Abstract
Pg fraction of humic acid is one of the components of humic substances extracted from soils and terrestrial sediments; it is characterized by the presence of green pigment. This green pigment derives from metabolites of many species of fungi and is widely distributed in world soils. In this paper, the vertical distribution of Pg absorption strength was obtained for the tephra soil sequence consisting of 6 representative tephras in southern Kyushu. The chronological sequence of Pg absorption strength, which covers ca. 100ka, shows good correspondence to the marine oxygen isotope curve up to stage 5a. From the analytical results of inorganic components, we clarified that Pg exists with iron oxides such as geothite and ferrihydrite. Strong fluctuation of Pg absorption was recognized in oxygen isotope stage 3, which was considered to correspond to the strength of past weathering and leaching process. This assumption was also supported by the distribution of silica/alumina ratio and alumina/mobile bases ratio in the profile. We confirm that Pg fraction of humic acid remains in tephra soil sequences as a signal of past environment, and indicates the strength of leaching controlled by the relative strength of precipitation and evaporation.
