Geochemical implications from Sr isotopes and K-Ar age determinations for the cook-austral islands chain

Sr isotopes and K-Ar ages were determined for volcanic rocks from three islands, Samoa, Rarotonga and Rurutu in the Austral-Gilbert-Marshall chain. We have established that Rurutu originated from the same “hot spot” as Tubuai and Raivavae in the Cook-Austral chain as indicated by its 11 m.y. K-Ar age and ( ) ratios. The ( ) ratios for Rurutu, Tubuai and Raivavae lie in the restricted range 0.7026–0.7035. K-Ar ages of all volcanic rocks from Samoa and Rarotonga are all less than 4 m.y. and their ( ) ratios are much higher than those in Rurutu, Tubuai and Raivavae. These findings suggest that Samoa and Rarotonga did not originate in a single hot spot for the Cook-Austral islands.     

Monitoring Quiescent Volcanoes by Diffuse CO2 Degassing: Case Study of Mt. Fuji, Japan

Since the 8th century, more than seventeen eruptions have been recorded for the Mt. Fuji volcano, with the most recent eruption occurring in 1707 (Hoei eruption). For the past 300 years the volcano has been in a quiescent stage and, since the early 1960s, has exhibited neither fumarolic nor thermal activity. However, the number of low-frequency earthquakes with a hypocentral depth of 10–20 km increased significantly beneath the northeastern flank of Mt. Fuji in 2000–2001, suggesting a possible resumption of magmatic activity. In this study, diffuse CO₂ efflux and thermal surveys were carried out in four areas of the volcano in 2001–2002 in order to detect possible signs of the upward movement of deep magma. At all survey points, the CO₂ efflux was below the detection limit with the exception of a few points with biological CO₂ emission, and ground temperatures at a depth of 20–30 cm were below ambient, indicating no surface manifestations of gas or heat emission. Should magma rise into the subsurface, the diffuse CO₂ efflux would be expected to increase, particularly along the tectonically weakened lineation on the Mt. Fuji volcano, allowing for the early detection of pre-eruptive degassing.     

Residual deformation characteristics of partially saturated sandy soils subjected to seismic excitation

Multiple series of triaxial tests were carried out on unsaturated sandy soils subjected to irregular seismic excitation. To observe the volume changes of unsaturated soil specimens during undrained loading and subsequent process of drainage, the inner cell was newly installed inside the large triaxial cell. The soil samples tested were recovered from the construction site of a liquefied natural gas storage facility located in Tokyo Bay area. In the tests, the initial conditions of soil specimens were specified with the degrees of saturation S_r of 50, 75 and 100%. The soil specimens were anisotropically consolidated, and subjected to undrained irregular seismic excitation. The time history of irregular seismic excitation was the one obtained from one of the recent earthquakes in Japan. The drainage line was then opened to let the excess pore water drain out of soil specimens. The volume changes and residual shear strains of unsaturated soil specimens thus obtained are presented in the present paper.     

Paleolimnological investigation of recent acidity changes in Sawanoike Pond, Kyoto, Japan

Sawanoike Pond, an artificial reservoir constructed ca. 150 yrs. ago, lies in an area geologically sensitive to acidification and contains water that is presently weakly acidic (pH 5.5; alkalinity 15 eq 1^-1). Sediments from a core obtained from the northern part and a core from the central part of the pond were analyzed for organic matter, total organic carbon, total nitrogen, natural and artificial radionuclides, pollen, diatoms, metals and spheroidal carbonaceous particles. A peaty silt layer at the bottom of the central core was replaced by a silt layer initially rich in humic substances, but became more mineral-rich near the surface. Although most of the dominant diatoms throughout the cores were acidophilous taxa, some acidobiontic species increased moderately in abundance after the middle 1960's. Diatom-inferred pH values for the northern core are relatively low before about 1930, then increase, reaching a maximum level between the 1930's and the late 1940's. They then decrease to the surface and reach minimum level during the middle 1980's. Diatom-inferred pH declined by 0.4-0.5 units in the northern core and 0.2-0.3 units in the central core since ca. 1910, when vegetation around the shore changed to a community similar to that of recent years. An increase in labile Pb, Zn, Cd and spheroidal carbonaceous particles, indicative of atmospheric deposition of fossil fuel combustion by-products, preceded the decline in inferred pH. The inferred pH decline of the pond could be an acidification due to acidic precipitation caused by fossil fuel combustion.     

