Holocene coastal wetland evolution in a tropical ecosystem: Sedimentary archives from South Kerala, India

This study focuses on the Late Holocene evolution of Ashtamudi Lake, utilizing a multiproxy analysis of borehole samples from Munroe Island in the Ashtamudi floodplain. Granulometric analysis reveals rapid shifts in sedimentary environments throughout the core, except at depths of 2–7 m and 14.5–18 m. High TOC/TN ratios in the Munroe Island core suggest an allochthonous source of organic carbon, while the predominance of C3 plants over C4 plants indicates a generally cool and wetclimate with occasional hot and sunny intervals. Conversely, low TOC/TN ratios at depths of 3.5, 4.5, 5, 13, and 21 m suggest episodes of aquatic phytoplankton activity, basin eutrophication, and brief periods of intense rainfall leading to nutrient influx. The presence of estuarine and marine molluscan shells at depths of 8 m and 21–26 m suggests a transition from a regression phase to normal marine conditions in the lower core, followed by marine transgression between 21–22.5 m, and regression thereafter. AMS radiocarbon dating at depths of 8.5 m and 23 m yielded ages of 2385 ± 39 years BP and 3702 ± 39 years BP, respectively, confirming a Late Holocene timeline. The occurrence of estuarine and marine fossil shells, coupled with geochemical and geochronological data, indicates that the coastal floodplains of Ashtamudi Lake evolved during the Late Holocene through a series of marine transgression-regression events and associated environmental changes.