Characterizing the Roles of Biogeochemical Cycling and Ocean Circulation in Regulating Marine Copper Distributions

Copper (Cu) is a key micronutrient for marine phytoplankton. Its oceanic biogeochemical cycle has elicited considerable attention due to dissolved Cu exhibiting a unique linear profile with depth. Several processes have been proposed for explaining this behavior. In this study, we characterize the relationships between the observed Cu, PO43-, and Si on a global scale. We find that the depth profiles of Cu resemble those of Si more than of PO43- in the global ocean. To understand their relationships, we couple the biogeochemical and internal circulation processes in a model, fitting optimal Cu:PO43- uptake ratios and remineralization length-scales to replicate the marine Cu distributions. The modeling results suggest that Cu uptake needs in the Southern Ocean are substantially higher than those in other oceanic regions. In addition, our modeling results indicate a deep Cu remineralization in the global ocean. We offer an alternative mechanism that relies on biogeochemical cycling and internal circulation to produce the linear depth profiles of dissolved Cu. Our results suggest that diatoms are likely the major phytoplankton dominating oceanic Cu cycling. Plain Language Summary While copper is known to be used as a trace nutrient by phytoplankton, its distribution in the ocean is very different from nutrients like phosphate and silicate. The material produced by phytoplankton in surface waters falls into the deep and regenerates phosphate and silicate. Both phosphate and silicate have mid-depth maxima. They also accumulate as waters move from the Atlantic into the Pacific. In comparison, the concentration of copper increases steadily with depth compared to phosphate and silicate. Copper also shows accumulation in the Pacific relative to the Atlantic. We show that these features of the copper distribution can be explained by allowing copper to be taken up by phytoplankton at the surface if the resulting material is allowed to fall all the way to the bottom and return to solution there. Furthermore, in order to fit the marine distribution of copper, its uptake in the Southern Ocean must be substantially higher than that in other oceanic regions. This fact, in addition to the fact that copper is better correlated with silicate than phosphate, leads us to hypothesize that diatoms in the Southern Ocean play a particularly important role in marine copper distributions.

Automated summary

Copper distribution in the ocean differs from other nutrients like phosphate and silicate. Our study suggests that diatoms in the Southern Ocean play a particularly important role in marine copper distributions. The uptake of copper in the Southern Ocean is substantially higher than in other oceanic regions.