5:15 PM - 7:15 PM
[MZZ42-P08] Removal fluxes of iron and manganese in the western subarctic Pacific surface waters by using Th-234 and U-238 disequilibrium.
Keywords:trace metals, ocean, residence time, Fe
Iron (Fe) and manganese (Mn) in seawater are essential micro-nutrients for marine phytoplankton. In the western subarctic Pacific, it is well known that iron-deficiency limits phytoplankton growth (Tsuda et al., 2003). Since Fe and Mn have similar sources in the ocean, they are frequently investigated simultaneously (e.g. Wong et al., 2022). From the marginal seas such as the Sea of Okhotsk and the western Bering Sea, large amounts of Fe are laterally transported through the intermediate layers of the North Pacific and finally supplied to the surface layers by winter mixing (Nishioka et al., 2020). However, Fe is a highly particle-reactive element in the oxic open ocean, and some of Fe is removed by sinking particles from the water column during the lateral transportation. Therefore, the Fe supply processes are complicated and have not been adequately revealed yet.
To estimate sinking particulate flux in the ocean, the 234Th-238U disequilibrium method has been used in the surface ocean. Th-234 (a half-life of 24.1 days) is highly particle-reactive, though U-238 behaves conservatively in the ocean. Adsorbed Th-234 onto sinking particles is removed from the surface layers more quickly than that created by U-238 decay. The rapidly sinking particles in the surface ocean produces a deficit of Th-234 in the surface waters which could be a good indicator of the scavenging rate of particle-reactive trace metals, like Fe and Mn (e.g. Hayes et al., 2018; Black et al., 2019; Black et al., 2020). The disequilibrium of Th-234 with U-238 reflects preceding processes on time scales of days to weeks.
In this study, we measured labile particulate Fe and Mn in the subarctic and subtropical western North Pacific during R.V. Hakuho-maru KH-22-7 and 23-2 cruises. During the research cruises, we also revealed the 234Th-238U disequilibrium and particulate Th-234 distributions in the surface layers at the same stations. By combining the particulate data of Fe, Mn and Th-234 and the 234Th-238U disequilibrium, we estimated the removal fluxes of Fe and Mn in the western subarctic and subtropical North Pacific.
The results indicated the highest removal fluxes of particulate Fe and Mn at the subarctic Pacific station. In this area, large amounts of biogenic particles could be generated by marine micro-organisms in the surface waters, and trace metals might be efficiently taken up by phytoplankton and/or removed by biogenic particles. The residence time of the trace metals can also be estimated from the fluxes and the average concentration of total dissolvable trace metals. The residence times of Fe and Mn in surface waters were estimated as 15 - 32 days and 40 - 387 days in the subtropical North Pacific, but 9.8 and 42.8 days in the subarctic Pacific, respectively. The residence time of Fe in surface waters ranged within those previously estimated by not only Th-234 based export measurements but also sediment-trap experiments (Black et al., 2020). The method using Th-234 and U-238 disequilibrium seems to be applicable for the removal flux estimations of Fe and Mn in surface waters.
To estimate sinking particulate flux in the ocean, the 234Th-238U disequilibrium method has been used in the surface ocean. Th-234 (a half-life of 24.1 days) is highly particle-reactive, though U-238 behaves conservatively in the ocean. Adsorbed Th-234 onto sinking particles is removed from the surface layers more quickly than that created by U-238 decay. The rapidly sinking particles in the surface ocean produces a deficit of Th-234 in the surface waters which could be a good indicator of the scavenging rate of particle-reactive trace metals, like Fe and Mn (e.g. Hayes et al., 2018; Black et al., 2019; Black et al., 2020). The disequilibrium of Th-234 with U-238 reflects preceding processes on time scales of days to weeks.
In this study, we measured labile particulate Fe and Mn in the subarctic and subtropical western North Pacific during R.V. Hakuho-maru KH-22-7 and 23-2 cruises. During the research cruises, we also revealed the 234Th-238U disequilibrium and particulate Th-234 distributions in the surface layers at the same stations. By combining the particulate data of Fe, Mn and Th-234 and the 234Th-238U disequilibrium, we estimated the removal fluxes of Fe and Mn in the western subarctic and subtropical North Pacific.
The results indicated the highest removal fluxes of particulate Fe and Mn at the subarctic Pacific station. In this area, large amounts of biogenic particles could be generated by marine micro-organisms in the surface waters, and trace metals might be efficiently taken up by phytoplankton and/or removed by biogenic particles. The residence time of the trace metals can also be estimated from the fluxes and the average concentration of total dissolvable trace metals. The residence times of Fe and Mn in surface waters were estimated as 15 - 32 days and 40 - 387 days in the subtropical North Pacific, but 9.8 and 42.8 days in the subarctic Pacific, respectively. The residence time of Fe in surface waters ranged within those previously estimated by not only Th-234 based export measurements but also sediment-trap experiments (Black et al., 2020). The method using Th-234 and U-238 disequilibrium seems to be applicable for the removal flux estimations of Fe and Mn in surface waters.