Abstract
Weathering rinds formed on terrace gravels buried during the last 300 ka were investigated to clarify the rate of chemical weathering and the process enhanced by redox reaction. The ages of terraces were estimated by tephro-stratigraphic method and 14C ages of plant tissues that were distributed in the southern part of Fukui Prefecture, central Japan. Gravels are sampled from terrace deposits that have different ages of sedimentation, 300 ka (U1), 120 ka (U2) and 50 ka (M), and the changes of physical and geochemical properties of the weathering rinds were examined. The results show that two types of weathering rinds occur. One is the 'iron-hydroxide edge developing type'; it is characterized by the enrichment of iron hydroxide formed at the front of oxidized zone in the weathering rinds. The other is the 'iron hydroxide gradually changing type'; gradual changes of iron hydroxide accumulation from gravel rims to the cores without specific enrichment at the front in the weathering rinds. The iron hydroxide accumulation observed in both types can be formed by the migration of ferrous ion under reducing subsurface conditions, and oxidized thereafter by the diffusion of oxygen from the terrace sediments. However, the detailed process of edge formation with little oxidant in the gravel matrices is still not clear. The process identified here may provide an analogue of secondary elemental migration process enhanced at any type of rock that proceeded due to water-rock interaction system in subsurface environment.
