S, O and Sr isotope systematics of active vent materials from the Mariana backarc basin spreading axis at 18°N

Stable isotope ratios of S, O and Sr have been measured for active vent materials which were first found and sampled in April 1987 from the Mariana backarc spreading axis at 18°N. Chimneys consisted mostly of barite with a lesser proportion of sulfide minerals such as sphalerite, galena, chalcopyrite and pyrite. Theδ³⁴S values of sphalerite and galena taken from several chimneys and various parts of a chimney showed a narrow range from 2.1 to 3.1‰, suggesting uniform conditions of fluid chemistry during chimney growth. The sulfur isotopic results imply a contribution of hydrogen sulfide reduced from seawater sulfate in the deep hydrothermal reaction zone, considering that fresh glasses of the Mariana Trough basalts haveδ³⁴S= −0.6 ± 0.3‰. Sulfur isotopic compositions of hydrogen sulfide in the high temperature vent fluids (δ³⁴S= 3.6–4.8‰) which are higher than those of the sulfide minerals suggest the secondary addition of hydrogen sulfide partially reduced from entrained seawater SO₄²⁻ at a basal part of the chimneys. This interpretation is consistent with theδ³⁴S values of barite (21–22‰) that are higher than those of seawater sulfate. The residence time of the entrained SO₄²⁻ was an order of an hour on a basis of oxygen isotopic disequilibrium of barite. Strontium isotopic variations of barite and vent waters indicated that Sr in barite was mostly derived from the Mariana Trough basalts with a slight contribution from Sr in circulating sea-water, and that 10–20% mixing of seawater with ascending hydrothermal fluids induced precipitation of barite at the sea-floor.     

Strontium, oxygen, and hydrogen isotope geochemistry of hydrothermally altered and weathered rocks in DSDP Hole 504B, Costa Rica Rift

DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones—Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows:

Zone	Depth(m)	Average87Sr/86Sr	Average δ18O (%o)	Average δD (%o)
I	275–550	0.7032	7.3	−63
II	550–890	0.7029	6.5	−45
III	890–1050	0.7035	5.6	−31
IV	1050–1350	0.7032	5.5	−36

Full-size table

Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 ± 20°C in Zones III and IV, which is consistent with secondary mineral assemblages.The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2‰, and 3‰, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR.Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions.     

P-wave velocity structure of the margin of the southeastern Tsushima Basin in the Japan Sea using ocean bottom seismometers and airguns

The Tsushima Basin is located in the southwestern Japan Sea, which is a back-arc basin in the northwestern Pacific. Although some geophysical surveys had been conducted to investigate the formation process of the Tsushima Basin, it remains unclear. In 2000, to clarify the formation process of the Tsushima Basin, the seismic velocity structure survey with ocean bottom seismometers and airguns was carried out at the southeastern Tsushima Basin and its margin, which are presumed to be the transition zone of the crustal structure of the southwestern Japan Island Arc. The crustal thickness under the southeastern Tsushima Basin is about 17 km including a 5 km thick sedimentary layer, and 20 km including a 1.5 km thick sedimentary layer under its margin. The whole crustal thickness and thickness of the upper part of the crust increase towards the southwestern Japan Island Arc. On the other hand, thickness of the lower part of the crust seems more uniform than that of the upper part. The crust in the southeastern Tsushima Basin has about 6 km/s layer with the large velocity gradient. Shallow structures of the continental bank show that the accumulation of the sediments started from lower Miocene in the southeastern Tsushima Basin. The crustal structure in southeastern Tsushima Basin is not the oceanic crust, which is formed ocean floor spreading or affected by mantle plume, but the rifted/extended island arc crust because magnitudes of the whole crustal and the upper part of the crustal thickening are larger than that of the lower part of the crustal thickening towards the southwestern Japan Island Arc. In the margin of the southeastern Tsushima Basin, high velocity material does not exist in the lowermost crust. For that reason, the margin is inferred to be a non-volcanic rifted margin. The asymmetric structure in the both margins of the southeastern and Korean Peninsula of the Tsushima Basin indicates that the formation process of the Tsushima Basin may be simple shear style rather than pure shear style.     

