Une reconstruction multi-proxy et comparaison des changements de paléomilieux de l’Holocène des estuaires d’Alvor et d’Alcantarilha (Portugal du sud)

The present study investigates the history of two estuaries on the Algarve coast, southern Portugal, through a multi-proxy approach that links sedimentology, geochemistry, palynology, microfaunal analyses and radiocarbon dating. These analyses provide an overview of the development of these estuaries over the last ∼7500 years. Palynological data reveal climate-driven vegetational shifts over the whole period. Microfauna, which is composed predominantly of foraminifers and ostracods, provides evidence for periods of marine and brackish water conditions, observed in both estuaries. Whereas the sediment record from the Alvor estuary shows a clear, nearly continuous silting-up sequence, the development of the Alcantarilha estuary is characterised by fluvial sedimentation, replaced by a marine environment at about 7300 yr calBP. Marine transgression is indicated by marine-brackish foraminifera and ostracod assemblages, while upper-marsh pollen decreased and wetland communities increased at the same time. A continuous shallowing of the estuaries followed between ∼6700 yr calBP and ∼5700 yr calBP in Alvor and 4500 yr calBP in Alcantarilha. This was interrupted by a distinct high-energy event, possibly a storm or tsunami, between 6400 and 5800 yr cal BP in Alvor; the event was not recorded in the Alcantarilha estuary. Afterwards, the silting-up restarted with clear similarities to the pre-event situation and continued until ∼3500/2000 yr calBP. This process wasinfluenced by the build-up ofsand barriersin front of the estuaries. Thisis also documented by a higher percentage of upper-marsh vegetation. The core from Alcantarilha estuary shows another layer of high-energy deposits, with mainly marine foraminifers and ostracods after 4500 yr calBP. The youngest parts of the two records are characterised by decreasing tidal influence and increasing anthropogenic impact, indicated by increasing values of cultivated plants and other human-induced changes of vegetation cover.Nous étudions l’histoire de deux estuaires de la côte d’Algarve (sud du Portugal) en utilisant des reconstructions multi-proxy (sédimentologie, géochimie, palynologie, micropaléontologie et datation au carbone 14). Basé sur ces derniers ∼7500 ans. Les données palynologiques décrivent des transformations florales que nous expliquons par des changements climatiques. L’évolution de la microfaune (foraminifères et ostracodes) est le reflet de l’alternance des environnements marins et saumâtres dans les deux carottes étudiées. La carotte d’Alvor montre clairement une séquence régressive. La carotte d’Alcantarilha est caractérisée par une sédimentation fluviatile à laquelle succède, vers 7300 ans calBP, une sédimentation marine littorale. La transgression est indiquée par des foraminifères et des ostracodes vivant dans des eaux saumâtres à marines. Pendant cette transgression, la proportion des pollens de l’étage supralittoral recule et celle de la zone humide augmente. Entre ∼6700 ans calBP et ∼5700 ans calBP à Alvor et 4500 ans calBP à Alcantarilha, la profondeur de la lagune diminue progressivement. Un évènement de haute énergie (tempête ou tsunami) est enregistré entre 6400 et 5800 ans cal BP sur le site d’Alvor mais pas celui d’Alcantarilha. Après cet évènement, la profondeur de la lagune régresse de nouveau jusqu’à 3500/2000 ans calBP. L’évolution de cette régression est soulignée par la formation d’un cordon littoral sur le front de l’estuaire, indiqué par l’augmentation de la proportion de la végétation de l’étage supralittoral. La carotte d’Alcantarilha montre une autre couche de sédiments, déposés en milieu de haute énergie, mise en évidence par les foraminifères et les ostracodes marins après 4500 ans calBP. La partie la plus jeune des carottes est marquée par la diminution de l’influence de la marée et une augmentation de l’impact anthropique (changement général de la végétation et accroissement des proportions des pollens des plantes cultivées).German Research Foundation (DFG; MA 1308/09)info:eu-repo/semantics/publishedVersio

Investigações geo-arqueológicas sobre a configuração do litoral algarvio durante o Holoceno

