Distribution of nitrogen-fixing bacteria in the Central Pacific Ocean

The plate culture method using the two formulae for non-nitrogenous media was adopted in this investigation for the purpose of counting and isolating nitrogen-fixing bacteria distributed in the open sea. Sea water samples were collected at eighteen different stations in the region of Lat. 50°N–15°S along Long. 155°W and two other stations in the Pacific Ocean. In order to compare with those samples from the open sea, water samples were also obtained at four stations in Suruga and Sagami Bays. Nitrogen-fixing bacteria appear to be widely but very unevenly distributed at all depths in sea water, in numbers approximately ranging from nil to 10⁴ per 100 ml of sea water, and denser vertical populations have been found in the area of Lat. 40°N and 5°N along Long. 155°W, even at depths from 2,000 to 3,000m. A conparatively denser population of bacteria was found in sea water from Suruga Bay and Sagami Bay. The bacteria associated with plankton were abundantly demonstrated, in numbers ranging from 10⁶ to 10⁸ per 1 ml settling volume of plankton, in many plankton samples collected at four stations in the southern parts of the Pacific Ocean. Almost all the bacteria isolated from the samples of blue green algal colonies,Trichodesmium, sp., were able to grow on nonnitrogeneous media.     

Seasonal appearance ofProchlorococcus in Suruga Bay,Japan in 1992–1993

Seasonal appearance ofProchlorococcus was studied by flow cytometry in Suruga Bay, Japan in 1992–1993.Prochlorococcus cells were in high concentrations (>1×10⁴ cells ml^–1) from July to October 1992 and September 1993, when the water temperature was over 20°C. The 16S rRNA of the isolated cells showed 98.5% sequence homology with that ofP. marinus (Sargasso strain), indicating that they are the same species. The former has a high divinyl-chlorophyll (DV-Chl.)a/b ratio similar to the Mediterranean strain and different from the Sargasso strain. Maximum concentration ofProchlorococcus at the surface water was 2.5×10⁴ cells ml^–1 in August 1992 and their DV-Chl.a accounted for 4.0% of the total chlorophylla. A decrease in cell density to less than 5×10³ cells ml^–1 was observed from December to May with an exceptional rise in January 1993. WhileProchlorococcus showed a maximum concentration of 3.6×10⁴ cells ml^–1 at 10 m depth in September 1992, phycoerythrin (PE)-richSynechococcus spp. were dominant with their maximum concentration of 2.2×10⁵ cells ml^–1 in the same water body. On the other hand, phycocyanin (PC)-richSynechococcus spp. and the larger phytoplankters showed maximum concentrations in the surface waters in May and June. BothProchlorococcus and PE-richSynechococcus showed their lowest concentrations in April. A significant positive correlation was obtained between cell concentrations of the PE-richSynechococcus andProchlorococcus.     

Toronto’s Little Portugal: gentrification and social relations among local entrepreneurs

This article clarifies neighborhood politics in Toronto’s Little Portugal Business Improvement Area (BIA), an ethnic neighborhood facing gentrification, by focusing on local entrepreneurs and the social relations among them. In the BIA, non-Portuguese businesses have replaced Portuguese businesses in the past decade; Portuguese and non-Portuguese entrepreneurs are now found in almost the same numbers. I visualized their social relations by using a diagram called a sociogram. Portuguese and non-Portuguese entrepreneurs are spatially inseparable but socially divided in the neighborhood. The BIA board, an organization for community development spearheaded by local entrepreneurs and property owners, has become predominantly non-Portuguese. The social relations among non-Portuguese entrepreneurs affect and are affected by the activities of this organization. While the Portuguese community could potentially mount resistance by mobilizing the collective power of the provincial ethnic alliance, the BIA is a form of quasi-government in Little Portugal; hence, the Portuguese community may have limited options for such resistance.     

