Zinc and silicon biogeochemical decoupling in the North Pacific Ocean

Abstract

In the ocean, zinc (Zn) is an important element for biological activity and biogeochemistry. The distribution of dissolved Zn in the global ocean is similar to that of silica (Si). Previous model-based experiments proposed the Southern Ocean hypothesis: high Zn/P uptake ratio by phytoplankton in the Southern Ocean leads to Zn-depleted surface water and this anomaly is transported into the interior ocean associated with mode water formation, resulting in a distribution similar to Si. However, recent observational data from the North Pacific showed that there is decoupling of Zn and Si: the correlation between Zn and Si breaks down in the North Pacific. This study investigates the process of the Zn cycle that causes the decoupling of Zn and Si in the North Pacific using a model. We conducted the model experiment with various Zn uptake speeds in the surface ocean, but it was not easy to reproduce Zn concentrations in the North Pacific, indicating that additional mechanisms are required to produce the decoupling of Zn and Si in the North Pacific. By considering additional Zn sources from the continental shelves of the Sea of Okhotsk and the Bering Sea, we found that high Zn concentration and the Zn–Si decoupling in the North Pacific were reproduced, consistent with observational data. Our result suggests that the Zn supply from the coastal regions in the North Pacific has an important role in causing the Zn–Si decoupling.