Pleistocene calcareous nannofossil stratigraphy for ODP Leg 177, Atlantic sector of the Southern Ocean

Seven Ocean Drilling Program (ODP) sites recovered during ODP Leg 177 in the Atlantic sector of the Southern Ocean were analyzed to study the Pleistocene calcareous nannofossil record. Calcareous nannofossil events previously described from intermediate and low latitudes were identified and calibrated with available geomagnetic and stable isotope stratigraphic data. In general, Pleistocene southern high latitude calcareous nannofossil events show synchronicity with those observed from warm and temperate latitudes. The first occurrence (FO) of Emiliania huxleyi and the last occurrence (LO) of Pseudoemiliania lacunosa are observed in marine isotope stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed at MIS 5. MIS 6 is characterized by an increase in G. muellerae and MIS 7 features a dramatic decrease in the proportion of Gephyrocapsa caribbeanica. This latter species began to increase its proportions from MIS 14 to 13. The LO of Reticulofenestra asanoi is observed within subchron C1r.1r and the FO of R. asanoi occurs at the top of C1r.2r. A reentry of medium-sized Gephyrocapsa can be identified in some cores during subchron C1r.1n. The LO of large morphotypes of Gephyrocapsa is well correlated through the studied area, and occurs during the middle-low part of subchron C1r.2r,synchronous with other oceanic regions. The FO of Calcidiscus macintyrei and FO of medium-sized Gephyrocapsa occur in the studied area close to 1.6 Ma.

本研究对南大西洋扇区大洋钻探计划（Ocean Drilling Program, ODP）第177航次回收的7个站位样品开展分析，以探究更新世钙质超微化石（calcareous nannofossil）记录。此前在中低纬度海域已报道的钙质超微化石事件，本次研究结合现有地磁地层与稳定同位素地层数据对其进行了识别与校准。总体而言，南半球高纬度海域的更新世钙质超微化石事件与暖温带纬度海域观测到的对应事件具有同步性。赫氏艾密里藻（Emiliania huxleyi）的首次出现（First Occurrence, FO）与中空拟洞球藻（Pseudoemiliania lacunosa）的末次出现（Last Occurrence, LO）分别见于海洋同位素阶段（Marine Isotope Stage, MIS）8和MIS 12。在MIS 5期，穆勒氏桥石藻（Gephyrocapsa muellerae）与赫氏艾密里藻的丰度出现反转。MIS 6期以穆勒氏桥石藻丰度升高为特征，而MIS 7期则表现为加勒比桥石藻（Gephyrocapsa caribbeanica）占比显著降低。该物种的占比自MIS 14至MIS 13期开始回升。网纹球藻（Reticulofenestra asanoi）的末次出现见于亚时（subchron）C1r.1r内部，其首次出现则位于C1r.2r顶界。在亚时（subchron）C1r.1n期的部分岩心中，可识别出中等体型桥石藻的重现现象。大型形态型桥石藻的末次出现在研究区范围内具有良好的地层可对比性，该事件发生于亚时C1r.2r的中下部，与其他大洋区域的该事件同步。麦氏钙盘藻（Calcidiscus macintyrei）的首次出现与中等体型桥石藻的首次出现均出现在研究区距今约1.6 Ma的地层中。