Collaborative Research: Evaluation of the Mid-Cretaceous Cool Tropics Paradox Using Isotopic Global Climate Models (GCMs) and Foraminiferal and Paleosol Siderite d18O Datasets

在地球历史上最温暖的时期，各种古气候代用指标表明，中高纬度地区的温度比现在高得多，而低纬度地区的温度与现在相似，甚至比现在更低。这些观测结果造成了一个气候悖论，因为行星尺度的纬向热输运通常随着纬向温度梯度的减小而减小。因此，高大气二氧化碳（CO2）含量可以解释热带外的温暖，但导致热带温度比代理证据预期的要高。这种冷热带悖论取决于古热带浮游有孔虫氧同位素（delta18O）值的解释，这需要海水delta18O估计来推断海水古温度。白垩纪海水三角洲18o通常被认为是全球均匀的，或者与现代相似，尽管过去温室世界的水文循环被认为与现在有很大不同。该奖项将有助于验证“凉爽热带悖论”源于白垩纪海水delta18O值的不确定性的假设。这将通过结合模型和数据比较方法来实现，使用耦合的海洋/大气环流模式（O/AGCM）和同位素大气环流模式（AGCM）来预测白垩纪表面-海水delta18O值以及温度。为此目的开发了一种异步建模策略，它受益于现有模型的优势，并避免了大量的模型开发。一个完全耦合的海洋大气模式（FOAM）将被用来模拟白垩纪中期的气候。同位素AGCM， GENESIS2，具有FOAM指定的海面温度，将预测delta18O表面通量，包括降水delta18O值。然后，来自GENESIS2的delta18O表面通量将用于运行具有示踪功能的delta18O分布版本（来自FOAM），并且将在模型之间迭代传递delta18O表面通量（来自GENESIS2），直到预测的海水delta18O值收敛。这项技术将用于预测当前气候下的海洋三角洲18o，以及二氧化碳浓度为4倍和10倍的白垩纪中期气候。该奖项还将通过收集和分析来自非洲、欧洲和南半球的古土壤橄榄石球粒以及来自阿拉斯加、不列颠哥伦比亚省和北大西洋的浮游有孔虫的同位素分析来填补现有的数据空白。这项研究将解决与未来温室气候高度相关的主题。通过对白垩纪表面delta18O值的预测和验证，这项工作将为以下问题提供重要见解：在大气CO2含量升高的条件下，地球的水文循环是否会发生根本性的变化，以及是否必须利用热带气候系统中的负反馈来限制热带海面温度。由于大气水汽的主要来源是热带海洋，因此对中白垩纪热带气候过程的关注将有助于更好地理解温室水文循环动力学。

During the warmest episodes of Earth history, various paleoclimate proxies indicate that mid-and high-latitude temperatures were considerably warmer than today, whereas low-latitude temperatures were similar to or even cooler than present. These observations create a climate paradox since planetary-scale latitudinal heat transport generally decreases with decreased latitudinal temperature gradients. As a result, high atmospheric carbon dioxide (CO2) contents can explain extra-tropical warmth, but lead to tropical temperatures that are warmer than expected from the proxy evidence. This cool tropics paradox hinges on interpretation of paleo-tropical planktonic foraminiferal oxygen isotope (delta18O) values, which require a seawater delta18O estimate to deduce seawater paleotemperature. Cretaceous seawater delta18O is generally assumed to have been globally uniform, or similar to the modern, though the hydrological cycle during this past greenhouse world is thought to have been considerably different from present. This award will help test the hypothesis that the cool tropics paradox stems from uncertainty of Cretaceous seawater delta18O values. This will be accomplished using a combined model and data comparison approach to predict Cretaceous surface-seawater delta18O values, as well as temperatures, using a coupled Ocean/Atmosphere General Circulation Model (O/AGCM) and an isotopic Atmospheric General Circulation Model (AGCM). An asynchronous modeling strategy has been developed for this purpose that benefits from existing model strengths, and avoids massive model development. A Fully Coupled Ocean Atmosphere Model (FOAM) will be used to simulate the mid-Cretaceous climate. An isotopic AGCM, GENESIS2, with sea-surface temperatures specified from FOAM will predict delta18O surface fluxes including precipitation delta18O values. The delta18O surface fluxes from GENESIS2 will then be used to run a tracer-capable version of delta18O distribution (from FOAM) and the delta18O surface fluxes (from GENESIS2) will be passed iteratively between models until predicted seawater delta18O values converge. This technique will be used to predict oceanic delta18O for the present climate, as well as mid-Cretaceous climates with 4x and 10x CO2 levels. This award will also fill in existing data gaps through the collection and isotopic analysis of paleosol siderite spherules from Africa, Europe, and the Southern Hemisphere, and the isotopic analysis of planktonic foraminiferal from Alaska, British Columbia, and the North Atlantic.This research will address topics that are highly relevant to a future greenhouse climate. Through the prediction and validation of Cretaceous surface delta18O values, the work will lead to important insights into whether or not Earth's hydrologic cycle is radically different under conditions of elevated atmospheric CO2 content, and whether or not negative feedbacks in the tropical climate system must be invoked to limit tropical sea-surface temperatures. Because the major source of atmospheric water vapor is the tropical ocean, attention to mid-Cretaceous tropical climate processes will provide a better understanding of greenhouse hydrologic cycle dynamics.