Eiichiro Nakayama

Fundamental studies for chemical speciation of iron in seawater with an improved analytical method

We improved the analytical methods for determining iron in seawater, based on our established procedure, and carried out fundamental studies for chemical speciation of iron. The blank value of the system used was ∼ 0.05 n M and the detection limit (3 SD) was 0.01 n M. To examine the dissolution of iron from suspended particles in seawater, four types of particles (aged iron colloid, biogenic particles, sediment particles and freshly deposited iron) were used. At a pH of ∼ 3, dissolution of iron from the easily leachable fraction of suspended particles was prompt, and the fraction of iron [total dissolvable iron, TD(Fe)] was thought to be the sum of labile particulate and dissolved iron. The iron fractions dissolved from suspended particles in the solutions were constant at pHs < 1.5 when solutions were heated in a microwave oven. The fraction of iron dissolved by this procedure was defined as the “leachable iron, L(Fe)”. Some known organic complexing agents were also studied as a model group to examine the possible effects of naturally occurring organic ligands on the recovery of iron with chelating resin preconcentration. Established methods were applied to analyses of seawater samples obtained from the western Southern Indian Ocean (SIO) and the East China Sea. Surface seawater samples from the SIO showed very low iron concentrations, which may be due to a lack of aeolian transport of mineral dust.

Chemical speciation of chromium in sea water: Part 2. Effects of Manganese Oxides and Reducible Organic Materials on the Redox Processes of Chromium

Abstract Experiments designed to clarify the role of manganese oxides in the oxidation process of Cr(III) to Cr(VI) in sea water are described. The theoretical redox equilibrium controlled only by dissolved oxygen is shown not to be always attained for the Cr(III)/ Cr(VI) system in practice. Some naturally occurring materials can reduce Cr(VI) under the conditions of natural sea water.

Chemical speciation of chromium in sea water: Part 1. Effect of Naturally Occurring Organic Materials on the Complex Formation of Chromium(III)

Abstract The effect of naturally occurring organic materials on the coprecipitation behaviour of trivalent chromium is studied to estimate the relative abundance of organic-bound chromium in sea water. Some amino acids and polyhydric organic acids can bind chromium(III) under the conditions prevalent in natural sea water. Some of the chromium(III) in sea water therefore probably occurs in organic species.

Chemical speciation of chromium in sea water: Part 3. The determination of chromium species

Abstract A method for the determination of chromium(III), chromium(VI) and organicallybound chromium in sea water is reported. It is confirmed that sea water contains about 9 × 10 -9 M dissolved chromium. This is shown to be divided as ca. 15% inorganic Cr(III), ca. 25% inorganic Cr(VI) and ca. 60% organically-bound chromium. It is suggested that the inconsistency of earlier results on the dominant chromium species and its concentration in sea water is largely due to the fact that organically bound chromium species were not considered.

Hydrothermal plumes in the eastern Manus Basin, Bismarck Sea: CH4, Mn, Al and pH anomalies

Abstract We present evidence for strong hydrothermal activity in the eastern Manus Basin (depth: 1700–2100 m), the existence of large scale triple-layered buoyant plumes at depths of ∼1100 m (“shallow plume”), ∼ 1700 m (“deep plume”), and ∼1400 m (“middle plume” with less extent than the other two plumes) that were revealed from water column anomalies of CH 4 , Mn, Al and pH observed in November to December 1990. Judging from the horizontal distribution of these parameters, the deep plume seems to originate from two distinct hydrothermal sites (eastern and western sites) in the research area, the eastern site being visually ascertained with deep-tow observations at the same time. The CH 4 /Mn ratio (mol mol −1 ) of the deep plume (0.02–0.05) is the lowest yet observed in hydrothermal plumes. The order of magnitude difference of CH 4 /Mn ratios between the shallow plume and the deep plume suggests that different kinds of fluid-rock interaction occurred to make the hydrothermal end members for the deep and shallow plumes. The shallow plume, which had an areal extent of more than 50 km, may be an episodic “megaplume”, because it was not recognized in the previous CH 4 profiles in 1986, and because it has a similar CH 4 /Mn ratio as the megaplume observed in the North Fiji Basin. We found that the eastern deep plume is characterized by enormously high aluminium concentrations (0.6– 1.5 μmol kg −1 ), pH anomalies (∼0.1) and high Al/Mn ratios (10–17). The endmember fluid for the eastern deep plume may have an unusually low pH value to dissolve this much aluminum during fluid-rock interaction, or this plume may originate from an eruption-influenced fluid.

