東赤道太平洋湧升流強度對所羅門海有孔蟲氮同位素的進動週期特徵之影響

Abstract

西赤道太平洋的表層水缺少硝酸鹽等主要營養鹽，卻具有來自高營養、受到部分同化作用 (partial assimilation) 影響之東赤道太平洋的氮同位素訊號。因赤道東風吹拂，赤道潛流在東赤道太平洋湧升，為表水帶來豐富的營養鹽。而此處由於生物作用僅消耗掉部分硝酸鹽，使得剩餘硝酸鹽氮同位素值 (δ15NNO3) 升高。這一東赤道太平洋的高氮同位素訊號，經過多次的生物提取與再礦化 (remineralization) 之循環，得以被記錄至有機氮中並向外傳送，也一部分造成了西赤道太平洋斜溫層的δ15NNO3高於全球平均。因此我們認為在古氣候變遷中，西赤道太平洋的δ15NNO3變化，主要是反映了東赤道太平洋高δ15NNO3硝酸鹽池的擴張或縮小。 本研究測量了取自所羅門海海洋岩芯MD05-2925 (9.3°S, 151.5°E, 水深1661 m) 的兩種浮游性有孔蟲–– Globigerinoides sacculifer和Globigerinoides ruber ––之有孔蟲氮同位素 (foraminifera-bound δ15N, FB-δ15N)，用以重建過去15.8萬年海洋古營養鹽環境。兩物種間的FB-δ15N相近，落於7‰ 至13‰ 之間。而岩芯頂部的FB-δ15N (9.2‰)，也近似於現今所羅門海表層水的主要硝酸鹽來源，即次表層水的δ15NNO3 (9.5‰)。因為氮同位素值在兩種有孔蟲間、有孔蟲與主要硝酸鹽來源間，都存在著相似性，使FB-δ15N作為重建過去海水δ15NNO3的指標 (proxy)，有著極高的可信度。FB-δ15N呈現進動週期特徵，在赤道高夏/秋日照量時期，同位素值上升。雖然沒有明顯的冰期/間冰期差異，FB-δ15N在紀錄中的兩次冰消期期間，都先下降至極小值(~8.3‰)，並隨後急遽上升。 FB-δ15N強烈的進動週期特徵，加上不明顯的冰期/間冰期變化，指示了低緯區變化具主導地位，同時，來自東太平洋最小含氧帶 (oxygen minimum zone) 的影響則十分微弱。現生水文資料與模擬顯示，所羅門海懸浮態顆粒氮的高同位素值，來自源於東赤道太平洋表層硝酸鹽池的有機氮之生成與再礦化。因此，我們認為所羅門海FB-δ15N的變化反映了東赤道太平洋表層營養環境的改變。例如：東赤道太平洋高δ15NNO3硝酸鹽池的擴張，造成了所羅門海的高FB-δ15N。於此同時，東赤道太平洋高δ15NNO3硝酸鹽池的大小，亦受控於當地風力驅動之湧升流的強弱，以及區域生產力的高低。連同東赤道太平洋古生產力的重建，我們的FB-δ15N強烈指示了在赤道高夏/秋日照量極大值時期，東赤道太平洋湧升流增強。此結果與前人的模擬研究相符，呈現出具進動週期特徵的長週期聖嬰—南方振盪現象 (ENSO) 演變。

The surface water in the Western Equatorial Pacific (WEP), although is depleted with nitrate and other major nutrients, bears isotopic signature of partial nitrate assimilation in the nutrient-rich Eastern Equatorial Pacific (EEP). Driven by the easterlies, Equatorial Under Current upwells to the surface of the EEP, where biological uptake only partially consumes nitrate and leaves the remaining nitrate enriched in its nitrogen isotopic composition. Through multiple cycles of uptake and remineralization, the elevated nitrogen isotopic composition is recorded by organic matter and is spread out to regions away from the EEP, partly causing the WEP thermocline nitrate δ15N (δ15NNO3) to be much more elevated than the global mean. We then expect the δ15NNO3 in the WEP to be sensitive to the expansion and shrinking of the EEP high δ15NNO3 region during past climatic cycles. This study reports foraminifera-bound δ15N (FB-δ15N) of two planktonic species – Globigerinoides sacculifer and Globigerinoides ruber – from marine sediment core MD05-2925 (9.3°S, 151.5°E, water depth 1661 m) in the Solomon Sea over the last 158 ka for paleo-nutrient-environment reconstruction. The FB-δ15N values, similar between the two species, fall between 7‰ to 13‰. The core-top FB-δ15N (9.2‰) is approximately equal to the modern subsurface δ15NNO3 (9.5‰), the dominate nitrate source to the Solomon Sea surface. The similarity between two species and with nitrate source indicates the high credibility of our proxy to represent past δ15NNO3 dynamics. The FB-δ15N values are significantly elevated during periods with high equatorial Summer/Fall insolation with a strong precessional pacing. While without a clear glacial/interglacial difference, at both terminations, the FB-δ15N hits lowest values (~8.3‰) before an abrupt increase. The lack of clear glacial/interglacial change, yet a strong precessional pacing in the Solomon Sea FB-δ15N record, implies a low latitude control and an insignificant influence from oxygen minimum zones in the eastern tropical Pacific. As modern hydrologic data and modeling exercises demonstrate that production and remineralization of organic matters produced from the remaining surface nitrate pool in the EEP explains the elevation in the Solomon Sea suspended particulate nitrogen, we argue that the variations in our FB-δ15N record reflect changes in the EEP surface nutrient status, such that expansions of the high δ15N nitrate pool cause the observed high FB-δ15N values. The expansion and shrinking of the EEP high δ15N nitrate pool are in turn controlled by wind-driven upwelling and biological productivity in the EEP. Together with reconstruction in the EEP paleoproductivity, our record strongly indicates enhanced EEP upwelling during periods with equatorial insolation maximum, consistent with previous modelling studies showing a strong precessional control on ENSO evolution.