浮游有孔蟲Globigerinoides fistulosus消失 與熱帶太平洋表水水文變化關係

Abstract

中上新世時（2.9 Ma）浮游有孔蟲Globigerinoides fistulosus從Globigerinoides sacculifer分支而出，到1.7 Ma絕滅，其絕滅面被廣泛應用為上新世—更新世界線指準面。前人研究指出東西赤道太平洋表水溫差梯度在1.7 Ma左右明顯變大，在此以西太平洋暖池南緣ODP 1115B岩芯作為材料，並將G. fistulosus視為G. sacculifer的型態變種之一，與G. sacculifer的幾種型態變異（normal form、kummer form與sac-like form）合併定義為｢G. sacculifer complex｣，測量G. sacculifer complex、G. ruber和N. dutertrei殼體中的氧碳同位素和鎂鈣元素比，詴圖推估G. fistulosus造殼深度，重建1.71到1.77 Ma西太平洋暖池表水水體結構，進而探討西熱帶太平洋地區G. fistulosus消失與表層垂直水層結構改變的關係。 以氧碳同位素和鎂鈣元素比對殼體大小分析結果推測，G. fistulosus的生命階段與G. sacculifer一樣受共生藻影響，其特殊型態應屬於G. sacculifer complex的最終階段，且在溫度更低的水層造殼。而西熱帶太平洋地區的垂直水溫結構在1.789到1.728 Ma間有相當大的改變：從厚度較薄的混合層、深度較淺的溫躍層（1.789-1.778 Ma），演變成較厚的混合層與深度較深的溫躍層（1.755-1.728 Ma），顯示西熱帶太平洋地區水文變化不僅限於表海水溫，更包含垂直水溫結構的改變，且逐步演變為類似今日西太平洋暖池的表層水垂直結構。 最後本研究利用雷諾數（Reynolds numbers）來討論G. fistulosus的消失原因，推測在上新世—更新世交界時東西赤道太平洋表水溫差變大，西太平洋暖池區域混合層變厚，垂直水溫梯度變緩且黏滯度變小，G. sacculifer complex為了維持固定的雷諾數，個體發生階段由成態退往幼態，導致個體發生最終階段的G. fistulosus消失。此生物事件指示了西太平洋暖池發育的重要階段，並且成為熱帶海洋地層更新統底界對比的良好指標。

At about 1.7 Ma the west-east sea surface temperature gradient in the equatorial Pacific changed and the thermocline depth was suggested being shallow in the eastern Pacifict. Meanwhile, 1.7 Ma marks the disappearance of Globigerinoides fistulosus, a planktonic foraminifera species descended from Globigerinoides sacculifer by adding finger-like projections on the flattening final chambers in the middle Pliocene (2.9 Ma). This disappearance was apparently related to the change of west-east sea surface temperature gradient in the tropical Pacific. Relationship between the disappearance of G. fistulosus and the thermocline depth fluctuation in western Pacific is proposed here. G. fistulosus was considered to be as a member of the ―G. sacculifer complex‖, which includes 4 morphotypes, normal form, kummer form, sac-like form, and fis form, geochemical proxy records of six different size fractions, were used to investigate the ontogenic characteristics of G. sacculifer complex and how G. fistulosus disppearred at 1.7 Ma. Paleo-sea surface temperature and surface-water hydrography from 1.789 to 1.728 Ma was reconstructed using stable isotope (δ18O, δ13C) and Mg/Ca ratios of G. sacculifer complex, G. ruber (250-355 μm, upper mixed layer dweller), and N. dutertrei (355-425 μm, upper thermocline dweller), collected from ODP Hole 1115B (ODP 180, 151°34‘E, 9°14‘S, Woodlark Rise, water depth 1149 m) at the southern periphery of western Pacific warm pool (WPWP). The δ13C values of G. sacculifer complex show an increasing trend with shell size in all the morphotypes (contributed by vital effect of symbiont). The results of δ18O value and Mg/Ca ratio of G. sacculifer complex suggest that the calcification depth of IV G. fistulosus might be deeper than that of all other morphotypes in the G. sacculifer complex, and the final particular chamber of G. fistulosus is the final stage of ontogenetic stage in G. sacculifer complex. A dramatic drop of Mg/Ca-derived ΔSST(G. ruber - N. dutertrei) from 1.767-1.758 Ma suggests that the difference of temperature between mixed layer and upper thermocline became smaller, and thermocline might become deeper when G. fistulosus became extinct. This finding implies that the hydrological variation of tropical western Pacific during disappearance of G. fisutlosus changed not only on sea surface temperature but also on the vertical water structure.Variation of Reynolds numbers suggests that the decrease in temperature gradient and decrease in viscosity in surface ocean during 1.7 Ma might trigger the paedomorphosis trend of G. sacculifer complex and cause the disapearance of G. fistulosus. This bio-event indicates an important step of evolution in the WPWP, and can be as a good Pliocene/Pleistocene boundary indicator in tropical marine sediments.