Elemental mobilisation and redistribution during the ferromanganese nodule formation in the sediments of Ganga plain, India

Abstract

Formation of ferromanganese nodules (FMNs) is an important pedogenic process which is commonly observed in the Ganga alluvium. We have studied geochemistry of the host alluvium and FMNs from the eastern Uttar Pradesh, India. The FMNs were categorized in three sizes (>5.6mm, 5.6 to 2mm, and 2 to 0.5mm). The major and trace elements present in the sediments and FMNs of three size ranges were analysed using XRF and ICP-OES. The geochemical composition of the sediments and FMNs were compared with the geochemical values of the Upper Continental Crust (UCC). The enrichment factor (EF) of the analysed elements were calculated for both the host sediments and the FMNs. Additionally, the Chemical index of weathering (CIA) was calculated to quantify the extent of chemical weathering of the sediments and FMNs in the alluvium. The sediments and FMNs both showed moderate chemical weathering. We propose that the weathering of aluminosilicate stages may have given Fe, Mn, and related trace elements for the nodule formation. Prolonged dry spells and monsoonal wet seasons in the Ganga plain region may be key contributors to the weathering, mobilization, precipitation, and redistribution of the Fe and Mn phases in nodules. Certain elements, such as Pb and Cr, which are considered to be harmful to the environment got sequestered in FMNs. This helped immobilization of these contaminants. Whereas, elements like P, Co, Zn, and Cu were also got sequestered during the formation of FMNs. Therefore, it has also impacted soil nutrient availability.