海平面變化對南海大陸棚生產力及脫氮作用的模式研究

Abstract

大陸棚地區中的海洋生產力極高，主要是由於該地區的海表層與海底距離差距不大，易使營養鹽在兩者之間快速循環，再加上河流、大陸邊緣湧升流所帶來的大量額外營養鹽，造成該地區孕育了將近二分之一的全球海洋生產力。另外大陸棚地區在海洋的氮循環中亦扮演了重要的角色，例如: 底棲脫氮作用便易發生於大陸棚上。生物死亡後的碎屑在沉降至海底時，一部分會被再礦化，剩餘的則會被埋藏至沈積物中，而在沈積物中的碎屑被分解的同時，會消耗氧氣，當氧氣被消耗至一定程度時，底棲脫氮作用便會發生，並將硝酸鹽轉化成氮氣，減少海洋系統中生物可利用的氮。故大陸棚的高生產力以及海表層與海底間的短差距，導致有較多有機物被埋藏在該地區並發生大量底棲脫氮作用。 然而在過去冰期與間冰期的循環中，海平面的快速變化影響大陸棚面積甚多，但其生產力以及全球氮循環的變化還未被研究透徹，因此本研究致力於海平面的變化對此兩者的影響。研究中我們使用了區域海洋模擬系統(Regional Oceanic Modeling System, ROMS)進行探討，此系統包含多種營養鹽的循環、數種海洋生物以及兩者之間的相互作用，並選定南中國海作為研究區域，主要是因為該區域是世界上最大的邊緣海之一，且大陸棚與海盆的面積比例相當高。我們進行了兩次模擬，第一個模擬代表現今海洋的狀況，第二個模擬只調降海平面120米而不變動其他参數設定，並進行兩者之間的比較，以得到海平面對生產力以及氮循環的影響。 我們的結果顯示在降低海平面120米的模擬中，因為大陸棚的消失，使海洋生產力(從799 下降至358太克碳每年)以及海洋的底棲脫氮作用(從2.7下降至0.4太克氮每年)都有著顯著的下降。儘管從河流的養分被帶至低海平面的大陸斜坡上，但因大陸斜坡在留置這些養分的效率不佳，再加上大陸邊緣湧升流的強度降低，使得邊緣海洋的生產力在低海平面時急劇下降，而在生產力下降的同時，有機顆粒碳的沉降量也減少，導致底棲脫氮的作用速率降低，這將對全球的氮循環影響甚巨。

Marginal ocean accounts for over 50% of the modern global ocean productivity, which to a large degree rely on the shallow continental shelves. Continental nutrient input in addition to enhanced vertical mixing at the continental break brings in new nutrients to support primary productivity. This is maintained by efficient nutrient recycling from productivity at the surface, remineralization at depth, and mixing back to the surface on the shallow continental shelves. Associated with the productive system, continental shelves are also important for the global nutrient cycle. One example is denitrification, the process that converts nitrate to N2 gas, thus removes biologically available nitrate from the ocean. Denitrification, when occurring in the sediments, is often associated with high organic fluxes, which drives sedimentary anoxia. It is thus considered that sedimentary denitrification (or benthic denitrification) mostly occurs in the continental shelf sediments, where high productivity in the surface ocean and shallow water column result in high organic fluxes into the sediments. Over the recent glacial/interglacial cycles over the Pleistocene, rapid changes in sea level stand across the terminations should have fundamentally altered the marginal ocean, yet the impacts on productivity and global nutrient cycles are not clearly understood. In this study, we intend to use South China Sea (SCS) as a case study, and explore the dynamic of the nutrient cycle and the surface productivity associated with sea level changes that represent present and Ice Age scenario, using Regional Oceanic Modeling System (ROMS), with multiple nutrient cycles and phytoplankton groups. SCS, one of the largest marginal seas in the world and one with the highest shelf/basin area ratio, loses almost all of its shallow shelves in the peak glacial due to sea level drop. Thus, it is an ideal location to understand shelf nutrient dynamics across different climatic regimes. In order to assess the direct influence of sea level changes, all the other forcing, including river input, temperature and wind fields, are maintained between the two simulations. Our results show a significant decrease in the primary production (from 799 to 358 Tg C/y) and benthic denitrification rate (from 2.7 to 0.4 Tg N/y) in the SCS at the low sea level case, associated with the loss of shallow continental shelves. Despite the extension of riverine nutrient input to the continental slope at the low sea level stands, the marginal ocean at low sea level stand lose its efficiency in trapping these nutrients, in addition to weakened nutrient mixing at the present-day continental breaks, both of which may contribute to the significant reduction in marginal ocean productivity. In the meantime, the decrease of benthic denitrification at low sea level stand could have caused a global impact in the marine nitrogen cycle.