Retention of metals in Sepetiba Bay (SE Brazil) conditioned by Climatic conditions and oceanic front placement

Abstract

Sepetiba Bay (SB) in Rio de Janeiro State, Brazil, plays a vital role for local fishermen, farmers, holidaymakers, and residents. However, these communities are facing a pressing problem, since the bay has been significantly polluted by sewage, urban and industrial effluents and maritime activities, particularly since the latter half of the 20th century. The inner area of the SB is heavily contaminated with metals and is experiencing processes of silting and eutrophication. The urgent need to address this pollution is one of the major concerns confronting humanity today. The transfer of pollutants from transitional waters to the ocean can disrupt biogeochemical cycles, disturb the balance of natural ecosystems, impact marine food webs, and pose risks to human health. This study examines the factors influencing the temporal record of natural and anthropogenic impacts in the outer region of SB, aiming to provide crucial insights for addressing this urgent issue. This will be achieved through a multi-proxy approach that includes analyses of grain size, mineralogy, geochemistry and silica phytoliths along the core SP16, collected between Itacuruça and Jaguanum islands, an intermediate region of Sepetiba Bay. The age model of this core was estimated using ²¹⁰Pb and ¹³⁷Cs methods. Although silica biomineralization has been frequently applied in continental areas of Brazil, studies conducted on environmental evolution in coastal sediments using silica phytoliths as bioindicators of sediment sources and paleoclimatic conditions are rare. The SP16 core with an average mass sedimentation rate of 0.26 ± 0.03 g cm⁻² yr⁻¹ records the period from 1878 to 2022. The findings indicate that the human interventions in the Guandu River basin, the most important in the region, rainfall regime, the positioning of oceanic fronts, and the stratification of water masses have influenced the sedimentological (including granulometric, mineralogical, and geochemical), oxygenation, and diagenetic characteristics of the sediments in the external area of SB. During periods of reduced rainfall, turbid river plumes are confined to the inner region of the SB, allowing oceanic fronts to develop close to the continent. These conditions enhance the retention of metals and organic matter in the internal area of the SB, which is currently experiencing significant metal contamination and eutrophication. Conversely, during periods of increased rainfall, the extension of the turbid river plumes to the outer region of the SB and the heightened stratification of water masses promote the accumulation of fine sediments in more distal zones. The results suggest that phytoliths can provide important information in paleoenvironmental reconstruction studies in coastal marine environments.