Submarine Hydrothermal Mineralization on the Izu-Bonin Arc, South of Japan: An Overview

Considerable effort has been expended in studying the Izu-Bonin Arc over the past 15 years. In particular, 43 dives of the Shinkai 2000 have been undertaken there to discover and evaluate the extent of submarine hydrothermal activity and mineraliza tion. Most effort has been focused on Myojin Knoll (23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives). The Izu-Bonin Arc is divided in two by the Sofugan Tectonic Line. Eight submarine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the arc is quite different from that of the southern sector. Volcanic rocks from the northern sector are more acidic than those from the southern sector. Evidence for submarine hydrothermal mineralization has been observed at four seamounts along the Izu-Bonin Arc (Myojin Knoll, Myojinsho, Suiyo Seamount, and Kaikata Caldera), and submarine hydrothermal activity is evident at another three seamounts along the arc (Kurose Hole, Mokuyo Seamount, and Doyo Seamount). The most extensive submarine hydrothermal mineral deposit so far located on the Izu-Bonin Arc is the Sunrise deposit at Myojin Knoll. This deposit, at least 400 m in diameter and 30 m high, is associated with black smoker venting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hydrothermal vents there was 278°C. Some of the sulfide chimneys contained as much as 49 μg / g Au and 3,400 μg / g Ag. The sunrise deposit is one of the largest submarine volcanic massive sulfide deposits so far discovered in midocean ridge, backarc, or arc settings and has an estimated mass of 9 x 10 6 t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide deposits may be found in other caldera along the Izu-Bonin Arc. The geological variability along the arc, the high seismicity, the occurrence of active volcanism and submarine hydrothermal venting, and a proven submarine hydrothermal mineral potential coupled with the proximity of the region to Japan suggest that the Izu-Bonin Arc could profitably serve as a natural laboratory for the long-term monitoring of the seafloor.     

Vertical profiles of nitrous oxide and dissolved oxygen in marine sediments

The whole core squeezing method was used to simultaneously obtain profiles of nitrous oxide (N₂O), nitrogenous nutrients, and dissolved oxygen in sediments of Koaziro Bay, Japan (coastal water), the East China Sea (marginal sea), and the central Pacific Ocean (open ocean). In the spring of Koaziro Bay, subsurface peaks of interstitial N₂O (0.5–3.5 cm depth) were observed, at which concentrations were higher than in the overlying water. This was also true for nitrate (NO₃⁻) and nitrite (NO₂⁻) profiles, suggesting that the transport of oxic overlying water to the depth through faunal burrows induced in situ N₂O production depending on nitrification. In the summer of Koaziro Bay, sediment concentrations of N₂O, NO₃⁻ and NO₂⁻ were lower than in the overlying water. In most East China Sea sediments, both N₂O and NO₃⁻ decreased sharply in the top 0.5–2 cm oxic layer (oxygen: 15–130 μM), which may have indicated N₂O and NO₃⁻ consumption by denitrification at anoxic microsites. N₂O peaks at subsurface depth (0.5–6.5 cm) implied in situ production of N₂O and/or its supply from the overlying water through faunal burrows. However, the occurrence of the latter process was not confirmed by the profiles of other constituents. In the central Pacific Ocean, the accumulation of N₂O and NO₃⁻ in the sediments likely resulted from nitrification. Nitrous oxide fluxes from the sediments, calculated using its gradient at the sediment–water interface and the molecular diffusion coefficient, were −45 to 6.9 nmolN m⁻² h⁻¹ in Koaziro Bay in the spring, −29 to −21 nmolN m⁻² h⁻¹ in the summer, −46 to 37 nmolN m⁻² h⁻¹ in the East China Sea, 0.17 to 0.23 nmolN m⁻² h⁻¹ in the equatorial Pacific, and <±0.2 nmolN m⁻² h⁻¹ in the subtropical North Pacific, respectively.     

