A geochemical and lead isotopic record from a small pond in a remote equatorial island, Fernando de Noronha, Brazil

Abstract

A 72 cm long core was collected from Lagoa da Viração (LV), a small pond in the Fernando de Noronha island, northern Brazil. Sediments from the lower section of the core (20—72 cm depth) contain essentially mineral matter, while in the upper section (0—20 cm depth) mineral matter is mixed with organic matter. Lithogenic conservative elements — Si, Al, Fe, Ti, Co, Cr, Cu, Ba, Ga, Hf, Nb, Ni, Y, V, Zn, Zr and REE — exhibit remarkably constant values throughout the core, with concentrations similar to those of the bedrock. The vertical distribution of soluble elements — Ca, Mg, Na, K, P, Mn and Sr — is also homogeneous, but these elements are systematically depleted in relation to the bedrock. LOI, TOC, Br, Se, Hg and Pb, although showing nearly constant values in the lower section of the core, are significantly enriched in the upper section. The concentration profiles of Br and Se suggest that they may be accounted for by natural processes, related to the slight affinity of these elements for organic matter. On the other hand, the elevated levels of Hg and Pb in recent sediments may be explained by their long-range atmospheric transport and deposition. Furthermore, the isotopic composition of Pb clearly indicates that anthropogenic sources contributed to the Pb burden in the uppermost pond sediments.