南大洋太平洋區域於晚中新世的古生產力變化

Abstract

海洋沉積物中的碳酸鈣及生物性蛋白石的質量累積速率(mass accumulation rate, MAR)，可以反映受氣候及洋流影響而變化的鈣質和矽質生物的生產力，以及海洋的保存能力。以往的研究發現，於晚中新世期間發生了強烈的氣候及環境變化。例如，於7.6-6.7 Ma的晚中新世碳同位素變化 (Late Miocene Carbon Isotope Shift, LMCIS)中，全球的底棲及浮游有孔蟲碳同位素有-1‰的變化。大約在同一時期，發生了晚中新世冷卻事件(Late Miocene Cooling, LMC)。大約於7至5.4 Ma，全球各地的海表面溫度都下降至接近現今的值。在晚中新世期間，陸地大小的冰帽只存在於南極洲，因此南極冰蓋強迫可能是造成於晚中新世的全球氣候劇烈變化的原因。南極環流(Antarctic Circumpolar Current, ACC)主導了南大洋的海表至深水，並對大氣環流作出響應，在全球的氣候和碳循環中有重要作用。然而，只有少數研究中探討ACC的太平洋區域，限制了我們對這個世界上最大的洋流系統的碳酸鹽和生物源蛋白石埋藏歷史的了解。我們的研究利用南大洋的太平洋區域中部和東部的碳酸鈣及生物性蛋白石埋藏，重建晚中新世的生產力。研究的樣本是來自綜合大洋鑽探計劃 383航次於南大洋太平洋區域的U1541及U1543站點的岩芯。 結果顯示，在8.3至6.0 Ma期間，U1541站點採集的碳酸鈣及生物性蛋白石量幾乎呈相反趨勢。整體而言，U1541於LMCIS開始後，碳酸鈣減少，蛋白石增加。於6.0至5.5 Ma期間，兩者都處於相對低值；而U1543站點則由蛋白石主導，碳酸鈣的埋藏量顯著地低。比較兩個站點的數據，U1541碳酸鈣MAR比U1543顯著地高。 我們推測南大洋的太平洋區域由生產力主導。另外，兩個地點的生產力變化趨勢，可能受到LMC時海表面溫度冷卻所影響，指示ACC的擴張，並影響湧升流的強度。ACC的擴張最後導致了ACC流速的減慢，以及6.0百萬年之後U1541的低生產量。兩個站點的表水營養受到不同的湧升流強度和地理條件所影響，導致兩個站點的生產力的差異。最後，ACC的太平洋區域的生產力與大西洋區域的趨勢相似，可能表示南大洋的生產力是由ACC控制。這些數據也表明結合晚中新世的陸地過程，全球高海洋生產力可能最終導致LMCIS。

Mass accumulation rate (MAR) of calcium carbonate (CaCO3) and biogenic opal in marine sediment reflect the variations in calcareous and siliceous productivity, and preservation of the ocean, influenced by climate and ocean circulations. During the late Miocene, Late Miocene carbon isotope shift (LMCIS), a long-term negative 1‰ shift globally, occurred between 7.6 and 6.7 million years ago (Ma). About the same period, late Miocene cooling (LMC) happened around 7 to 5.4 Ma, which culminated with ocean temperatures dipping to near-modern values. The Antarctic ice sheet forcing might be responsible for drastic changes globally during the late Miocene, as continental-sized ice sheets only exist in Antarctica during this period of time. The Southern Ocean, where the Antarctic Circumpolar Current (ACC) dominates the surface-deep water, and responds to atmospheric circulations, plays an important role in the global climate and carbon cycle. However, only a few investigations look into the Pacific sector of the ACC, limiting our understanding of the productivity histories of this world’s largest zonal current system. Our study reconstructs the biogenic productivity using the sedimentary carbonate and biogenic opal content in the central and eastern Pacific sector of Southern Ocean. Major samples are from sites U1541 and U1543 obtained during the International Ocean Discovery Program (IODP) Expedition 383. Our results show that at site U1541 (central Pacific sector of the ACC), CaCO3 and opal fluxes sustain a negative relationship between 8.3 to 6.0 Ma. Overall, the CaCO3 flux decreases while the opal increases during the onset of LMCIS. Both CaCO3 and opal fluxes remain low during 6.0 to 5.5 Ma. At site U1543 (east Pacific sector of the ACC), opal burial dominates, while CaCO3 flux is significantly low throughout the period. Comparing the data in the two sites, site U1541 has a significantly higher CaCO3 flux and relatively lower opal flux than U1543. We speculate that the Pacific sector of Southern Ocean is productivity-dominated. Overall, the increasing opal in both sites might be the consequence of sea surface temperature cooling during the LMC, and indicate the ACC expansion, which affects the strength of upwelling. This eventually leads to the slowing of ACC and the low burial fluxes at site U1541 after 6.0 Ma. The regional productivity differences between the two sites may be due to the different nutrient availability caused by upwelling strength and spatial differences. On the other hand, the productivity changes in the Pacific sector of the ACC coincides with the Atlantic sector, which may indicate that the Southern Ocean productivity is mainly controlled by the ACC. This also indicates that LMCIS may be related to the high global primary productivity, with the influence of the terrestrial processes.