The paradox of high-amplitude inter(glacial) variability across the Oligo-Miocene transition tackled using spectacular new deep-sea sediment archives

二氧化碳是一种强大的温室气体，自工业革命开始以来（大约250年的时间内），其在地球大气中的浓度增加了约35%，达到了以冰芯气泡测量的过去80万年中任何时候都要高的水平。如果人为（人为）向大气排放的二氧化碳按照预测的速率，那么到2100年，二氧化碳浓度将达到自渐新世（3400万至2300万年前）以来地球上从未见过的水平。当时，地球被认为比今天更温暖，格陵兰岛是真正的绿色，南极东部冰盖（EAIS）的盛衰导致了高幅度的海平面变化（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。我们的重点可以说是这个区间最极端的事件渐新世-中新世过渡，OMT,(~24至22 Ma)，并向我们提出了两个基本悖论：(1)已发表的记录表明，OMT以一个突出的全球气候“瞬变”为特征，涉及南极冰盖的扩张和退缩，从现今南极东部冰盖的一半到整个冰盖，EAIS，配置，再到400-kyr，叠加高振幅高频轨道振荡。然而，数值分析表明，在二氧化碳水平没有大变化的情况下（在现有记录中只有轻微的变化），大型EAIS一旦形成，就应该是稳定的，因为与冰盖几何形状相关的强滞后特性[为了推动2至4公里厚的冰盖的实质性融化，雪线必须上升到大气中，远远超过最初开始冰生长的原始基岩表面]。(2)在这些记录中，地球绕太阳轨道的复杂变化对推断出的冰期-间冰期旋回的影响的印记是明确无误的。然而，来自不同地点的时间等效氧同位素序列显示出轨道变化的频率根本不同——如果这种循环是由大型大陆冰盖的生长和衰变所强迫的，那么这一结果就与氧同位素系统学的基本原理相矛盾。解决这一矛盾的主要因素是缺乏合适的深海沉积剖面。为了解决这一问题，我们建议利用IODP第342远征队（2012年6 - 7月；Wilson，联合首席科学家；Liebrand，地层对比员，见第1部分，支持案例）从纽芬兰边缘沉积物漂移中恢复的新的深海沉积物档案。这些沉积物积聚得异常迅速，直接在今天的深西部边界流的流动路径上，它们富含粘土，并含有保存完好的惊人的钙质微化石和分类良好的角状沙粒大小的岩屑颗粒。我们试图解决的一些问题是：在沉积速率高、微化石保存得非常完好的地方，OMT不同的氧同位素数据系列的可重复性如何？在同时期的高纬度北大西洋，靠近北半球冰盖、深水对流和海冰形成的地点，使该地区成为当今气候系统中如此重要的因素，气候变率的本质是什么？是现有的二氧化碳记录未能捕捉到渐新世至中新世变化的全部幅度，还是这些模式可能严重高估了南极东部冰盖的稳定性？在我们的前期研究记录中，CaCO3含量的增加以及有机碳埋藏和再氧化事件对OMT核心的推断有什么意义？我们的工作将为全球气候、冰盖和碳循环在温暖世界中的相互作用方式提供新的线索。

Carbon dioxide, CO2, is a powerful greenhouse gas and its concentration in Earth's atmosphere has increased by around 35% since the start of the Industrial Revolution (in a ca. 250 yr time period) to a level that is higher than at any time in the past 800 thousand years as measured in air bubbles from ice cores. If man-made (anthropogenic) CO2 emissions to the atmosphere follow projected rates, then by 2100, concentrations will reach values not seen on Earth since the Oligocene epoch ~34 to 23 million years ago (Ma). Back then, Earth is thought to have been 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. Our focus is arguably the most extreme event of this interval the Oligocene-Miocene transition, OMT, (~24 to 22 Ma) and presents us with a fundamental paradox on two counts: (1) Published records indicate that the OMT was marked by a prominent 'transient' in global climate involving the inferred expansion and then retreat of Antarctic ice sheets from about half to full present-day East Antarctic ice sheet, EAIS, configuration and back again in ~400-kyr with superimposed high amplitude higher frequency orbital oscillations. Yet numerical analysis suggests that, in the absence of big changes in CO2 levels (only modest change is apparent in existing records), once formed, a large EAIS should be stable because of strong hysteresis properties associated with ice sheet geometry [to drive substantial melting of a 2 to 4 km-thick ice sheet, the snow line must ascend high into the atmosphere, well beyond the original bedrock surface where ice growth was first initiated]. (2) The imprint in these records of the influence of intricate changes Earth's orbit around the Sun on inferred glacial-interglacial cycles is unmistakable. Yet, time-equivalent oxygen isotope series from different sites show fundamentally different frequencies of orbital change- a result that contradicts the basic principles of oxygen isotope systematics if the cycles are forced by the growth and decay of large continental ice sheets. The main factor that has limited progress in tackling this paradox has been a lack of suitable deep-sea sedimentary sections on which to work.We propose to tackle this problem by exploiting new deep-sea sediment archives recovered from sediment drifts on the Newfoundland margin, NW Atlantic by IODP, Expedition 342 (Jun-Jul 2012; Wilson, Co-Chief Scientist; Liebrand, Stratigraphic Correlator, see Part 1, Case for Support). These sediments accumulated unusually quickly, directly in the flow path of the present day Deepwestern Boundary Current, they are clay-rich and host spectacularly well-preserved calcareous microfossils and well-sorted angular sand-sized lithic grains. Some of the questions that we seek to address:How reproducible are the different published oxygen isotope data series for the OMT at a site where sedimentation rates are high and microfossils are spectacularly well-preserved?What was the nature of climate variability in the contemporaneous high-latitude North Atlantic where proximity to northern hemisphere ice sheets, sites of deep-water convection and sea ice formation make the region such an important agent in the climate system today?Do existing CO2 records fail to capture the full amplitude of Oligo-Miocene change or do the models perhaps grossly over estimate the stability of the East Antarctic Ice Sheet? What is the significance of the increases in CaCO3 content and organic carbon burial and then re-oxidation event inferred for the core of the OMT in our pilot study records?Our work will shed new light on the ways in which global climate, ice sheets and the carbon cycle interact in a warm world.