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，各种古气候代理表明，中纬度和高纬度的温度比现在要高得多，而低纬度的温度与现在相似，甚至比现在更冷。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量输送通常会随着纬向温度梯度的降低而减少。因此，大气中二氧化碳（CO2）含量高可以解释热带以外的温暖，但导致热带温度高于替代证据的预期。这种凉爽的热带悖论取决于古热带亚热带有孔虫氧同位素（δ 18 O）值的解释，这需要海水δ 18 O估计推断海水古温度。白垩纪海水δ 18 O通常被认为是全球统一的，或与现代相似，尽管过去温室世界的水文循环被认为与现在有很大不同。这个奖项将有助于测试的假设，凉爽的热带悖论源于白垩纪海水delta 18 O值的不确定性。这将使用一个组合的模型和数据比较的方法来预测白垩纪的表面海水delta 18 O值，以及温度，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）。异步建模策略已开发用于此目的，受益于现有的模型的优势，并避免大规模的模型开发。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。一个同位素AGCM，GENESIS 2，指定从泡沫的海面温度将预测delta 18 O表面通量，包括降水delta 18 O值。来自GENESIS 2的delta 18 O表面通量将用于运行delta 18 O分布（来自FOAM）的示踪剂版本，delta 18 O表面通量（来自GENESIS 2）将在模型之间迭代传递，直到预测的海水delta 18 O值收敛。这项技术将用于预测海洋三角洲18 O目前的气候，以及白垩纪中期气候与4倍和10倍的CO2水平。该奖项还将通过收集和同位素分析来自非洲，欧洲和南半球的古土壤菱铁矿球粒，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。通过预测和验证白垩纪表面δ 18 O值，这项工作将导致重要的见解地球的水文循环是否是从根本上不同的条件下，大气中的二氧化碳含量升高，以及是否在热带气候系统的负反馈必须调用限制热带海表温度。由于大气水蒸气的主要来源是热带海洋，注意白垩纪中期的热带气候过程将提供一个更好的了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.