U-series dating of Lake Nyos maar basalts,Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam

From previously published ¹⁴C and K–Ar data, the age of formation of Lake Nyos maar in Cameroon is still in dispute. Lake Nyos exploded in 1986, releasing CO₂ that killed 1750 people and over 3000 cattle. Here we report results of the first measurements of major elements, trace elements and U-series disequilibria in ten basanites/trachy-basalts and two olivine tholeiites from Lake Nyos. It is the first time tholeiites are described in Lake Nyos. But for the tholeiites which are in ²³⁸U–²³⁰Th equilibrium, all the other samples possess ²³⁸U–²³⁰Th disequilibrium with 15 to 28% enrichment of ²³⁰Th over ²³⁸U. The (²²⁶Ra/²³⁰Th) activity ratios of these samples indicate small (2 to 4%) but significant ²²⁶Ra excesses. U–Th systematics and evidence from oxygen isotopes of the basalts and Lake Nyos granitic quartz separates show that the U-series disequilibria in these samples are source-based and not due to crustal contamination or post-eruptive alteration. Enrichment of ²³⁰Th is strong prima facie evidence that Lake Nyos is younger than 350 ka. The ²³⁰Th–²²⁶Ra age of Nyos samples calculated with the (²²⁶Ra/²³⁰Th) ratio for zero-age Mt. Cameroon samples is 3.7 ± 0.5 ka, although this is a lower limit as the actual age is estimated to be older than 5 ka, based on the measured mean ²³⁰Th/²³⁸U activity ratio. The general stability of the Lake Nyos pyroclastic dam is a cause for concern, but judging from its ²³⁰Th–²²⁶Ra formation age, we do not think that in the absence of a big rock fall or landslide into the lake, a big earthquake or volcanic eruption close to the lake, collapse of the dam from erosion alone is as imminent and alarming as has been suggested.     

Seismic performance evaluation of single damped‐outrigger system incorporating buckling‐restrained braces

The outrigger system is an effective means of controlling the seismic response of core‐tube type tall buildings by mobilizing the axial stiffness of the perimeter columns. This study investigates the damped‐outrigger, incorporating the buckling‐restrained brace (BRB) as energy dissipation device (BRB‐outrigger system). The building's seismic responses are expected to be effectively reduced because of the high BRB elastic stiffness during minor earthquakes and through the stable energy dissipation mechanism of the BRB during large earthquakes. The seismic behavior of the BRB‐outrigger system was investigated by performing a spectral analysis considering the equivalent damping to incorporate the effects of BRB inelastic deformation. Nonlinear response history analyses were performed to verify the spectral analysis results. The analytical models with building heights of 64, 128, and 256 m were utilized to investigate the optimal outrigger elevation and the relationships between the outrigger truss flexural stiffness, BRB axial stiffness, and perimeter column axial stiffness to achieve the minimum roof drift and acceleration responses. The method of determining the BRB yield deformation and its effect on overall seismic performance were also investigated. The study concludes with a design recommendation for the single BRB‐outrigger system.     

Assessment of shallow groundwater in Lake Nyos catchment (Cameroon,Central-Africa): implications for hydrogeochemical controls and uses

In 1986, carbon dioxide gas exploded from Lake Nyos and killed about 1,800 people. After that disaster, various administrative and research activities have been conducted to mitigate subsequent disasters. However, none of those endeavors have characterized the groundwater chemistry to identify hydrogeochemical processes that control the water chemistry, and the quality of the water for domestic and agricultural uses that support the lives of un-official resettlers around Lake Nyos. Conventional hydrochemical techniques coupled with statistical and graphical analysis were therefore employed to establish the baseline hydrochemical conditions, assess processes controlling solutes distribution in shallow groundwater in the Lake Nyos catchment and explore its usability. Groundwater samples were analyzed for their physical and chemical properties. The wide ranges of electrical conductivity and total dissolved solid values reveal the heterogeneous distribution of groundwater within the watershed. The relative abundance of major dissolved species was Ca > Mg > Na > K for cations and HCO₃ >>> Cl > SO₄ > NO₃ for anions. Piper diagram classified almost all water samples into mixed CaMg–HCO₃ water type. Major ion geochemistry reveals that, in addition to silicates weathering (water–rock interaction), ion exchange processes regulate the groundwater chemistry. Principal component analysis supports the occurrence of water rock interaction. Hierarchical cluster analysis showed that the chemistry of groundwater in the study area is controlled by three main factors, and suggests no hydraulic connectivity between deep lake water and groundwater in the catchment. The quality assessment of the groundwater showed that groundwater parameters are within the acceptable limit of the World Health Organization and Nigeria guidelines for drinking and domestic uses, and water found to be good for irrigation.     

