Geochemical and diatom records of recent changes in depositional environment of a tropical wetland, central west coast of India

Abstract

Two mudflat sediment cores collected from a sub-channel (S-61) and the main channel (S-60) of a tropical wetland, along central west coast of India, were investigated for recent changes in depositional environment using geochemical (sediment grain size, total organic carbon, total nitrogen, carbon/nitrogen ratio (TOC/TN), selected metals and pH) and diatom records. The sub-channel (S-61) represents river environment which opens into main channel (S-60) which represents marine environment being close to the sea. We report a transition from river-dominated to marine-dominated depositional environment over the years. The relatively higher sand percentage together with elevated carbon/nitrogen ratio and predominance of freshwater diatoms suggested greater river runoff in the past while marine influence is supported by decrease in TOC/TN ratio and increased dominance of marine diatoms in recent years. In the main channel (S-60), distribution of metals is regulated by organic matter (total nitrogen and total organic carbon) while in the sub-channel (S-61), Fe-Mn oxyhydroxides play a significant role in trace metal distribution. The highest numbers of diatoms were recorded in the sub-channel (S-61) which is also characterized by higher total nitrogen concentration. Geochemical and diatom signatures thus have helped to infer spatial and temporal variations in depositional environment.