以浮游性有孔蟲群落與地球化學代用指標重建中部南太平洋在330至240萬年前的古海洋環境變化

Abstract

在過去幾十年內，由於工業化社會的不斷發展，全球大氣中二氧化碳濃度顯著提升，並且對地球氣候系統造成了深遠的影響。除了作為溫室氣體對全球顯著升溫的影響以外，二氧化碳濃度的變化也與全球碳循環的調節息息相關。因此，了解地球系統與全球碳循環的調節是急迫且重要的議題。根據調查，南大洋地區為現今人類活動產生的二氧化碳的主要匯集之處，與全球氣候系統的變化密切相關。南極地區受西風帶控制的南極繞極流（Antarctic Circumpolar Current, ACC），與全球經向翻轉流（Global Meridional Overturning Circulation, GMOC） 共同連接全球各大洋盆，將熱量、物質與海水中儲存的二氧化碳傳輸至不同緯度與深度的海洋中。因而，探討地質時期南極繞極流的演變，對於理解未來氣候系統中海氣互動與全球碳循環的潛在變化具有關鍵意義。 為了評估南極繞極流的上層水體對氣候系統變化的反應以及在氣候變遷時其針對水團與碳循環的調節，我在研究中選擇分析國際大洋發現計劃（InternationalOcean Discovery Program） 在太平洋副南極區所採的沉積物岩芯樣本。研究所使用的樣本年代範圍橫跨了上新世（Pliocene） 中期至更新世（Pleistocene） 早期，在這段時期全球氣候系統在北半球冰蓋擴張的影響下經歷了由暖變冷的顯著變化。此事件目前被認為主要透過巴拿馬地峽關閉影響GMOC 變化、全球二氧化碳下降以及北半球水氣增加，進而導致北半球陸地冰蓋逐漸形成。但針對此事件中大氣二氧化碳如何被海洋儲存，以及南大洋在此事件中的角色卻仍缺乏相關研究。 我利用浮游性有孔蟲生物群落分析以及針對浮游性有孔蟲與沉積物樣本的地球化學分析來探討該區上層水團與碳酸鈣系統的變化。本研究中主要的浮游性有孔蟲可以分為以下幾種環境指示種：Neogloboquadrina pachyderma （冷水環境）、Globoconella spp. （溫躍層；G. puncticulata 與G. inflata）、以及Globigerina bulloides （湧升流及富集營養鹽）。 在本研究中我觀察到溫躍層指示種的豐度在過去330 至240 萬年間共下降了近90％，取而代之的則是冷水指示種N. pachyderma（增加了85％）。浮游性有孔蟲生物群落分析結果指示了海水分層被破壞以及冷水團增強的證據。而地球化學分析的結果則透過有孔蟲δ18O 的增加、Mg/Ca（有孔蟲所居棲之水體溫度）的降低、以及次表層與表層δ13C 梯度的增加指示了海冰擴張、冷水團形成、以及湧升的富含12C 的深層水所帶來的影響在過去330–240 萬年間有所提升。而綜合副南極區主導時期（270 萬年前） 有孔蟲破片比的下降與當時碳酸鈣產量的大量增加而言，顯示了此時深海的碳酸鈣保存能力上升。 除了長期變化的趨勢外，本研究的生物群落分析、地球化學分析、以及沉積物元素分析結果都在260 萬年前過後顯示了隨著4 萬1 千年週期冰期–間冰期的變化。而在這段期間地球化學的分析結果更是透過大幅度的冰期–間冰期變化指示了中部南太平洋的水團性質開始由全球冰期–間冰期旋迴所主導。 本研究結果指出中部南太平洋副極區的上層海水結構在經歷了由全球氣候持續的冷卻以及南極海冰擴張的影響所主導的南大洋鋒面北移後，由明顯層化（well-stratified） 的副熱帶型態轉變成去分層化（destratified） 的副南極型態。分層結構被破壞的上層海水以及顯著增加的表層海水初級生產力使大氣中更多的二氧化碳得以被光合作用與下沉之水團捕捉並且傳輸至海洋內部及海水更深層。除此之外，提升的碳酸鈣保存能力以及下沉之水團的產率增強使海洋對於二氧化碳的儲存能力以及深海的碳酸鈣封存能力也有所提升。而這些南大洋的變化則直接或間接地為接下來的更新世全球冷卻趨勢提供貢獻。

In recent years, Earth’s climate system has been profoundly affected by rising atmospheric CO2 due to industrial activities. The global elevated CO2 levels not only drive temperature rise via the greenhouse effect but also alter the global carbon cycle. Therefore, understanding how Earth responds to CO2 fluctuations is important and urgent for assessing climate system dynamics and carbon cycle regulation. According to the investigations, the Southern Ocean (SO) is highly correlated with the variations of the global climate system as it is a major anthropogenic CO2 sink. The Antarctic Circumpolar Current (ACC) and the Global Meridional Overturning Circulation connect the water basins and transport heat and the ventilated CO2 over the ocean interior. Hence, interpreting the variations of the ACC during geological periods is crucial to evaluating the changes in the ocean, atmosphere, and the global carbon cycle of the future climate system. In order to evaluate how the upper SO of the ACC responds to climate changes and generates the water masses and the carbon cycle, I analyze the sediment core samples from the International Ocean Discovery Program (IODP) Expedition 383 Site U1541 at the central South Pacific during the warm-to-cold Plio-Pleistocene transition (3.30-2.40 Ma) dominated by the Northern Hemisphere Glaciation (NHG) using planktonic foraminiferal assemblages and geochemical proxies. Key planktonic foraminiferal groups include Neogloboquadrina pachyderma (cold-water indicator), Globoconella spp. (thermocline indicators, G. puncticulata and G. inflata), and Globigerina bulloides (nutrientenrichment indicator). Through the closure of the Panama Gateway and its link to the GMOC variations, the NHG is mainly considered to drive the global decline of the CO2 and the gradual growth of the Northern Hemisphere continental ice sheet due to the increase in the water vapour. However, how the CO2 is stored in the ocean and the role of the SO in this event are still lacking research. I find that the thermocline species abundance has exhibited a 90% decrease throughout 3.30-2.40 Ma, while the cold-water indicator N. pachyderma (increasing over 85%) takes place afterward. Planktonic faunal assemblage records reveal evidence of the destratification and the enhanced formation of the cold water masses. The geochemical results indicate the increased influence of the sea ice extension, cold surface water formation, and the upwelled nutrient-rich deep water through the rising δ18O, decreasing Mg/Ca, and the increased δ13C gradient between the subsurface and surface oceans. The decreased fragmentation index indicates that the calcite preservation ability has increased when the research site is dominated by the Subantarctic water after 2.70 Ma, accompanied by increased calcite production. Besides the long-term variations, I also find that the faunal, geochemical, and sedimentary records have exhibited significant variations following the 41-kyr glacial-interglacial (G/IG) periods after 2.6 Ma. The geochemical proxies show large-amplitude fluctuations, suggesting the dominance of the G/IG cycles in the water mass properties during this interval. This research suggests that the upper ocean has transitioned from a well-stratified subtropical pattern into a destratified Subantarctic pattern after the northward migration of the SO frontal system due to the gradual global cooling and Antarctic sea ice extension. The destratified upper ocean and the increased surface productivity allow more CO2 to be captured and transported into the ocean interior. Moreover, the increased calcite preservation ability and the strengthened sinking water mass formation increase the efficiency of CO2 storage and deep ocean calcite sequestration. These variations of the SO likely contribute to the ongoing Pleistocene global cooling trend.