Intense chemical weathering in southwest Japan during the Pliocene warm period

Abstract

Bulk chemical composition and mineral composition of the Pliocene sediments in southwest Japan reveal the effects of source rock compositions and grain sizes, as well as the relationship between sedimentary environments and the degree of chemical weathering in the Pliocene warm period. A wide variation in the degree of chemical weathering, source rock compositions and grain sizes was observed with vertical and spatial changes in the sedimentary environment. Coarse-grained sediments, deposited in sandy channels and their floodplains, indicate a lower but wide range of CIA values. This result suggests that the degree of chemical weathering can be modified by changes in grain size due to hydrologic processes. In contrast, stagnant water sediments, composed of homogeneous fine-grained sediments, display a wide variety of REE compositions and Al2O3/SiO2 ratios mainly caused by variation in source rock compositions and grain sizes. However, the CIA values of the stagnant water sediments are over 90, such that these sediments display a tight cluster near the Al2O3 apex on the Al2O3-(CaO + Na2O)-K2O ternary diagram. This study demonstrates CIA values are generally above 90, which is due to the increased leaching of Ca, Na, and K with exposure to under intense chemical weathering conditions even if the source rocks and grain sizes are variable. Regardless of the wide variety of source rock compositions and grain sizes, REE and kaolinite-rich clay mineral concentrations in these sediments are indicative of the intense weathering conditions that prevailed in the Pliocene period (3–4 Ma) in southwest Japan.