Models on Snowball Earth and Cambrian explosion: A synopsis

During the late Proterozoic from 1000 to 542 Ma, the Earth is thought to have been frozen at least during two times: in the Sturtian (715–680 Ma) and in the Marinoan (680–635 Ma) global glaciations. Following the Marinoan Snowball Earth, large multi-cellular animals of the Ediacara fauna flourished as a prelude to the Phanerozoic world. Here we summarize the most popular models on the cause and cessation of Snowball Earth. Episodic decrease of greenhouse gas occurs through the effect of erosion and weathering promoted by either mountain building or by an increase in the coastlines during the break-up of supercontinents. Effects on the globe caused by true polar wander, eruption of voluminous flood basalts, or dramatic reduction in planetary obliquity can also lead to ice ages and mass extinction. A radically revised concept based on Earth's magnetic intensity has also been proposed, which explains the true polar wander through a quasi-polar dynamo model. The ‘switch-on’ and ‘switch-off’ of the Earth's strong dynamo can lead to the onset and disappearance of the Snowball Earth. The galactic model infers that gamma ray burst associated with starburst creates huge amounts of clouds which would cut off sun rays and freeze the Earth.The Snowball Earth event is considered to have exerted a significant control on the subsequent revolutionary changes in the evolution of life forms. Although according to the biological clock, extensive re-organisation of genome is thought to have been completed by around 900 Ma, the evolution of modern life in Cambrian occurred only after the geochemical bridge was in place with elevated oxygen and nutrient levels in lakes that developed within continental rifts where the hydrothermal system in the granitic basement created the chemical environment enriched in Ca²⁺, Fe²⁺, V, Mo, HCO₃, phosphate and other elements required for building the skeleton and bone of the first modern animals. With cosmic radiation exerting a significant control on the mutation, the Neoproterozoic Earth history illustrates the possible link from Galaxy to the genome level.     

Temporal changes in electrical resistivity at Sakurajima volcano from continuous magnetotelluric observations

Continuous magnetotelluric (MT) measurements were conducted from May 2008 to July 2009 at Sakurajima, one of the most active volcanoes in Japan. Two observation sites were established at locations 3.3 km east and 3 km west–northwest of the summit crater. At both observation sites, the high-quality component of the impedance tensor (Zyx) showed variations in apparent resistivity of approximately ± 20% and phase change of ± 2°, which continued for 20–180 days in the frequency range between 320 and 4 Hz. The start of the period of changes in apparent resistivity approximately coincided with the start of uplift in the direction of the summit crater, as observed by a tiltmeter, which is one of the most reliable pieces of equipment with which to detect magma intrusion beneath a volcano. A 2D inversion of MT impedance suggests that the resistivity change occurred at a depth around sea level. One of the possible implications of the present finding is that the degassed volatiles migrated not only vertically through the conduit but also laterally through a fracture network, mixing with shallow groundwater beneath sea level and thereby causing the observed resistivity change.     

Major and trace element abundances in volcanic glass shards in visible tephras in SG93 and SG06 drillcore samples from Lake Suigetsu,central Japan,obtained using femtosecond LA–ICP–MS

The major and trace element concentrations of volcanic glass shards from visible tephra layers in the SG93 and SG06 cores from Lake Suigetsu, central Japan, were determined by femtosecond laser ablation–inductively coupled plasma–mass spectrometry. The glass-shard analyses, together with the petrographic properties of the tephra samples, allow the Suigetsu tephra layers to be broadly classified into tephras derived from calderas on Kyushu Island, and from Daisen and Sambe volcanoes in the Chugoku district of southwest Japan. The layers correlated with tephras from Kuju caldera and Daisen volcano, and with the younger Sambe tephras, have adakitic elemental features. A Suigetsu tephra sample correlated with the Sambe−Kisuki tephra based on petrographic properties has an elemental pattern similar to that of the Toya tephra from Hokkaido Island, northeast Japan. This match implies that tephras from northeast Japan, as well as Kyushu–Chugoku tephras, are possible correlatives of the Suigetsu tephra layers. Both petrographic properties and major–trace element data of volcanic glass shards are essential for robust tephra correlations, and hierarchical cluster analysis proved additionally useful in statistically evaluating relationships among the tephras.     

