| 摘要: | 大氣氣溶膠沉降中的微量金屬可促進浮游植物的生長、並改變海洋浮游生物群落結構，進而影響海洋微量金屬的生物地球化學過程。在西北太平洋中，人為氣溶膠沉降更為海洋提供了大量高溶解性的微量金屬，而其在海洋生地化循環所扮演的角色與可能造成的影響與日俱增。為了研究微量金屬的來源及循環過程，本研究首先於西菲律賓海和鄰近東海黑潮區域的表水中進行顆粒態微量金屬分佈之初探，並由顆粒態微量金屬的元素比發現人為氣溶膠是表水中顆粒態微量金屬的主要來源。且這些金屬大多都是吸附或聚集於在表水中的顆粒上。藉由統整文獻資料，本研究發現全球的顆粒態微量金屬濃度與大氣氣溶膠沉降通量呈正相關，證實了氣溶膠沉降是影響表水中顆粒態微量金屬組成的主要因子。於研究的微量金屬中，鋅具有一個有別於其他金屬且全球一致的趨勢，即不同洋區中的顆粒態鋅磷比高於其已知藻類細胞內之鋅磷比，暗示了顆粒吸附及清除過程（scavenging）對鋅循環的重要性。
因此，有鑒於鋅於氣溶膠中的高濃度及具有被吸附清除的特性，鋅是研究海洋中人為氣溶膠金屬循環過程的理想代表元素。為了進一步研究海洋鋅循環，本研究建立了一套測量鋅同位素組成的技術，並結合已有的微量金屬分析技術以獲得更多資訊。為了研究鋅在西北太平洋中的來源和循環機制，本研究接著量測了不同性質的海洋樣品，包含海水、懸浮顆粒、沈降顆粒和氣溶膠中的鋅同位素組成。在西北太平洋的表層海水中，人為氣溶膠沉降的高通量導致溶解態鋅濃度相對高於其他海域觀察到的濃度。關於鋅同位素的分化作用，Rayleigh fractionation model 亦支持吸附清除是調節表水中鋅循環的主要機制，且吸附清除作用傾向將重的鋅同位素吸附於顆粒上，使溶解態的鋅同位素組成變輕。藉由統整全球溶解態鋅濃度及同位素資料，本研究發現氣溶膠沉降所供應的鋅不僅提高了海水中的鋅濃度，且增強了吸附清除作用對海水中鋅同位素的分化，使觀測到的同位素值比其他氣溶膠沉降通量較低的海域中觀察到的值更
加輕。此外，本研究於顆粒中觀察到的鋅同位素組成亦支持了顆粒清除吸附作用對表層海洋中鋅循環的重要性。
於深層海水中，本研究使用鋅同位素做質量平衡計算，發現人為氣溶膠鋅佔南海沉降顆粒態鋅總量的70%左右。雖然南海只是世界上最大的邊緣海域之一，但人為氣溶膠鋅的影響已至深海。除此之外，在北太平洋中，藉由統整本研究中的結果及文獻中的溶解態鋅同位素數據，發現太平洋西側的深水鋅同位素組成略輕於太平洋東側。此一趨勢可歸因於兩種機制運行的結果，包括太平洋西側輕同位素源的輸入及東側輕同位素的移除。在北太平洋中，人為氣溶膠沉降和潛在的底水沉積源輸入可能是輕同位素的來源，而藉由質量平衡計算發現底水沉積源輸入貢獻更甚於人為氣溶膠沉降，人為氣溶膠沉降對溶解態深水的影響相對有限。此外，硫化鋅沉澱可能是於太平洋東側輕同位素的移除機制，因為於太平洋東側的深水中最少含氧濃度較低且低溶氧水層較厚。透過顆粒態微量金屬和鋅同位素組成，本研究揭示了氣溶膠沉降在調節全球海洋表水中微量金屬分佈的主導角色，並證實了吸附清除作用對海水中鋅循環的重要性。此外、人為氣溶膠對海洋的影響不僅只在邊緣海且極可能延伸到一般的開放大洋、亦可能已經觸及深海。;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. |
