A multi-proxy evidence for the transition from estuarine mangroves to deltaic freshwater marshes, Southeastern Brazil, due to climatic and sea-level changes during the late Holocene

Abstract

The present study investigates a paleo-estuary at the Doce River Delta, southeastern Brazil, through a multi-proxy approach that links palynology, diatoms, sedimentology and geochemistry analyses (i.e., Total C, Total N, δ13C and δ15N). These analyses, temporally synchronized with five radiocarbon ages, revealed environmental changes from marine to continental over the last ∼7550years. The studied sedimentary succession recorded the upward transition from estuarine channel (until ~7550calyr BP) to estuarine central basin (>~7550 to ~5250calyr BP) deposits, the latter containing increased mangrove vegetation, marine and marine/brackish water diatoms. The range of geochemical values (δ13C=−30–−10‰, δ15N=2−8‰ and C/N=4–40) also indicate marine/estuarine organic matter and C3 terrestrial plants to that time interval. A following period recorded two phases: lake/ria (~5250 to ~400calyr BP) and fluvial channel (~400calyr BP until modern age). During this stage, mangroves were replaced by trees/shrubs and herbs/grasses due to the disconnection with the marine realm. As a result, the corresponding sediments contain only organic matter sourced from freshwater and C3 terrestrial plants (δ13C=−29–−26‰, δ15N=0−8‰ and C/N=10–45). The equilibrium between fluvial sediment supply and relative sea-level changes during the Holocene controlled the morphologic and vegetation changes in the studied littoral. The estuary became established during the early Holocene as a resulted of a eustatic sea-level rise, when the fluvial sediment supply to the coast was relatively lower due to a dry period. However, during the late Holocene, the climatic force was more significant to the development of coastal morphology due to a wet period that caused an increase in sandy sediment supply to coastal system. Then, the increase of fluvial discharge associated to a relative sea-level fall caused a marine regression and shrinkage of mangroves during the late Holocene. The evaluation of mangrove dynamics according to climatic and sea-level changes mainly during the late Holocene is essential for the understanding of their survival ability under future scenarios, with a probable accelerated sea-level rise and intensification of extreme climatic events in southeastern Brazil for the next century.