Discovery of enigmatic toroidal carbonate concretions on the Rio Grande Rise (Southwestern Atlantic Ocean)

Autor(es):Mostrar menos -

Millo, Christian ^[1] ; Vieira do Nascimento e Silva, Matheus H. ^[1] ; de Mello, Renata Moura ^[2] ; Leckie, R. Mark ^[3] ; Benites, Mariana ^[1] ; Fonseca Giannini, Paulo C. ^[4] ; Boggiani, Paulo C. ^[4] ; Bosence, Dan ^[5] ; Lusty, Paul A. J. ^[6] ; Murton, Bramley J. ^[7] ; Jovane, Luigi ^[1]

Afiliação do(s) autor(es)

^[1] Univ Sao Paulo, Inst Oceanog, Praca Oceanog 191, BR-05508900 Sao Paulo, SP - Brazil

^[2] PETROBRAS Res & Dev Ctr, Av Horacio Macedo 950, BR-21941915 Rio De Janeiro, RJ - Brazil

^[3] Univ Massachusetts, Dept Geosci, 627 N Pleasant St, Amherst, MA 01003 - USA

^[4] Univ Sao Paulo, Inst Geociencias, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

^[5] Royal Holloway Univ London, Dept Earth Sci, Egham Hill, Egham TW20 0EX, Surrey - England

^[6] British Geol Survey, Environm Sci Ctr, Nottingham NG12 5GG - England

^[7] Natl Oceanog Ctr, Waterfront Campus, Southampton SO14 3ZH, Hants - England

• Tipo de documento: Artigo Científico
• Fonte: MARINE GEOLOGY; v. 443, JAN 2022.
• Citações Web of Science: 0

Resumo

We present enigmatic toroidal carbonate concretions retrieved from 700 m water depth from two sites in the upper plateau of the Rio Grande Rise. The concretions have a diameter of similar to 15 cm and a central hole of similar to 5 cm, and were observed on top of loose bioclastic sand over an area of similar to 30 m(2) at 0.5-1.5 m from one another. They consist of brown, porous, bioclastic grainstone, lacking internal structures. Grains consist of sand (< 3% coarse, 30% medium, 35% fine, 25% very fine), composed mainly by planktonic foraminiferal tests, and < 10% lime mud. The observed foraminiferal species indicate initial deposition of the sand in an open ocean setting. Biostratigraphy suggests an age no older than Pleistocene. Petrographic thin sections and SEM reveal that the fossiliferous grainstone contains intraclastic micritic cement and isopachous rim cement made of bladed magnesian calcite. delta O-18 values range from +1.5 to +3.3 parts per thousand (V-PDB) and increase with the degree of cementation, while delta C-13 ranges from +0.5 to +2.3 parts per thousand irrespective of cementation. The cementation of the grainstones is likely to have taken place in the marine phreatic environment. Carbonate precipitation induced by methane oxidation or (subaerial) meteoric diagenesis are ruled out based on both cement fabric and isotopic composition. Plausible causes for the toroidal shape of these structures could be: 1) sediment excavation by organisms, or 2) cementation within biofilms around burrows, followed by selective seafloor erosion. However, unveiling the actual formation mechanisms warrants further investigation. (AU)

• Processo FAPESP: 14/50820-7 - Marine ferromanganese deposits: a major resource of E-tech elements
• Beneficiário: Frederico Pereira Brandini
• Modalidade de apoio: Auxílio à Pesquisa - Temático