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Caracterização das populações de foraminíferos e ostracodos nos depósitos de alta energia de Almargem e do Martinhal : tsunami versus tempestade
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Resumo(s)
O estudo dos sedimentos estuarino-lagunares de Almargem e Martinhal permitiram
distinguir depósitos sedimentares de tsunami de depósitos de tempestade, a partir da análise
microfaunística.
Foram analisadas 31 amostras representativas das condições de sedimentação
associadas ao ambiente estuarino-lagunar, ao depósito do tsunami de 1º Nov.1755 e a depósitos
de tempestades. Nestas amostras foi possível identificar 10 530 foraminíferos, representando
mais de 30 géneros e de 50 espécies, e 5 721ostracodos, repartidos por mais de 20 géneros e de
25 espécies.
Os dois tipos de depósitos de alta energia podem distinguir-se considerando a
percentagem de carapaças de foraminíferos fracturadas e com marcas de abrasão (>2% nos
depósitos de tempestade), de espécies marinhas vs. espécies de alto sapal e de espécies
planctónicas (>13% no depósito de tsunami). A “população” de ostracodos contribui para a
separação destes 2 tipos de depósitos analisando a percentagem de indivíduos marinhos,
(ausentes nos depósitos de tempestade) e o número de espécies (2 vezes maior na base do
depósito de tsunami, relativamente aos depósitos de tempestade).
No paul do Martinhal as espécies margino-marinhas estão presentes em todas as
amostras, o que associado ao reduzido número de foraminíferos e de ostracodos tornou a análise
dos depósitos de alta energia mais desafiadora. A análise estatística multivariada permitiu, no
entanto, distinguir estes dois tipos de depósitos. Apesar de muito reduzida, a “população” de
ostracodos, também revela que as condições marinhas estiveram presentes ao longo de todo o
registo.
Estes dados sugerem que, devido à presença de uma barreira pouco elevada no Martinhal, a
ocorrência de galgamentos oceânicos foi mais frequente do que em Almargem, onde o contexto
geomorfológico (barreira com cotas mais altas) permitiu o maior isolamento ambiental da
laguna de Almargem e a distinção entre os dois tipos de depósitos de alta energia.
In Almargem and Martinhal high-energy deposits have been studied to distinguish between tsunami and storm sedimentary deposits based on its microfossil population. Thirty-one samples representing the sedimentation conditions associated with the estuarine-lagoon environment, the tsunami deposit of 1st of Nov.1755 and storm deposits were analyzed. In these samples it was possible to obtain 10 530 foraminifera, representing more than 30 genera and 50 species, and 5 721 ostracods, divided into more than 20 genera and 25 species. The two types of high-energy deposits can be distinguished by considering the percentage of fractured foraminifera carapaces or with abrasion marks (>2% in storm deposits), marine vs. high marsh species and planktonic species (>13% in the tsunami deposit). The ostracod population contributes to the separation of these 2 types of deposits from the percentage of marine individuals, (absent in storm deposits) and the number of species (twice as large at the base of the tsunami deposit, relative to storm deposits). In the marshland of Martinhal, marine-marine species are present in all samples, which, associated with the reduced number of foraminifera and ostracods, made the analysis of highenergy deposits more challenging. Its multivariate statistical analysis allowed, however, to distinguish these two types of deposits. Despite being very small, the ostracod population also reveals that marine conditions were present throughout the entire record. These data suggest that, due to the presence of a low barrier at Martinhal, the occurrence of ocean overtopping was more frequent than at Almargem, where the geomorphological context (barrier with higher levels) allowed greater environmental isolation of the Almargem lagoon and the distinction between the two types of high-energy deposits.
In Almargem and Martinhal high-energy deposits have been studied to distinguish between tsunami and storm sedimentary deposits based on its microfossil population. Thirty-one samples representing the sedimentation conditions associated with the estuarine-lagoon environment, the tsunami deposit of 1st of Nov.1755 and storm deposits were analyzed. In these samples it was possible to obtain 10 530 foraminifera, representing more than 30 genera and 50 species, and 5 721 ostracods, divided into more than 20 genera and 25 species. The two types of high-energy deposits can be distinguished by considering the percentage of fractured foraminifera carapaces or with abrasion marks (>2% in storm deposits), marine vs. high marsh species and planktonic species (>13% in the tsunami deposit). The ostracod population contributes to the separation of these 2 types of deposits from the percentage of marine individuals, (absent in storm deposits) and the number of species (twice as large at the base of the tsunami deposit, relative to storm deposits). In the marshland of Martinhal, marine-marine species are present in all samples, which, associated with the reduced number of foraminifera and ostracods, made the analysis of highenergy deposits more challenging. Its multivariate statistical analysis allowed, however, to distinguish these two types of deposits. Despite being very small, the ostracod population also reveals that marine conditions were present throughout the entire record. These data suggest that, due to the presence of a low barrier at Martinhal, the occurrence of ocean overtopping was more frequent than at Almargem, where the geomorphological context (barrier with higher levels) allowed greater environmental isolation of the Almargem lagoon and the distinction between the two types of high-energy deposits.
Descrição
Tese de Mestrado, Geologia do Ambiente, Riscos Geológicos e Ordenamento do Território, 2023, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Eventos extremos Depósito de tsunami Depósito de tempestade Micropaleontologia Algarve Teses de mestrado - 2023