Geochemical and Sr–Nd isotopic variations in palaeoflood deposits at mainstem–tributary junction, western India: Implications on late Holocene flood events

Element geochemistry and Sr–Nd isotopes have been used to characterise the palaeoflood sediments at a tributary junction in lower Narmada Valley archiving the Little Ice Age (LIA) and to determine the present day sources for the mainstem and the tributary sediments. The major and trace element concentrations show minor variations in values post 360CalyrBP. The 87Sr/86Sr and εNd values of palaeoflood sediments range from 0.7122 to 0.7191 and −2.89 to +6.28 respectively. The bed sediments from the mainstem river (Narmada) and its tributary (Karjan) yield 87Sr/86Sr values of 0.72394 and 0.7113 and εNd values of −7.18 and +6.09 respectively. Potential sources of palaeoflood sediments have been investigated from Sr and Nd isotope compositions and mixing of the mainstem as well as the tributary sediments is suggested. Pronounced contribution from the Deccan Basalt during the early part of the Little Ice Age is inferred. The more radiogenic Nd values indicate a contribution from the weathering of younger Deccan basalts, especially the Poladpur formations. The control of weathering intensity and mineral sorting is observed in ‘higher than the source rock’ 87Sr/86Sr ratios of the sediments. The major and trace element variations during the last 700years are not conclusive although, isotopic signatures of the flood sediments help in constraining the flux from the tributaries. Sr and Nd isotopic composition of the palaeoflood sediments can be used to track the monsoon intensity and pattern but their concentrations in a fluvial system remain primarily controlled by mineral sorting processes and mixing of sources.