Tidal exchange through Naruto,Akashi and Kitan Straits

In order to re-evaluate the water volume exchange through Naruto Strait, we have performed a numerical experiment (nonlinear barotropic model including the actual depth of water and the details of shoreline) where trajectories of a number of labeled particles are calculated during a full cycle of the M₂ tidal current. The ratio of water volume exchanged through Naruto Strait to that through Akashi Strait is found to be twice as large as the previously estimated value. The calculated water exchange rate is 104 % for Naruto Strait, 52 % for Akashi Strait and 28 % for Kitan Strait. In the case where the tide-induced residual current is excluded from the calculated velocity field (i.e. considering only the M₂ current), the calculated exchange rate maintains the level of 68 % for Naruto Strait, 18 % for Akashi Strait and 11 % for Kitan Strait, respectively. The mechanism of tidal exchange through these three straits is discussed, and it is shown that a suitable exchange rate is obtained by starting the calculation of trajectories of labeled particles at the time of either a maximum ebb- or flood-current.     

Analysis of the global relationship of biennial variation of sea surface temperature and air-sea heat flux using satellite data

We investigated the phase difference and the cross correlation coefficient between the band-pass filtered biennial variations of sea surface temperature (SST) and air-sea heat flux estimated by the monthly mean 2°×2° satellite data of Advanced Very High Resolution Radiometer (AVHRR) and Special Sensor Microwave/Imager (SSM/I) from July 1987 to June 1991. Judging from the phase difference, it can be determined whether the biennial variation of SST is controlled by local thermal air-sea interaction or oceanic processes of horizontal transport. When the local air-sea heat flux controls the biennial variation of SST, the phase of SST advances /2 (6 months) against that of the air-sea heat flux. In contrast, when the biennial variation of SST is controlled by the oceanic process, the phase difference between the SST and the air-sea heat flux becomes 0 or (12 months). In this case, two types of the phase differences are determined, depending on which variability of SST and air-sea heat flux is larger. The close thermal air-sea interaction is noticeable in the tropics and in the western boundary current region. The phase difference of /2 appears mainly in the north Pacific, the southeast Indian Ocean, and the western tropical Pacific; zero in the eastern tropical Pacific and the northeast and equatorial Atlantic; and that of in the central equatorial Pacific and north of the intertropical convergence zone (ITCZ) of the Atlantic. Phase differences of 0, , or /2 are possible in the western boundary current regions. This fact indicates that each current plays a different role to the biennial variation of SST. It is inferred that SST anomalies in the tropics are mutually correlated, and the process in which marked SST anomalies in the tropics are transferred to the remote area was probed. In the equatorial Pacific, the SST anomaly is transferred by the long planetary wave. On the other hand, it is found from the phase relationship and the horizontal correlation of SST that the SST anomaly in the central and western equatorial Pacific is connected through atmospheric mediation. It is suggested that the biennial variation of SST in the eastern Indian Ocean is affected by heat transport due to the Indonesian throughflow from the western tropical Pacific. It is found that the mentioned pattern of the interannual variation of SST in the tropical Atlantic as a dipole is not tenable.     

Bispectra of wind-waves and wave-wave interaction

Bispectra of wind-waves in wind tunnels were calculated in order to understand the characteristics of the nonlinear wave-wave interaction in actual wind-wave field. It is shown that the nonlinearity in wind-waves increases in magnitude with the development of wind-waves and that the characteristics of nonlinearity in wind-waves in the early stage of development differ from those in the late stage. It is shown that the bispectra are classified into five types (IV), and that the bispectral type changes from the type I to the type V as the wind-waves develop from the stage of the initial-wavelets to that of the sea-waves. The relations between frequencies of the component waves interacting each other are discussed in each bispectral type.     

Nonlinearity of the horizontal velocity field under wind-waves

Using the data from a wind-tunnel experiment, bispectra of orbital-motions of wind-waves and of turbulence are calculated, and nonlinear interaction of wind-waves with turbulence in water are discussed. The wind-waves or the orbital motions of them, at first, produce the turbulence coherent with themselves, and then this coherent turbulence changes to the turbulence noncoherent with wind-waves. Nonlinearity of the velocity fluctuations in the surface layer in water under wind-waves is almost due to the nonlinear energy transfer from the orbital motions of wind-waves to the coherent turbulence. The vertical variations of the power spectra and of the normalized bispectra suggest that the wind driven currents near the surface play an important role in the process from orbital motion of wind-waves to noncoherent turbulence.     

