Controls of Copper in the Ocean

Copper (Cu) is a biologically-important trace metal for marine plankton, but it is also toxic at high concentrations. Understanding the global distribution of Cu and the processes controlling its cycling in the ocean is important for understanding how the distribution of this important element can respond to climate change, but is hampered by a paucity of observations. Here, we use available observations of dissolved copper, an artificial neural network, and an ocean circulation inverse model, to derive the first global estimate of the 3-dimensional distribution and cycling of Cu in the ocean. We find that surface-ocean dissolved Cu concentrations are highest in the Southern Ocean, where strong biological uptake suggests that plankton assimilate Cu to cope with iron limitation. In the low-latitude surface ocean, Cu is everywhere present at moderate concentrations, and biological uptake is weak. In fact, we diagnose a net source of Cu to the oligotrophic subtropical gyres that is not associated with dust deposition, which we hypothesize is due to photochemical reactions. Below the surface, Cu is scavenged onto sinking particles, contributing to an increase in the flux of particulate Cu with depth. This removal of Cu by scavenging is counteracted by a source of Cu from ocean sediments, which sustains a gradual increase in the concentration of dissolved Cu with depth.