利用多種浮游有孔蟲種屬之氧同位素及鎂鈣古溫度指示過去2.5萬年來黑潮變化

Abstract

黑潮為一強勁的西方邊界流，可將來自低緯度的熱傳輸到中高緯度。因此西北太平洋的區域氣候受到黑潮影響。現代海洋觀測結果顯示次表層/溫躍層的水溫可能比海表溫度更能指示黑潮的變化。然而，過去研究鮮少利用位在沖繩海槽不同緯度的多個站位的次表層溫度記錄來重建過去黑潮的變化。因此，為了克服上述缺點，本篇研究使用從沖繩海槽北部和琉球島弧東部採集的三個岩心中重建過去有孔蟲Pulleniatina obliquiloculata和Neogloboquadrina dutertrei 的δ18O和Mg/Ca來探討過去2.5萬年來沖繩海槽地區的上層水文變化。此外，我們還整合過去已發表的地球化學紀錄以及高解析度模擬的結果來提出並討論過去黑潮在冰期間冰期的變化。我們的溫度重建結果顯示出P. obliquiloculata和N. dutertrei指示的溫度隨時間變化的趨勢有所不同，可能是由於N. dutertrei的棲息深度隨著氣候條件變化而改變所導致。我們的鹽度重建結果顯示冰期間冰期之間沒有明顯的變化，這和模擬結果並不一致，可能是由於此鹽度重建的方法有很大的誤差導致。P. obliquiloculata的Mg/Ca溫度的冰期冷卻程度的空間分布顯示在末次冰盛期間黑潮有向東南方向遷移。但是此現象無法在G. ruber的Mg/Ca海表溫度中觀察到，可能是由於黑潮在表層的變化沒有很顯著又或者是採樣地點的選擇所影響。本研究著重在以多個站位來重建過去黑潮的優勢以及了解代用指標季節性的重要。

The regional climate in the northwest Pacific is greatly influenced by the Kuroshio Current (KC), a strong western boundary current that brings an enormous amount of heat from the Equatorial Pacific to the middle latitudes of the western Pacific. Modern oceanographic data show that the subsurface/thermocline waters may be more indicative of changes in the KC than the sea surface temperature. However, few studies in the literature have utilized subsurface temperature records from multiple sites along the latitudinal range of the Okinawa Trough (OT) to reconstruct past changes in the KC. Therefore, to overcome the aforementioned shortcoming, here I generated Mg/Ca ratios and δ18O data of thermocline-dwelling foraminifera, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, using three sediment cores retrieved from the northern OT and the east of the Ryukyu Arc to reconstruct past hydrographic changes over the past 25 ka. These datasets, together with published geochemical records, as well as high-resolution ocean model output, provide a glacial-interglacial perspective of the KC variations and its link to the regional climate. Our results showed discrepancies in the temperature evolution between P. obliquiloculata and N. dutertrei, which may be due to differences in habitat ecology, as N. dutertrei might have changed its habitat depth range in response to climatic conditions. Our salinity reconstructions show no coherent changes over the past 25 ka, which is different from the model simulation, plausibly due to the large uncertainties in the approach. A southeastward migration of an intensified KC during the LGM was suggested by the regional patterns of P. obliquiloculata Mg/Ca temperatures. However, this scenario is not evident in the sea surface temperature pattern derived from G. ruber, possibly due to the limited surface expression of KC changes and/or the choice of sites. This study highlights the advantage of a multi-site approach as well as the need to consider proxy seasonality in the reconstruction of past KC variability.