Terrestrial responses to millennial and sub-millennial climate variability in the Early Glacial and implications for the hydrological cycle

虽然很多工作都集中在冰川的终结上，但人们对从温暖到凉爽的气候状态的转变知之甚少。因此，从6.7 - 8.5万年前（ka）末次间冰期复体到早更冰期的过渡，为深入了解轨道和短期气候变率与冰盖生长之间的相互作用提供了一个重要的研究案例。这一间隔的特征是一系列千年尺度的气候振荡，而海平面下降了约50米，反映了北部高纬度地区冰盖的扩张。要使冰原向寒冷地区提供温暖潮湿的空气是必需的，这样冰的积累就超过了消融。重建的格陵兰岛温度在此期间显示了一系列变暖事件（间冰期），但大气甲烷浓度的相应变化被抑制，表明排放减少和水文循环减弱。与此同时，来自冰芯的新证据显示，在这段时间间隔内，格陵兰岛的温度存在百年振荡。这些变化伴随着大气甲烷浓度的变化，强调了这些气候变化的巨大地理范围。该项目旨在更好地限制陆地环境和水文循环的变化以及67-85 ka期间气候系统中关键参数之间的联系。这将通过生成高分辨率的花粉记录来实现，该记录将与来自葡萄牙南部边缘的同一参考海洋序列中的沉积学和古海洋学代用物联系起来。近年来，葡萄牙南部边缘已成为追踪千年尺度变化并将海洋和陆地记录联系起来的关键区域。一个关键方面是该地区的地理环境，在这里，塔霍河和狭窄的大陆架的共同作用导致包括花粉在内的陆地物质迅速输送到深海环境。同一序列内的联合花粉、沉积物和古海洋学代用分析可以直接比较海洋和陆地的变化，绕过时间尺度和相关性的不确定性。具体而言，该项目旨在建立：(1)中低纬度地区在间断期的水循环强度：考虑到南欧植被的变化主要受大陆水文平衡变化的控制，该间断期的花粉记录将提供一个独立的水循环强度变化指标，可以与大气甲烷记录推断的水循环强度变化进行比较。(2)植被对次千年事件的响应及其对侵蚀的影响：虽然在葡萄牙边缘的工作已经确定了植被对千年尺度变率的直接响应，但次千年的变化以前没有文献记载。参考海洋岩芯中沉积物组成变化的一项新的超高分辨率记录现在揭示了次千年事件的存在，与冰芯记录的变化同时发生。通过提高这些事件期间花粉记录的分辨率，该项目将研究沉积物组成的振荡是否伴随着植被变化。反过来，这将检验这样一种假设，即沉积物组成的快速变化反映了来自塔霍河的陆源供应的变化，这最终是由水文平衡和植被覆盖范围的相互作用控制的。这些结果将有助于理解在过渡到完全冰川条件时冰盖生长的水分来源以及陆地环境对短期气候变率的响应。

While much work has focused on glacial terminations, less is known about the transitions from warmer to cooler climate states. As such, the transition from the Last Interglacial Complex to the Early Pleniglacial, 67-85 thousand years ago (ka), represents an important case study for gaining insights into the interaction of orbital and short-term climate variability and ice sheet growth. This interval is characterized by a series of millennial-scale climate oscillations, while sea level fell by about 50m, reflecting the expansion of ice sheets in northern high latitudes. For ice sheets to expand a supply of warm, moist air to cold regions is required, so that ice accumulation exceeds ablation. Reconstructed Greenland temperatures show a series of warm events (interstadials) during this period, but corresponding changes in atmospheric methane concentration are subdued, pointing to reduced emissions and a weakening of the hydrological cycle. In parallel, new evidence from ice cores has revealed the presence of centennial oscillations in Greenland temperatures during this interval. These are accompanied by variations in atmospheric methane concentrations, underlining the large geographical extent of these climate changes. This project aims to deliver better constraints on changes in terrestrial environments and the hydrological cycle and on links between key parameters in the climate system during the interval 67-85 ka. This will be achieved by generating a high-resolution pollen record, linked with sedimentological and palaeoceanographic proxies within the same reference marine sequence from the S Portuguese margin. In recent years, the S. Portuguese margin has emerged as a critical area for tracing millennial-scale variability and linking marine and terrestrial records. A key aspect is the geographical setting of the area where the combined effects of the Tagus River and a narrow continental shelf lead to the rapid delivery of terrestrial material, including pollen, to the deep-sea environment. Joint pollen, sediment and palaeoceanographic proxy analysis within the same sequence allows a direct comparison between ocean and terrestrial changes, bypassing timescale and correlation uncertainties. More specifically, the project aims to establish: (1) the strength of the hydrological cycle at mid-to-low latitudes during interstadials: given that changes in vegetation in southern Europe are mainly controlled by variations in the continental hydrological balance, a pollen record for this interval will provide an independent index of changes in the strength of the hydrological cycle, which can be compared with that inferred from the atmospheric methane record. (2) the response of vegetation to sub-millennial events and impact on erosion: while work on the Portuguese margin has established the immediate response of vegetation to millennial-scale variability, sub-millennial variations have not been previously documented. A new ultra high-resolution record of changes in sediment composition in the reference marine core has now revealed the presence of sub-millennial events, coeval with variations in ice core records. By increasing the resolution of the pollen record during these events, the project will investigate whether the oscillations in sediment composition were accompanied by vegetation changes. This, in turn, will test the hypothesis that the rapid changes in sediment composition reflect variations in the terrigenous supply from the Tagus River, which are ultimately controlled by the interplay of the hydrological balance and extent of vegetation cover. The results will have implications for understanding moisture sources for ice sheet growth during transitions into full glacial conditions and the response of terrestrial environments to short-term climate variability.