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

全球气候对不断增加的大气二氧化碳（CO2）水平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（人为）向大气排放的二氧化碳按照预估速率排放，到2100年，二氧化碳浓度将达到自2300万至3400万年前渐新世以来地球上从未见过的水平。当时，地质学家推断地球比现在更温暖，格陵兰岛是真正的绿色，南极东部冰盖（EAIS）的盛期和凋零导致了海平面的高幅度变化（约40米）。这些惊人的观察结果有力地推动了我们对过去气候系统的理解。这一建议的重点是一个重要的，但尚未得到充分研究的时间间隔（~26 ~ 28 Ma），在这个时间间隔内，已发表的古气候记录表明整个渐新世以来南极冰量和高纬度温度的最大重复变化（10万年时间尺度）。我们提出的研究将产生地质数据来验证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，确认动态南极冰盖的存在将提出一个重大的科学问题，因为对冰盖行为的数值分析表明，在CO2水平没有大变化的情况下，由于与冰盖几何形状相关的强滞后特性，大型南极冰盖一旦形成就应该是稳定的。因此提出了几个重要的问题：早期南极冰盖对二氧化碳变化的适应能力如何？这些数值模式是否对渐新世以及由此延伸到今天的南极东部冰盖的稳定性给出了错误的认识？渐新世CO2变率是否比现有的重建结果大得多？在渐新世，冰盖是否可能存在于南极洲之外？制约这些问题进展的主要因素是缺乏合适的沉积剖面。我们建议分别利用综合海洋钻探计划远征318和342期间从南极和纽芬兰边缘恢复的新的深海沉积物档案，我们的研究小组在其中发挥了重要作用（见第1部分，支持案例）。我们的项目将使用(i)南极岩心来测试南极东部边缘动态行为（前进和后退）的侵蚀产物，以及（ii）纽芬兰岩心来测试北半球高纬度气候条件是否有利于冰盖生长。有趣的是，纽芬兰边缘的岩心中含有大量明显的角砂大小的岩石碎片，这些碎片被解释为冰筏的起源——暗示着渐新世附近存在某种形式的冰。我们将通过南极边缘碎屑同位素地球化学的新研究和北大西洋高纬度地区有机地球化学温度代用品的应用来完成我们的工作。对我们的方法至关重要的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，以及全球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.