Short-term waxing and waning of Antarctic ice sheets during the late Oligocene

从ODP站点689（Maud Rise，Weddell海）和北大西洋IODP站点U1406获得的冰量重建显示出高度动态的南极冰盖（AIS），其特征在于相对于现代冰量高达70%的轨道控制高振幅波动。拟议项目的主要目标是严格测试这些海平面/冰量重建晚渐新世，并获得一个机械的理解，导致观察到的冰盖动态的过程。这些目标将通过生成渐新世晚期特定时间间隔（磁时C8n.2n; 25.304-25.987 Ma）的古气候代用记录来实现，以实现尽可能高的地层精度。要生成的代用记录包括三种独立的方法，以便对现有的海平面/冰量重建进行灵敏度测试：（i）基于底栖生物微生物地球化学（Mg/Ca和稳定同位素），（ii）来自散装沉积物样品碎屑部分的Nd同位素，以及（iii）来自散装沉积物样品自生部分的Pb同位素。前两种方法已经成为项目第一阶段的重点。对于第二个项目阶段，重点将放在方法（iii）上，即对来自689号站点的跨Chron C8n.2n的自生Pb同位素记录进行基因分析，以解释径流信号的来源变化，这些径流信号来自不同的源岩，这些源岩可用于风化过程，作为渐新世晚期冰盖覆盖范围扩大或缩小的函数（即，在大冰量期间以沿海为主的侵蚀与在小AIS间隔期间以内陆为主的侵蚀），如第一项目阶段新生成的有孔虫冰量估计所示。在这里提出的项目（网站689）的重点网站提供了一个强大的磁性地层框架内的高时间分辨率的工作机会。此外，它在追踪南极冰盖变化对向南大洋输送碎屑和风化产物的影响方面具有战略地位。拟议的记录将允许详细了解新生代气候演变关键间隔（即，渐新世冰室气候上升）。此外，这些数据将允许一个复杂的比较背后的驱动机制冰川/间冰期周期在两个根本不同的间隔地球的历史，其气候过程：单极冰川渐新世和两极冰川晚更新世。

Available ice-volume reconstructions from ODP Site 689 (Maud Rise, Weddell Sea) and from North Atlantic IODP Site U1406 show a highly dynamic Antarctic ice sheet (AIS) characterized by orbitally-controlled high-amplitude fluctuations of up to 70% relative to modern ice volume. The principal objectives of the proposed project are to critically test these sea-level/ice-volume reconstructions for the late Oligocene and to obtain a mechanistic understanding of the processes that led to the observed ice-sheet dynamics. These objectives will be reached through the generation of paleoclimate proxy records for a specific time interval of the late Oligocene (magnetochron C8n.2n; 25.304–25.987 Ma) to allow to work on highest possible stratigraphic precision. Proxy records to be generated comprise three independent approaches to have a sensitive test of the available sea-level/ice-volume reconstructions: (i) based on benthic fora¬mini¬fe¬ral geochemistry (Mg/Ca and stable isotopes), (ii) Nd isotopes from the detrital fraction of bulk-sediment samples, and (iii) Pb isotopes from the authigenic fraction of bulk-sediment samples. The first two approaches were already in the focus of the first project phase. For the second project phase, the focus will be on approach (iii), the gene¬rat¬¬ion of an authigenic Pb-isotope record across Chron C8n.2n from Site 689 to decipher provenance changes of the runoff signal resulting from different source rocks that are made available for weathering processes as a function of expanding or diminishing ice-sheet coverage in the late Oligocene (i.e., coastal dominated erosion during large ice volume vs. inland-dominated erosion during intervals of a small AIS) as it is suggested by the newly generated foraminiferal-based ice-volume estimates of the first project phase. The site focused upon in the here proposed project (Site 689) provides the opportunity to work at a high temporal resolution within a robust magnetostratigraphic framework. Moreover, it is located in a strategic position with regard to tracing the influence of changing Antarctic ice sheets on the delivery of detritus and weathering products to the Southern Ocean. The proposed records will allow detailed insight into the evolution of the cryosphere for a critical interval of Cenozoic climate evolution (i.e., the Oligocene rise of the icehouse climate). Moreover, the data will allow a sophisticated comparison of the driving mechanisms behind glacial/interglacial cycles during two fundamentally different intervals of Earth’s history with respect to their climatic processes: the unipolar-glaciated Oligocene and the bipolar-glaciated Late Pleistocene.