西北太平洋過去十八萬年來深層海水的環流變化

Abstract

底棲性有孔蟲殼體的δ13C數值顯示現今深層海水的最小值(-0.2‰)於北太平洋附近地區，但在上次冰盛期(Last Glacial Maximum, LGM)時，全球大洋的最小值(-0.9‰)卻出現在南大西洋深水層。前人研究認為係因為冰期時北太平洋有額外注入的低營養鹽水團所致 [Curry et al., 1988; Duplessy et al., 1988; Herguera et al., 1992; Keigwin, 1998]，但Toggweiler et al. [2006]則認為在冰期時太平洋深層海水主要仍受北大西洋源區來的水團影響，北太平洋注入的水團影響有限。多年來對於影響大洋深層水的δ13C最小值遷移的因素眾說紛紜。 在西北太平洋地區，有孔蟲碳酸鈣殼體不易良好保存，鑽井有限，迄今對於古深部洋流變化的相關研究年限不超過3.5萬年前，雖在Shatsky Rise海底高原地區有部分岩芯有孔蟲氧碳同位素數據可達約18萬年前，但其討論仍著重在表層黑潮及親潮水團的變化，較少論及深層水團(>2000公尺)。本研究期望透過位於西北太平洋的海底高原Shatsky Rise所鑽取的岩芯ODP 1210A(32o13’N, 158o15’E；水深2573.6 m)，藉由分析岩芯中底棲性有孔蟲Uvigerina peregrina (300-425 μm)氧碳同位素數值，討論18萬年以來西北太平洋深層海水的變化。 從底棲性有孔蟲的氧碳同位素結果比較中得知Shatsky Rise海底高原上的三根岩芯ODP 1210A、NGC102、S-2 (岩芯水深分別約為2574公尺、2612公尺和3017公尺) ，在過去18萬年來均位於相同的水團中──繞極深層水(Circumpolar Deep Water, CDW)。進一步將所整理的八根西太平洋岩芯底棲性有孔蟲δ13C數值相比較，可知西太平洋地區約2500公尺以下的深層海水，在過去18萬年來冰期-間冰期主要皆受從大西洋而來的繞極深層水（CDW）影響，該水團注入太平洋海盆後成為深層西部邊界流(Deep Western Boundary Current, DWBC)往北運輸；從西南太平洋與西北太平洋岩芯的碳同位素數據比較中可見，在冰期時西北太平洋的δ13C數值並沒有高於西南太平洋的δ13C數值，顯示出冰期時2500公尺以下的深層海水並沒有如前人研究中所說的受到冰期北太平洋中層水的影響。 本研究認為西太平洋位於2500-4000公尺深層海水的δ13C數值係由60%的NADW與40%的AABW水團混合，並加上再礦化常數的海水訊號[Lisiecki, 2010]而成，水團從西南往西北太平洋輸送過程中，有機物不斷分解，故須加上碳的再礦化常數，δ13C數值會逐漸變負，西赤道及西北太平洋地區的再礦化常數約為-0.5‰，而西南太平洋地區的再礦化常數為-0.2‰；另一方面，冰期時從西南太平洋注入的深層海水速度較快，使得西南太平洋與西赤道/西北太平洋地區的δ13C數值差異較間冰期時來得小。 總結來說，本研究認為過去18萬年以來，太平洋深層海水主要是受大西洋來源水團的影響，故可排除冰期時北太平洋因有額外注入的低營養鹽水團，使得大洋深層水的δ13C最小值遷移的因素。

At present the deep ocean carbon isotope minima (-0.2‰) is in the North Pacific, whereas during the Last Glacial Maximum (LGM) the minima (-0.9‰) was in the South Atlantic. Several previous studies pointed out that this shift was caused by additional freshly-formed waters in the North Pacific [Curry et al., 1988; Duplessy et al., 1988; Herguera et al., 1992; Keigwin, 1998], but Toggweiler et al. [2006] suggested the Pacific deep water was affected mainly by the deep water source from North Atlantic. In northwestern Pacific, the previous studies covered only the history of deep-ocean circulation patterns within the past 35 kyr, although a few records on the Shatsky Rise discussed surface water mass circulations over the past 180 kyr. To extend our understanding of the northwestern Pacific deep water mass circulation, we analyzed carbon and oxygen isotopes (δ13C and δ18O) of benthic foraminifera Uvigerina peregrina (300-425 μm) of Hole ODP 1210A (32o13.4’N, 158o15.6’E; water depth 2573.6 m) cored from Shatsky Rise. An age model for the past 180 kyr was established by 14C dating data and U. peregrina δ18O curve correlated to the LR04 global benthic foraminifera δ18O stack [Lisiecki and Raymo, 2005]. The results of benthic foraminifera δ18O and δ13C of three cores (ODP 1210A、NGC102、S-2) on Shatsky Rise showed they have been in the same water mass, Circumpolar Deep Water (CDW), during the past 180 kyr. We compared eight δ13C records of the western Pacific and found they have similar δ13C patterns during the past 180 kyr, implying that the deep water below 2500 m in the western Pacific were influenced by the Deep Western Boundary Current (DWBC). Because the δ13C records of northwestern Pacific did not show higher δ13C values than that in the southwestern Pacific during the glacial periods, we consider that the Glacial North Atlantic Intermediate Water (GNAIW) did not influence the deep waters below 2500 m in the northwestern Pacific. Our study point out that the δ13C records of 2500-4000 m deep water in western Pacific was composed by 60% NADW and 40% AABW modified with a reminerlization constant [Lisiecki, 2010]. As the water mass was transported from the southwest to northwest Pacific, more organic matters were decomposed and the δ13C of seawater decreased. Due to this remineralization, the adjusting constant in the formula for the western equatorial and northwestern Pacific δ13C is -0.5‰, and for the southwestern Pacific is -0.2‰. Besides, the transportation of deep waters from the southwestern Pacific into the Pacific Basin was faster in glacial times than in the interglacial times. This increase of flow rate of the deep water resulted in a smaller difference between the δ13C of the western equatorial/northwestern Pacific and the δ13C of the southwestern Pacific during the glacial times. In summary, we consider the Pacific deep water was affected mainly by the deep water sources from Atlantic during the past 180 kyr. The shift of the deep ocean δ13C minima was not caused by additional freshly-formed waters in the North Pacific.