Effects of the middle Holocene high sea‐level stand and climate on Amazonian mangroves

Abstract

ABSTRACTThe high sea‐level stand during the mid‐Holocene is a benchmark in mangrove dynamics along the north‐east/south‐east coast of Brazil and provides a reference point for landward and seaward mangrove migrations corresponding to changes in relative sea level (RSL). However, evidence of the impacts associated with RSL fall on the northern Brazilian coast is scarce. Multi‐proxy data from the highest tidal flats of the Bragança Peninsula in northern Brazil revealed modern herbaceous areas were occupied by mangroves Rhizophora and Avicennia from ~6250 to ~5850 cal a bp, and only Avicennia between ~5850 and ~5000 cal a bp. The same tidal flats were vegetation‐free between ~5000 and ~4300 cal a bp. A combination of a high sea‐level stand (0.6 ± 0.1 m) at ~5000 cal a bp and a dry early–middle Holocene in the Amazon probably caused an increase in porewater salinity of tidal flats, which resulted in a mangrove succession from Rhizophora to Avicennia dominance. RSL fall accentuated this process, contributing to mangrove degradation between ~5000 and ~4300 cal a bp. RSL fall, and a wetter period over the past ~4300 cal a bp caused a mangrove migration from highest to lowest flats, followed by expansion of herbaceous vegetation on the highest flats.