Origin of waters responsible for serpentinization of the Izu-Ogasawara-Mariana forearc seamounts in view of hydrogen and oxygen isotope ratios

Many serpentinite seamounts occur over a region 20–120 km west of the trench axis in the Izu-Ogasawara-Mariana forearc regions. The hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization: seawater and water derived from dehydration of the descending slab. Serpentine from two Mariana and two Torishima samples with microscopically ductile and sheared texture (sheared-type) have lowerδD(−63to−52‰) and slightly higherδ18O values (+6.1 to +8.2‰) than that of other nine Ogasawara samples with mesh texture (mesh-type) (δD= −43to−49‰ andδ¹⁸O= +5.8to+6.7‰). This suggests that the sheared-type serpentine with lowerδD and slightly higherδ¹⁸O values was formed within the wedge mantle by interaction with water derived from a descending slab. The sheared texture is likely to have been produced during diapiric uplift. The unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor, resulting in formation of the mesh-type serpentine with higherδD values.     

Disaster prevention,disaster preparedness and local community resilience within the context of disaster risk management in Cameroon

This paper presents a simulation of three components of near-field ground shaking recorded during the main shock at three stations of the September 16, 1978, Tabas (M _w = 7.4), Iran, earthquake, close to the causative fault. A hybrid method composed of a discrete wavenumber method developed by Bouchon (Bouchon in Bull Seismol Soc Am 71:959–971, 1981; Cotton and Coutant in Geophys J Int 128:676–688, 1997) and a stochastic finite-fault modeling based on a dynamic corner frequency proposed by Motazedian and Atkinson (Bull Seismol Soc Am 95:995–1010, 2005), modified by Assatourians and Atkinson (Bull Seismol Soc Am 97:935–1949, 2007), is used for generating the seismograms at low (0.1–1.0 Hz) and high frequencies (1.0–20.0 Hz), respectively. The results are validated by comparing the simulated peak acceleration, peak velocity, peak displacement, Arias intensity, the integral of velocity squared, Fourier spectrum and acceleration response spectrum on a frequency-by-frequency basis, the shape of the normalized integrals of acceleration and velocity squared, and the cross-correlation with the observed time-series data. Each characteristic is compared on a scale from 0 to 10, with 10 being perfect agreement. Also, the results are validated by comparing the simulated ground motions with the modified Mercalli intensity observations reported by reconnaissance teams and showed reasonable agreement. The results of the present study imply that the damage distribution pattern of the 1978 Tabas earthquake can be explained by the source directivity effect.     

Behaviors of 210Pb and 210Po around hydrothermal vents in the Okinawa Trough

Vertical profiles for the uranium-series radioisotopes 210Pb and 210Po were obtained at thetwo hydrothermal vent sites, the Iheya Ridge and the Minami-Ensei Knoll, in the Mid-Okinawa Trough in 1993 and 1994, respectively. In 1995, both radioisotopes were measured at the Minami-Ensei Knoll a-gain. At the Iheya Ridge, where the hydrothermal activity is not active as reflected by the CH_4, and 222Rn data, both the total 210Pb and 210Po activities show deficiency relative to their parents, and the mean residence time of 210Pb and 210Po is approximately equal to 20 and 2-5 a, respectively. At the Minami-Ensei Knoll, which is characterized by black smokers, the total 210Pb(0.167 × 10~(-3)-2.5 × 10~(-3)_ Bq/kg) around the plumes is deficient relative to 226Ra but the total 210Po activities (1.83 × 10 ~(-3) - 2.83 × 10~(-3) Bq/kg) are in excess relative to 210Pb. The 210Po activities are higher than those in the East China Sea and the Okinawa Trough and excess 210Po has been found. The 210Pb/226Ra and 210Po/     

