[A-OS17] Chemical and Biological Oceanography

[AOS17-04] A modeling study on distribution of dissolved Zinc in the North Pacific

*Kiminori Sugino¹, Akira Oka¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:GEOTRACES, Zinc, Silica, Ocean biogeochemical model, Marine chemistry

In the ocean, Zinc is a trace metal and exists in smaller amounts than the more abundant elements. Zn, like Fe, has been recognized as an important element for biological activities and biogeochemistry in the ocean. It has been clarified from studies in the 1980s that the distribution of dissolved Zn in the global ocean is similar to that of Si. However, while Si distribution in the ocean is controlled by diatom shell made of opal, Zn exists only about 1-3% in diatom shell. Since Zn is incorporated into organic matter, its distribution should be similar to that of Phosphate, a representative nutrient; however, this is not the case. Therefore, the mechanism of coupling between Zn and Si has been discussed. In recent years, the GEOTRACES program has led to a rapid increase in observational data on dissolved Zn in the ocean, which has accelerated the discussion of the dissolved Zn cycle process in the ocean. From model-based experiments, Vance et al.(2017) proposed the Southern Ocean hypothesis; excessive Zn uptake by phytoplankton in Southern Ocean causes Zn-depleted surface water masses and this anomaly is transported northward into the interior ocean, resulting in a distribution similar to that of Si. In this model-based experiment, the Zn was assumed to be coupled with the P model, and was independent of the Si model. The results showed that the correlation between Zn and Si was better than the correlation between Zn and P, despite the fact that Zn was associated with P and the model was independent of Si, supporting the Southern Ocean hypothesis. However, observational data from Kim et al.(2017) showed that the correlation between Zn and Si was broken in the North Pacific. This was not discussed in Vance et al.(2017), which focused on the coupling relationship between Zn and Si in the global ocean. Based on this finding that the Southern Ocean hypothesis of Vance et al.(2017) is not sufficient for the distribution of Zn in the North Pacific, this study aimed to clarify the process of Zn cycle that causes the decoupling of Zn and Si distributions in the North Pacific by using a model. In our model, the basic model setting of Zn was the same as that of Vance et al.(2017). Additional experiments based on the Southern Ocean hypothesis which assumed different Zn uptake rates from previous studies were also conducted. However, no improvement in the distribution of Zn in the North Pacific was observed in any case, indicating that additional processes are required to reproduce the high Zn concentration in the North Pacific. We conducted the next experiment based on the discussion in Kim et al.(2017) where the trace metal source from the North Pacific continental shelf is incorporated. We tested whether Zn-Si decoupling appears in the North Pacific by applying large Zn source onto the continental shelves of the Sea of Okhotsk and the Bering Sea. We found that Zn-Si decoupling was reproduced in all cases wherever the source of Zn was specified. In conclusion, our studies suggest that high Zn outflow from the coastal regions in the North Pacific might cause the Zn-Si decoupling in the North Pacific.