西太平洋暖池MD97–2140岩芯120萬至60萬年前之古海洋學研究:有孔蟲殼體穩定碳氧同位素與鎂鈣比值

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2021

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本研究分析西太平洋暖池中心MD97–2140岩芯(2°02 N, 141°45 E,水深2547公尺),岩芯深度1700–2700公分(約120萬年至60萬年前)次表層浮游有孔蟲 Neogloboquadrina dutertrei(355–425 μm)之穩定碳、氧同位素成分與Mg/Ca比值,與三種底棲性有孔蟲Uvigerina spp.、Cibicidoides wuellerstorfi與Pyrgo spp.(>250 μm)之穩定碳、氧同位素成分,以重建中更新世變遷時西赤道太平洋次表層水及深層水的古海洋環境紀錄。 研究結果顯示,N. dutertrei的Mg/Ca比值(1.23–2.52mmol/mol)轉換之次表層水溫介於14.2–22.2 ℃間(N=100),在120萬至90萬年前之間次表水層溫度最高溫與最低溫差約為6 ℃,而90萬至60萬年前之間的最高溫與最低溫差異則增加至8 ℃。而120萬至90萬年前之間,表層水有孔蟲Globigerinoides ruber(de Garidel-Thoron et al., 2005)與次表層水N. dutertrei兩者的Mg/Ca溫度,在MIS 19–15間之溫差為8.5 ℃,相較於MIS 35–28間之溫度差7.7℃為大,可能反映出斜溫層變淺的現象。 N. dutertrei殼體δ18O數值介於 -1.3至0.9 ‰(VPDB)(N=100), MIS16–23(約90萬年前)的冰期段落的δ18O數值比MIS24–36冰期段落的δ18O數值漸增約0.5‰,可能反映了冰川體積增加的趨勢;另MIS23間冰期的殼體δ18O數值較先前之間冰期的殼體δ18O數值大,亦暗示著此間冰期較少冰川融化的現象。而藉由N. dutertrei殼體δ18O數值與Mg/Ca比值所計算的次表層水δ18Osw數值,發現次表層海水δ18Osw於MIS23至MIS16期間,冰期δ18Osw數值出現漸增情況,也暗示陸冰體積可能有增加的現象。於MIS 23之後,次表層水δ18Osw數值有較多大於表層水之δ18Osw數值的狀況,可能源自表水層受到蒸發量減少或降雨增加之大氣狀態改變之影響。上述資料顯示中更新世變遷事件於西太平洋暖池海洋與大氣狀態的變化,而這些海氣變化皆可能是反應出地球冰期–間冰期循環週期自4.1萬年旋回轉變為10萬年旋回的影響。N. dutertrei殼體δ13C數值介於0.6至1.8 ‰之間,在中更新世變遷期間距今90 萬年前左右,與理論斜溫層應有的δ13C數值相對減少,可能支持斜溫層有變淺現象;又或者反映浮游植物以50 萬年消長的低谷期,因此數值較小。底棲性有孔蟲的δ13C數值在中更新世變遷期間變化較無規律性,其中Uvigerina spp.的δ13C數值與位於南太平洋的ODP 1123岩芯的Uvigerina spp.的δ13C數值在距今120–60萬年前間之數值多數相符,顯示此處底水環境反映來源的底水水團繞極深層水之δ13C狀態。
This study analyzes stable carbon and oxygen isotope compositions and Mg/Ca ratios of sub-surface (N = 100) and benthic foraminifera (N = 67) from core MD97–2140 located in Western Pacific Warm Pool (2°02 N, 141°45 E; water depth 2547 m) to resconstruct the paleoceanography during Mid-Pleistocene.Based on the Mg/Ca contents of N. dutertrei (from 1.23–2.52mmol/mol; 355–425 μm), the sub-surface seawater temperatures are estimated between 14.2℃ and 22.2℃. Temperature differences are respectively 6℃ and 8℃ prior to MIS 23 and posterior to MIS 23. According to the G. ruber (de Garidel-Thoron et al., 2005) and N. dutertrei Mg/Ca temperatures, shallowing of thermocline depth is observed in Mid-Pleistocene Transition (MIS 21–15).δ18O values of N. dutertrei shells are between -1.3–0.9 ‰ (VPDB). The overall δ18O values of N. dutertrei shells incerased about 0.5‰ for the glacial stages after 900 ka may indicate increase of ice volume. Adopted the Mg/Ca temperatures, the calculated sub-surface seawater δ18Osw values based on N. dutertrei δ18O values also showing greater δ18Osw values and thus provide further evidence for increament of ice volume after 900 ka. In addition, the δ18O values of MIS 23 was greater than those of interglacial periods prior to MIS23 indicating that less continental ice was melted in MIS 23.Sub-surface water δ18Osw values were higher than those of surface water after MIS 23 might imply that the surface water δ18Osw values were influenced by the weaken evaporation rate or more rainfall at this area. The Western Pacific Warm Pool ocean and atmosphere condition transformed in MPT time interval might be the caused by change in the glacial–interglacial cycle period from 41 kyr to 100 kyr.δ13C values of N. dutertrei shells are between 0.6‰ and 1.8 ‰. The δ13C values after MIS 23 are lower than those before MIS23 may indicate shallowing of thermocline as well. The δ13C records of Uvigerina spp. (>250 μm) of this study have comparable patten and values with those of core ODP 1123 located in southern pacific (Elderfield et al., 2012), implying the bottom water was mainly influence by Circumpolar Deep Water (CDW).

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中更新世變遷, 西太平洋暖池, 有孔蟲穩定碳氧同位素與鎂鈣比值, Mid-Pleistocene Transition (MPT), Western Pacific Warm Pool (WPWP), Foraminiferal carbon and oxygen isotopes and Mg/Ca ratios

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