| Summary: | 博士 === 國立中央大學 === 國際研究生博士學位學程 === 107 === Trace metals from atmospheric aerosol deposition can enhance the growth of phytoplankton and modify plankton community structure in the ocean, thus changing marine trace metal biogeochemistry. In the Northwestern Pacific Ocean (NWPO), anthropogenic aerosols deposition further provides a significant amount of highly soluble trace metals to the ocean. We first investigated particulate trace metal distribution in the surface water of the Western Philippine Sea and the Kuroshio region adjacent to the East China Sea to study their sources and cycling processes. Applying elemental ratios, we found that anthropogenic aerosols were the dominant source of particulate trace metals in these oceanic regions. And most of these metals may be extracellularly adsorbed and/or aggregated on the particles in the surface water. Compiling the data observed globally, we found that particulate trace metal concentrations are positively associated with aerosol deposition fluxes in the surface ocean, validating the dominant role of aerosol deposition in controlling trace metal composition. Among the studied metals, Zn possesses a global pattern that particulate Zn/P ratios are consistently higher than the proposed intracellular Zn/P quotas, suggesting the importance of extracellular scavenging of Zn.
Zn thus is an ideal representative element to study the cycling processes of anthropogenic aerosol metals in the water column due to its high concentrations in the aerosols and its feature for scavenging. To further study marine Zn cycling, we developed a novel technique for measuring Zn isotopic composition and combined with regular trace metal analysis to obtain more information. We then determined Zn isotopic compositions in different marine samples, seawater, suspended particles, sinking particles, and aerosols, to investigate its sources and cycling processes in the NWPO. In the surface ocean of the NWPO, dissolved Zn concentrations are higher than the ones observed in other oceanic regions because of high anthropogenic aerosol deposition fluxes. In terms of Zn isotopic fractionation, Rayleigh fractionation model approaches supported scavenging is the dominant process regulating Zn in the surface ocean, and scavenging tends to fractionate heavy Zn to the particulate phase, leaving the ambient dissolved phase isotopically light. Globally, the extra Zn from aerosol deposition not only elevates its dissolved concentrations but also enhances Zn isotopic fractionation of scavenging to leave seawater isotopically lighter than the values observed in the regions with low aerosol deposition fluxes. Our direct observation of Zn isotopic composition in particles also demonstrated the importance of scavenging on fractionating Zn in the surface ocean.
In the deep ocean, we found anthropogenic aerosol Zn accounts for around 70% of the total Zn in the sinking particles collected by moored sediment traps in the South China Sea, which is constrained by a mass balance calculation of Zn isotopic composition. The impact of anthropogenic aerosol Zn has reached the deep water of the South China Sea. In the deep water of the North Pacific Ocean (NPO), dissolved Zn isotopic composition in the west end was found to be lighter than the values observed in the east end. The light dissolved Zn isotopic composition can be attributed to the isotopically light inputs from anthropogenic aerosol deposition and potential benthic input to the deep water. A box model estimate showed that benthic input may be the major source of light Zn input and anthropogenic aerosols may play a minor role in changing dissolved Zn isotopic composition. In addition, the heavy Zn isotopic composition in the east end might be attributed to the removal of isotopically light Zn by Zn sulfide precipitation since the dissolved oxygen concentration is lower and the oxygen minimum zone is thicker in the east end than in the west end of NPO. The spatial variation of Zn isotopic composition may be attributed to the combination effect of the input and the removal of isotopically light Zn in the west and east end of NPO, respectively.
Witnessed by the evidences from particulate trace metal composition and Zn isotopic composition obtained in this dissertation study, we revealed the dominant role of aerosol deposition on regulating trace metal distribution in the surface ocean globally and confirmed the importance of scavenging on regulating marine Zn cycle. Furthermore, the impact of anthropogenic aerosols may have also reached to the deep water of the marginal sea, and highly likely to the open ocean in the NWPO.
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