Neogene and Quaternary paleoenvironmental history at DSDP Leg 71 Holes

The Neogene and Quaternary sedimentary record of Leg 71 and previously drilled sequences from the Southern Ocean reveal evidence of a major late Miocene change of oceanic and glacial conditions in the southern high latitudes during paleomagnetic Chron 9. The characteristics of late Miocene sedimentation and in particular the study of erosional patterns and ice-rafted debris suggest the following conclusions.
1) In the late Miocene, the Polar Front first migrated to the northern latitudes of the Southern Ocean and surface water temperatures became similar to those of today.
2) Extensive ice shelves or ice tongues were not present along the Antarctic margin until late Chron 9 (about 9.0 Ma).
3) Before Chron 9, West Antarctica was occupied by an archipelago and the West Antarctic Sea.
4) Extensive ice shelves formed in the West Antarctic region, eventually coalescing and thickening to form the grounded West Antarctic ice sheet by Chron 9.
5) The newly formed West Antarctic ice sheet was probably unstable and frequently became an ungrounded ice shelf, thus accounting for the scarcity of late Miocene ice-rafted debris.
6) Extensive erosion or nondeposition of sediment was probably the result of increased Antarctic Bottom Water (AABW) formation in the West Antarctic region during the initial formation of extensive West Antarctic ice shelves and during periods when the West Antarctic ice sheet was ungrounded.
7) In the Southwest Atlantic, AABW velocity waned during the latest Miocene. During the late Gilbert Chron a major and permanent change occurred in the pattern of ice-rafting to the South Atlantic, and after 4.35 Ma the increased IRD accumulation rate and frequency of major episodes of IRD accumulation suggest increased stability of the West Antarctic ice sheet.
In addition, radiolarian faunas of Hole 514 record at least eight migrations of the Polar Front to the north of the site during the past 4.07 m.y. An apparent increase in the frequency of Polar Front migrations occurred about 2.7-2.6 Ma, possibly in response to oceanic change induced by fluctuations in glacial conditions of the Northern Hemisphere.

南大洋第71航次（Leg 71）及此前已钻探岩芯的新近纪（Neogene）与第四纪（Quaternary）沉积记录，揭示了古地磁 Chron 9（paleomagnetic Chron 9）时期，南极高纬度海域的海洋与冰川环境发生了重大晚中新世（late Miocene）转变的证据。晚中新世沉积特征，尤其是对侵蚀模式与冰筏碎屑（ice-rafted debris）的研究，得出了如下结论：
1) 晚中新世时期，极锋（Polar Front）首次向南大洋中高纬以北区域迁移，表层海水温度与现代相近。
2) 直至古地磁 Chron 9 晚期（约9.0百万年前），南极大陆边缘才出现大规模冰架（ice shelf）或冰舌（ice tongue）。
3) 古地磁 Chron 9 之前，西南极地区为群岛与西南极海所覆盖。
4) 西南极地区形成了大规模冰架，最终在古地磁 Chron 9 时期通过冰架合并与增厚，形成了接地的西南极冰盖（West Antarctic ice sheet）。
5) 新形成的西南极冰盖大概率处于不稳定状态，频繁退化为非接地冰架，这也解释了晚中新世冰筏碎屑较为稀少的现象。
6) 大规模沉积物侵蚀或无沉积现象，大概率是因为在大规模西南极冰架初始形成阶段，以及西南极冰盖处于非接地状态的时期，西南极地区的南极底层水（Antarctic Bottom Water, AABW）生成作用增强所致。
7) 在西南大西洋海域，晚中新世末期南极底层水的流速有所减缓。在吉尔伯特 Chron （Gilbert Chron）晚期，南大西洋的冰筏沉积模式发生了重大且永久性的改变；而在4.35百万年之后，冰筏碎屑累积速率的提升与大型冰筏碎屑累积事件频次的增加，表明西南极冰盖的稳定性有所增强。
此外，514钻井（Hole 514）的放射虫（radiolarian）动物群记录了过去407万年中，极锋至少8次向该钻井位点以北迁移的事件。极锋迁移频次在约2.7~2.6百万年前出现明显提升，这可能是对北半球冰川环境波动所引发的海洋环境变化的响应。