Commentary on “Insights into the Major Processes Driving the Global Distribution of Copper in the Ocean From a Global 3‐D Model” by Camille Richon and Alessandro Tagliabue

The distribution of copper in the oceanic water column is simple but surprisingly difficult to explain. It is biologically active, but its distribution is remarkably similar to thorium‐ 230, whose removal is dominated by reversible scavenging throughout the water column. In this issue, Richon and Tagliabue present results of a global three‐dimensional physical biogeochemical model that shows that reversible scavenging is indeed the major mechanism responsible for Cu distribution. The model provides a high degree of resolution, revealing regional differences in Cu distribution. Model predictions compared remarkably well to the large database from the international GEOTRACES. That success highlights the value of detailed sectional surveys in understanding the controls on trace element distributions in the oceans. The model shows that Cu diverges from true nutrient‐like elements like zinc because organic complexation limits biological uptake by phytoplankton. However, their model also shows that some fraction of organic complexes are biologically available. Perhaps the most provocative outcome of the study is their claim that riverine fluxes of copper are much higher than previous estimates, resulting in a tenfold decrease in Cu residence time. The difference arises from a different way of estimating Cu fluxes than previous workers. They calculated Cu fluxes from estimates of Fe fluxes in a previous model study by applying a universal Cu:Fe ratio. This assumption may prove controversial but demonstrates that more information is required to decrease uncertainties in river flux data to model trace metals in the oceans and to exploit the opportunities created by GEOTRACES.