The making and breaking of supercontinents: Some speculations based on superplumes, super downwelling and the role of tectosphere

The mechanisms of formation and disruption of supercontinents have been topics of debate. Based on the Y-shaped topology, we identify two major types of subduction zones on the globe: the Circum-Pacific subduction zone and the Tethyan subduction zone. We propose that the process of formation of supercontinents is controlled by super downwelling that develops through double-sided subduction zones as seen in the present day western Pacific, and also as endorsed by both geologic history and P-wave whole mantle tomography. The super-downwelling swallows all material like a black hole in the outer space, pulling together continents into a tight assembly. The fate of supercontinents is dictated by superplumes (super-upwelling) which break apart the continental assemblies. We evaluate the configuration of major supercontinents through Earth history and propose the tectonic framework leading to the future supercontinent Amasia 250 million years from present, with the present day Western Pacific region as its frontier. We propose that the tectosphere which functions as the buoyant keel of continental crust plays a crucial role in the supercontinental cycle, including continental fragmentation, dispersion and amalgamation. The continental crust is generally very thin, only about one tenth of the thickness of the tectosphere. If the rigidity and buoyancy is derived from the tectosphere, with the granitic upper crust playing only a negligible role, then supercontinent cycle may reflect the dispersion and amalgamation of the tectosphere. Therefore, supercontinent cycle may correspond to super-tectosphere cycle.     

A speculation on the structure of the D″ layer: The growth of anti-crust at the core–mantle boundary through the subduction history of the Earth

The growth curve of the continental crust shows that large amounts of continental crust formed in the early part of the Earth history are missing. In order to test a hypothesis that the former crust was subducted to the deep mantle, we performed phase assemblage analysis in the systems of mid-oceanic ridge basalt (MORB), anorthosite, and tonalite–trondhjemite–granite (TTG) down to the core–mantle boundary (CMB) conditions. Results show that all these materials can be subducted to the CMB leading to the development of a compositional layering in the D″ layer. We speculate that there could be five layers of FeO-enriched melt from partial melting of MORB, MORB crust, anorthosite, TTG, and slab or mantle peridotite in ascending order. Although the polymorphic transformation of perovskite to post-perovskite in (Mg,Fe)SiO₃ may explain the seismic discontinuity at the top of the D″ layer (D″ discontinuity), the effects of solid solution on the sharpness of the transformation suggest that the compositional layering is more plausible for the origin of the D″ discontinuity. The D″ layer can be an “anti-crust” made up mostly of TTG + anorthosite derived from the former continental crust. Tectonic style of the anti-crust at the CMB is similar to that at the surface. At both places, chemically distinct layers are density stratified and are also characterized by the processes of accretion, magmatism, and metasomatism.     

Counter-clockwise prograde P–T path in collisional orogeny and water subduction at the Precambrian–Cambrian boundary: The ultrahigh-pressure pelitic schist in the Kokchetav massif,northern Kazakhstan

A prograde pressure–temperature (P–T) path is estimated for pelitic schists from the latest Precambrian Kokchetav ultrahigh-pressure massif, Kazakhstan, using compositional zoning and mineral inclusions in coarse-grained and inclusion-rich garnets. Ti-bearing inclusions are abundant in garnet and display a zonal distribution. Ilmenite occurs in the inner-core, where most of it makes a composite inclusion with rutile, whereas monomineralic rutile occurs in the outer-core to mantle domains. In the rim region both ilmenite and rutile are present, although in small amounts. Application of the ilmenite-garnet thermometer yields a systematic temperature increase towards rim from 500 to 750 °C. The pressure-sensitive reaction: 3 Fe-Ilm (in Ilm) + Ky + 2 Qtz = 3 Rt + Alm (in Grt) yielded pressures of 1.2–1.3 GPa for the outer-core inclusions.A petrogenetic grid in the K₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O model system was used to estimate the equilibrium compositions of the garnet. The change of the grossular component along the model P–T path expected from the forward modelling is close to the observed compositional profile of the outer-core to rim domains. No constraint is available from thermobarometry in the inner-core; however, the forward modelling of garnet zoning provides information on the early stage of the P–T path during the garnet growth.The estimated P–T path is counter-clockwise in the prograde stage with a steep bend at around 700 °C and 1.2–1.5 GPa. This is similar to the metamorphic P–T gradient of the Kokchetav massif. This result contrasts markedly with the traditional clockwise P–T path in many collisional metamorphic terranes, and is regarded to represent a subduction geotherm at the Precambrian–Cambrian boundary. The P–T path proposed in this study also supports the models for the recovery of the “snowball Earth” from late-Proterozoic glaciation through effect of water in the solid Earth mantle.