Assinatura geoquímica da matéria orgânica na Baía de Todos os Santos: evidências do antropoceno e suas implicações ambientais

Abstract

Significant changes in the transport or composition of organic matter (OM) can lead to alterations in trophic environments, the degree of lability, the release of adsorbed contaminants, as well as the storage and biogeochemical cycling of carbon. In this context, Baía de Todos os Santos (BTS) has been subject to multiple human activities with the potential to modify the nature and content of sedimentary OM: oil and chemical industries, and to a lesser extent, urbanization. Reconstructing historical variations in carbon, nitrogen, their respective isotopes, hydrocarbons (aliphatic, polycyclic aromatic, and alkylated), and black carbon (BC) based on sedimentary records is significant for understanding long-term pollutant emissions, tracing sources, and establishing effective pollution control strategies. By comparing two sediment cores collected in the northern region of the BTS, historical variations were reconstructed and compared with local socioeconomic development. Overall, the results indicate that the temporal profiles of OM deposited in cores can vary significantly on a small spatial scale. In core T2, collected further offshore, the analyzed parameters indicate better environmental conditions with natural sources of OM. The concentrations of total organic carbon (TOC) and total nitrogen (TN) ranged from 0.66% to 1.98% and 0.12% to 0.19%, respectively. The isotopic mixing model showed an average marine contribution of 70.23%, relatively uniform, with the exception of the period between the 1930s and 1980s, when there was an increase in the marine contribution possibly associated with the expansion of an algal bed. The low levels of ALCTOT (0.25 to 2.23 μg g-1), ΣALI (0.36 to 2.90 μg g-1), HPATOT (7.62 to 354.45 ng g-1), HPAALK (1.11–77.50 ng g-1), associated with the absence of UCM, pristane, and phytane, indicate little or no influence of contamination by anthropogenic sources in the OM areas. The diagnostic indices indicated pyrolytic sources as the main contributors of PAHs in the T2 core. The BC contents ranged from 0.0800 to 0.440 mg g-1 and were produced from the incomplete combustion of C4 type biomass. In core T4, the concentrations of total organic carbon (TOC) (1.17 to 2.23%) and total nitrogen (TN) (0.13 to 0.26%) increased over time but still indicate an oligotrophic system. The isotopic mixing model revealed changes in the proportions of the dominant sources of organic matter (OM), with a gradual increase in terrestrial (allochthonous) contribution from the 1950s onwards, likely associated with changes in land use. From the 1970s onwards, hydrocarbon levels increased abruptly, reaching high pollution levels, despite the reduction observed in recent decades. During this period, there was also a shift in BC sources, with contributions from biomass burning being replaced by vehicle emissions. From 1980 onwards, a clear petrogenic signature was observed with the presence of an unresolved complex mixture (nd to 349.94 μg g-1), increased concentrations of ΣALI (0.35 to 380.34 μg g-1), ALCTOT (0.25 to 5.84 μg g-1), HPATOT (3.05 to 1096.37 ng g-1), and HPAAlk (0.40 to 329.23 ng g-1). From the 1990s onwards, there was a more pronounced increase in BC concentrations. The effects of implementing environmental policies to improve water quality in the BTS began to be reflected in sediment quality at the beginning of the 21st century.