Dynamics of the Oligocene cryosphere: mid-to-high latitude climate variability and ice sheet stability

全球气候对大气中二氧化碳（CO2）含量增加的敏感性是人类目前面临的最大问题之一。量化地球气候系统对过去地质时期二氧化碳水平变化的敏感性是减少未来气候预测不确定性的一种方法。如果人为排放到大气中的CO2遵循预测的速度，到2100年，浓度将达到地球上自2300万至3400万年前的渐新世以来从未见过的值。当时，地质学家推断地球比今天更温暖，具有真正的绿色格陵兰岛和不断变化的东南极冰盖（EAIS），导致海平面变化幅度很大（约40米）。这些令人吃惊的观测结果提供了一个强有力的激励，以提高我们对过去气候系统运作的理解。这一提议的重点是一个重要的，但研究不足的时间间隔（约26至28 Ma），已发表的古气候记录表明，在整个渐新世时期，南极冰量和高纬度温度发生了最大的重复变化（10万年时间尺度）。我们提出的研究将产生地质数据，以测试渐新世高纬度气候不稳定性的解释，并进一步阐明冰盖和温度变化的性质。然而，验证动态南极冰盖的存在将是一个重大的科学问题，因为冰盖行为的数值分析表明，在CO2水平没有大的变化的情况下，一个大的南极冰盖应该是稳定的，一旦形成，因为与冰盖几何形状相关的强滞后特性。早期南极冰盖对二氧化碳变化的适应力如何？2.数值模型是否对渐新世和现今东南极冰盖的稳定性产生了错误的认识？3.渐新世CO2的变异性是否比现有重建中显示的要大得多？4.在渐新世南极洲以外的地方有没有可能存在冰盖？限制在解决这些问题方面取得进展的主要因素是缺乏可供研究的适当沉积剖面。我们建议利用综合大洋钻探计划第318和342次考察期间分别从南极和纽芬兰边缘回收的新深海沉积物档案，我们的调查小组在这两次考察中发挥了重要作用（见第1部分，支持案例）。我们的项目将使用（i）南极岩心来测试东南极边缘动态行为（前进和后退）的侵蚀产物，和（ii）纽芬兰岩心来测试北方半球的高纬度气候条件是否有利于冰盖生长。有趣的是，来自纽芬兰边缘的岩芯中含有大量明显的角砂大小的岩屑，这些岩屑被解释为冰筏起源，暗示渐新世附近存在某种形式的冰。我们的工作将通过对南极边缘碎屑同位素地球化学的新调查和在高纬度北大西洋应用有机地球化学温度代用指标来完成。我们的方法的关键是生成高分辨率数据集，这些数据集可以精确地确定日期并相互关联，以及地球仪周围的其他高分辨率数据集。

The sensitivity of global climate to increasing atmospheric carbon dioxide (CO2) levels is one of the biggest issues currently facing humanity. Quantifying the sensitivity of the Earth's climate system to changes in CO2 levels in the geologic past is one way of reducing the uncertainty in future climate predictions. If man-made (anthropogenic) CO2 emissions to the atmosphere follow projected rates, by 2100 concentrations will reach values not seen on Earth since the Oligocene epoch ~23 to 34 million years ago (Ma). Back then, geologists infer that Earth was warmer than today, featuring a genuinely green Greenland and a waxing and waning East Antarctic Ice Sheet (EAIS) that drove high amplitude sea level change (~40 m). These startling observations provide a powerful incentive to improve our understanding of the workings of that past climate system.The focus of this proposal is on an important, but understudied, interval of time (~26 to 28 Ma) for which published palaeoclimate records indicate the biggest repeated (100 thousand-year time scale) changes in Antarctic ice volume and high-latitude temperatures of the entire Oligocene epoch. Our proposed study will generate geological data to both test this interpretation of Oligocene high-latitude climate instability and further elucidate the nature of ice-sheet and temperature variability. Validation of the existence of dynamic Antarctic ice sheets, however, would present a major scientific problem because numerical analysis of ice sheet behavior suggests that, in the absence of big changes in CO2 levels, a large Antarctic ice sheet should be stable once formed because of strong hysteresis properties associated with ice sheet geometry.Several important questions are therefore raised:1. How resilient were the early Antarctic ice sheets to CO2 change?2. Do the numerical models give a false sense of the stability of both the Oligocene and, by extension, present day East Antarctic Ice Sheet?3. Was Oligocene CO2 variability much greater than indicated in existing reconstructions?4. Is it possible that ice sheets existed beyond Antarctica during the Oligocene?The main factor that has limited progress in tackling these questions has been a lack of suitable sedimentary sections on which to work. We propose to exploit new deep-sea sediment archives recovered from the Antarctic and Newfoundland margins during Integrated Ocean Drilling Program Expeditions 318 and 342, respectively, on which our investigator team played significant roles (see Part 1, Case for Support). Our project will use (i) the Antarctic cores to test for the erosive products of dynamic behaviour (advance and retreat) on the East Antarctic margin, and (ii) the Newfoundland cores to test if high-latitude climatic conditions in the Northern Hemisphere were conducive to ice-sheet growth. Intriguingly, the drill cores from the Newfoundland margin contain abundant conspicuous angular sand sized lithic fragments that have been interpreted to be of ice-rafted origin-hinting at the presence of some form of nearby ice in the Oligocene. Our work will be accomplished through novel investigation of detrital isotope geochemistry on the Antarctic margin and application of organic geochemical temperature proxies in the high-latitude North Atlantic. Critical to our approach will be generation of high-resolution datasets that can be precisely dated and correlated to one another, as well as other high-resolution datasets around the globe.