Major and trace elements in sediments from Viração Logoon, Fernando de Noronha Archipelago, Brazil

Abstract

Fernando de Noronha is an isolated group of 21 volcanic islands and located in the South Equatorial Atlantic, approximately 545 km from Recife, PE, Brazil. This archipelago is a protected and isolated habitat with restrict access. Viração Lagoon, a small lake located at Fernando de Noronha Island, 20 meters above the sea level, is a pristine lake located in the main island. The local drainage is restricted and ephemeral, and no connection with the general insular drainage is observed. Two sediment cores (27 and 41 cm-Iong) were extracted in this lagoon in 2000, in order to determine the chemical and mineralogical composition of the sediments. The sediments are made up of smectite, and Fe and Ti oxides. Trace elements As, Ba , Br, Co, Cr, Cs, Rb, Sb , Sc, To , Th , U, Zn and rare earths (Ce, Eu , La , Lu, Nd, Sm, Tb and Vb) were determined by instrumental neutron activation analysis (lNAA). All samples, previously dried at 60oC, were passed through a 0.065 mm sieve. The cores were sliced at every two centimeters. For the multi elemental analyses approximately 200 mg of sediment (duplicate samples) and about 150 mg of reference material and synthetic standards were accurately weighted and sealed in pre-cleaned double polyethylene bags, for irradiation ., Single and multielement synthetic standards were prepared by pipetting convenient aliquots of standard solutions (SPEX CERTIPREP) onto small sheets of Whatman no 41 filter paper. Sediment samples, reference material and synthetic standards were irradiated for 16 hours, under a thermal neutron flux of 1012 n cm-1 s- l in the IEA-R 1 nuclear reactor at Instituto de Pesquisas Energeticas e Nucleares fi IPEN. Two series of counting were made: the first after one-week decay and the second, after 15-20 days. The counting time was 2 hours for each sample and reference material, and 30 min for each synthetic standard. Gamma spectrometry was performed using a Canberra gamma X hyperpure Ge detector and associated electronics, with' a resolution of 0.88 keV and 1.90 keV for 57Co and 60Co, respectively. The validation of this methodology was performed by means of Buffalo River Sediment SRM 2704 (NISD , Soil-7 (lAEA) and BEN (Basalt-IWG-GID reference material analysis. The major elements were determined by XRF technique at IG/USP and the validation was carried out by means of JB-1 a and JG-1 a (Geological Survey of Japan) reference materials analysis. Their concentrations are similar to those reported for NASC (North American Shale Composite), except for Ba, REE , Cr, Zr and Zn , with higher concentrations. Enrichment factors for major and trace elements were calculated , using the composition of the surrounding rock (source of the sediments) as a normalizer. Results show that Cr, U, Th, Rb, and Zn are enriched and Hf, Co, Eu and Tb are depleted relative to the rock.