Publicação
Late holocene environmental change at the Quiaios-Tocha coastal plain
| Resumo: | This thesis concerns the palynological investigation of organic sediment series of coastal mire deposits along a transect of sites from the sea to the inland old dune fields, including lagoons, wetlands and lakes. The results have been integrated in an extended view of landscape evolution, taking into consideration many sources of information such as those from coastal geomorphology, sedimentology, malacology and finally from regional history. For the last millennia (Middle/Late Holocene) patterns of local and regional vegetation change, concerning the vascular plants and the wetland micro-communities (fungi/algae) have been described and partly dated, giving rise to a scheme of palaeohydrological evolution of the waterbodies in the context of palaeoecological evolution. This is primarily palaeovegetational and palaeohydrological in character, but has been used to give new insight into coastal evolution in general such as phases of aeolian activity and dune building, earlier coast line and related sea level. In addition aspects of human impact have been considered. The obtained data shows that during Prehistoric times the region was covered with a forest of pine (Pinus spp.) and oak (Quercus spp.). Some time between 4000 and 1600 years BP the region was progressively deforested and a semi-natural heathland developed dominated by species from the Ericaceae, probably maintained by cutting, burning and grazing. Later on this heathland and dune vegetation was degraded by an intense destabilisation and immigration of dunes and a subsequent desertification. This phenomenon was connected to the deteriorating climate during the Little Ice Age (LIA) and possibly a consequence of an increasing over-grazing towards modern times. This eventually led to reforestation efforts initiated in the vicinity c 300 years ago and locally on the coastal fringe to the west of the lakes, as late as between 1924 and 1940. Palaeolimnological implications of the palynological records of hygrophilous taxa and microcommunities of algae (and fungi), such as Pediastrum, gave new and interesting information about the history of the studied lakes. The record of local-scale vegetation patterns was utilized to reconstruct the evolution of the environment in and around the lakes. Initially the sites were wetland areas similar to the dune slacks and seasonal pools found in the region at present. Formation of the two investigated lakes, Lake Vela and Lake Braças as permanent open water lakes, occurred relatively synchronously between 1723 and 1866 AD in response to the same regional geo-hydrological and climatic forcing. Biotic and abiotic dissimilarities between the two lakes seem to have existed from the time of lake formation until today, with one of the lakes (Vela) being more alkaline and more eutrophic than the other. Contemporary knowledge of the ecological requirements of vascular plants and algae permits an attempt to elucidate these differences. The dissimilarities were probably caused by variations in water flux- direction and velocity and in agricultural intensity in the vicinity of the lakes. In this work an apparently close relation between lake formation and aeolian activity and accumulation of sand dunes at the western banks of the lakes, was found. The three generations of dunes identified for the region by geomorphologists, were observed as truncated dunes, parabolic dunes and transverse/oblique dunes. The last generation associated with an intense migration of sand until the western banks of the present lakes, is presumably closely linked to lake formation at the Vela/Braças sites. At the westernmost part of the littoral fringe lagoonal deposits were encountered. Analysis of these deposits suggest an end to the Flandrian transgression at a sea level similar to the present one c 5000 years BP at which time the coast line was c 1 km further inland compared to the actual situation. The following progradation of the coast line, that occurred c 4000 years ago, may have been related to the formation of a protective sand barrier giving rise to the establishment of brackish water lagoons to the east of the barrier. |
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| Autores principais: | Danielsen, Randi |
| Ano: | 2010 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | This thesis concerns the palynological investigation of organic sediment series of coastal mire deposits along a transect of sites from the sea to the inland old dune fields, including lagoons, wetlands and lakes. The results have been integrated in an extended view of landscape evolution, taking into consideration many sources of information such as those from coastal geomorphology, sedimentology, malacology and finally from regional history. For the last millennia (Middle/Late Holocene) patterns of local and regional vegetation change, concerning the vascular plants and the wetland micro-communities (fungi/algae) have been described and partly dated, giving rise to a scheme of palaeohydrological evolution of the waterbodies in the context of palaeoecological evolution. This is primarily palaeovegetational and palaeohydrological in character, but has been used to give new insight into coastal evolution in general such as phases of aeolian activity and dune building, earlier coast line and related sea level. In addition aspects of human impact have been considered. The obtained data shows that during Prehistoric times the region was covered with a forest of pine (Pinus spp.) and oak (Quercus spp.). Some time between 4000 and 1600 years BP the region was progressively deforested and a semi-natural heathland developed dominated by species from the Ericaceae, probably maintained by cutting, burning and grazing. Later on this heathland and dune vegetation was degraded by an intense destabilisation and immigration of dunes and a subsequent desertification. This phenomenon was connected to the deteriorating climate during the Little Ice Age (LIA) and possibly a consequence of an increasing over-grazing towards modern times. This eventually led to reforestation efforts initiated in the vicinity c 300 years ago and locally on the coastal fringe to the west of the lakes, as late as between 1924 and 1940. Palaeolimnological implications of the palynological records of hygrophilous taxa and microcommunities of algae (and fungi), such as Pediastrum, gave new and interesting information about the history of the studied lakes. The record of local-scale vegetation patterns was utilized to reconstruct the evolution of the environment in and around the lakes. Initially the sites were wetland areas similar to the dune slacks and seasonal pools found in the region at present. Formation of the two investigated lakes, Lake Vela and Lake Braças as permanent open water lakes, occurred relatively synchronously between 1723 and 1866 AD in response to the same regional geo-hydrological and climatic forcing. Biotic and abiotic dissimilarities between the two lakes seem to have existed from the time of lake formation until today, with one of the lakes (Vela) being more alkaline and more eutrophic than the other. Contemporary knowledge of the ecological requirements of vascular plants and algae permits an attempt to elucidate these differences. The dissimilarities were probably caused by variations in water flux- direction and velocity and in agricultural intensity in the vicinity of the lakes. In this work an apparently close relation between lake formation and aeolian activity and accumulation of sand dunes at the western banks of the lakes, was found. The three generations of dunes identified for the region by geomorphologists, were observed as truncated dunes, parabolic dunes and transverse/oblique dunes. The last generation associated with an intense migration of sand until the western banks of the present lakes, is presumably closely linked to lake formation at the Vela/Braças sites. At the westernmost part of the littoral fringe lagoonal deposits were encountered. Analysis of these deposits suggest an end to the Flandrian transgression at a sea level similar to the present one c 5000 years BP at which time the coast line was c 1 km further inland compared to the actual situation. The following progradation of the coast line, that occurred c 4000 years ago, may have been related to the formation of a protective sand barrier giving rise to the establishment of brackish water lagoons to the east of the barrier. |
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