Determination and distribution of iodide- and total-iodine in the North Pacific Ocean - by using a new automated electrochemical method

Abstract A new automated analytical method for determining inorganic iodine species in seawater as iodide- and total-iodine (iodide+iodate), by using a flow-through electrode system, has been applied to the seawater samples collected from the adjacent seas of Japan and the western and central North Pacific Ocean. It was found that a considerable amount of iodide-iodine is distributed not only in the euphotic zone, but also in deep waters of the Japan Sea and the Japan Deep and in directly overlying waters on the sea floor of the open ocean. This fact suggests that the reduction of iodate to iodide by biological activity in seawater occurs when nitrate is consumed by nitrate respiration in the presence of a large amount of easily oxidizable organic matter, which may be in particulate form, rather than when nitrate is deficient, as is generally accepted, because these waters contain the usual amount of nitrate for deep waters.

Hydrothermal plumes at the Rodriguez triple junction, Indian ridge

Water column anomalies of light transmission, Mn, Fe, Al and CH4 concentrations were searched in the central, southeastern and southwestern Indian Ridge segments centered on the Rodriguez Triple Junction (RTJ) (∼25°32′S, ∼70°02′E), for the purpose of locating hydrothermally active areas, in July to August 1993. We found an active zone in the central Indian Ridge segment (25°18–20′S) approximately 12 miles north of the RTJ, where significant hydrothermal plumes were observed at 2,200–2,400 m depth. Intensive tow-yo observations using a CTD rosette multi-sampling system equipped with a transmissometer revealed that the plumes show temporal as well as spatial variations. Discrete water samples within the plumes were enriched in Mn, Fe, and CH4, with maximum concentrations of 9.8 nM, 40.2 nM and 3.3 nM, respectively. Judging from the spatial and chemical characteristics of the plumes, especially from transmission anomalies andC/H4Mn ratios, we speculate that the hydrothermal venting site might be not in the rift valley but on the eastern off-axis zone, several miles distant from the rift valley.

Distribution of trace bioelements in the subarctic North Pacific Ocean and the Bering Sea (the R/V Hakuho Maru Cruise KH-97-2)

A column concentration-high resolution inductively coupled plasma mass spectrometry (ICP-MS) determination was applied to measure the total dissolved concentrations of Fe, Co, Ni, Cu and Zn in seawater collected from the subarctic North Pacific (~45°N) and the Bering Sea in July–September 1997. Total adsorbable Mn was determined on board by column electrolysis preconcentration and chemiluminescence detection. The vertical profiles for Fe, Ni and Zn were nutrient-like. The deep water concentration of Fe was ~0.5 nM in the northeast Pacific (18°-140°W) and increased to ~1 nM in the northwest Pacific (161°E) and ~2 nM in the Bering Sea (57°N, 180°E). The deep water concentrations for Ni and Zn in the Bering Sea were also 1.3–2 times higher than in the North Pacific. The profiles for Co and Cu were examined in the subarctic North Pacific, and results obtained were consistent with previous reports. There was a significant correlation between the concentrations of Co and Mn except for surface mixed layer. The profiles for total adsorbable Mn were similar to the reported profiles for total dissolvable Mn. The deep water concentration of Mn in the Bering Sea was also 4 times higher than in the North Pacific. Iron and zinc were depleted in surface water of the subarctic North Pacific. The relationship between these trace elements and nutrients suggests that these elements could be a limiting factor of phytoplankton productivity. In the Bering Sea, surface water contained ~0.3 nM of Fe. The Zn concentration, which was less than the detection limit in surface water, increased at shallower depths (~30 m) compared with the subarctic North Pacific. These results imply a higher flux of Fe and Zn to surface water in the Bering Sea. This in turn may cause the ecosystem in the Bering Sea characterized by a dominance of diatoms and high regenerated production.

Tungsten in North pacific waters

Tungsten in oceanic waters has only been analysed in a few coastal areas. Here, we report the first tungsten depth profiles in oceanic waters. Tungsten concentration in the north Pacific is constant at 53–60 pmol l−1 (normalized to a salinity of 35‰) from the surface to the bottom. Similar vertical profiles of tungsten and molybdenum probably result from the fact that both elements are hardly adsorbed at all onto organisms and particulate organic matter in surface waters. However, although the crustal abundance of tungsten and molybdenum are nearly equal, the concentration of tungsten is ∼ 11800 of that of molybdenum in oceanic waters. This indicates that tungsten is removed from seawater much more rapidly than molybdenum, by adsorption onto ferric hydroxide, manganese oxide and clay minerals.

Development of a deep-sea in situ Mn analyzer and its application for hydrothermal plume observation

Abstract This paper presents the first in situ flow-through chemical analyzer using a chemiluminescence (CL) method in the deep sea to a depth of 5200 m. The analyzer, called GAMOS (Geochemical Anomalies MOnitoring System), determines the concentration of dissolved manganese continuously using a H 2 O 2 –luminol CL method. A detection limit of 0.23 nM was obtained. The GAMOS was used successfully for hydrothermal plume observation.