High-Resolution Acoustic Mapping of Gas Charged Sediments and Living Benthic Foraminifera Assemblages From the Ne Region of the Guanabara Bay (Rj, Brazil)

Abstract

. 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

Sources of Pollutants in the Northern/Northeast Area of Guanabara Bay (Se, Brazil) Since the Late Nineteenth Century Using Lead Isotopes and Metal Concentrations

“…These values indicate a tendency to increase the accumulation rate toward the recent. The analyzed cores are located in an area of low hydrodynamism (Amador, 1980), where presently tidal currents are not intense, favoring the accumulation of finegrained sediments enriched in organic materials (Delavy et al, 2016;Alves Martins et al, 2018).…”

“…The high levels of organic matter, such as those presented in the inner part of the bay and in more sheltered areas, are in general related to reducing sedimentary conditions and consequently lower rate of degradation (e.g. Delavy et al, 2016;Alves Martins et al, 2018).…”

“…The opposite pattern of Ca and TOC contents should indicate carbonate dissolution (poor preservation) due to the production of acids following organic matter degradation (e.g. Delavy et al, 2016;Martins et al, 2016 a, b). On the other hand, in coastal environments mollusks and foraminifera with carbonate shells and tests are relatively abundant.…”

“…The reduction of Ca values may also indicate a decline in the presence of these organisms due to processes of oxygen depletion in the bottom water and sediment, due to eutrophication processes (e.g. Delavy et al, 2016;Baptista Neto et al, 2017). In these sediments, it is frequent to observe the presence of living foraminifera with poorly calcified and dissolved tests (Martins et al, 2016 a, b).…”

“…Another effect of high accumulation of organic matter in the inner area of GB is the presence of gas seeping (Catanzaro et al, 2004;Delavy et al, 2016). The release of biogenic methane from the sediments was identified in some areas of the northern region of GB (Catanzaro et al, 2004;Delavy et al, 2016).…”