Mechanisms of glacial/interglacial changes during the late Oligocene

该项目的主要目的是从机制上了解在IODP遗址U1406(纽芬兰近海，IODP考察队第342次)晚渐新世期间导致冰川/间冰期(G/I)循环形态差异的过程。该站点现有的亚轨道分辨率底栖有孔虫氧同位素数据集在~25.6~23.9 Ma的时间间隔揭示了两种G/I旋回的形态。突出的形态是U形(“控制区间”)，在中更新世过渡期(MPT)之前的冰川中广为人知。另一种形态的特征是底栖有孔虫的氧同位素值缓慢增加，然后迅速下降(“目标区间”)。这种模式从根本上不同于其他渐新世冰期旋回的U型，但明显地让人想起晚更新世冰川/间冰期旋回的锯齿状性质。有鉴于此，已经生成了“目标”和“控制”区间的海底有孔虫δ18O和镁/钙记录，并用于约束观测到的G/IG旋回的冰量和海平面波动的幅度和持续时间。对于“目标”间隔和“控制”间隔，BWT都出奇地恒定，取值范围一般在2到4°C之间。更令人惊讶的是，这两个间隔在BWT中都缺乏明确的G/IG循环，这表明在海底Delta18O数据中，BWT波动对观测到的G/IG循环的贡献很小。当海水的δ18O被计算并换算为海平面时，这一点变得更加明显。在两个研究间隔之间观测到的海平面估计的差异表明，南极洲的冰量大幅减少。如果与倾角相比，海平面/冰量在“控制间隔”期间的变化似乎对倾角的反应几乎是线性的。相反，“目标间隔”期间的海平面/冰量变化发生在倾角较弱的时间段。这表明，在研究间隔期间，南极冰量对潜在倾角强迫的潜在非线性阈值响应。然而，这种阈值是否真的存在，以及这种潜在阈值行为的确切气候条件是什么，目前还不能用手头的记录来回答。因此，建议扩展新的站点U1406高分辨率记录，以覆盖26-24.5 mA的时间间隔(即在可用Delta18O记录中以显著的“阶跃”为特征的时间间隔)。在第一个项目阶段成功采用的方法之后，将通过调查底栖有孔虫的镁/钙和稳定同位素来实现这一点。

The principal objective of this project is to obtain a mechanistic understanding of the processes that led to the differences in the morphology of glacial/interglacial (G/I) cycles during the Late Oligocene of IODP Site U1406 (off Newfoundland, IODP Expedition 342). An existing sub-orbital resolution benthic foraminiferal oxygen-isotope dataset from this site for the time interval ~25.6 to 23.9 Ma reveals two morphologies of G/I cycles. The prominent morphology is U-shaped ("control interval") and well-known from glacials before the Middle-Pleistocene Transition (MPT). The other morphology is characterized by a slow increase of benthic foraminiferal oxygen isotope values followed by a rapid decrease (“target interval”). This pattern is fundamentally different from the U-like shape of other Oligocene glacial cycles, but conspicuously reminiscent of the sawtooth nature of Late Pleistocene glacial/interglacial cycles. In light of the above, benthic foraminiferal δ18O and Mg/Ca records for the „target“ and "Control" intervals have been generated and used to constrain the magnitudes and durations of ice-volume and sea-level fluctuations for the observed G/IG cycles. For both the "target" and the "Control" intervals, BWT are surprisingly constant, with values that range generally between 2 and 4 °C. Even more surprisingly, both intervals lack a clear G/IG cycle in BWT, which suggests that the contribution of BWT fluctuations to the observed G/IG cycles in the benthic delta18O data is small. This becomes even more evident when the δ18O of seawater is calculated and converted to sea level. The observed difference in sea-level estimates between the two study intervals argues for a substantial ice-volume loss on Antarctica. If compared to obliquity, sea-level/ice-volume change during the "control interval" seems to respond nearly linearly to obliquity. Sea-level/ice-volume change during the “target interval”, in contrast, occurred during a time of weaker obliquity. This suggests a potential non-linear threshold response of Antarctic ice volume to the underlying obliquity forcing during the study interval. Whether such a threshold truly existed and what the exact climatic conditions for this potential threshold behavior were, however, cannot be answered with the records at hand so far. Therefore, it is proposed to expand the new Site U1406 high-resolution records to cover the time interval 26–24.5 Ma (i.e. the time interval characterized by significant "steps" in the available delta18O record. Following the approach successfully employed during the first project phase, this will be achieved via the investigation of benthic foraminiferal Mg/Ca and stable isotopes.