7º Simpósio sobre a Margem Ibérica Atlântica – MIA 2012
16-20 de Dezembro de 2012, Lisboa
Evolução recente da restinga da Ponta da Passadeira: um exemplo de retrogradação em ambiente estuarino
Metal geochemistry in sediment core from the SW Portuguese continental shelf Geoquímica de metais de uma sondagem da plataforma continental SW Portuguesa
Recent evolution of the Ponta da Passadeira spit: a retrograding barrier in estuarine environment
M. Caetano (1), T. Drago (1), A. Oliveira (2), J. F. Duarte (2), F. Naughton (1), S. Gomes (1), J. M. Jouanneau (3), C. Vale (1)
M. C. Freitas (1,2), A. Amorim (1), C. Andrade (1,2), T. Silva (1,2), C. Pinto (1,3) (1) (2) (3)
(1)
Centro de Geologia da Universidade de Lisboa, Bloco C6, 3º Piso, Campo Grande, 1749-016 Lisboa, Portugal.
[email protected] Departamento de Geologia da Faculdade de Ciências da Universidade de Lisboa, Bloco C6, 3º Piso, Campo Grande, 1749-016 Lisboa, Portugal. Agência Portuguesa do Ambiente, I.P., Av. Almirante Gago Coutinho, 30, 1049-066 Lisboa, Portugal.
(2) (3)
IPMA, Av. Brasília, 1449-006 Lisboa – Portugal.
[email protected] Instituto Hidrográfico, Rua das Trinas, 49, 1200 Lisboa, Portugal. Universidade Bordéus I, 33405 Talence-Cedex, France.
To assess the temporal variation of the terrestrial contribution from rivers of the south Iberian coast linked with the paleoenvironmental evolution of the southern continental shelf, sedimentological and geochemical analyses were preformed in a 5 m core. Dating were also performed and three major sedimentary units were defined. The oldest (499456cm) which is also the coarsest, is clearly individualized by the higher values of As/Al, Cd/Al, U/Al and Y/Al. The second (130 -456 cm) is rather uniform (sandy mud) with an Al content between 5 and 5.8 %. A peak concentration of Cu/Al, As/Al and Pb/Al was found at circa 200 cm probably indicating evidences of roman mining exploitation. Deeper in the core (303cm) an enhancement of Cr/Al and Cd/Al ratios and decrease of Fe content was found. The upper unit, constituted by muddy sediments is characterized by a gradual decrease of Al (from 7.3 to 5.3 %). These sediments showed an increase of Zn/Al, Cu/Al, Cr/Al, As/Al and Pb/Al ratios from 40 cm depth until the topmost layer, reflecting an antrophogenic influence with the more recent mining of the Iberian Pyrite Belt. Concentrations of total REE (Rare Earth Elements) followed the Al variation pointing that depth variation is ruled by sediment nature. The REE normalized patterns for the European Shale in the upper sedimentary unit showed an enrichment of LREE in comparison to HREE. This proportion decreases with depth where an almost flat profile was found in bottom sediments probably due to a dilution effect from the increase of carbonate biogenic particles.
Ponta da Passadeira is a sand spit, formed ca. 7 200 cal BP in the inner Tagus Estuary. At that time, a pine forest colonized the dune and back-barrier surface. At the end of the Neolithic (ca. 5 000 cal BP), a community dedicated to ceramic production settled in, as testified by ovens and waste materials found in the barrier sands. Along the sheltered side of the spit, a salt marsh developed, possibly supplying the raw material for that industry. Short cores performed in the spit, associated to geomorphological, sedimentological and archaeological studies, allowed to characterize the Holocene evolution of this barrier. Results indicate that the spit experienced erosion and rolled over the adjacent saltmarsh in consequence of sea-level rise and exhaustion of sediment sources. This promoted exposure of former marsh outcrops in the present-day barrier beach. Comparison of present-day tidal parameters and mean sea level data with the elevations at which the ceramic ovens and pine stumps are found, suggest that sea level must have rosen of about 100m during the last 7000 years. Palavras chave: estuário do Tejo, ocupação humana, Neolítico, geoarqueologia, nível do mar. Keywords: Tagus estuary, human occupation, Neolithic, geoarchaeology, sea level.
Palavras chave: plataforma continental sul portuguesa, geoquímica de metais, Holocénico. Keywords: southern continental shelf, metal geochemistry, Holocene. Acknowledgments: This is a contribution of POPEI project (PDCT/MAR/55618/2004. The authors wish to thanks to Monge Soares, for the 14C ages calibration, to Joana Pinheiro for sedimentological lab analyses and also, to the colleagues of Hydrographic Institute for the sea mission and sampling.
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