Biogeoquímica molecular da matéria orgânica em testemunho do Lago Jacundá, Amazônia brasileira

Abstract

The Amazon represents one of the most important biomes in the biogeochemical cycle of carbon. Despite the fundamental role that floodplain lakes in the accumulation and sink of organic carbon, their organic matter is little explored at the molecular level. A geochemical study was carried out on Jacundá Lake, belonging to the Tapajós River, one of the most important clearwater rivers in the Brazilian Amazon Several classes of molecular biomarkers (n-alkanes, acyclic isoprenoids, hopanoids, higher plant triterpenoids, sterols, acyclic acids and alcohols) were evaluated aiming to understand the interactions of different sources of organic matter and the diagenetic processes that govern its degradation and preservation in the region, and compound-specific carbon isotopic composition added information regarding the tropic environment. The results indicate a pristine environment with major contribution of terrestrial plants as primary producers based on n-alkanes (TAR>10) and sterol constitution (C27/C29<. 0.3) and farorable preservation of allochhonous material. On the other hand, a high secondary production derived from microbial activity was observed based on hopanoid (triterpenes/hopenes<1.0) and carboxylic acids (TAR<1.0). The isotopic composition of individual compounds (CSIA) revealed depleted δ13C values (diploptene = -53.5‰) revealing incorporation of microbial carbon in the trophic chain. The study of the initial diagenesis in the sediment points to an extremely anoxic environment from the first centimeter of the core, resulting from the permanent stratification of the water column, therefore representing a hot point of organic carbon sink and degradatio within the Amazon basin. The information obtained in this work combined individual molecular organic geochemistry assessment of biomarkers and carbon isotopic geochemistry and allowed a strengthened comprehension on the organic matter dynamics in the core, using analytical tools of elevated chemical specificity, enriching the biogeochemical knowledge on the Amazon region.