Genetic differentiation of the soft shore barnacle Fistulobalanus albicostatus (Cirripedia: Thoracica: Balanomorpha) in the West Pacific

This study examined the phylogeography of the barnacle Fistulobalanus albicostatus, which inhabits mangroves and estuarine shores in the West Pacific. Differentiation in the mitochondrial cytochrome c oxidase subunit I (COI) and 12S ribosomal RNA (12S) genes of 401 specimens of F. albicostatus was examined in samples from 16 locations in the West Pacific, ranging from Honshu to Southern China. Our results revealed that F. albicostatus comprises two major clades exhibiting a COI divergence ranging from 1.25% to 2.8%. Clade A demonstrated the widest distribution, ranging from Japan to China, and was divided into three subclades occurring in the South China Sea (A1), Okinawa (A2), and Honshu, Korea and Qingdao (A3). Clade B was determined to be endemic to Okinawa; i.e. two endemic lineages occur in this island. Thus, F. albicostatus resembles several inter‐tidal species in having clades that are endemic to Okinawan waters. Nevertheless, in contrast to the rocky inter‐tidal barnacles Tetraclita spp. and Chthamalus malayensis, F. albicostatus was not found to be separated into continental and oceanic populations, but instead is divided into northern and southern clades, probably because of the Yangtze River discharge, which limits gene flow between the northern and southern populations.     

Oxygen isotope study of the Kamioka Zn-Pb skarn deposits,Central Japan

The Kamioka Zn-Pb deposits consist of clinopyroxene-rich skarns replacing limestone lenses in Hida gneissose rocks. Paleozoic metabasite, early Jurassic Funatsu granitic rocks and late Cretaceous porphyritic dikes and stocks, are the known igneous rocks in the mining area. Oxygen isotopic compositions of eight separates each of clinopyroxene and quartz from the deposits (Table 1), and some minerals and rocks from the major rock units around the deposits (Table 2), have been analyzed. The estimated oxygen isotopic ratios of the fluids responsible for the formation of the deposits, range as low as -4 to +3 (SMOW). These values are considerably lower than expected from magmatic waters, and demonstrate that none of the igneous rocks in the area could be a direct source of the skarn- and ore-forming fluids. The data instead show that the deposits are formed by a huge convective circulation of solutions of meteoric water origin, promoted by a hidden batholithic intrusion at the time of the late Cretaceous.     

Ion Chromatographic Determination of Fluorine and Chlorine in Silicate Rocks Following Alkaline Fusion

A simple and accurate method to determine fluorine and chlorine contents in small amounts (∼ 30 mg) in rock has been developed using ion chromatography after extraction by alkaline fusion. Powdered sample was mixed with sodium carbonate and zinc oxide at a mass ratio of 1:3:1, and was fused in an electric furnace at 900 °C for 30-40 minutes. An aqueous solution obtained by dissolving the fused silicate rock was diluted to the appropriate concentration of sodium carbonate (< ∼ 24 mmol l^-1) to minimise the tailing effect on F^- during ion chromatography caused by the large amount of carbonate species originating from the flux. Fluorine and chlorine contents were then determined by a standard additions method. Based on the relative standard deviation of the backgrounds, detection limits of both fluorine and chlorine were ∼ 4 μg g^-1, when 30 mg test portions were fused and diluted by a factor of 1200. We also report new fluorine and chlorine contents in nine GSJ (Geological Survey of Japan) reference materials, including peridotite (JP-1), granite (JG-1a), basalts (JB-1b, 2 and 3), andesites (JA-1 and 2) and rhyolites (JR-1 and 2). Fluorine and chlorine contents in the reference materials in this study were consistent with previously reported values. Reproducibilities were < 10 % for samples with F and Cl concentrations of > 20 μg g-1 and < 20 % with F and Cl < 20 μg g^-1.