Hajime Obata

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.

Response of equatorial Pacific phytoplankton to subnanomolar Fe enrichment

In the equatorial Pacific Ocean along 160° W, surface-water samples with natural plankton communities were placed in incubation bottles to which subnanomolar levels of Fe were added under ultraclean conditions. Addition of 0.1-0.8 nM Fe to seawater samples containing high NO 3 increased stocks of Chl a and POC, NO 3 consumption and net growth rate of phytoplankton in incubation bottles relative to the controls. A large increase in the Chl a concentration of large- (> 10 μm) and medium-size (3-10 μm) fractions was observed in the Fe-enriched samples. POC concentrations doubled even with 0.1 nM Fe at the equator. The net growth rates of large- and medium-size phytoplankton increased systematically with added Fe concentration. The dissolved Fe concentration in the incubation bottles, which was determined on board by flow injection analysis, decreased significantly during the first 3 days of incubation. However, 50-90% of the added Fe remained in the dissolved fraction (< 0.2 μm) at the end of the experiments. These results indicate that changes in subnanomolar Fe levels affect the equatorial phytoplankton communities by promoting the growth of large phytoplankton.

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.

Measurement of fluoride ion in the river-water flowing into Lake Biwa

To study the effect of biotite granite on the concentration of fluoride ion in the river-water around it, the concentration of the ion in several river-water samples flowing into Lake Biwa, in Shiga Prefecture, Japan, was measured by using ion chromatography. Several biotite granite plutons around Lake Biwa were contributed to relatively high concentration of fluoride ion in river-water around the plutons. By this study and previously reported result on the river-water flowing in the Rokko Granite, in Hyogo Prefecture, Japan, it was supposed that the degree of the distribution depends on the age of each pluton and the extent of its weathering.