Elemental distribution of coastal sea and stream sediments in the island-arc region of Japan and mass transfer processes from terrestrial to marine environments

A total of 49 elements have been identified in 338 coastal sea sediment samples collected from an area situated off the Ise-Tokai region of Japan for a nationwide marine geochemical mapping project. The spatial distribution patterns of the elemental concentrations in coastal seas along with the existing geochemical maps in terrestrial areas were used to define the natural geochemical background variation, mass transport, and contamination processes. The elemental concentrations of coastal sea sediments are determined primarily by particle size and regional differences. Most elemental concentrations increase with a decrease in particle size. Some elements such as Ca, Mn, and Yb are found to exist in large quantities in coarse particles containing calcareous shells, Fe–Mn oxides, and felsic volcanic sediments. Regional differences reflect the mass transfer process from terrestrial areas to coastal seas and the influence of the local marine geology. An analysis of variance (ANOVA) reveals that for many elements, the particle size effect is predominant over regional difference. The mean chemical compositions of coastal sea sediments are similar to those of stream sediments in adjacent terrestrial areas and in the upper crust of Japan. This observation supports the fact that coastal sea sediments have certainly originated from terrestrial materials. However, the spatial distributions of elemental concentrations are not always continuous between the land and coastal seas. The scale of mass movement observed in marine geochemical maps occurs at a distance of 20 km from the river mouth. A detailed examination of the spatial distribution patterns of K (K₂O) and Cr concentrations suggests that terrestrial materials supplied through rivers are deposited near the shore initially, and then gravity-driven processes shift the sediments deeper into the basin. Contamination with heavy metals such as Zn, Cd and Pb was observed in coastal bays surrounded by urban and industrial areas. It is noteworthy that the areas with the highest concentration of these elements usually do not occur near the shore (not near the contamination source) but at the center of the bay. Unexpected low concentrations of Zn, Cd and Pb near shore may either be due to a decreased anthropogenic load in the most recent sediments or to dilution by unpolluted flood sediments.     

Distribution depth of the transforming stage larvae of myctophid fishes in the subtropical–tropical waters of the western North Pacific

We describe the day–night vertical distribution patterns of 18 species or types of myctophid fish larvae at the transforming stage based on discrete depth sampling from the surface down to 1000-m depth in the subtropical–tropical western North Pacific. A total of 551 transforming stage larvae were collected at the 19 sampling stations. Except for the Diaphus species and Notolychnus valdiviae, all of the transforming stage larvae (including genera Benthosema, Bolinichthys, Centrobranchus, Ceratoscopelus, Diogenichthys, Hygophum, Lampanyctus, Lobianchia, Myctophum, Symbolophorus, and Triphoturus) were collected in the lower mesopelagic zone from 600- to 900-m depth during both day and night, showing no diel vertical migration (DVM). On the contrary, the Diaphus species and N. valdiviae larvae undergo DVM during the transforming stage, occurring below 200-m layer during the daytime and migrating up to the upper 150-m layer at night, i.e., they show earlier adaptation to juvenile–adult behaviors. Most myctophid fish larvae are known to undertake substantial ontogenetic vertical migration (OVM) from the epipelagic to mesopelagic zones during their early life stage. Although considerable sampling effort was carried out in this study, transforming larvae, except for the above two migratory ones, were not collected in the epipelagic and upper mesopelagic zones, strongly suggesting that their sinking speed would be high. It would be advantageous for survival to spend their highly vulnerable transforming stage in the lower mesopelagic zone, where predation pressures are lower and physical conditions are more stable than in the upper layers.     

