Kimihiko OKI’s research while affiliated with Kagoshima University and other places

Drill core, known as the Taniyama boring core, from the mouth of the Nagata River in Kogoshima city, southwest Japan, contains five pyroclastic flow deposits. The Kakuto, Kobayashi, and Hiwaki (Shimokado) pyroclastic flow deposits were identified through particle composition analysis and the refractive index of glass shards in the flow deposits. However, two of the deposits do not correlate with the deposits reported in previous studies. Many pyroclastic flow deposits occur in Kagoshima City and the surrounding areas. Some of these deposits have not been analyzed in terms of lithological features such as the major element compositions and trace element contents of volcanic glass shards. In addition, many deposits have not been clearly defined stratigraphically. It is necessary to review the major element compositions and trace element ratios of volcanic glass shards within these deposits to reconstruct the tephrostratigraphy of southern Kyushu. In this study, five pyroclastic flow deposits, including the Kakuto, Oda, Kobayashi, and Hiwaki, pyroclastic flow deposits were sampled, and the major and trace element compositions of volcanic glass in the deposits were analyzed by Energy Dispersive X-ray spectrometry (EDX) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP–MS). The results show that the Kakuto, Oda, Kobayashi, and Hiwaki pyroclastic flow deposits can be identified from the major element composition of volcanic glass, whereas in the core, one pyroclastic flow deposit remains unidentified, although these deposits show diagnostic trace element compositions.

Submarine fumaroles are located in the 200-m deep basin of the northern part of Kagoshima Bay. To estimate the influence of mercury emitted from these fumaroles, the seasonal changes in total mercury (T-Hg), reactive mercury (RM), dissolved gaseous mercury (DGM), and monomethylmercury (MMHg) in water were studied.Seawater samples were collected from five separate locations in the northern part from May, 2011 to May, 2012 using a CTD-Rosette water sampler; the samples were taken every 50 m, starting from the surface.T-Hg, DGM, RM, and MMHg in the seawater columns (n=132) were in the range of 0.05-3.04ngL-1, 0.003-1.08ngL-1, 0.006-1.47ngL-1, 0.02-0.69ngL-1 for an entire sample. Although the average T-Hg values at the surface and middle layers (0-100m) for each sample showed no significant seasonal variation throughout the year, those at the deeper layer (150-200m) were high from summer to autumn and low in winter. The seasonal changes and vertical distributions of DGM, RM, and MMHg concentrations were similar to those of T-Hg. A thermocline was observed from spring to autumn and ceased in winter. Because the thermocline prohibits vertical circulation of seawater, the fumarole impact may increase during those months.

To estimate the current contamination levels in Minamata Bay, the vertical and horizontal distributions of mercury in the sediment were investigated. Sediment core samples were collected in 2002, 2006, 2008, and 2010 at 12 locations by gravity core sampling in Minamata Bay and Fukuro Bay, which is located in the southern part of Minamata Bay. The average total mercury concentration during each year in the surface sediment was 2.47–3.34 and 3.50–4.66 mg kg- 1 for Minamata Bay and Fukuro Bay, respectively; significant variation in the values was not observed during the study period. The total mercury concentration in Fukuro Bay increased with increasing depth and reached a maximum at 8–14 cm from the surface and decreased with increasing depth in the deeper layer. In Minamata Bay, the total mercury concentration did not change significantly from the surface to a depth of 10 cm and the values were considerably higher than the background level. In the lower layers of the long cores taken from both areas, the total mercury concentration decreased with depth, and the deepest layers exibited relatively uniform low values. These values can be considered to represent the background concentration in the absence of anthropogenic influence. The depth of the sediment affected by the effluent was estimated to be 20 ± 7 cm and 33 ± 4 cm for Minamata Bay and Fukuro Bay, respectively. Furthermore, it was estimated from the average concentration and volume of the contaminated layer that approximately 1 ton of discharged mercury remained in the sediment of the two bays.

Since 1981, we have been engaged in field studies of the habitat of Nautilus pompilius in the Philippines (1981 and 1982) and in the Fiji Islands (1982 and 1983) (Hayasaka et al., 1982; Hayasaka, 1983, 1985). The main purpose of these studies was to obtain basic data on the habitat of N. pompilius in the Philippines and in Fiji, which are at opposite ends of the vast distribution range this species. Although the overall project is still in progress, the results of study to date are summarized in this chapter.

Total mercury (T-Hg) and methylmercury (MeHg) concentrations were determined in water at the bottom, in the suspended solid, and in the surface sediment of Minamata Bay to assess the remobilization of mercury from the sediment into the water column. The water and sediment samples were taken from the bottom of the bay using a gravity core sampler at nine locations in October 2002 and six locations in April 2005. The average concentration of T-Hg and the proportion of MeHg in the sediment were 3.71 ± 1.90 mg/kg and 0.27 ± 0.28%, respectively. The water contained 1.80 ± 1.00 ng L− 1 of T-Hg, which was higher than the value reported for the upper-middle depth of Minamata Bay. The results suggest that the sediment is an important source of mercury in the water of Minamata Bay. The percentage of MeHg in the water at the bottom was 50.7 ± 24.6%, also considerably higher than in the upper-middle layer of the water column, suggesting that MeHg may be the predominant form of mercury released from the sediment into the water. The percentage of MeHg was lower in the resuspended sediment than in the surface sediment. The amount of mercury eluted from the sediment into the water was estimated at 0.46 kg and 0.11 kg per year for T-Hg and MeHg, respectively.

