마지막 빙하기 이후 중앙적도태평양의 고해양학적 진화

Abstract

To understand the oceanographic evolution of central equatorial Pacific since the last glacial maximum, planktic foraminifera records from a sediment core MC 931 (6°40′N, 177°28′W) were obtained including their faunal assemblages and the stable oxygen isotope (δ18O) and Mg/Ca compositions of three species Globigerinoides sacculifer, Pulleniatina obliquiloculata, and Globorotalia tumida that dwell in the mixed layer, upper thermocline, and lower thermocline, respectively. The upper thermocline temperature increased by 3°C during the last deglaciation (18-12 ka) with a simultaneous decrease of δ18O while the mixed layer and lower thermocline temperatures varied within a narrow range from 18 ka onwards. Such changes can be explained by an enhanced mixing of the upper ocean and/or a reduced habitat depth separation between P. obliquiloculata and G. sacculifer during 18-12 ka, the last deglaciation. The faunal assemblage of planktic foraminifera during the last deglaciation resembles the modern assemblages at subtropical central Pacific sites where the strong trade winds prevail. We suggest that the study site, presently under the strong influence of the Intertropical Convergence Zone (ITCZ)-North Equatorial Countercurrent, had been influenced by the northeasterly Trades and NEC during the 18-12 ka interval. This interpretation is consistent with previous documentation of a more southerly location of the ITCZ durie stable oxygen isotope (δ18O) and Mg/Ca compositions of three species Globigerinoides sacculifer, Pulleniatina obliquiloculata, and Globorotalia tumida that dwell in the mixed layer, upper thermocline, and lower thermocline, respectively. The upper thermocline temperature increased by 3°C during the last deglaciation (18-12 ka) with a simultaneous decrease of δ18O while the mixed layer and lower thermocline temperatures varied within a narrow range from 18 ka onwards. Such changes can be explained by an enhanced mixing of the upper ocean and/or a reduced habitat depth separation between P. obliquiloculata and G. sacculifer during 18-12 ka, the last deglaciation. The faunal assemblage of planktic foraminifera during the last deglaciation resembles the modern assemblages at subtropical central Pacific sites where the strong trade winds prevail. We suggest that the study site, presently under the strong influence of the Intertropical Convergence Zone (ITCZ)-North Equatorial Countercurrent, had been influenced by the northeasterly Trades and NEC during the 18-12 ka interval. This interpretation is consistent with previous documentation of a more southerly location of the ITCZ duri