Vol.3 No.4 2011

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Research paper : Investigation of the distribution of elements of the whole of Japan and their applications (N. Imai)−273−Synthesiology - English edition Vol.3 No.4 (2011) to a hotel by the team and laid out to dry. They almost always dried completely overnight. In this way, about 60–100 samples per person were collected during trips conducted over two weeks, and 600–800 samples were collected overall.5.1.2 Geochemical map of the whole country[2]When the geochemical map of the whole country was being made, reducing the human load for work in handling vastly numerous samples was the most important problem, as we had learned when producing the geochemical map of the northern Kanto region. If all the sampling work were outsourced, then the necessary time and effort for us would have decreased. However, the cost would become too enormous to realize it as a normal research project. To save costs, the samples were to be collected by ourselves to the greatest extent possible. It was also important that the time-consuming sampling process be simplified. For that reason, work such as filtering was not performed in the field and no other work than collecting the sample was conducted. In this way, the costs were reduced, even if the work was outsourced. Consequently, the drying and sieving were not conducted in the field as in the case of the northern Kanto region map compilation. Fine-grained sand of about 2 kg was sampled using a shovel or bottom sampler. Moreover, the sampling point was set to the place where a large river intersected a large road: a location that could be reached easily. However, it took much time to move to the next sampling site because the sampling density was coarse and the distance separating sampling points was great. Especially for deep valleys in mountainous areas, even if it was apparently possible to collect a sample easily when viewed on a map, the actual river was often far under a cliff. Even in such cases, there was usually a small path running down to the river for use by sport anglers. Nevertheless, the sampling was sometimes impossible depending on the circumstances of the field. Eventually, about half of the total of 3024 samples were collected by ourselves. Otherwise, samples were taken by outsourcing. In all cases, dryness and sieving were done separately in a laboratory. However, the sample processing was efficient because the process allowed preparation of many samples at one time.5.1.3 Sampling of sea-bottom sediments[3]The most daunting problem was sampling to produce the geochemical map of the sea. The marine sediments were necessarily collected using a ship. Therefore, it was much too difficult to do ourselves, unlike the sample collection that was conducted on land. Especially, when the depth was more than several hundreds of meters, sample collection was impossible without a large specialized ship. It also entailed huge costs. We were able to reduce the costs greatly using marine sediment samples that the GSJ had collected in the past. In the survey, a marine research vessel of more than 2000 t was used for the surrounding seas of Japan whose depth was up to about 3000 m. For this study, the samples were newly collected in the sea for which we had no sample. Because the project had tight budget limitations, the depth of the seas in which we could collect the samples was at most 100–200 m. However, the areas with seas less than 200 m deep were sufficiently wide to constitute a good sampling area, especially in the west of Japan, where the continental shelf is developed well. There we were able to cover the large area of the sea easily. The marine sediment samples were collected using a glove bottom sampler (Smith–Macintyre type bottom sampler) that was installed on the survey ship. Sample processing was done fundamentally in the same way as for the samples from the land. The marine samples were 4905. Figure 2 depicts how the collected samples were raised from the sea bottom.5.2 Chemical analysisIn this study, because it was necessary to analyze numerous samples, sample processing and analysis were automated and standardized as much as possible. In the geochemical map of the northern Kanto region, 26 elements were analyzed using ICP emission spectrometry and neutron activation analysis. For the geochemical map of the whole country, 53 elements were measured using ICP mass spectrometry and atomic absorption spectrometry. In both cases, an auto-sampler and automated measurement systems were used to the greatest extent possible. To simplify the sample treatment, stream sediments of less than 80 mesh were analyzed without powdering. In most cases, the analytical results obtained for samples that had not been powdered were the same as those obtained for powdered samples. Moreover, the stream sediment was analyzed after extracting magnetic minerals such as iron sands using a magnet. Heavy minerals are sometimes concentrated by the fractionation of river water. The purpose of extracting magnetic minerals is to avoid letting the map become an unnatural geochemical map. Especially in regions where many iron sands exist, the geochemical map becomes an iron sand map. Samples were decomposed using mixed acids of nitric acid, perchloric acid, hydrofluoric acid, and 0.1N hydrochloric acid.5.3 Drawing of the geochemical mapIn drawing a continuous geochemical map from geographically discrete sampling points, it is necessary to interpolate in areas for which data do not exist. The watershed analysis was made by GIS to determine the upper watershed area for each sampling point of stream sediment. The elemental concentration for each area is assumed to be the same. To simplify the calculation, the watershed area was converted to square mesh and the concentration data were allocated as the same value to each mesh. Then the geochemical map was drawn by interpolating the mesh data. In this study, the geochemical map was made without consideration of the movement of the marine sediment by seawater or the oceanic current because a more realistic portrayal quickly became too complicated to calculate.

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