Mercury-contaminated effluent was discharged into Minamata Bay from a chemical plant over a period of approximately 40 years until 1968. In October 1977, the Minamata Bay Pollution Prevention Project was initiated to dispose of sedimentary sludge containing mercury concentrations higher than 25 mg kg(-1). In March 1990, the project was completed. In an effort to estimate current contamination in the bay, the vertical and horizontal distributions of mercury in sediment were investigated. Sediment core samples were collected on June 26, 2002 at 16 locations in Minamata Bay and Fukuro Bay located in the southern part of Minamata Bay. The sediment in Fukuro Bay had not been dredged. The total mercury concentration in surface sediment was 1.4-4.3 mg kg(-1) (2.9+/-0.9 mg kg(-1), n=9) for the dredged area of Minamata Bay and 0.3-4.8 mg kg(-1) (3.6+/-1.6 mg kg(-1), n=4) for Fukuro Bay. In the lower layers of long cores taken from both areas, the total mercury concentration decreased with depth and finally showed relatively uniform low values. These values can be considered to represent the background concentration absent of anthropogenic influence, which was estimated for the study area to be 0.068+/-0.012 mg kg(-1) (n=10). From the surface, the total mercury concentration in Fukuro Bay increased with depth and reached a maximum at 8-14 cm. In Minamata Bay, several centimeters from the surface the total mercury concentration did not change significantly having considerably higher values than the background level. At six stations, the methylmercury concentration was determined. Although the vertical variations were similar to those for total mercury, Fukuro Bay sediment showed a higher concentration of methylmercury than Minamata Bay sediment.

Mercury-contaminated effluent was discharged into Minamata Bay from a chemical plant over a 20-year period until 1965 (from 1958 to 1959, effluent was discharged into Minamata River), causing Minamata disease. In an effort to characterize the extent of the contamination in the Yatsushiro Sea, the vertical and horizontal distributions of mercury in sediment were investigated. Sediment was sampled at 62 locations in the southern part of the sea from 4 to 6 March 1996. In the lower layers of the long cores of sediment, the total amount of mercury was at a relatively uniform low concentration. We interpret these low values to represent the background concentration absent of anthropogenic influence. The background value thus estimated for the Yatsushiro Sea was 0.059±0.013 mg kg−1 (mean±S.D., n=51). The highest concentration in each sample ranged from 0.086 to 3.46 mg kg−1 (mean, 0.57 mg kg−1). The higher values were obtained at stations near Minamata Bay and the Minamata River (the sources of the pollution). Concentrations decreased with distance from the source. An inspection of the vertical profiles of mercury concentration in cores suggested that the deposited mercury had not been fixed in sediment but had been transported, despite 30 years having past since the last discharge of contaminated effluent. At nine stations, extractable inorganic and organic mercury concentrations were determined differentially. Inorganic mercury is the predominant species in sediment and organic mercury comprising approximately 1% of the total.

The ratio of the number of living specimens to the total number of living specimens and empty tests (L/Tl value) of benthic foraminifers in the bottom surface sediments has been used as an index of relative sedimentation rate at each sampling station. In an attempt to clarify whether or not the L/Tl values can be applied to shallow and semi-closed inland seas, where high energy current system persist, we analyzed the benthic foraminiferal assemblages in the topmost sediments of 73 core samples collected from 74 stations in southern Yatsushiro Kai (Sea). The L/Tl values were calculated and compared with the sedimentation rates, which were independently estimated by the vertical changes of mercury contents in the cores. It has been shown that the L/Tl values near the straits influenced by strong bottom currents were higher than the expected sedimentation rates, suggesting that the foraminiferal tests had been transported and/or destructed after death. The L/Tl values were also higher in the area of fine- to very fine-grained sand substrata surrounding the straits, a result which can be explained by the post-mortem destruction of the empty tests especially of the species that have a thin test. Furthermore, the L/Tl values in nearshore areas were lower than expected, probably reflecting the extraordinary rainfalls in the last few years that supplied an unusual amount of sediments through rivers. These results indicate that the L/Tl values need to be used with caution when applying to the sediments of shallow inland seas.

The main purpose of this study is to clarify the relation between characteristics of sediments and environmental conditions, based on the analysis of core samples from the South Yatsushiro Kai (Sea) which is located off the west coast of central Kyushu Island, Japan.Bottom sediment samples were collected at 74 stations in the southern part of the Yatsushiro Kai using gravity corers in March 1996. 62 core samples were used for the mechanical analysis by the settling-tube method, and Q-mode cluster analysis was carried out based on the proportion of grain size distribution (Wentworth size classes). 61 core samples were available to estimate the sedimentation rates (during c. 50 years) which were recognized from the first appearance of high mercury contents of cores. On the other hand, the topmost one centimeter of 73 core samples, which is approximately 10cc of wet sediments, was utilized for foraminiferal study.The general trend of the bottom current system in the area was discussed, based on the mechanical analysis data of bottom sediments, the distribution of sedimentation rates and the distributions of the number of individuals of planktic and living benthic foraminifera. Based on the results mentioned above, the South Yatsushiro Kai is subdivided into the following five areas: 1) the area near the straits characterized by coarse sediments under the influence of strong tidal and bottom currents; 2) the southern part and the area off the mouth of the Minamata River, both of which are characterized by relatively coarse sediments under the influence of longshore currents and poorly sorted sediments supplied by the Komenotsu and Minamata Rivers; 3) the northwestern and eastern parts characterized by fine-grained sediments accumulated under conditions of rather stagnant water masses; 4) the central area, characterized by relatively fine sediments and a large number of planktic empty tests deposited on the sea bottom under the condition of rather weak tidal current systems; 5) the northern part characterized by gravels or gravelly sands which were derived from shallow coastal areas.Judging from the distribution pattern of the maximum mercury content at some horizons of each core, the fine-grained sediments polluted by mercury were transported northeastward and southward by weak longshore currents and spread toward north and west across the northern and southern parts of the South Yatsushiro Kai.