40Ar/39Ar ages and zircon petrochronology for the rear arc of the Izu-Bonin-Marianas intra-oceanic subduction zone

Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and ⁴⁰Ar/³⁹Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with ⁴⁰Ar/³⁹Ar and shipboard ages to a depth of ~1390 m below sea floor (mbsf) where Igneous Unit Ig 1 yielded an ⁴⁰Ar/³⁹Ar age of 12.9 ± 0.3 Ma (all errors 2σ). One single zircon (age 15.4 ± 1.0 Ma) was recovered from the deepest lithostratigraphic unit drilled, Unit VII (1459.80–1806.5 mbsf). Site U1437 zircon trace element compositions are distinct from those of oceanic and continental arc environments and differ from those generated in thick oceanic crust (Iceland-type) where low-δ¹⁸O evolved melts are produced via re-melting of hydrothermally altered mafic rocks. Ti-in-zircon model temperatures are lower than for mid-ocean ridge rocks, in agreement with low zircon saturation temperatures, suggestive of low-temperature, hydrous melt sources. Zircon oxygen (δ¹⁸O = 3.3–6.0‰) and hafnium (εHf = + 10–+16) isotopic compositions indicate asthenospheric mantle sources. Trace element and isotopic differences between zircon from Site U1437 rear-arc rocks and the Hadean detrital zircon population suggest that preserved Hadean zircon crystals were probably generated in an environment different from modern oceanic convergent margins underlain by depleted mantle.     

Photosynthesis-irradiance relationship of phytoplankton and primary production in the vicinity of Kuroshio warm core ring in spring

The light-saturated maximum value (P ^B _max) and initial slope (α) of the photosynthesis-irradiance (P-E) curve were examined in a warm streamer, a cold streamer and a warm core ring off the Sanriku area in the subarctic western North Pacific Ocean during an ADEOS/OCTS Sanriku field campaign in early May 1997. BothP ^B _max and α were within the ranges of temperate populations. A regional difference was apparent inP ^B _max: populations in the warm streamer tended to show higher value ranging between 1.92 and 4.74 mgC (mgChla)⁻¹h⁻¹ than those in the cold streamer and the warm core ring (1.35–2.87 mgC (mgChla)⁻¹h⁻¹). A depth variation was also observed in α in both the warm streamer and the warm core ring: shallow populations tended to have lower α than deep populations. The depth variations in bothP ^B _max and α resulted in a lower light intensity of the light saturation in a deeper population than that of a shallower one. These depth-related variations in the P-E parameters were likely a manifestation of “shade-adaptation” of photosynthesis. Photoinhibition was not observed over in situ surface light intensity varying below ca 1600 μmol photon m⁻²s⁻¹. Water-column primary productivity was biooptically estimated to be 233 to 949 mgC m⁻²d⁻¹ using vertical distributions of the P-E parameters, chlorophylla, phytoplankton light absorption and underwater irradiance. Applicability of surface data sets for estimation of water-column productivity is discussed.     

Introduction to “Tsunami Science Four Years After the 2004 Indian Ocean Tsunami, Part II: Observation and Data Analysis”

In this introduction we briefly summarize the fourteen contributions to Part II of this special issue on Tsunami Science Four Years After the 2004 Indian Ocean Tsunami. These papers are representative of the new tsunami science being conducted since the occurrence of that tragic event. Most of these were presented at the session: Tsunami Generation and Hazard, of the International Union of Geodesy and Geophysics XXIV General Assembly held at Perugia, Italy, in July of 2007. That session included over one hundred presentations on a wide range of topics in tsunami research. The papers grouped into Part II, and introduced here, cover field observations of recent tsunami’s, modern studies of historical events, coastal sea-level observations and case studies in tsunami data analysis.     

