2018-08-012024-05-17https://scholars.lib.ntu.edu.tw/handle/123456789/695787摘要：氮總量和氮循環控制大部份中低緯地區海洋的生產力，儘管人們已經認識到氮的輸入（固氮作用）和損失（反硝化作用）之間應存在長期平衡，但對氮循環中反饋作用的靈敏度知之甚少，因此，目前還不清楚氮在海洋中的含量是否可能在較短時間內發生巨大變化，也無法預期人類活動給海洋表面增加輸入的氮是否會對海洋的生產力造成長期的影響，了解這些自然和人為過程是如何改變氮循環以及海洋的生產力是一個重要的氣候和地質問題。在該計劃中，我建議用中國南海作為一個案例研究，以解決對全球海洋氮循環認識的兩個關鍵問題：1） 氮的輸入和輸出在冰期間冰期是否緊密相伴以平衡氮的總量變化？ 2）我們是否可以觀察和量化近幾十年來人為作用對海洋表層氮輸入量增加的影響？針對第一個問題，我們提出利用有孔蟲氮同位素重建南中國海在過去86 萬年固氮作用的變化，通過將該記錄將與海平面變化、反硝化作用變化、表層海水溫度變化、以及粉塵通量的變化做相關性研究，來測試控制固氮作用的主要因素及其敏感性。針對第二個問題，我們將通過測量從東沙環礁採集的珊瑚骨骼中的氮同位素，來觀測及估算近50 年來，伴隨東南亞工業快速發展，人為活動對海洋所帶來的氮含量的增加。<br> Abstract: The nitrogen budget and cycle controls the productivity of the oceans. While it has been recognized that a long term balance is required between inputs (dominantly N fixation) and losses (dominantly denitrification), the sensitivity of the feedbacks that stabilize nitrogen inventory is poorly understood. As a result, it is unclear whether considerable temporal changes could have happened in the past and future. One example of such perturbation is the ongoing increase in anthropogenic nitrogen fluxes to the open ocean. Understanding how natural and anthropogenic processes change the nitrogen cycle in the open ocean and how this would change the fertility of the ocean is an important climatic and geological question. Here I proposed to use the South China Sea as a case study to address two key questions that have global implications: 1) how closely are the N inputs and losses coupled in the geological past? And 2) can we observe and quantify the recent increase of anthropogenic input of N to the open water in the recent decades? To address the first question, we propose to reconstruct N fixation changes in the South China Sea over 8 glacial cycles using the δ15N of foraminifera-bound N (FB-δ15N) at site MD97-2142 from the continental slope off Palawan Island. This record will be compared with sea level reconstructions, water column denitrification record, local temperature changes, and dust fluxes changes to test the controlling factors on N fixation rate and their sensitivities. We hypothesize that the N fixation in the South China Sea may be closely coupled, and thus synchronized, with changes in shelf denitrification over the glacial/interglacial cycles. To record recent changes in the anthropogenic nitrogen input to the open South China Sea, we will generate coral skeleton-bound δ15N on collected Porites cores from Dongsha Atoll, covering the last 50 years, a period of rapid industrial development in the South East Asia. If anthropogenic sources of nutrient (e.g., biomass burning or fertilizers) become increasingly important to the surrounding ocean, we might be able to observe the changes by tracing the nitrogen isotope changes over time.