黑潮上游區全新世古海洋學研究

Abstract

過去對於黑潮全新世古海洋學的探討，岩心材料多半侷限於沖繩海槽內，且研究方法上倚賴傳統的浮游有孔蟲轉換函數與氧碳同位素，使得過去對於全新世一些古生態與古海洋記錄的解釋有所爭議。本研究分析、比對黑潮上游區（菲律賓海至台灣東部外海）與南沖繩海槽的海洋岩芯記錄，並同時利用傳統（浮游有孔蟲群聚、多種類浮游有孔蟲氧碳同位素）與新近（鎂鈣比）的古海洋指標，企圖釐清沖繩海槽內古海洋訊號的意義，進而從中探索黑潮的歷史。首先透過對全新世中期「普林蟲銳減事件」的研究，了解浮游有孔蟲古生態在黑潮上游區的變化。結果顯示「普林蟲銳減事件」的地理範圍並不限於邊緣海內，而是遍及西北太平洋地區；但是各項古海洋指標均顯示，在此ㄧ大範圍古生態事件發生的同時，並無對應的古水文變化。本研究推測該事件發源於赤道太平洋地區，並對該事件提出族群生態學的可能解釋。此外，浮游有孔蟲群聚組合深受沖繩海槽內複雜的水文作用影響，不適合作為黑潮的指標。本研究進一步利用化學指標分析古海表溫與古海表鹽度的變化，結果顯示南沖繩海槽全新世時期海表溫並無顯著變化，但海表鹽度逐漸降低。此一表水變淡的趨勢與東亞季風的消長趨勢不符，卻與西赤道太平洋的表水鹽度變化一致。因此，本研究推測北赤道洋流—黑潮系統的表層水鹽度在全新世以來逐漸變淡，但不排除太平洋海盆在全新世以來海水逐漸變淡的可能性。總結前文，全新世時期南沖繩海槽內的古海洋變化與赤道太平洋有著海洋遙相關。

I have studied a suite of deep-sea cores (MD01-2403, OR715-21 and MD98-2188) located in the upper reach of the Kuroshio (from the Philippine Sea to off eastern coast of Taiwan) and the Southern Okinawa Trough (SOT) using both traditional methods (foraminiferal assemblages and stable isotopes) and a newly developed paleothermometer (Mg/Ca ratio). The goal of this research is to investigate paleoceanographic relationships among the East Asian monsoon (EAM), Equatorial Pacific (EP) and SOT during the Holocene, and to further unravel the Holocene history of the Kuroshio. During the past decade, paleoceanographers have reported the so-called 'Pulleniatina Minimum Event (PME)' at 4.5-3.0 ka from the Okinawa Trough and South China Sea. The paleoenvironmental and ecological hypotheses for the PME were tested using our data in the upper reach area and the SOT. The three cores all witness the PME, implying this event is widespread in the western North Pacific. The results of Mg/Ca-based SSTs and thermocline hydrography reconstructed by multispecies isotopic analyses reveal nothing anomalous in the paleoceanographic proxies associated with the PME. Relative abundance of Neogloboquadrina dutertrei did not shows reciprocal pattern to that of P. obliquiloculata in the upper reach area. We proposed a biological hypothesis (epidemic mass mortality) originating in the EP that resulted in the PME. This new hypothesis may be testable by approaches in population dynamics. This PME study has two implications for paleoceanography. First, the discrepancy between the Mg/Ca-derived SSTs and fauna-based SSTs during the PME period is discussed. Second, P. obilquiloculata is not a faithful indicator for the Kuroshio in the Holocene epoch. To discuss the Holocene SST front variability in the East China Sea, we present the surface water d18O (d18Osw) record in the SOT (MD01-2403), estimated from d18O of Globigerinoides ruber and Ma/Ca-based SST of Globigerinoides sacculifer. The Mg/Ca-based SSTs are quite stable throughout the Holocene, whereas a long-term decrease in d18Osw of ~0.5 o/oo was found over the past 8,000 yrs, equivalent to a decrease of 1 psu in salinity calculated from the local empirical equation: d18Osw=0.51S-17.28 (R2 = 0.81, n=37). We compare the d18Osw record from the SOT with climatic records of Greenland GISP2 ice core d18O, stalagmite d18O of the Dongge Cave in China, and d18Osw of MD98-2181 in the western terminus of the North Equatorial Current. The decreasing trend in SSS in the SOT is inconsistent with the record of Asian monsoon strength of the South China, but agrees better with the d18Osw record from the western tropical Pacific. Our study indicates that the decrease of d18Osw in the SOT might reflect either the decrease of SSS in the North Equatorial Current-Kuroshio system or to a larger extent, a basin-wide change in salinity of the Pacific Ocean. By examining various paleoceanographic proxies of the upper reach of the Kuroshio, we evaluated previous interpretations and provided new perspectives on the Holocene paleoceanography of the Kuroshio. The extensive distribution of the PME and the freshening trend in the SOT imply an oceanographic teleconnection between the SOT and the EP.