Sedimentary geochemical record of historical anthropogenic activities affecting Guanabara Bay (Brazil) environmental quality

A sediment core from Guanabara Bay (Brazil) was analyzed for ²¹⁰Pb dating, grain size, total organic carbon (TOC) and total nitrogen, carbon stable isotope ratio (δ¹³C), nitrogen stable isotope ratio (δ¹⁵N) and the metals Fe, Mn, Ni, Co, Cu, Pb, V and Zn, to assess the influence of land use changes on the aquatic system in a region for which large industrial and urban development is expected in the next few decades. To obtain baseline data for improving the monitoring of the expected increase in anthropogenic impacts from surrounding drainage basins, a multivariate analysis of data from different sediment layers was carried out to evaluate the dated sediment record. The geochemical data suggested three different sedimentary phases along the last 200 years. Before the 1880s, the highest clay and TOC contents were observed, where the C/N ratios and the δ¹³C values suggested a mixture of algal and terrestrial organic matter and the lowest concentrations of Co, Cu, Pb, V and Zn, for which background levels were estimated (4.6, 2.7, 14.9, 24.3 and 70.2 mg kg⁻¹, respectively). From the 1880s to the 1950s, the metal concentrations and sand particles increased, but no change in organic matter quality was observed, reflecting a period of land use change, still without significant sewage input. After the 1950s, the sedimentation rate increased from 0.42 to 0.77 cm year⁻¹ and increasing urban sewage input was evidenced by lower C/N ratios, higher δ¹⁵N, decrease of Fe and Mn concentrations and increased fluxes of metals and TOC, which showed a good relationship with population growth data.