Estratigrafia do mercúrio (Hg) como indicador de mudanças paleoambientais

Abstract

The transitions occurring between different geological periods are marked by dramatic changes in the geological record, biological crises, sea level fluctuations and/or changes in atmospheric composition. In order to understand and interpret the scenarios of these changes, as well as the paleoenvironmental conditions of formation of the sedimentary strata, there is an increasing use of geochemical and isotopic tracers. The low concentrations of mercury (Hg) found naturally on the surface of the Earth over several geological periods makes this element an important environmental tracer to identify accumulation peaks in sediments and may be provisionally related to different natural processes such as volcanism, weathering, climate change and/or extreme environmental crises. In this perspective, the present work presents initial results on the stratigraphy of Hg in sedimentary rocks, collected in the Araripe Basin. The Araripe Basin, located in northeastern Brazil, is known worldwide for its diverse fossiliferous content. In this region, 27 samples were collected in october/2016 and may/2018, in different constituent formations Rio da Batateira, Crato, Ipubi and Romualdo, sections of the Araripe Basin, that belong the Cretaceous. The samples were duly conditioned and transported to Laboratory of Coastal Biogeochemistry in Labomar/UFC, preserved in hermetically sealed bottles in a dry environment, conditioned at 20°C until analyzed. The Hg content was determined using the method of acid digestion described in the literature, the extracts being quantified by Atomic Absorption Spectrometry with Generation of Cold Vapor. All analyses were performed in duplicate, while an estuarine sediment reference material (NIST 1646A) was analysed to evaluate the accuracy and recovery of the method. To date, in addition to the Hg analysis, total organic carbon (TOC) analysis was performed using a method described by Yeomans and Bremner (1988), adapted by Mendonça and Matos (2005). Rio de Batateira, Crato, Ipubi and Romualdo formations presented the highest Hg peaks. In addition, the increase in Hg content in the aforementioned formations does not show a correlation with the TOC content, suggesting that the Hg anomalies resulted from a greater entry of atmospheric Hg species. The information obtained brings to light the necessary understanding of the variations in the chemical composition patterns of the Araripe Basin, still largely unknown, and it is necessary to expand the field of investigation that allows understanding the paleoenvironmental and paleoclimatic changes on a regional scale.