Middle Eocene to late Oligocene calcareous nannofossils of ODP Hole 120-748B

A major deterioration in global climate occurred through the Eocene-Oligocene time interval, characterized by long-term cooling in both terrestrial and marine environments. During this long-term cooling trend, however, recent studies have documented several short-lived warming and cooling phases. In order to further investigate high-latitude climate during these events, we developed a high-resolution calcareous nannofossil record from ODP Site 748 Hole B for the interval spanning the late middle Eocene to the late Oligocene (~42 to 26 Ma). The primary goals of this study were to construct a detailed biostratigraphic record and to use nannofossil assemblage variations to interpret short-term changes in surface-water temperature and nutrient conditions. The principal nannofossil assemblage variations are identified using a temperate-warm-water taxa index (Twwt), from which three warming and five cooling events are identified within the middle Eocene to the earliest Oligocene interval. Among these climatic trends, the cooling event at ~39 Ma (Cooling Event B) is recorded here for the first time. Variations in fine-fraction d18O values at Site 748 are associated with changes in the Twwt index, supporting the idea that significant short-term variability in surface-water conditions occurred in the Kerguelen Plateau area during the middle and late Eocene. Furthermore, ODP Site 748 calcareous nannofossil paleoecology confirms the utility of these microfossils for biostratigraphic, paleoclimatic, and paleoceanographic reconstructions at Southern Ocean sites during the Paleogene.始新世-渐新世（Eocene-Oligocene）时期发生了全球性气候的重大恶化，其特征为陆地与海洋环境均出现长期降温趋势。然而在这一长期降温进程中，近年研究已记录到多幕短暂的增温和降温阶段。为进一步探究上述事件期间的高纬度气候特征，我们基于大洋钻探计划（Ocean Drilling Program, ODP）748站位B孔，构建了跨越中始新世晚期至渐新世晚期（约4200万至2600万年前）的高分辨率钙质超微化石（calcareous nannofossil）记录。本研究的核心目标为构建精细的生物地层记录，并利用超微化石组合的变化特征，解析表层海水温度与营养盐条件的短期演变。通过温暖水类群指数（Twwt）识别主要的超微化石组合变化，我们在中始新世至渐新世早期地层中识别出3次增温事件与5次降温事件。其中，约39 Ma处的降温事件（降温事件B，Cooling Event B）为本次研究首次记录。该站位748的细组分氧同位素δ¹⁸O值变化与Twwt指数呈现显著关联，这一结果佐证了凯尔盖朗高原（Kerguelen Plateau）区域在中、晚始新世期间，表层海水环境存在显著的短期波动。此外，对大洋钻探计划748站位钙质超微化石的古生态分析证实，这类微体化石可有效应用于古近纪（Paleogene）南大洋海域的生物地层、古气候与古海洋重建工作。