Obliquity pacing of the expansion and contraction of the Western Pacific Warm Pool during the past 740,000 years

根據海洋鑽探計畫（Ocean Drilling Program, ODP）位於所羅門海（Solomon Sea）的1115B岩芯（151o34’E, 9o14’S, 水深1149米）中浮游性有孔蟲Globigerinoides sacculifer鎂鈣元素比（Mg/Ca elemental ratios）分析，我們重建了西南赤道太平洋地區七十四萬年來連續的古海水表面溫度（Paleo-sea surface temperature）記錄。ODP 1115B岩芯位於年平均海表溫28℃的邊緣，為一絕佳監測暖池南北分佈範圍的站位，在此我們重新調整年代模式後經由計算得到了西太平洋暖池南北溫度差的紀錄，將其視為暖池範圍改變的代用指標；並利用前人已發表的赤道太平洋西側與東側的古海表溫度記錄得到太平洋東西側的溫度差。 74萬年來暖池南北溫差與太平洋東西溫差的演變可約略分為四期：（1）74-58萬年、（2）58-32萬年、（3）32-21萬年以及（4）21-0萬年。（1）74-58萬年，此段時期中，暖池南北與太平洋東西之溫度差改變幅度小，但並沒有同步，此時在北大西洋生物生產力上升，全球底棲性以及浮游性有孔蟲碳同位素值皆開始上升，太平洋碳酸鈣沈積速率開始降彽但大西洋開始上升，暗示著一個全球碳儲藏庫的重新分布事件（global carbon reservoir redistribution event）的開始。（2）58-32萬年，暖池南北與太平洋東西側溫差改變幅度增強並且呈現同步的趨勢，暖池南北溫度差出現四萬一千年周期，但是與地軸傾角呈正相關；全球底棲與浮游有孔蟲碳同位素值達到頂峰，並在海洋氧同位素期13之後開始下降，全球性的碳酸鈣溶解事件，大規模的現代珊湖礁群生長，間冰期二氧化碳平均濃度升高，全球冰帽擴張都在這一時段中發生。（3）32-21萬年：此階段早期, 32-25萬年, 仍延續上一期的趨勢，但太平洋東西溫度差開始呈現長期的下降，而暖池南北的溫度差卻開始增加。到此階段晚期, 25-21萬年, 暖池南北的溫度差開始與地軸傾角呈現良好的負相關，而太平洋東西側的溫度差達到最彽並且持續到下一期。（4）21萬年以來暖池南北溫差與南緯三十度與六十度之年平均日照量差，呈現良好的負相關，日照量差越大，暖池南北溫度差越小，反之亦然，並且都具有四萬一千年周期，據此推論，當地球自轉傾角較小時，中緯度與高緯度地區之日照量差增大，致使南半球中緯度海環 (gyre) 環流增強，並導致更多的暖水團增疊至暖池區域，使暖池範圍增大，暖池邊緣地區海表溫升高。在這段時間中，暖池南北溫度差與太平洋東西兩側溫度差並沒有同步改變，根據東西兩側鈣質超微化石群集研究指出，這一段時間內太平洋地區貿易風減弱，導致湧升流減少，東西溫度差變小，與此觀察吻合。中-高緯度日照量的差值變大會增加跨緯度間水氣傳送的強度，過去25萬年來的南極冰芯（Vostok）的重氫過剩值（deuterium excess values, d），與地軸傾角有良好的負相關，暗示與水氣來源區及來源區的海水表面溫度有關。25萬年來的暖池範圍改變，的確與南極冰芯重氫過剩值的紀錄有良好的對比。 由上述的比較，我們認為暖池範圍的確受控於天文軌道力的改變，但氣候系統內部的振盪與調整亦會調整暖池範圍的改變周期；全球碳儲藏庫與暖池大小亦有關連，推測與生物生產力改變，大規模造礁事件，造成海洋中超過冰期-間冰期旋回尺度的長時間碳儲藏庫改變有關。Magnesium/calcium ratios of planktonic foraminifer Globigerinoides. sacculifer in Ocean Drilling Project Leg 180 Core 1115B (ODP 180, 151o34’E, 9o14’S, water depth 1149 m) from the Solomon Sea were used to reconstruct paleo-sea surface temperature (paleo-SST) records in the western Pacific warm pool (WPWP) area over the past 740 kyrs. ODP 1115B locates at the marginal area of the annual 28℃ isotherms, thus severs as an excellent monitor to detect past warm pool extent variations. We can derive center-southern margin SST (ΔSSTC-S) gradient as an indicator of warm pool extent change, and to compare it with previous studies of western and eastern SST gradient (ΔSSTW-E) to yield a more comprehensive understanding the change in past warm pool extent variations. The dynamic history of the WPWP can be subdivided into 4 Intervals: (1) 740-580 ka, (2) 580-320 ka, (3) 320-210 ka, and (4) 210-0 ka. (1) 740-580 ka: ΔSSTW-E and ΔSSTC-S fluctuated with relatively small amplitudes but did not covary at the same pacing. Northern Atlantic surface productivity increased, global benthic and planktonic carbon isotope values increased, and carbonate accumulation rate in eastern Pacific decreased but increased in Atlantic. These changes mark the beginning of a carbon reservoir redistribution event. (2) 580-320 ka: ΔSSTW-E and ΔSSTC-S covaried with each other and show positive correlation with obliquity. Both SST gradients reached the peak values with global benthic and planktonic carbon isotope values at Marine Isotope Stage 13 and gradually decreased then. Building of modern coral reef, strong carbonate dissolution, and establishment of modern interglacial CO2 level all took place at this Interval. (3) 320-210 ka: in the early period of this Interval, 250-320 ka, both SST gradients show similar pattern of the previous interval, following by long-term decreasing trends of both gradients. (4) 210-0 ka: ΔSSTW-E decreased through this interval, in consistent with previous study results of coccolith floral assemblages of the western and eastern equatorial Pacific. ΔSSTC-S shows good negative correlation with obliquity cycles. Annual insolation gradient between 30o and 60oS drives south Pacific gyre circulation. When insolation gradient is large, the enhancement of gyre circulation may be induced which resulting in large-scale warm pool with profound of warm water and a decreasing ΔSSTC-S. The implication is supported by Vostok ice core deuterium excess (d) data, reflecting moisture source changes and SST of source areas.Chapter 1 Introduction 01 Chapter 2 Material and Methods Chapter 3 Results and Discussions 2.1 ODP Site 1115B 07 2.2 Modern Oceanography 12 2.3 Species 13 2.4 Sample preparation 15 2.5 Stable Isotope Analysis 16 2.6 Mg/Ca analysis 18 2.7 Age Model 21 3.1 Stable Isotope and Mg/Ca Ratios Results of ODP 1115B 23 3.2 Seawater oxygen isotopic values 24 3.3 Comparison to Other Equatorial Pacific Records 25 3.4 Orbital Forcing on Warm Pool Extent Variations 33 3.5 Long-term Climate System Oscillation and Warm Pool 41 Chapter 4 Conclusions 48 Reference 50 Appendix a. Data Table 57 b. Principal 1. Planktonic foraminiferal Mg/Ca ratios as SST proxy 6