Sedimentary mercury cycling in recent upwelling systems

The sedimentary database of mercury (Hg) in modern upwelling sediments from the Humboldt and Benguela current systems is sparse, yet this element is a prime indicator of anthropogenic perturbations of the marine realm. Mercury has various natural and anthropogenic sources, occurs in different species, and internal recycling processes before final burial renders the interpretation of the Hg accumulation process challenging. Here, we present data of total Hg (along with Al, P, Zr, organic carbon and Pb, another anthropogenic indicator) for ²¹⁰Pb-dated continental margin sediments from Peru, Chile and Namibia from within and below their respective oxygen minimum zones (OMZs). All sediments exhibit upcore authigenic enrichments of Hg in the upper 20 - 30 cm, which suggests that Hg has an anthropogenic source, similar to Pb. Moreover, the Hg enrichments are highest off Peru with up to 800 ppb authigenic Hg in the early ‘80s, followed by those off Chile (~ 150 ppb Hg) and Namibia (~ 80 ppb Hg). This 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. The data further suggest that Hg is trapped by organic particles, which settle quickly through O₂-deficient waters. In contrast to Pb, which is rapidly removed from the water column at OMZ sites, Hg is also exported to the deep sea environment (> 1000 m water depth below OMZs). This is likely due to recycling processes before final Hg burial. Authigenic Hg enrichments in Peruvian sediments that have negligible authigenic Pb contents suggests the presence of Hg inputs since pre-industrial times (before ~ 1900 AD), which is in line with previous findings from Peruvian lakes and the Galapagos Islands. By contrast, anthropogenic Hg is only visible in near coastal Namibian sediments since the last ~ 70 years. Overall, our data indicate that upwelling sediments are valuable archives for the preservation of anthropogenic signals given the favorable boundary conditions (high productivity, high oxygen deficiency and high sedimentation rates).