ALBERT

Description: The Araguaia Belt, the northern branch of Neoproterozoic Tocantins tectonic province, developed during West Gondwana amalgamation as a result of collision between the Amazon and West African and/or Sao Francisco/Congo cratons. The external zone of the belt consists of ophiolitic slices and fragments, sedimentary rocks derived from magmatic arc sources, volcanic rocks, and part of a passive continental margin with low-grade metamorphic rocks, while the internal zone corresponds to a pile of low- to medium-grade metasedimentary rocks. The largest and best preserved ophiolitic bodies occur in the southern part of the belt, where the Quatipuru and Morro do Agostinho ophiolites are composed predominantly of mantle peridotites (mainly residual harzburgite) representing the base of the Moho transition zone. They contain chromitite pods, dunitic lensoid bodies and a suite of maficultramafic dykes and/or sills resulting from partial melting, magma impregnation and diapiric up-rise. A SmNd isochron age of 757{+/-}49 Ma indicates oceanic crust formation during the Early Neoproterozoic. NW African correlatives of the Araguaia Belt, the MauritanideBassarideRokelide belt, show similarities with respect to lithostratigraphic units, the ages of basement and supracrustal rocks, the presence of Neoproterozoic ophiolitic slices and fragments, suture zones characterized by high gravity anomalies and centrifugal tectonic vergence. We conclude that these belts were probably formed around the same Neoproterozoic ocean or several small coeval oceans.

Description: A long term experiment was conducted in a primary forest area in Amazonia, with continuous in-situ measurements of aerosol optical properties between February 2008 and April 2011, comprising, to our knowledge, the longest database ever in the Amazon Basin. Two major classes of aerosol particles, with significantly different optical properties were identified: coarse mode predominant biogenic aerosols in the wet season (January–June), naturally released by the forest metabolism, and fine mode dominated biomass burning aerosols in the dry season (July–December), transported from regional fires. Dry particle median scattering coefficients at the wavelength of 550 nm increased from 6.3 Mm−1 to 22 Mm−1, whereas absorption at 637 nm increased from 0.5 Mm−1 to 2.8 Mm−1 from wet to dry season. Most of the scattering in the dry season was attributed to the predominance of fine mode (PM2) particles (40–80% of PM10 mass), while the enhanced absorption coefficients are attributed to the presence of light absorbing aerosols from biomass burning. As both scattering and absorption increased in the dry season, the single scattering albedo (SSA) did not show a significant seasonal variability, in average 0.86 ± 0.08 at 637 nm for dry aerosols. Measured particle optical properties were used to estimate the aerosol forcing efficiency at the top of the atmosphere. Results indicate that in this primary forest site the radiative balance was dominated by the cloud cover, particularly in the wet season. Due to the high cloud fractions, the aerosol forcing efficiency absolute values were below −3.5 W m−2 in 70% of the wet season days and in 46% of the dry season days. Besides the seasonal variation, the influence of out-of-Basin aerosol sources was observed occasionally. Periods of influence of the Manaus urban plume were detected, characterized by a consistent increase on particle scattering (factor 2.5) and absorption coefficients (factor 5). Episodes of biomass burning and mineral dust particles advected from Africa were observed between January and April, characterized by enhanced concentrations of crustal elements (Al, Si, Ti, Fe) and potassium in the fine mode. During these episodes, median particle absorption coefficients increased by a factor of 2, whereas median SSA values decreased by 7%, in comparison to wet season conditions.

Description: A long term experiment was conducted in a pristine area in the Amazon forest, with continuous in situ measurements of aerosol optical properties between February 2008 and April 2011, comprising, to our knowledge, the longest database ever in Amazonia. Two types of aerosol particles, with significantly different optical properties were identified: coarse mode predominant biogenic aerosols in the wet season (January–June), naturally released by the forest metabolism, and fine mode dominated biomass burning aerosols in the dry season (July–December), transported from regional fires. Dry particle median scattering coefficients at the wavelength of 550 nm increased from 6.3 Mm−1 to 22 Mm−1, whereas absorption at 637 nm increased from 0.5 Mm−1 to 2.8 Mm−1 from wet to dry season. Most of the scattering in the dry season was attributed to the predominance of fine mode particles (40–80% of PM10 mass), while the enhanced absorption coefficients are attributed to the presence of light absorbing aerosols from biomass burning. As both scattering and absorption increased in the dry season, the single scattering albedo (SSA) did not show a significant seasonal variability, in average 0.86 ± 0.08 at 637 nm for dry particles. Measured particle optical properties were used to estimate the aerosol forcing efficiency at the top of the atmosphere. Results indicate that in this pristine forest site the radiative balance was dominated by the cloud cover, or, in other words, the aerosol indirect effect predominated over the direct effect, particularly in the wet season. Due to the high cloud fractions, the aerosol forcing efficiency was below −3.5 W m−2 in 70% of the wet season days and in 46% of the dry season days. These values are lower than the ones reported in the literature, which are based on remote sensing data. Besides the seasonal variation, the influence of external aerosol sources was observed occasionally. Periods of influence of the Manaus urban plume were detected, characterized by a consistent increase on particle scattering (factor 2.5) and absorption coefficients (factor 5). Episodes of biomass burning and mineral dust particles advection from Africa were observed between January and April, characterized by enhanced concentrations of fine mode (PM2.0), crustal elements (Al, Si, Ti, Fe) and potassium. During these episodes, median particle absorption coefficients increased by a factor of 2, whereas median SSA values decreased by 7%, in comparison to wet season conditions.

Description: The Araguaia Belt, the northern branch of Neoproterozoic Tocantins tectonic province, developed during West Gondwana amalgamation as a result of collision between the Amazon and West African and/or São Francisco/Congo cratons. The external zone of the belt consists of ophiolitic slices and fragments, sedimentary rocks derived from magmatic arc sources, volcanic rocks, and part of a passive continental margin with low-grade metamorphic rocks, while the internal zone corresponds to a pile of low- to medium-grade metasedimentary rocks. The largest and best preserved ophiolitic bodies occur in the southern part of the belt, where the Quatipuru and Morro do Agostinho ophiolites are composed predominantly of mantle peridotites (mainly residual harzburgite) representing the base of the Moho transition zone. They contain chromitite pods, dunitic lensoid bodies and a suite of mafic–ultramafic dykes and/or sills resulting from partial melting, magma impregnation and diapiric up-rise. A Sm–Nd isochron age of 757±49 Ma indicates oceanic crust formation during the Early Neoproterozoic. NW African correlatives of the Araguaia Belt, the Mauritanide–Bassaride–Rokelide belt, show similarities with respect to lithostratigraphic units, the ages of basement and supracrustal rocks, the presence of Neoproterozoic ophiolitic slices and fragments, suture zones characterized by high gravity anomalies and centrifugal tectonic vergence. We conclude that these belts were probably formed around the same Neoproterozoic ocean or several small coeval oceans.