Geochemistry of ferromanganese nodules in the soil of Okinawa Island, Japan: Paleoclimatic implications and discrimination scheme of the nodule origins

Shimajiri–Mahji refers to dark-red soil covering Ryukyu limestone that is characterized by buried ferromanganese (Fe–Mn) nodules, and it is widely distributed in Okinawa Island, southwestern Japan. The formation environment of the Fe–Mn nodules is considered closely related to the pedogenetic process of Shimajiri–Mahji because the mineral assemblage of the nodules is similar to that of the surrounding soil. There are two competing hypotheses about the formation environment of the Fe–Mn nodules: marine or terrestrial. In this study, the chemical composition of the Fe–Mn nodules was analyzed to determine their origin. The low Ni and Cu contents and the positive Ce and negative Y anomalies of the Fe–Mn nodules are inconsistent with the geochemical features associated with formation in the deep ocean and shallow seas or lakes, respectively. In contrast, the geochemical features of the Fe–Mn nodules correspond to formation in terrestrial soil, especially in terra rossa. An elemental map of the Mn/Fe ratio and computed tomography results demonstrated that the Fe–Mn nodules have a layered structure, which indicates a temporal change in the redox potential of Shimajiri–Mahji. This may be attributed to changes in the soil moisture owing to the variable intensity of the East Asian monsoon and climate change. A new discrimination diagram based on Al/Na and Co/Rb ratios was developed for classifying the formation environments of Fe–Mn nodules, which may help improve understanding of the formation history of soils and sediments containing them.