Middle Eocene-lower Miocene calcareous nannofossil magnetobiochronology of ODP Holes 699A and 703A in the subantarctic South Atlantic

The distribution of calcareous nannofossils is documented for the middle Eocene through lowermost Miocene cores from Ocean Drilling Program Holes 699A and 703A in the subantarctic South Atlantic. The detailed nannofossil biostratigraphies established, in combination with published magnetostratigraphic data, have provided a fairly detailed age model for each hole. This study suggests that the middle Eocene through lowermost Miocene section from Hole 699A is virtually complete. A major hiatus has been identified in Hole 703A in the earliest Oligocene, coincident with n abrupt cooling in the Southern Ocean. Comparison of the nannofossil datum ages calibrated with magnetostratigraphy in the two holes with those from mid and southern high latitudes demonstrates synchroneity or diachroneity for the following nannofossil datums: (1) The last occurrence (LO) of Reticulofenestra bisecta is a consistent and reliable biostratigraphic marker for the Oligocene/Miocene boundary from mid- to high latitudes but not in extreme high latitudes; (2) similarly, the LO of Chiasmolithus altus has a consistent age of about 26.8 Ma in the Southern Ocean except in the extreme high latitudes where the datum appears to be substantially younger; (3) the LO of Reticulofenestra umbilica is about 32.9 Ma in the Southern Ocean; (4) the LO of Isthmolithus recurvus is reliable and consistent from mid through high latitudes and correlates with the lower part of Subchron C12R (~34.4 Ma); (5) the LO of Reticulofenestra oamaruensis has a consistent age of 36.0 Ma at all four Southern Ocean sites that have yielded a lower Oligocene magnetostratigraphy; (6) the first occurrence (FO) of R. oamaruensis is at 38.4 Ma in the Southern Ocean; and (7) the FO of I. recurvus shows some age variations from mid to high latitudes and the age range is 38.5-39.0 Ma at the five Southern Ocean sites.

本数据集记录了南大西洋亚极地海区大洋钻探计划（Ocean Drilling Program）699A孔与703A孔中，始新世中期至中新世最早期岩芯的钙质超微化石（calcareous nannofossils）分布情况。研究建立了详细的超微化石生物地层序列，结合已发表的磁地层数据，为两个钻孔各自构建了较为精细的年代模型。本研究表明，699A孔的始新世中期至中新世最早期地层近乎完整；而703A孔在渐新世早期存在一处显著沉积间断，该间断与南大洋的一次突发性降温事件相吻合。将两个钻孔中经磁地层校准的超微化石标志层年代，与中、南高纬度地区的同类数据进行对比，可明确以下7类超微化石标志层的等时性或穿时性特征：1. 网状钙球（Reticulofenestra bisecta）的末次出现（last occurrence, LO）可作为中至高纬度地区渐新世/中新世边界的可靠且一致的生物地层标志，但在极端高纬度海区并不适用；2. 高刺钙盘石（Chiasmolithus altus）的末次出现年龄在南大洋大部分区域约为26.8 Ma，但在极端高纬度海区该标志层的出现时间明显偏晚；3. 脐网钙球（Reticulofenestra umbilica）的末次出现年龄在南大洋约为32.9 Ma；4. 等片石（Isthmolithus recurvus）的末次出现在中至高纬度海区均为可靠且一致的标志层，其年代与亚磁极性时Subchron C12R的下部（约34.4 Ma）相对应；5. 奥马鲁网状钙球（Reticulofenestra oamaruensis）的末次出现年龄在所有4个产出渐新世早期磁地层数据的南大洋站位中均为36.0 Ma，一致性良好；6. 奥马鲁网状钙球的首现（first occurrence, FO）在南大洋的年龄为38.4 Ma；7. 等片石的首现年龄在中至高纬度海区存在一定差异，在5个南大洋站位中的年龄范围为38.5~39.0 Ma。