Dating of Japanese Quaternary tephras by ESR and luminescence methods

Electron spin resonance (ESR) and red thermoluminescence (RTL) methods were applied to quartz of eight Japanese Quaternary tephras ranging from 30 to 900 kyr, which have independent ages from other dating methods. The ESR ages were consistent for younger samples, while those from the Ti–Li center are older than those for the Al center for older samples. RTL ages are consistent with the age references and are roughly consistent with Al center ages. The dose response of the Ti–Li center after heating at 260 °C for 15 min implies that thermally unstable component is created in Ti–Li center by irradiation, leading to possible overestimations in the dose values.     

Accurate Isotopic and Concentration Analyses of Small Amounts of Pb Using Isotope Dilution Coupled with the Double Spike Technique

A new method has been developed for the simultaneous determination of Pb abundance and Pb isotopic composition with high precision and accuracy for small test portion masses by thermal ionisation mass spectrometry. In this method, a ²⁰⁵pb-²⁰⁴pb double spike is added to samples prior to the chemical separation of Pb, and the isotopic composition of the spike-sample mixture is determined rigorously by the double spike technique using a ²⁰⁷Pb-²⁰⁴Pb spike. The isotopic composition and concentration of Pb in the sample are then obtained by utilising the principle of isotope dilution. Using this technique, replicate determinations of Pb from NIST SRM 981 and GSJ JP-1 (peridotite; 0.07 μg g⁻¹ Pb) were performed. The measured concentration and isotopic data were identical, within uncertainty, to published data or to data that were determined independently in this study. The application of this method to U-Pb dating and the determination of the "initial" Pb isotopic composition was also tested. Lead isotopic compositions and the concentrations of Pb, Th and U were determined for a single batch of samples, through the addition of ²⁰⁵pb-²⁰⁴pb, ²³⁰Th and ²³⁵U spikes to samples prior to chemical separation. Also in these experiments, we confirmed that this routine gives accurate data for Pb, Th and U concentrations and Pb isotopic compositions.     

Estimation of geometric route distance from its topological distance: application to narrow road networks in Tokyo

The structure of road networks has been investigated in accordance with the development of GIScience. By classifying road networks into wide and narrow ones, we can define the route as the path from the route’s origin (also called the root) on a wide road network to a narrow road segment which consists of the sequence of narrow road segments arranged by ascending order of the number of steps of adjacency to its root. The length of the route can be defined with the following geometric and topological terms: the route distance, measuring the length along the route and the depth, counting the number of road segments on the route. The depth plays the important role of being a substitute for the route distance in modelling road networks as a planar graph. Since road networks clearly exhibit irregular patterns and road segment lengths are non-uniform, it is considered appropriate to adopt a stochastic approach rather than a deterministic one to analyse the route distance. However, the relationship between the route distance and its depth has not been sufficiently investigated stochastically. Therefore, the research question is how can we estimate the route distance from its depth? Based on an empirical study in the Tokyo metropolitan region, it was found that (1) the statistical distribution of the route distance can be formulated as an Erlang distribution whose parameters are its depth and the inverse of the mean length of narrow road segments, and (2) this length is constant and close to 40 m. Therefore, we can estimate the route distance from only one parameter, the depth. Also, as a practical application, accessibility to the kth depth link in terms of firefighting was evaluated because the maximum length of the extension of fire hoses is approximately 200 m. It was found that (1) even if k?≤?5, the probability that the route distance to the kth depth link is equal to or longer than 200 m ranges from 0 to 0.45; and (2) if k?≥?8, the probability is approximately 1. These indicate the limitation of the deterministic approach because, on the basis of complete grid patterns (with intervals of 40 m between intersections), k?=?5 corresponds to a distance of 200 m from wide road networks and the route to the 5th depth link can be covered with fire hoses. Moreover, it was found that the connectivity of wide road networks is higher than that of narrow ones in terms of the smaller ratio of cul-de-sacs and the larger ratio of four-way intersections. These answers contribute substantially not only to constructing a science of cities that provides a simple model and specifies the most important parameter, but also to our understanding of the structure of narrow road networks within several hundred metres of wide road networks.