Collaborative Research: The role of pCO2 in the astronomically-paced climatic cycles of the Miocene

地球的地质记录是由地球绕太阳轨道的几何形状的周期性变化决定的，通常称为米兰科维奇周期（以第一个预测轨道变化的塞尔维亚数学家的名字命名）。在过去约一百万年里，这些轨道变化导致全球气候在寒冷的冰河时期（当时大冰盖覆盖了北美大部分地区）和温暖的间冰期（气候与今天相似）之间波动。对南极冰芯中古代大气气泡的测量表明，这些冰河时代周期与大气二氧化碳（CO2）水平的显着波动相一致。这表明二氧​​化碳温室效应在轨道时间尺度上影响气候变化。然而，对来自冰芯的大气二氧化碳的估计仅涵盖了过去80万年，因此大气二氧化碳是否在地球历史早期的米兰科维奇气候强迫中发挥了作用这一问题仍然悬而未决。这项研究旨在填补这一重大知识空白，通过应用地球化学方法以前所未有的分辨率重建古代二氧化碳，该时间间隔内有详细记录的米兰科维奇循环〜1400万年前，当时全球气候变暖，大陆与今天相比处于不同位置。由此产生的高分辨率二氧化碳记录将与尖端的地球系统模型相结合，以测试有关米兰科维奇循环如何控制地球碳循环和气候的假设。总之，该项目期间进行的新地球化学记录和建模有望加深对温暖气候期间大气二氧化碳作用的理解——鉴于目前大气二氧化碳含量不断增加，这是一个具有重大社会重要性的问题。研究结果将通过在广泛阅读的科学期刊上发表的出版物传达给科学界，并通过创作和分发一本探索地球历史上大气二氧化碳与气候之间相互作用的图画书传达给小学生。更具体地说，该项目的目标是使用有孔虫硼同位素代理系统以及中新世中期全球温度和碳循环动力学的匹配代理记录，生成第一个高分辨率（每约5000年1个样本）的大气pCO2记录。该时间间隔被选为比现代地球温暖的例子，其背景 pCO2 略高，大陆冰盖较小，气候和碳循环中具有明确的米兰科维奇循环。主要的高分辨率硼同位素记录将在站点 926（大西洋塞阿拉海隆）生成，补充的较低分辨率数据将在站点 608（北大西洋）和 806（西太平洋）生成。硼同位素测量将包括开发和严格分析测试一种新的微升华方法，用于从碳酸盐样品中分离硼，该方法有望实现更小的样品尺寸和更快的通量。在 926 号站点，硼同位素将得到浮游和底栖有孔虫的碳同位素（限制碳循环）、氧同位素（作为古温度和冰体积的测量）和微量金属浓度（包括 Mg/Ca（古温度替代物）和 B/Ca（新兴碳酸盐化学替代物））的测量的补充。这些组合数据集将用于重建海洋温度和碳循环动力学，并通过与基于硼同位素的二氧化碳记录进行比较，可以估计轨道时间尺度上变化的二氧化碳水平的温度敏感性。新记录将用于指导中等复杂性地球系统模型（cGENIE）中米兰科维奇气候强迫的模型模拟，为负责米兰科维奇控制气候和碳循环的机制和反馈提供定量约束。对比中新世和更新世米兰科维奇循环将能够直接评估大型大陆冰盖在调节轨道强迫 pCO2 变化的动力学和作用方面的作用。

Earth's geologic record is paced by cyclical variations in the geometry of our planet's orbit around the sun, generally called Milankovitch cycles (named for the Serbian mathematician who first predicted the orbital variations). Over the last ~1 million years, these orbital variations caused global climatic oscillations between cold Ice Ages, when large ice sheets covered much of North America, and warmer Interglacial periods with climates similar to today's. Measurements of air bubbles of ancient atmosphere trapped in Antarctic ice cores has shown that these Ice Age cycles coincide with significant swings in atmospheric carbon dioxide (CO2) levels. This suggests that the CO2 greenhouse effect plays a role climate change on orbital timescales. However, estimates of atmospheric CO2 from ice cores cover only the last 800,000 years, leaving open the question of whether atmospheric CO2 played a role in the Milankovitch forcing of climate during earlier times in Earth history. This research aims to fill this major knowledge gap by applying geochemical methods to reconstruct ancient CO2 at unprecedented resolution during a time interval with well-documented Milankovitch cycles ~14 million years ago, when global climate was warmer, and the continents were in different positions compared to today. The resulting high-resolution CO2 record will be coupled to a cutting-edge Earth System model to test hypotheses regarding how Milankovitch cycles control Earth's carbon cycle and climate. Together, new geochemical records and modeling undertaken during this project promise to refine understanding of the role of atmospheric CO2 during warm climates -- an issue of significant societal importance in light of currently increasing atmospheric CO2. Results will be communicated to the scientific community by publications in widely-read scientific journals, and to grade school students through by the creation and distribution of an illustrated book exploring the interaction between atmospheric CO2 and climate in Earth history.More specifically, this project aims to generate the first high-resolution (1 sample per ~5 thousand years) records of atmospheric pCO2 using the foraminiferal boron isotope proxy system coupled with matching proxy records of global temperature and carbon cycle dynamics in the mid-Miocene. This time interval was selected as an example of a warmer-than modern Earth with slightly higher background pCO2, smaller continental ice sheets, and well-defined Milankovitch cycles in climate and carbon cycling. The primary, high-resolution boron isotope record will be generated at Site 926 (Ceara Rise, Atlantic Ocean) with supplementary, lower resolution data generated at Sites 608 (North Atlantic) and 806 (Western Pacific). Boron isotope measurements will include the development and rigorous analytical testing of a new microsublimation method for boron separation from carbonate samples, which offers the prospect of smaller sample sizes and faster throughput. At Site 926, boron isotopes will be complemented by measurements of carbon isotopes (placing constraints on carbon cycling), oxygen isotopes (as a measure of paleotemperature and ice volume) and trace metal concentration including Mg/Ca (a paleotemperature proxy) and B/Ca (an emerging carbonate chemistry proxy) on both planktic and benthic foraminifera. These combined datasets will be used to reconstruct ocean temperatures and carbon cycle dynamics, and by comparison to the boron isotope-based CO2 record, allow estimates of temperature sensitivity to changing CO2 levels on orbital timescales. The new records will be used to guide model simulations of Milankovitch climate forcing in an Earth system model of intermediate complexity (cGENIE), providing quantitative constraints on the mechanisms and feedbacks responsible for the Milankovitch control of climate and carbon cycling. Contrasting mid-Miocene and Pleistocene Milankovitch cycles will enable a direct assessment of the role of large continental ice-sheets in modulating the dynamics, and role, of orbitally-forced pCO2 variations.