Concentração de mercúrio e selênio na Plataforma Costeira do Sudeste do Brasil

Abstract

Recent studies have addressed the increase in mercury concentrations in the South Atlantic over the last 600 years in the deep waters of this oceanic basin. Added to this growing problem, Mercury has high toxicity, in addition to its ability to biomagnify along the trophic web. Considering the large area of the Brazilian territorial sea and little information about this element in the South Atlantic, the present study aims to present data on the distribution of Hg on the platform of Southeast Brazil and its relationship with the main water masses in the region. . Thus, this work will address two topics: (i) Distribution of Mercury and Selenium (Se) in the Biological Pump under the influence of upwelling on the Cabo Frio-RJ platform. For this, 79 samples of plankton (fractions > 20μm, >64 μm , >150 μm and >300 μm) were collected on the Cabo Frio Continental Shelf during 7 oceanographic cruises in 2012. Hg was determined by atomic absorption by generation of cold vapor and Se by ICP-MS; (ii) Distribution of Mercury in the water masses in the South Atlantic Ocean (24ºs to 20ºs), Brazilian Exclusive Economic Zone. For this, water was collected on two cruises, one from November 5 to 6, 2016 aboard the Ocean Stalwart ship, and another from December 5 to 20, 2016, at 17 points water was collected in depth profiles. The determination of HgT (non-filtered) in water was carried out according to the US EPA 1631 method. These data make up the Resurgence Project database, of the Petrobras Geochemistry network, and were interpreted for this dissertation. The results of Hg in marine plankton show higher values in the zooplankton fraction (0.138 μg.g-1) than in the phytoplankton fraction (0.064 μg.g-1), and demonstrate a process of biomagnification and trophic transfer. Se values, however, show higher concentrations in phytoplankton and lower concentrations in zooplankton. These results were not enough to demonstrate the antagonism between Se and Hg, but high concentrations of Se are observed in relation to Hg, which may be related to the role of Se as a Hg inhibitor, as reported in other studies for the region of the South Atlantic. The Hg:Al vs P:Al ratio in plankton by mesh sizes compared to crust value (Hg:Al), obtained an above average ratio of sediment and crust, which indicates biogenic/anthropogenic sources, and resurgence itself, in addition to the remineralization of this element in the water column, influenced by the phenomenon itself. The circulation process of this metal responded to the physical and chemical conditions imposed by the ACAS, where the values of HgT and Se were higher under the action of this mass in the study region. The water masses also present high Hg values in the study region, with the highest averages of 6.5pM, and indicate a strong contribution of the oceanic circulation to the high concentrations of Hg found, in addition to the continental discharge, and the time of residence of this metal in older masses. TOC appears to be an important proxy for Hg concentration in marine waters. It may be responsible not only for the remineralization of Hg, but it may also associate with this element and moderate its speciation, mainly in background environments. The biological pump seems to influence, even on a small scale, the transfer of these metals to the sediment, mainly due to sea snow, which redistributes and recirculates these elements in the water column.