Collaborative Proposal: Tectonic degassing as a possible solution to the Miocene climate enigma

大约2000万年前，地球比现在暖和得多，南极几乎没有冰。然而，大约1400万年前发生了一次神秘的气候转变，从那以后地球经历了一系列的降温事件。数百万年的持续冷却最终导致了今天的两极冰窖气候的出现。长期以来，科学家们一直对什么过程可能导致冷却感到困惑。一种观点认为，喜马拉雅山脉的上升可能加速了侵蚀，增加了化学风化的速度，这是一种吸收大气中二氧化碳的过程。其他科学家认为，温度下降可能是由于地球深处二氧化碳供应的减缓造成的。这个观点认为，海洋地壳岩石的形成速度在大约1400万年前减慢了。这将导致从地球内部释放到大气中的二氧化碳减少。温室气体供应的减少导致了全球气温的下降。这项研究将通过提供对过去全球温度的改进估计，帮助解决这个冷却之谜。这些估计将来自对深海沉积物中保存的有机分子的分析和气候模型的模拟。新的全球温度重建将有助于确定造山和地球内部二氧化碳释放对碳循环变化的相对贡献。这项研究将更好地说明地球上的构造过程，包括陆地和海底，是如何影响长期全球气候的。该项目更广泛的影响包括支持两所机构的博士后研究人员，支持布朗大学的研究生和本科生研究人员，并通过布朗大学现有的项目支持K-12重点外展活动。从中新世中期（~14 Ma）开始，地球经历了10-12摄氏度的持续降温，结束了自中生代以来普遍存在的温暖气候。关于中新世气候转变的一个主要谜团是，它是由减少的二氧化碳脱气（源）还是增强的风化去除（汇）驱动的。先前的研究假设海底的扩张速度随着时间的推移相对恒定，表明二氧化碳的减少（以及全球变冷）是由化学风化作用的增强引起的。这可能是由于喜马拉雅山脉的隆起或热带海洋大陆的出现，在世界上最温暖，最潮湿的地区之一，容易风化的火山物质暴露出来。然而，最近的海洋表面温度重建显示，中新世中期比以前认为的要温暖得多，这就提出了一个难题：仅仅是风化作用的减少是否足以维持大规模的变暖；在暖中中新世条件下，可能需要增强的二氧化碳通量来平衡预期的二氧化碳风化汇。最近有证据表明，自15ma以来，全球地壳生产速率降低了约30%，这对风化假说提出了挑战。该项目将产生新的生物标志物海洋表面温度估算，覆盖全球中新世，填补当前数据集的时空空白。除了代理分析，该研究还将开发新的中新世气候模拟，采样广泛的模型物理和边界条件，以重现中新世大尺度温度和水文循环。模型模拟将用于探测源/汇配置与中新世温度和碳循环中的质量平衡相兼容。通过综合模型数据信息，该项目将改进中新世气候的重建，最终将允许估计不同源汇项（即喜马拉雅山、海洋大陆和与海底扩张相关的二氧化碳脱气）在驱动中新世温度和大气二氧化碳演化中的相对贡献。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

About 20 million years ago, the Earth was much warmer than today and the Antarctic was nearly ice-free. However, a mysterious climatic transition happened around 14 million years ago and the Earth went through a series of cooling events since then. The persistent cooling over millions of years ultimately led to the emergence of the bi-polar icehouse climate of today. Scientists have long been puzzled about what processes could have caused the cooling. One idea is that the rise of the Himalayas mountains may have sped up erosion and increased the rate of chemical weathering, a process that draws down atmospheric carbon dioxide. Other scientists suggest that the cooling may have been caused by a slowdown in the supply of carbon dioxide from deep inside the earth. The idea is that the formation rate of oceanic crustal rocks decelerated about 14 million years ago. That would have caused a decrease in the release of carbon dioxide from the Earth's interior into the atmosphere. The decrease in the supply of that greenhouse gas then led to the decrease in global temperatures. This study will help address this cooling mystery by providing improved estimates of past global temperatures. Those estimates will come from analyses of organic molecules preserved in deep-sea sediments and from climate model simulations. The new global temperature reconstruction will help determine the relative contributions of mountain building and carbon dioxide release from the Earth's interior to changes in the carbon cycle. The study will provide a better picture of how tectonic processes on Earth, both on land and at the sea floor, influence long-term global climate. The project broader impacts include support for a postdoctoral researcher at both institutions, support for a graduate student and undergraduate researchers at Brown University, and support for K-12 focused outreach activity through an existing program at Brown University.Beginning in the Middle Miocene (~14 Ma), the Earth experienced sustained cooling of 10-12 degrees C that ended the generally warm climate that had prevailed since the Mesozoic. A major enigma about this Miocene climate transition is whether it is driven by reduced carbon dioxide degassing (source) or enhanced weathering removal (sink). Assuming a relatively constant seafloor spreading rate over time, previous studies suggest that the carbon dioxide drawdown (and the global cooling) was caused by enhanced chemical weathering. That could have been due to either the uplift of the Himalayas or the emergence of the tropical maritime continent, which exposed easily weathered volcanic material in one of the warmest, wettest areas of the world. Recent sea surface temperature reconstructions, however, reveal that the Middle Miocene was much warmer than previously thought, raising the puzzle of whether reduced weathering alone is sufficient to sustain the large warming; an enhanced carbon dioxide flux is probably required to balance the expected weathering sink of carbon dioxide under warm Middle Miocene conditions. This study is motivated by a challenge to the weathering hypothesis based on recent evidence of a ~30% reduction in global crustal production rate since 15 Ma. This project will generate new biomarker sea surface temperature estimates with global coverage for the Miocene, filling temporal and spatial gaps of current datasets. Along with proxy analysis, the study will also develop new Miocene climate simulations sampling a wide range of model physics and boundary conditions to reproduce the Miocene large-scale temperature and hydrological cycle. The model simulations will be used to probe source/sink configurations compatible with Miocene temperatures and mass balance in the carbon cycle. By synthesizing the model-data information, the project will develop an improved reconstruction of the Miocene climate which will ultimately allow for estimation of the relative contribution of different source-sink terms (i.e., the Himalayas, the maritime continent, and the carbon dioxide degassing associated with seafloor spreading) in driving the Miocene temperature and atmospheric carbon dioxide evolution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.