Os efeitos da subida do nível do mar sobre os manguezais do litoral sul da Bahia durante o Holoceno

Abstract

This work integrates the palinological, sedimentological and geomorphological data with radiocarbon dating, as well as δ13C and C/N from the sedimentary organic matter to identify the impact of changes in sea level and climate during the Holocene in Jucuruçu River’s mangroves, near the city of Prado, South Bahia. A sediment core with 4.5 m depth was sampled from a fluvial valley, 23 km away from the modern coastline. The base of sediment core presents age of 7450 cal yrs BP. The data reveal two important phases characterized by 1) an estuarine system with tidal flats colonized by mangroves. Its sedimentary organic matter is mainly sourced from estuarine algae during the early and middle Holocene; and 2) in the second phase, the mangroves shrank and herbaceous vegetation expanded. The δ13C and C/N values show an increase in contribution of terrestrial C3 plants. These phases identified in this study are in agreement with the relative sea-level (RSL) rise during the early and middle Holocene, and its subsequent fall during the late Holocene. In addition, the influence of weather patterns proposed for Holocene may be identified throughout the studied sediment core. Probably, changes in depositional environment and dominant vegetation at the study site were caused by the combined action of changes in RSL and fluvial discharge. According to the model proposed in this work, during the early and middle Holocene occurred a RSL rise that caused a marine incursion along the studied fluvial valley. The dry period occurred during the early and middle Holocene produced a decrease in the fluvial discharge and contributed to this marine transgression. However, during the late Holocene occurred a RSL fall and a wet period. It favored the marine regression and consequently, the mangroves migrated to tidal flats attached to the edge of lagoons near the modern coastline. The geomorphologic and vegetation evolution is in agreement with the mid-Holocene RSL maximum above present RSL and subsequent fall to the present time. Keywords: climate; Holocene; relative sea-level; South of Bahia; vegetation.