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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Watershed analysis for geochemical mapping in Japan based on a hydrologic model: The concentrations of 53 elements and the dominant lithology in a drainage basin

Atsuyuki Ohta, Noboru Imai, Takashi Okai, Mitsuo Manaka, Ran Kubota, Atsunori Nakamura, Yoshiko Tachibana
Geochemical Journal, Vol. 55, No. 2, P. 59-88, 2021

ABSTRACT

Recent interest in geochemical maps produced for environmental assessment has led to multi-purpose applications of such maps in various other fields including provenance studies for agriculture and archaeology. Research subjects of these provenance studies closely relate to surficial rocks. Fine stream sediment samples used for Japanese geochemical mapping originate from parent rocks present in the corresponding drainage basins. Consequently, as well as the chemical composition of stream sediment samples, information regarding the parent lithology is imperative for the applications mentioned above. In this work, the exposed areas of different lithologies in river drainage basins were calculated using a digitalized geological map and watershed polygons obtained using a hydrologic model. Stream sediment samples were classified into 18 different types in a larger-scale classification and into 92 more specific types in a smaller-scale classification based on the following assumption: when a specific rock type crops out over more than 50% of the watershed area, it is the dominant rock type controlling the chemical composition of stream sediments. This assumption was confirmed to be reasonable using a one-way analysis of variance. Chemical Index Alteration values suggest that the chemical weathering process is not dominant in the formation of stream sediments from their source rocks. Accordingly, the chemical compositions of stream sediments are approximately comparable to that of their source rocks. Finally, the percentages of the different lithologies in the watershed areas were tabulated for all stream sediment samples (n = 3024). The median values of 53 elemental concentrations were calculated for samples classified according to the dominant lithology. This new database lists the chemical compositions and the dominant lithology in the watershed area for each sample and constitutes an extensive source of information relevant to understanding of the chemical, mineralogical, and petrological variety of surface materials in this island arc setting, which helps to respond to existing academic or industrial requests.

KEYWORDS

geochemical map, geographic information system, watershed analysis, hydrologic model, stream sediment

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