As part of a research project funded by the German Science Foundation (DFG. Deutsche Forschungsgemeinschaft), which took place from 2006 until 2010, traditional archaeological and modern geo-archaeological methods were used to study the changes taking place during the Holocene within the estuaries on the south coast of Portugal, the Algarve. Priority was placed on ascertaining if the development of the coastline between the Neolithic Period and the Middle Ages was more the result of natural causes (transgression, high energy events and climate) or human impact (settlement, land-use, erosion). A clear starting point was provided by the previously undertaken geomorphic research done at the Cerro da Vila (Vilamoura), where it has been possible to reconstruct the living conditions in and around the Roman harbour village once located there. Building on the results of this “on site” investigation, the first point of research was to analyse the influence of the hinterland (“off site”) on the hydrology of the coastal areas. Meanwhile the situation in selected estuaries neighbouring the Cerro da Vila and the related drainage areas was studied in order to identify the fundamental developments of the entire western Algarve coast (Barlavento). By comparing the different effected geo-archaeological archives in the Algarve it should become possible to differentiate erosional phases caused by changes in climate or high energy events from mostly local factors, which are induced by anthropogenic land-use; therein lies the hypothesis. The palynological studies were especially useful here in determining the human influence on the land use of the Algarve coast. The aim of the accompanying archaeological study was to draw chronologically differentiated distribution maps showing the development of human settlement between the Neolithic Period and the Middle Ages. The results of this interdisciplinary study show that the silting of the large estuaries on the southern coast of Portugal can be seen as a result of anthropogenic erosion that had already begun in the Bronze Age and reached its peak during the Roman Period. Furthermore geo-archaeological investigation shows a very strong impact, with several effects on the settlement patterns of the region (harbours, fishing-villages, murex-production, etc.), during the time of the Roman Empire.Integrado num projecto financiado pela Deutsche Forschungsgemeinschaft (DFG, Fundação Alemã de Investigação), foram investigados entre 2005 e 2010, com os métodos da arqueologia tradicional e da geo-arqueologia interdisciplinar, a evolução e a colmatação de diversos estuários e lagunas na actual costa sul de Portugal, no Algarve, durante o Holoceno recente (7000–1000 BP). O principal objectivo deste estudo foi o de esclarecer qual o impacto que as influências antropogénicas e climáticas tiveram sobre a evolução da costa, desde o Neolítico até à Época Medieval. O ponto de partida baseou-se nos estudos geomorfológicos anteriormente realizados nos arredores do sítio arqueológico do Cerro da Vila (Vilamoura), com o objectivo de obter uma reconstituição das condições ambientais desta estação portuária romana. Tendo por base estas primeiras informações “on site”, a prioridade foi analisar a influência das mudanças ocorridas durante o holoceno-recente na rede hidrográfica costeira (rios, ribeiras, etc.). Ao mesmo tempo, alguns estuários vizinhos do Cerro da Vila e suas redes — todos na zona ocidental do Algarve (Barlavento) — foram alvos de estudo, com o objectivo de identificar e definir melhor a configuração da costa sul de Portugal. Foi a hipótese base deste projecto que, com a análise comparativa dos diversos arquivos geo-arqueológicos identificados na zona do Algarve, possibilitou a individualização dos efeitos sub-regionais (climáticos, subida do nível do mar) dos efeitos locais (uso do solo) referentes à erosão do solo. Em especial, as investigações palinológicas possibilitaram a obtenção de excelentes resultados em relação à reconstituição da influência humana e da agricultura no litoral algarvio no passado. Os trabalhos arqueológicos que se realizaram em paralelo com estes estudos das geociências, tiveram o objectivo de elaborar diversos mapas de distribuição da ocupação humana do Algarve para as várias etapas da Pré-História e História, entre o Neolítico e a Época Medieval, especialmente nas zonas mais afectadas pelas mudanças na erosão e na colmatação dos estuários e lagunas. O resultado deste estudo possibilitou um melhor conhecimento sobre o processo de colmatação dos estuários do Sul de Portugal como principal evidência do notável impacto humano nesta zona, desde a Idade do Bronze e com um clímax na Época Romana. Estas alteraçőes do paleo-ambiente tiveram, evidentemente efeitos muito profundos no padrão de assentamento durante esta última fase de ocupação roman