Ocean current generated sedimentary facies in the Osumi Strait, South of Kyushu, Japan

An investigation of the surface sediments and bedforms in the Osumi Strait, located between Kyushu and Tanegashima, south of Kyushu, Japan, was carried out. The distribution of some characteristics of the surface sediments and bedforms is clarified.In the Osumi Strait, the surface sediments tend to become finer in size, better sorted and lower in specific gravity from southwest to northeast. The bedform distribution shows a systematic change in the same direction. This direction is the same as the direction of sediment transportation and of the current flowing through the Strait. It is considered that these changes in bedforms and sediment properties are formed by the decrease in the energy of the current. The current generating bedforms and sediment distribution is the Osumi Branch Current, one of the branches of the Kuroshio. The sediment transportation is active under the present hydraulic conditions.The sedimentary facies developed in the Osumi Strait is controlled by a unidirectional ocean current. The ocean current is one of the important factors for sedimentary processes where strong ocean current prevails along or near the continental shelf such as around the Japanese Islands.     

Recent Occurrences of Dinophysis fortii (Dinophyceae) in the Okkirai Bay, Sanriku, Northern Japan, and Related Environmental Factors

Occurrence of Dinophysis fortii, a causative organism of diarrhetic shellfish poisoning, in the Okkirai Bay, Sanriku was surveyed in 1995, 1996, 1998 and 1999. In each year, its major occurrence was detected from the late May or early June and continued until the late June or early July. Seawater temperature, salinity and nutrients measurements suggested that inflows of offshore water into the bay played key role on the first major occurrence of D. fortii. With an analysis of continuous temperature data in the Otsuchi Bay which locates north of the Okkirai Bay, this influent was considered to be intermittent inflow of the offshore water by internal tidal waves which propagated from north to south. First occurrence peak of D. fortii was synchronous with phycobilin containing microalgae, synechococcoid cyanobacteria and cryptomonad, in all years. In vivo fluorescence measurement of D. fortii cells in 1995 and 96 showed that the cells in these microalgal-rich water contained more phycobilin pigment than those in the microalgal-poor water. The result may support a hypothesis that D. fortii acquires phycobilin by an uptake of these microalgae. After the first major occurrence in the bay, D. fortii sometimes occurred in rather inshore waters where showed elevated ammonium level possibly due to increased heterotrophic activity. Together with another finding that D. fortii is mixotrophic, it could be assumed that the environment being suited to heterotrophic nutrition also stimulates D. fortii growth in the bay.     

Aspects of rhodium marine chemistry

Rhodium, one of the least abundant of the Pt-group elements in the earth's crust, has the second highest concentration in seawater. This may be a consequence of strong complexing, perhaps kinetically restrained with time constants of the order of centuries to millenia. The element shows a nutrient-like distribution in waters of the Pacific Ocean. Pelagic and coastal sediments have Rh concentrations close to crustal values. There is a remarkable enrichment of Rh in ferromanganese minerals that cannot be explained by oxidative capture. Unlike the other Pt-group elements, Rh is enriched in the high temperature hydrothermal sulfide deposits as well as phosphorites.     

Decomposition of Phytoplankton in Seawater. Part I: Kinetic Analysis of the Effect of Organic Matter Concentration

Decomposition experiments were conducted on cultured phytoplankton (Skeletonema costatum) in seawater containing decomposer and consumer of size less than 500 μm. We determined the decomposition rates of bulk particulate organic matter (POM), the ratio of labile to semi-refractory fractions in the POM, and the POM carbon/ nitrogen (C/N) ratio during decomposition. To identify the kinetic mechanisms involved in the reactions of different order (e.g., first- and second-order), we studied the sensitivity of reaction rates to the initial concentration of POM, ranging from 2.4 to 71 mg-C L⁻¹. The results showed that decomposition consists of two first-order reactions: decomposition of labile and of semi-refractory particulate organic carbon (POC). The decomposition rate constants found for labile (0.13 day⁻¹ at 20°C), and semi-refractory POC (0.008 day⁻¹ at 20°C), and the carbon weight ratio of semi-refractory POC (13% at 20°C), were insensitive to the initial organic matter concentration. The time-dependence of the C/N ratio was also independent of this initial concentration. The decomposition rate constants and the content of semi-refractory POC did not change, regardless of the absence or presence of 25–500 μm organisms in natural seawater. This revised version was published online in July 2006 with corrections to the Cover Date.