9:30 AM - 9:45 AM
[AOS16-03] Study on biogeochemical processes controlling the distribution of Zinc in the North Pacific Ocean
Keywords:GEOTRACES, Zinc, Silica, Trace element, Ocean tracer model, Ocean general circulation model
Zinc (Zn) in the ocean is a trace metal and the distribution is controlled by the biological processes, and therefore it is considered a biogeochemically important element. The ratio of Zn to silicon (Si) is known to become almost constant in the ocean, indicating a coupling relationship. However, the processes controlling the distribution of Zn and Si are different. The particle form of Zn is an organic particle, while that of Si is diatom opal. The way their decomposition occurs is different. Therefore, the cause of the coupling of Zn and Si distributions in the global ocean globally has long been discussed.
In recent years, the mechanism of the coupling begins to be discussed in detail due to the increase and the improvement of observation and accumulation of data. Vance et al. (2017) proposed the Southern Ocean hypothesis that the similarity of Zn and Si distribution spreads from the Southern Ocean surface to the global ocean. The hypothesis is based on the diatom Zn uptake characteristics and the water mass formation process in the Southern Ocean. However, it has been reported that this coupling relationship breaks down in the North Pacific (Janssen and Cullen 2015; Kim et al. 2017). The Zn-Si decoupling is observed in the middle water, which is not on a straight line but convex upward. The cause of this decoupling has been discussed in various studies, including one by Janssen and Cullen (2015) that argues that it is due to the formation of Zn sulfides, another by Kim et al. (2017) that discusses the supply of Zn from the continental shelf, and another by Vance et al. (2019) that proposes that the difference in regeneration scales of Zn and Si becomes evident in the North Pacific. However, none of them have been quantitatively verified or discussed in detail. The processes that cause Zn-Si decoupling in the North Pacific are not clear.
The objective of this study is to understand the mechanisms that cause Zn-Si decoupling in the North Pacific with an ocean general circulation model. Sensitivity experiments were conducted in order to investigate the below main hypotheses. One is a continental shelf supply experiment. It examined the possibility that Zn supply from the continental shelf could cause the decoupling (Kim et al. 2017). Another is an ocean circulation field change experiment. It examined the possibility of the influence of different regeneration scales of Zn and Si (Vance et al. 2019).
A control experiment with the same parameter settings as in Vance et al. (2017) reproduced the results of the global Zn-Si coupling relationship as in the previous study. However, the North Pacific decoupling was not reproduced. The continental shelf supply experiment reproduced the Zn-Si decoupling similar to the observation. This result suggested that Zn supply from the continental shelf could be responsible for the decoupling as discussed by Kim et al. (2017). The ocean circulation field change experiment showed that decoupling can be reproduced in different circulation fields without explicitly considering additional processes. We compared the circulation fields in which decoupling is reproduced with those in which it is not. It was confirmed that differences in the regenerated Zn led to differences in the reproducibility of decoupling. This suggests that in the North Pacific away from the Southern Ocean, the difference in the Zn and Si circulation process is appearing as discussed by Vance et al. (2019).
In the real North Pacific, the contribution of continental shelf supply and Zn regeneration processes to Zn-Si decoupling was estimated. From this estimation, it is estimated that more than 70% of the Zn concentration above 1000m where the decoupling is prominent is due to Zn regeneration, suggesting that Zn regeneration is the main factor causing decoupling. On the other hand, it is estimated that about 30% of the Zn concentration below 2000 m cannot be explained without an additional source of Zn. In other words, we conclude that although continental shelf supply of Zn is not the main cause of Zn-Si decoupling, the possibility of that process cannot be denied. However, uncertainties are still existing, such as the lack of consideration for the reversible scavenging process. In the future, it is required to further investigation of the Zn circulation processes in the North Pacific.
In recent years, the mechanism of the coupling begins to be discussed in detail due to the increase and the improvement of observation and accumulation of data. Vance et al. (2017) proposed the Southern Ocean hypothesis that the similarity of Zn and Si distribution spreads from the Southern Ocean surface to the global ocean. The hypothesis is based on the diatom Zn uptake characteristics and the water mass formation process in the Southern Ocean. However, it has been reported that this coupling relationship breaks down in the North Pacific (Janssen and Cullen 2015; Kim et al. 2017). The Zn-Si decoupling is observed in the middle water, which is not on a straight line but convex upward. The cause of this decoupling has been discussed in various studies, including one by Janssen and Cullen (2015) that argues that it is due to the formation of Zn sulfides, another by Kim et al. (2017) that discusses the supply of Zn from the continental shelf, and another by Vance et al. (2019) that proposes that the difference in regeneration scales of Zn and Si becomes evident in the North Pacific. However, none of them have been quantitatively verified or discussed in detail. The processes that cause Zn-Si decoupling in the North Pacific are not clear.
The objective of this study is to understand the mechanisms that cause Zn-Si decoupling in the North Pacific with an ocean general circulation model. Sensitivity experiments were conducted in order to investigate the below main hypotheses. One is a continental shelf supply experiment. It examined the possibility that Zn supply from the continental shelf could cause the decoupling (Kim et al. 2017). Another is an ocean circulation field change experiment. It examined the possibility of the influence of different regeneration scales of Zn and Si (Vance et al. 2019).
A control experiment with the same parameter settings as in Vance et al. (2017) reproduced the results of the global Zn-Si coupling relationship as in the previous study. However, the North Pacific decoupling was not reproduced. The continental shelf supply experiment reproduced the Zn-Si decoupling similar to the observation. This result suggested that Zn supply from the continental shelf could be responsible for the decoupling as discussed by Kim et al. (2017). The ocean circulation field change experiment showed that decoupling can be reproduced in different circulation fields without explicitly considering additional processes. We compared the circulation fields in which decoupling is reproduced with those in which it is not. It was confirmed that differences in the regenerated Zn led to differences in the reproducibility of decoupling. This suggests that in the North Pacific away from the Southern Ocean, the difference in the Zn and Si circulation process is appearing as discussed by Vance et al. (2019).
In the real North Pacific, the contribution of continental shelf supply and Zn regeneration processes to Zn-Si decoupling was estimated. From this estimation, it is estimated that more than 70% of the Zn concentration above 1000m where the decoupling is prominent is due to Zn regeneration, suggesting that Zn regeneration is the main factor causing decoupling. On the other hand, it is estimated that about 30% of the Zn concentration below 2000 m cannot be explained without an additional source of Zn. In other words, we conclude that although continental shelf supply of Zn is not the main cause of Zn-Si decoupling, the possibility of that process cannot be denied. However, uncertainties are still existing, such as the lack of consideration for the reversible scavenging process. In the future, it is required to further investigation of the Zn circulation processes in the North Pacific.