A evolução dos manguezais nos litorais Nordeste e Sul brasileiros durante o Holoceno

Abstract

It is possible that climate changes and sea level fluctuations (allogenic processes) are and will cause major changes in mangrove dynamics. However, other driving forces may be significantly affecting this system. Distinguishing allogenic and autogenic influence on mangroves is a challenging question, because mechanisms related to the natural dynamics of depositional environments (autogenic processes) have strong influences on the establishment and degradation of mangroves. Thus, impacts on mangroves caused by autogenic processes may be erroneously attributed to allogenic mechanisms. Therefore, it is imperative to identify the ‘fingerprint’ of global changes in modern mangrove dynamics. This thesis integrates palynological, geochemical, sedimentological data, 14C dating of sedimentary organic matter, geomorphological and vegetation data in order to evaluate the influence of autogenic and allogeneic processes on Brazilian mangroves during the Holocene. Tropical estuaries from Rio Grande do Norte and southern Bahia, and subtropical estuaries in northern and southern Santa Catarina estates with different climatic, geomorphological and oceanographic characteristics were studied. The Relative Sea-Level (RSL) along the Rio Grande do Norte reached modern level and stabilized at about 7,000 cal yr BP, allowing the mangrove establishment at the edges of the Ceará-Mirim River estuary until the nowadays. However, changes in the spatial distribution of mangroves have occurred since then due to channels dynamics in the region (autogenic processes). Considering the mangroves of the Jucuruçu River in southern Bahia, their horizontal and vertical distribution were controlled by the interactions of the changes in the RSL and fluvial discharge. Therefore, the dynamics of these estuarine mangroves during the Holocene was mainly controlled by changes in sea level and precipitation that affected fluvial discharge. These allogeneic mechanisms were the main drivers of the dynamics of these mangroves. However, during the last 600 years, factors intrinsic to the depositional system gained relevance by controlling the establishment and migration of mangroves by deposition and erosion of muddy tidal flats, abandonment and reactivation of channels (autogenic processes). In the case of Santa Catarina mangroves, the RSL rise up to the middle Holocene was decisive for the establishment of tidal flats appropriate for the expansion of saltmarshes. However, mangroves did not tolerate the Holocene low temperatures in the southern littoral of Santa Catarina. The pollen data indicate the establishment of mangroves with Laguncularia around 1,700 cal yr BP, followed by Avicennia and lastly Rhizophora trees, a cold less tolerant genus, around 650 cal yr BP in São Francisco do Sul, north of Santa Catarina. The mangroves of Laguna, south of Santa Catarina, composed of Laguncularia and Avicennia, were established in the current southern limit of the South American mangroves only in the last decades. No evidence was found for the presence of mangroves in Laguna during the Holocene. The establishment of these mangroves in the region probably started during the Anthropocene, as a consequence of the increase in minimum winter temperatures in southern Brazil. Considering changes in precipitation rates over drainage basins that feed mangrove estuaries, as well as trends in RSL rise and temperatures until the end of the 21st century, tropical estuarine mangroves will likely migrate to higher topographically sectors in the interior of the river valleys, where its extension will depend on the volume of river discharge interacting with the RSL rise. Subtropical mangroves are expected to expand to more temperate zones as minimum winter temperatures increase. This process should cause an increase in the diversity of mangrove species, such as the introduction of the Rhizophora genus in the current southern limit of the mangroves, positioned in Laguna-SC. However, in the case of high rates of RSL rise, the relatively new subtropical mangroves are also expected to migrate to topographically higher sectors of the coast.