Sedimentary mercury cycling in recent upwelling systems

Mercury (Hg) is a prime indicator of anthropogenic perturbations yet the database of Hg in modern upwelling sediments from the Humboldt and Benguela current systems is sparse. Mercury has natural and anthropogenic sources, occurs in different species, and recycling processes before final burial complicates the interpretation of Hg accumulation.

Here, we present data of total Hg along with Al, Zr, organic carbon and Pb (a more straightforward and well-studied anthropogenic indicator) for ²¹⁰Pb-dated sediments from Peru, Chile and Namibia from within and below their respective oxygen minimum zones (OMZs). Almost all sediments exhibit upcore enrichments of Hg in the upper ∼ 30 cm suggesting an anthropogenic source for Hg, similar to Pb. Peak Hg enrichments are highest off Peru (800 µg kg^-1) in the early 1980s followed by less pronounced maxima off Chile (150 µg kg^-1) and Namibia (80 µg kg^-1). This difference is likely due to a high number of industrial and pre-industrial mining sites in Peru, which is less important in Chile and essentially missing in Namibia. Mercury also accumulates in sediments below OMZs supporting previous indications of abyssal Hg exports while Pb only accumulates in OMZ sediments.

Mercury enrichments in Peruvian sediments that have negligible excess Pb contents suggest metallurgic Hg inputs since pre-industrial times (before CE 1900). By contrast, anthropogenic Hg is detectable at one coastal Namibian site, while Hg in the other Namibian sediments may be natural rather than anthropogenic. Our data suggest that both environments within and below recent OMZs are Hg accumulation hotspots.