The growth of the Doce River Delta in northeastern Brazil indicated by sedimentary facies and diatoms

Abstract

The combination of diatom analyses with sedimentary facies has been limited in studies aimed at reconstructing paleoenvironments and the history of sea-level fluctuations in coastal settings. The present study integrated facies and diatom analyses, together with 14C dating and geomorphological information derived from remote sensing, to analyze the Quaternary deposits of a paleoestuary nearby the Doce River Delta in northeastern Brazil. The goal was to demonstrate the potential of diatoms as a paleoenvironmental proxy and improve the history of Holocene sea-level fluctuations in that region. The deposits, ranging from 7425–7509 to 1313–405 cal yr bp, were arranged in an overall regressive succession recorded by the upward superposition of estuarine channel, estuarine central basin, lake/ria and fluvial channel/marsh facies associations. Marine, marine/brackish and freshwater diatoms were found in these deposits. The highest concentration of diatoms was associated with the low-energy estuarine central basin deposits formed when the relative sea level was high. In the mid-Holocene, ca. 5000 cal yr bp, the coastline began its progradation due to a relative drop in sea level. This change in coastal dynamics was marked by a decrease in the concentration of diatoms and the prevalence of freshwater species over marine and marine/brackish water species. As a result of the Doce River Delta progradation, the studied estuarine channel was closed due to the accumulation of sand as beach ridges and barriers at the river mouth, a process that ultimately led to the estuary being replaced by a lake/ria.