Geochemical characterization and assessment of contamination in mud depocenters from the southern Brazilian shelf

Abstract

Continental shelves hold ecological, environmental and economic importance. Mud depocenters found on continent shelves are able to adsorb and incorporate organic and inorganic pollutants into the surface of their grains, creating an effective record of anthropogenic influence and environmental changes in a region. The aim of this study is to perform a chemical characterization of mud depocenters on the southern Brazilian shelf, providing an inventory of elements and evaluation of their distribution and potential contamination or enrichment. The sediments from seven short cores were analyzed by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) and X-Ray Fluorescence (XRF). The high correlation of metals such as Fe, K, Ti and V with PC 1 of PCA indicated the predominance of a terrigenous signal in the region. This signal was more evident in the cores collected further south and closer to the continent, because they were better related to PC 1 in the scatter plot of PCA. Furthermore, they presented the highest concentration of terrigenous elements, Fe/Ca and Fe/K ratios. The results corroborate the importance of Rio de la Plata discharge and local inputs, such as Patos Lagoon, to the southern Brazilian shelf. A period of decreased terrigenous input, identified by low element ratios from one of the cores, correlated with a drought that occurred in Rio Grande do Sul in 1943. The results of the pollution indices with almost all enrichment factors (EF) below 2, modified degree of contamination index (mCd) below 1.2, sediment quality guideline quotient (SQG-Q) around 0.1, and the elements with concentrations below those established by Brazilian regulation as safe, demonstrated the absence of potential risks to the biota and contamination levels by Cr, Cu, Ni, Pb and Zn. Nonetheless, increases in some EF vertical profiles after 1928 and 1988, which may be associated with the beginning of the Industrial Revolution in Brazil and the establishment of Neoliberal politics in Brazil and Latin America, respectively, and the differentiated results for As, suggest that the influence of anthropogenic activities in the sediments of the region must not be neglected.