Mechanisms of glacial/interglacial changes during the late Oligocene

该项目的主要目标是获得导致晚渐新世IODP站点U1406（纽芬兰，IODP远征342）冰川/间冰期（G/I）旋回形态差异的过程的机制理解。现有的亚轨道分辨率底栖有孔虫氧同位素数据集（时间间隔~25.6 ~ 23.9 Ma）显示了两种G/I旋回形态。其突出的形态为u型（“控制区间”），在中更新世过渡（MPT）之前的冰川时期就有。另一种形态的特征是底栖有孔虫氧同位素值缓慢上升，然后迅速下降（“目标区间”）。这种模式与其他渐新世冰期旋回的u型有本质的不同，但与晚更新世冰期/间冰期旋回的锯齿状特征明显相似。有鉴于此，已经生成了“目标”和“控制”区间的底栖有孔虫δ18O和Mg/Ca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”和“控制”区间，BWT都是惊人的恒定，其值通常在2到4°C之间。更令人惊讶的是，这两个区间在底栖三角洲18o数据中都缺乏明确的G/IG周期，这表明BWT波动对观测到的G/IG周期的贡献很小。当计算海水的δ18O并转换为海平面时，这一点就更加明显了。观测到的两个研究间隔之间海平面估计值的差异表明，南极洲的冰量大量减少。如果与倾角相比，“控制区间”内海平面/冰量变化似乎对倾角几乎呈线性响应。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期。这表明，在研究区间内，南极冰体积对潜在的倾斜强迫可能存在非线性阈值响应。然而，这样一个阈值是否真的存在，以及这种潜在阈值行为的确切气候条件是什么，目前还无法用手头的记录来回答。因此，建议将新的Site U1406高分辨率记录扩展到26-24.5 Ma的时间区间（即现有delta18O记录中具有显著“步长”特征的时间区间）。在项目第一阶段成功采用的方法之后，将通过对底栖有孔虫Mg/Ca和稳定同位素的研究来实现这一目标。

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.