A study on the disappearance of Globigerinoides fistulosus andts relationship to the hydrological change of the tropical Pacific
Date Issued
2008
Date
2008
Author(s)
Chen, Chen-Yin
Abstract
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.aleo-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 ofV. 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.
Subjects
Western Pacific Warm Pool
oxygen and carbon isotope
Mg/Ca ratios
Reynolds Number
Pliocene/Pleistocene boundary
SDGs
Type
thesis
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