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)，地球大气中二氧化碳的浓度增加了约35%，以冰芯产生的气泡衡量，达到了过去80万年来最高的水平。如果人类(人为)排放到大气中的二氧化碳按照预计的速度排放，那么到2100年，二氧化碳浓度将达到自3400万至2300万年前(Ma)渐新世以来地球上从未见过的值。当时，地球被认为比今天更温暖，具有真正绿色的格陵兰和东南极冰盖(EAIS)的消长，这推动了海平面的高幅度变化(约40米)。这些令人震惊的观察结果为我们更好地理解过去气候系统的运行机制提供了强大的动力。我们的焦点可以说是这一时期最极端的事件--渐新世-中新世转变(OMT，~24至22 Ma)，并在两个方面向我们展示了一个基本的悖论：(1)已发表的记录表明，OMT的特点是全球气候中的一个显著的‘瞬变’，包括南极冰盖从大约半个今天的东南极冰盖扩张到完整，EAIS，构型，并在~400-KYR叠加了高幅度的高频轨道振荡。然而，数值分析表明，在二氧化碳水平没有大的变化(现有记录中只有轻微变化)的情况下，一旦形成，大型的EAIS应该是稳定的，因为与冰盖几何相关的强烈滞后特性[为了推动2到4公里厚的冰盖的实质性融化，雪线必须上升到大气中，远远超出最初开始冰生长的原始基岩表面]。(2)这些记录中关于地球围绕太阳的轨道的复杂变化对推断的冰期-间冰期周期的影响的印记是明确的。然而，来自不同地点的时间相当的氧同位素序列显示出根本不同的轨道变化频率--如果循环是由大型大陆冰盖的生长和衰退所迫使的，这一结果与氧同位素系统学的基本原理相矛盾。限制在解决这一矛盾方面取得进展的主要因素是缺乏合适的深海沉积区段可供研究。我们建议通过开发新的深海沉积物档案来解决这一问题，该档案是由IODP，第342次远征，从大西洋西北部纽芬兰边缘的沉积物漂流中发现的(2012年6月至7月；联合首席科学家威尔逊；地层对比专家利布兰德，见第1部分，支持案例)。这些沉积物以异常快速的速度直接聚集在今天的深西部边界流的流动路径中，它们是粘土丰富的，保存完好的钙质微体化石和分选良好的角状砂粒大小的岩屑颗粒。我们试图解决的一些问题是：在一个沉积速率很高、微体化石保存得非常完好的地点，不同的OMT公布的氧同位素数据系列的重复性有多大？在北大西洋同期的高纬度地区，由于靠近北半球冰盖、深水对流和海冰形成，气候变化的本质是什么，使该地区成为当今气候系统中如此重要的因素？现有的二氧化碳记录未能捕捉到渐中新世变化的全部幅度，还是这些模型可能严重高估了南极东部冰盖的稳定性？在我们的初步研究记录中，对OMT核心推断的CaCO3含量和有机碳埋藏然后再氧化事件的意义是什么？我们的工作将为全球气候、冰盖和碳循环在温暖世界中相互作用的方式提供新的线索。

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.