Distribution of calcareous nannofossils in the lower sequence of the CIROS-1 sediment core (Table 2)@en

Semiquantitative data of calcareous nannofossil species abundance were collected for 31 samples from 380.00 to 696.61 m below sea floor (mbsf) in the CIROS-1 core from the western McMurdo Sound (Ross Sea). This core has provided the most complete record of Paleogene glacial history of Antarctica. Only half of the samples yielded calcareous nannofossils and species diversity is generally low. However, several samples contain diverse assemblages and nannofossil zonal markers. Isthmolithus recurvus was found between 406.57 and 681.16 mbsf. This species has an age range of ~35-39 Ma in the mid to high latitudes, as calibrated previously by magnetostratigraphy at a number of DSDP/ODP sites. The interval from 406.57 to 681.16 mbsf can be assigned to the late Eocene-earliest Oligocene (~35-39 Ma), and the sample at 391.85 mbsf, which still contains Reticulofenestra hillae and Reticulofenestra umbilica but does not contain I. recurvus, is identified as early Oligocene age (~33-34 Ma). This represents an age refinement of the previous nannofossil biostratigraphy, where the interval from 385.77 to 690.40 mbsf was dated as middle Eocene-early Oligocene. Comparison of the nannofossil assemblages in the CIROS-1 core with those in a similar glaciomarine sequence recovered in Prydz Bay (East Antarctica) and those at deep-sea ODP Sites 738 and 744, where lowermost Oligocene ice-rafted debris (IRD) were found, suggests that the nannofossils in the CIROS-1 core samples examined are in situ. The semiquantitative nannofossil data also suggest that Isthmolithus recurvus and Reticulofenestra davies¸ are most tolerant of cold waters within the late Eocene-early Oligocene nannofloras.

针对采自罗斯海西部麦克默多湾CIROS-1钻孔岩心、水深范围380.00至696.61米深海下（meters below sea floor, mbsf）的31个样品，研究人员获取了其钙质超微化石（calcareous nannofossil）物种丰度的半定量数据。该岩心是目前获取的南极古近纪冰川历史最完整的沉积记录。仅半数样品产出钙质超微化石，且物种多样性普遍偏低。但部分样品含有多样的化石组合以及钙质超微化石分带标志种。狭桥石（Isthmolithus recurvus）产于406.57至681.16米深海下层段；该种在中高纬度地区的产出时代范围约为35~39 Ma，此前已通过多个深海钻探计划（Deep Sea Drilling Project, DSDP）/大洋钻探计划（Ocean Drilling Program, ODP）钻孔的磁地层学工作完成时代校准。406.57至681.16米深海下层段可划归为始新世晚期-渐新世早期（~35~39 Ma）；而391.85米深海下层位的样品中仍产出希拉网面球石（Reticulofenestra hillae）和脐孔网面球石（Reticulofenestra umbilica），但未发现狭桥石，其时代被鉴定为渐新世早期（~33~34 Ma）。这一结果修正了此前的钙质超微化石生物地层学年代框架：原研究将385.77至690.40米深海下层段的时代定为始新世中期-渐新世早期。将CIROS-1岩心的钙质超微化石组合，与东南极普里兹湾获取的相似冰海相沉积序列中的组合，以及产出渐新世最早期冰筏碎屑（ice-rafted debris, IRD）的深海ODP 738和744钻孔的组合进行对比后可知，本次研究的CIROS-1岩心样品中的钙质超微化石均为原位沉积。半定量钙质超微化石数据还显示，在始新世晚期-渐新世早期的超微化石植物群中，狭桥石（Isthmolithus recurvus）和戴维斯网面球石（Reticulofenestra davies）对冷水环境的耐受性最强。