HIGH-RESOLUTION ACOUSTIC MAPPING OF GAS CHARGED SEDIMENTS AND LIVING BENTHIC FORAMINIFERA ASSEMBLAGES FROM THE NE REGION OF THE GUANABARA BAY (RJ, BRAZIL)

Authors

  • Francielli Paula Delavy Programa de Capacitação Institucional, Museu Paraense Emílio Goeldi, Coordenação de Botânica. Av. Perimetral, Terra Firme, CEP 66077-830, Belém, PA
  • Alberto Garcia de Figueiredo Jr Universidade Federal Fluminense, Departamento de Geologia, Lagemar. Av. Milton Tavares de Souza, s/n, CEP 24210-340, Niterói, RJ
  • Maria Virgínia Alves Martins Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 2020A, Maracanã. 20550-013 Rio de Janeiro, RJ
  • René Rodrigues Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 2020A, Maracanã. 20550-013 Rio de Janeiro
  • Egberto Pereira Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 2020A, Maracanã. 20550-013 Rio de Janeiro, RJ
  • Marco António Ruivo de Castro e Brito Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, Maracanã. 20550-013 Rio de Janeiro, RJ
  • Maria Clara Machado da Fonseca Laboratory of Micropaleontology Universidade do Estado do Rio de Janeiro,LMP-UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 4031F, Maracanã. 20550-013 Rio de Janeiro, RJ
  • Lazaro Luiz Mattos Laut Universidade Federal do Estado do Rio de Janeiro - UNIRIO, Laboratório de Micropaleontologia - LabMicro, Av. Pasteur, 458, IBIO/CCET sala 500 Urca, 22.240-490, Rio de Janeiro
  • Sérgio Bergamaschi Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 2020A, Maracanã. 20550-013 Rio de Janeiro, RJ
  • Paulo Miranda Universidade de Aveiro, GeoBioTec, Departamento de Geociências, Campus de Santiago, 3810-193 Aveiro
  • Maria Antonieta da Conceição Rodrigues Universidade do Estado do Rio de Janeiro - UERJ, Faculdade de Geologia, Departamento de Estratigrafia e Paleontologia. Av. São Francisco Xavier, 524, sala 2020A, Maracanã. 20550-013 Rio de Janeiro, RJ

DOI:

https://doi.org/10.12957/jse.2016.26281

Keywords:

Shallow gas. Physical properties. TOC. Rock-Eval pyrolysis. Stable isotopes. Living benthic foraminifera.

Abstract

This work was performed in the NE region of the Guanabara Bay, a highly impacted Brazilian coastal system, located in Rio de Janeiro State. It aimed to: i) identify and map the areas with occurrence of gas in the sediment, as well as its acoustic signature; ii) characterize the physical properties of the sediments and; iii) document the response of microbenthic organisms (living benthic foraminifera) to changes in quantity and quality of organic matter. Seismic surveys at the frequency of 12 kHz identified a large area with about 50% gas charged sediments in the study area.

The main acoustic signatures of the shallow gas were black shadow and gas blanket. In addition, features related to gas seepages to the water column (acoustic plumes and pockmarks) and gas percolation within the sediments (intra-sedimentary plumes, turbidity pinnacles) were also identified. The gas has a biogenic origin and results from the high sedimentation rate between 0.03 to 0.9 cm.year-1 and from the decomposition of large amount of organic matter (10-20%). Vertical distribution of gas ranges from few centimeters to 9 m below the water-sediments interface. These occurrences are related to both gas migration from lower sedimentary layers to Holocene muds above, and to recent generation in near-surface sediments as the area display favorable conditions for gas production. Cores ranging from 150-240 cm in length have predominantly muddy sediments and variations in the P-wave velocity followed the changes in sediment density, controlled mainly by the presence of gas in sediments, bioclasts accumulation, textural variation and percentage of organic matter.

The TOC content and Rock-Eval pyrolysis parameters evaluated in nine surface sediment samples indicate that good to excellent amount of organic matter associated with moderate to good source potential for gas production is present in the study area. In these areas living benthic foraminifera are of reduced diversity and density. The assemblages are largely dominated by Ammonia tepida. Statistical results evidence that areas of intense gas release affect the benthic faunas since the benthic foraminiferal assemblages are reduced in diversity and density.

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Published

2016-10-31

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Section

Research article