Composição isotópica de carbono e oxigênio em ostracodes de depósitos Neógenos da Formação Solimões, estado do Amazonas

Abstract

The Neogene period was marked by important global events such as subsidence, climate seasonality and changes in sea level. In South America, the uplift of the Andean Cordillera was the main event affecting the river system and, consequently, climate and biota of the Amazon during the Miocene. The knowledge of the depositional history contributes to the understanding of many environmental changes occurring in the Miocene. The question exists about possible marine incursions in the Miocene deposits of Western Amazonia, is a favorable scenario for the use of isotope analyze in the shells of ostracods from Solimões Formation. This work is a contribution for paleoenvironmental interpretations of data using the isotopic ratio of C and O in the ostracod shells from the Neogene Solimões Formation aiming to correlation with adjacent areas. The samples are from two cores (1-AS-31-AM e 1-AS-34-AM) and three outcrops (Morada Nova, Aquidabã e Torre da Lua) located in the southwestern state of Amazonas. Among the registered ostracofauna, it was observed that the most representative species for this study belong to the genus Cyprideis (Cyprideis pebasae and Cyprideis machadoi), given its occurrence both in outcrops and cores, as well as the preservation condition they were found. In general, the isotope analysis showed a distribution with only low values for 13C/12C and 18O/16O ratios. The lower depletion of δ18O the sample TL03 of Torre da Lua outcrop and the depth 138,20m of 1AS-31-AM core, reflecting increased evaporation. In these intervals cores increased evaporation occurred over the top where we found the less negative values. However the remaining samples from outcrops and cores presented greater depletion where the decrease of δ13C values possibly indicates humid environment and a reduction in the productivity of these species or organic matter preservation decay and the δ18O values registered lower evaporation.