Hydrothermal Formation of Lath-Shaped Trioctahedral Smectite with High Crystallinity

Lath-shaped and highly crystalline trioctahedral smectites were synthesized under hydrothermal conditions. The quenched glasses with stoichiometrically dehydrated Na-smectite compositions were treated at （a） 500℃ and 100 MPa for 1 and 11 days; and （b） 300℃ and 100 MPa for 7 days. The crystallinity and particle size of products were dependent on the chemical composition of the starting glass, synthetic temperature and duration of hydrothermal treatment. The high structural ordering and large dimensions of the products were confirmed from the sharpness of XRD peaks; and hydration behavior under controlled relative humidity. Transmission electron microscopy was also performed for the characterization of the particle size of product. Particle sizes vary from ca. 10 nm to a few pm by changing the chemical compositions of the starting materials. The product with the highest structural ordering and largest dimension was obtained from Nao.33（Mg1.83Al0.67）Si4O11 glass treated at 500℃ and 100 MPa for 1 day. The obtained results also confirmed the metastability and compositional dependency in the formation of highly crystalline trioctahedral smectite at hydrothermal conditions.     

Nitrate-nitrogen isotopic patterns in surface waters of the western and central equatorial pacific

To understand the processes transporting nitrate to the surface layer of the western and central equatorial Pacific, we measured the nitrogen isotopic ratio of nitrate (δ ¹⁵NO ₃ ⁻ ), which is a very useful tracer of the source of nitrate, above 200 m depth in this region in December 1999. δ ¹⁵NO ₃ ⁻ is higher (about 13.0‰) in the surface water than in the subsurface water (where it is about 6.5‰) due to isotopic fractionation during nitrate uptake by phytoplankton. The δ ¹⁵NO ₃ ⁻ value has a roughly linear relationship with the natural logarithm of nitrate concentration (ln[NO ₃ ⁻ ]). However, for values above 150 m depth, the intercept of this linear relationship varies with position from east to west. On the other hand, the data at 200 m depth at all observation stations are concentrated around a single point (ln[NO ₃ ⁻ ] = 2.5 and δ ¹⁵NO ₃ ⁻ = 6.5‰) and do not fit the linear relationships for the shallower values. To examine the meaning of the observed distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration we developed a box model including nitrogen and nitrogen isotopic cycles. By reproducing the observed relationship between δ ¹⁵NO ₃ ⁻ and nitrate concentration using this model we found that most nitrate is transported horizontally from the eastern equatorial Pacific. We also conducted case studies and investigated the effects of differences in pathways of nitrate transport on the distributions of δ ¹⁵NO ₃ ⁻ and nitrate concentration. From these studies we concluded that the observed linear relationships between δ ¹⁵NO ₃ ⁻ and ln[NO ₃ ⁻ ], having a common slope around 6‰ but different intercepts at each station, are evidence of the significant horizontal transport of nitrate to the surface water in this area.     

Diffuse CO2 emission rate from Pululahua and the lake-filled Cuicocha calderas,Ecuador

We report herein the first results of two soil CO₂ efflux surveys carried out at Cuicocha lake-filled and Pululahua caldera volcanic systems, Ecuador. A total of 172 and 217 soil CO₂ efflux measurements were taken at the surface environment of Pululahua and Cuicocha calderas respectively, by means of the “accumulation chamber” method during the summer of 2006 to constrain the total CO₂ output from the studied area. Soil CO₂ efflux values ranged from non-detectable up to 48.5 and 141.7 g m^{− 2} d^{− 1} for Cuicocha and Pululahua calderas respectively. In addition, probability graphs were used to distinguish the existence of different geochemical populations. Sequential Gaussian Simulation was used to construct an average map for 100 simulations and to compute the total CO₂ emission at each studied area: 106 and 270 t d^{− 1} (metric tons per day) for Cuicocha (13.3 km²) and Pululahua (27.6 km²) volcanoes respectively.