. High-resolution acoustic mapping of gas charged sediments and living benthic foraminifera assemblages from the NE region of the Guanabara Bay (RJ, Brazil). Journal of Sedimentary Environments, 1 (3): 360-384.
AbstractThis 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 (intrasedimentary plumes, turbidity pinnacles) were also identified. The gas has a biogenic origin and results from the high acumulation 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.
Journal of Sedimentary Environments
The disposal of waste and sewage by anthropic activities in Guanabara Bay (GB), Rio de Janeiro State (Brazil) is negatively affected this coastal ecosystem over the last few decades. This research intends to document how organic matter and metal enrichment affect the benthic organisms. Physicochemical, textural, geochemical and microfaunal data (foraminifera) are analyzed in nine stations in the NE sector of Guanabara Bay. The size and structure of the living
. Late Holocene closure of a barrier beach in Sepetiba Bay and its environmental impact (Rio de Janeiro, Brazil). Journal of Sedimentary Environments, 2 (1): 65-80.
AbstractCore T4 (with 286 cm long) collected in the inner part of Sepetiba Bay is a record of late Holocene evolution of Sepetiba Bay. This core was described and sampled every 2 cm. Textural, geochemical and foraminifera data were analyzed, and two radiocarbon dates were obtained. Estimated ages indicate that core T4 represents the last ≈4,000 years calibrated before present (yrs cal BP). Results of this core suggest that coarse sediments, recorded in the period between ≈4,000-3,400 yrs cal BP, were probably associated with high hydrodynamic conditions in the study area. These sediments should be deposited at an incipient phase of the Marambaia Beach Barrier evolution. The development of this sand barrier resulted in the confinement of the inner region of Sepetiba Bay, leading to the deposition of muddy sediments rich in organic matter. Organic matter enrichment increased especially in the last ≈500 yrs cal BP. Foraminifera assemblages and carbonates content indicate that the studied region should have been affected in the last ≈3,400 yrs cal BP by changes in the water depth column. An increase of the water column depth was recorded for example between ≈850-350 yrs BP. The results obtained in the core T4 show that the confinement and eutrophication process of the internal zone of Sepetiba Bay has been occurring since ≈3,400 yrs cal BP and is being caused by other factors such as geomorphological and climatic changes and human activities.
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