the case with a shallow-angle plate boundary, retrograde slab motion occurs to generate a shallow-angle deep slab.     

Remote effects of mixed layer development on ocean acidification in the subsurface layers of the North Pacific

Using the outputs of projections under the highest emission scenario of the representative concentration pathways performed by Earth system models (ESMs), we evaluate the ocean acidification rates of subsurface layers of the western North Pacific, where the strongest sink of atmospheric CO₂ is found in the mid-latitudes. The low potential vorticity water mass called the North Pacific Subtropical Mode Water (STMW) shows large dissolved inorganic carbon (DIC) concentration increase, and is advected southwestward, so that, in the sea to the south of Japan, DIC concentration increases and ocean acidification occurs faster than in adjacent regions. In the STMW of the Izu-Ogasawara region, the ocean acidification occurs with a pH decrease of ~0.004 year^?1 , a much higher rate than the previously estimated global average (0.0023 year^?1), so that the pH decreases by 0.3–0.4 during the twenty-first century and the saturation state of calcite (Ω_Ca) decreases from ~4.8 down to ~2.4. We find that the ESMs with a deeper mixed layer in the Kuroshio Extension region show a larger increase in DIC concentration within the Izu-Ogasawara region and within the Ryukyu Islands region. Comparing model results with the mixed layer depth obtained from the Argo dataset, we estimate that DIC concentration at a depth of ~200 m increases by 1.4–1.6 μmol kg^?1 year^?1 in the Izu-Ogasawara region and by 1.1–1.4 μmol kg^?1 year^?1 in the Ryukyu Islands region toward the end of this century.     

Possible change in distribution of seaweed, Sargassum horneri, in northeast Asia under A2 scenario of global warming and consequent effect on some fish

Global warming effects on seaweed beds are already perceptible. Their geographical distributions greatly depend on water temperatures. To predict future geographical distributions of brown alga, Sargassum horneri, forming large beds in the northwestern Pacific, we referred to future monthly surface water temperatures at about 1.1° of longitude and 0.6° of latitude in February and August in 2050 and 2100 simulated by 12 organizations under an A2 scenario of global warming. The southern limit of S. horneri distribution is expected to keep moving northward such that it may broadly disappear from Honshu Island, the Chinese coast, and Korean Peninsula in 2100, when tropical Sargassum species such as Sargassum tenuifolium may not completely replace S. horneri. Thus, their forests in 2100 do not substitute those of S. horneri in 2000. Fishes using the beds and seaweed rafts consisting of S. horneri in East China Sea suffer these disappearances.     

A review of the NEMURO and NEMURO.FISH models and their application to marine ecosystem investigations

The evolution of the North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO) family of models to study marine ecosystems is reviewed. Applications throughout the North Pacific have shown the models to be robust and to be able to reproduce 1D, 2D and 3D components of nutrient, carbon cycle and biogeochemical cycles as well as aspects of the lower trophic levels ecosystem (phyto- and zooplankton). NEMURO For Including Saury and Herring, an extension that includes higher trophic levels, can be run uncoupled or coupled to NEMURO. In the uncoupled mode, the growth and weight of an individual fish is computed using plankton densities simulated by NEMURO but with no feedback between fish consumption and plankton mortality. In the coupled mode, the feeding, growth and weight of a representative fish are computed, and prey removals due to feeding by fish appear as mortality terms on the prey. The NEMURO family of models continues to evolve, including effects of the microbial loop and iron limitation at lower trophic levels, and full life cycle, multi-species and multi-generational simulations at higher trophic levels. We outline perspectives for future end-to-end modeling efforts that can be used to study marine ecosystems in response to global environmental change.     

Myojin Rift, Izu–Bonin Arc as the Modern Analog of Hokuroku Basin, Northeast Japan: Geotectonic Significance of the New Hydrothermal Deposit in the Back-Arc Rift

A huge hydrothermal field, named the "Hakurei Sulfide Deposit" (HSD) was discovered in the North Myojin Rift (NMR), Izu–Bonin Back-Arc Rift (BAR) during the 2003 survey cruise of R/V Hakurei-maru No.2 . This paper investigates the geotectonic features and the tectonic setting of ore deposits between the NMR and the Hokuroku Basin, which is representative of kuroko fields in Japan. The topographic features of the NMR and the Hokuroku Basin are similar. Both have a clear ring structure surrounded by faults and the east–west width is almost the same. Many kuroko deposits were formed on the extrusion centers of the five pre-mineral acidic volcanic complexes, located in a loop inside the Hokuroku Basin. In the case of the NMR, seven submarine volcanoes are also located in a loop, and the HSD formed inside the summit caldera of Bayonnaise Knoll, which is one of the seven volcanoes. These topographic similarities highlight that the NMR is a modern analog of the Hokuroku Basin. Identifying such similarities is extremely useful when prospecting kuroko deposits on land equivalents as well as on the other segments of the Izu–Bonin BAR. The probability of finding kuroko deposits on land is expected to increase when the following are identified: (i) location of back-arc rift and the volcanic front; (ii) direction of the arc–trench system and intra-rift faults (and/or fracture zone); (iii) position of submarine volcanoes surrounding a back-arc rift; and (iv) intersections of a caldera fault and intra-rift fault (and/or fracture zone) inside the summit caldera of submarine volcanoes. Within these aforementioned points a ring structure, acidic volcanic complexes that circle the circuit and submarine calderas along the volcanic front, are an important indication of submarine hydrothermal deposits.     

Sensitivity analysis of a coastal marine ecosystem

An ecosystem with four compartments, i.e. PO₄-P, phytoplankton, zooplankton and detritus, in a coastal region was considered. Sensitivity analysis of a phosphorus flow model in well-mixed water without currents concludes that (1) the maximal photosynthetic rate,V _m , in Michaelis-Menten relation plays an important role in the distribution of biomass among compartments but the half saturation constant,K _s , is not so important, (2) the natural death rate of phytoplankton is important for the ecosystem, (3) the natural death rate of zooplankton is also effective on the ecosystem.A numerical experiment was also performed on the ecosystem with four compartments are also studied using a dynamical barotropic model of tidal currents of Mikawa Bay (Japan). Diffusion coefficient in diffusion equation plays the role of a linear smoothing parameter in the horizontal distribution of compartment. On the other hand, perturbations of biological parameters cause nonlinear variations in the horizontal distribution of compartment.     

Criterion for stability of phytoplankton patchiness using a Liapunov method

A criterion for the stability of plankton patchiness is developed using a Liapunov function. Plankton growth, advection and diffusion in a closed area are considered. A critical length scale for the stability does not exist but a more common criterion which contains the growth rateα, diffusion coefficientA _l and arbitrary function of growthf can be introduced in an integral form.