resumo
Mud volcanoes are feature of the coastal margins where anaerobic oxidation of methane triggers geochemical signals. Elemental composition, percentage of fine particles and loss on ignition were determined in sediment layers of eleven gravity cores retrieved from four mud volcanoes (Sagres, Bonjardim, Soloviev and Porto) and three undefined structures located on the deep Portuguese margin of the Gulf of Cadiz. Calcium was positively correlated to Sr and inversely to Al as well as to most of the trace elements. Vertical profiles of Ba, Cd and As concentrations, and their ratios to Al, in Porto and Soloviev showed pronounced enhancements in the top 50-cm depth. Sub-surface enhancements were less pronounced in other mud volcanoes and were absent in sediments from the structures. These profiles were interpreted as diagenetic enrichments related to the anaerobic oxidation of methane originated from upward methane-rich fluxes. The observed barium fronts were most likely caused by the presence of barite which precipitated at the sulphate-methane transition zone. Cd and As enrichments have probably resulted from successive dissolution/precipitation of sulphides in response to vertical shifts of redox boundaries.
palavras-chave
EASTERN MEDITERRANEAN SEA; CONTINENTAL-MARGIN SEDIMENTS; GAS HYDRATE; COLD SEEPS; AUTHIGENIC CARBONATES; SULFATE REDUCTION; SUBDUCTION ZONES; DISSOLVED BARIUM; BLAKE RIDGE; OF-MEXICO
categoria
Oceanography
autores
Carvalho, L; Monteiro, R; Figueira, P; Mieiro, C; Almeida, J; Pereira, E; Magalhaes, V; Pinheiro, L; Vale, C
nossos autores
Joana C. Almeida
Investigador
Paula Figueira
Técnico de Laboratórioagradecimentos
This work was supported by the Strategic Funding of CESAM (UID/AMB/50017/2013), CIIMAR (CORAL project), CICECO (UID/CTM/50011/2013), Euromargins MVSeis Project (01-LEC-EMA24F) and through national funds provided by FCT/MEC and the co-funding by the FEDER, within the PT2020 Partnership Agreement and Compete 2020. Rui Monteiro (SFRH/BD/108535/2015) and Claudia Mieiro (SFRH/BPD/100740/2014) acknowledge their grants to FCT. The manuscript benefited significantly from comments by Fritz Neuweiler, two anonymous reviewers and the editor Igor M. Belkin.