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 subchannel (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.

Mercury enrichment in anthrosols and adjacent coastal sediments at a Classic Maya site, Marco Gonzalez, Belize

“…To provide more information on changing depositional environments at the margin of MG, Big Pool core samples were selected for diatom analysis. Diatom (Bacillariophyceae) fossil records are used in transitional coastal settings to identify fluctuations between marine, freshwater and terrestrial conditions and impacts of human activity during coastal wetland development (Pande et al, 2015; Parsons et al, 2006; Vos & de Wolf, 1993). Diatoms are used as ecological indicators of significant metal contamination in aquatic environments (Morin et al, 2012) but at low concentrations, co‐effects of nutrients, salinity and substrate dominate coastal diatom assemblages (Desianti et al, 2017, 2019; Pande et al, 2015).…”

“…Diatom (Bacillariophyceae) fossil records are used in transitional coastal settings to identify fluctuations between marine, freshwater and terrestrial conditions and impacts of human activity during coastal wetland development (Pande et al, 2015; Parsons et al, 2006; Vos & de Wolf, 1993). Diatoms are used as ecological indicators of significant metal contamination in aquatic environments (Morin et al, 2012) but at low concentrations, co‐effects of nutrients, salinity and substrate dominate coastal diatom assemblages (Desianti et al, 2017, 2019; Pande et al, 2015). Sediment samples (0.1‐g dry mass) were placed in 20 ml of 30% H 2 O 2 and heated in a beaker on a hot plate (90°C, ~2 h) to oxidise organic material.…”