Controls on Dissolved Cu Concentrations and Isotopes in the North Atlantic: The Importance of Continental Margins

Abstract

Copper (Cu) is a marine micronutrient whose distribution and budget remain incompletely understood. Here, we present a section of dissolved Cu isotope compositions (δ⁶⁵Cu) across the North Atlantic (GEOVIDE cruise, GEOTRACES GA01). High δ⁶⁵Cu are observed in surface waters and co-vary with carbon uptake rates, indicating light Cu removal by biological activity or complexation of heavy Cu by organic ligands. Beneath the surface, low δ⁶⁵Cu may be partially caused by remineralization. Below 1,500 m, an increase in δ⁶⁵Cu points to removal by particulate scavenging. At greater depths, reversible scavenging, driven by high vertical particulate exports, could explain the increase in Cu concentrations between the surface and deep ocean, mostly in the eastern part of the transect. Investigation of external sources and sinks reveals that anthropogenic aerosols and benthic processes locally supply isotopically light Cu to the ocean, whilst hydrothermal activity above the Reykjanes ridge does not seem to represent a significant source. A striking feature is the low δ⁶⁵Cu observed between 300 and 1,500 m from the Iberian margin to the Icelandic basin, which coincides with elevated non-conservative dissolved neodymium fractions (Nd_xs). This comparison suggests that margin inputs are a source of light Cu to the ocean, and that this Cu can be transported over long distances. The Iberian margin is a hotspot of internal tides and their energy triggers sediment resuspension, leading to particle dissolution and Cu release. These results suggest that continental margins contribute significantly to the missing source of light Cu in the ocean.