15:30 〜 15:45
[MGI34-01] 流域に占める主要岩相に応じて分類を行った河川堆積物の地球化学データセットについて
★招待講演
キーワード：地球化学図、地理情報システム、水理解析、分散分析、河川堆積物、起源分析
Recently, Japanese geochemical maps have found a wide application, such as provenance study of agricultural production, migration analyses in archaeology, forensic investigation, and the estimation of geoneutrino fluxes. To fulfill these requirements, we report the newly-compiled dominant lithology in watersheds where stream sediment samples were collected for Japanese geochemical mapping based on a hydrologic model. Median values of 53 elemental concentrations of stream sediments classified according to the parent lithology have been calculated (Ohta et al., 2021).
[Introduction]
Geochemical maps, the spatial distribution of elements, provide the natural background levels of elemental concentrations on the Earth’s surface, which is useful to for assessing pollution and to explore undiscovered metalliferous deposits. The Geological Survey of Japan, AIST has created nationwide geochemical maps using 3024 fine stream sediment samples and 4905 marine sediment samples, for 53 elements (Imai et al., 2004, 2010). The maps are designed for environmental assessment and for the investigation of diffusion processes of materials from the land to the sea. All data are available on the online database (https://gbank.gsj.jp/geochemmap/).
[Hydrologic analysis]
Stream sediments dominantly comprise the weathering products of rocks in the catchment area upstream from the sampling site. The drainage basin for each sampling location was obtained using GIS and 50m-DEM data. The areal distribution of each lithology in the digital geologic map was calculated for each watershed: the exposed area for each lithology was estimated. We presume that when a specific rock type crops out over more than half of the drainage basin area (> 50 %), it is representative of surface rock types in the watershed and is the dominant control on elemental abundances in stream sediments. Although this is a rough approximation, we found that there are no significant differences among elemental concentrations determined by different threshold values (60%, 70%, 80%, and 90%) with a few exceptions.
[Discussion]
A one-way ANOVA was applied to examine the influence of coexisting lithology and differences in the mineralogical composition of source rocks on the chemical composition of stream sediment. For example, high-pressure type metamorphic rocks are composed mainly of mafic, pelitic, and psammitic schists. Stream sediment derived from mafic schist has different mineralogical and chemical compositions to that of pelitic and psammitic ones, which coexists in the watershed. We found that the differences in mineral contents and the different source rocks significantly affect the chemical composition of stream sediments. However, these effects are smaller than the effect of heterogeneity of the sample media in the riverbed.
Finally, the lithologies of interest are classified into the 13 categories (e.g., sedimentary rock, mafic and felsic volcanic rock, and granitic rock) in a larger-scale classification, and into the 92 more specific types (classified by source rock type, formation age, tectonic belt, and formation name) in a smaller-scale classification. The median values of elemental concentrations in stream sediments classified by the dominant lithology are consistent with those of the parent rocks. The result suggests that the formation of clastic materials from source rocks is not particularly affected by the chemical weathering process. The chemical compositions of stream sediments and information regarding their parent lithology are imperative for provenance studies.
References
Imai, N. et al. (2010) Geochemical Map of Sea and Land of Japan
Imai, N. et al. (2004) Geochemical map of Japan.
Ohta, A. et al. (2021) Geochem. J. 55, 59–88. 10.2343/geochemj.2.0618.
[Introduction]
Geochemical maps, the spatial distribution of elements, provide the natural background levels of elemental concentrations on the Earth’s surface, which is useful to for assessing pollution and to explore undiscovered metalliferous deposits. The Geological Survey of Japan, AIST has created nationwide geochemical maps using 3024 fine stream sediment samples and 4905 marine sediment samples, for 53 elements (Imai et al., 2004, 2010). The maps are designed for environmental assessment and for the investigation of diffusion processes of materials from the land to the sea. All data are available on the online database (https://gbank.gsj.jp/geochemmap/).
[Hydrologic analysis]
Stream sediments dominantly comprise the weathering products of rocks in the catchment area upstream from the sampling site. The drainage basin for each sampling location was obtained using GIS and 50m-DEM data. The areal distribution of each lithology in the digital geologic map was calculated for each watershed: the exposed area for each lithology was estimated. We presume that when a specific rock type crops out over more than half of the drainage basin area (> 50 %), it is representative of surface rock types in the watershed and is the dominant control on elemental abundances in stream sediments. Although this is a rough approximation, we found that there are no significant differences among elemental concentrations determined by different threshold values (60%, 70%, 80%, and 90%) with a few exceptions.
[Discussion]
A one-way ANOVA was applied to examine the influence of coexisting lithology and differences in the mineralogical composition of source rocks on the chemical composition of stream sediment. For example, high-pressure type metamorphic rocks are composed mainly of mafic, pelitic, and psammitic schists. Stream sediment derived from mafic schist has different mineralogical and chemical compositions to that of pelitic and psammitic ones, which coexists in the watershed. We found that the differences in mineral contents and the different source rocks significantly affect the chemical composition of stream sediments. However, these effects are smaller than the effect of heterogeneity of the sample media in the riverbed.
Finally, the lithologies of interest are classified into the 13 categories (e.g., sedimentary rock, mafic and felsic volcanic rock, and granitic rock) in a larger-scale classification, and into the 92 more specific types (classified by source rock type, formation age, tectonic belt, and formation name) in a smaller-scale classification. The median values of elemental concentrations in stream sediments classified by the dominant lithology are consistent with those of the parent rocks. The result suggests that the formation of clastic materials from source rocks is not particularly affected by the chemical weathering process. The chemical compositions of stream sediments and information regarding their parent lithology are imperative for provenance studies.
References
Imai, N. et al. (2010) Geochemical Map of Sea and Land of Japan
Imai, N. et al. (2004) Geochemical map of Japan.
Ohta, A. et al. (2021) Geochem. J. 55, 59–88. 10.2343/geochemj.2.0618.