CAREER: Habitability of the Hadean Earth - A South African perspective

地球是唯一已知有生命存在的行星，但地球何时变得适合居住仍存在争议。最初，地球是不适合居住的，因为它被岩浆海洋覆盖，经常受到巨大的撞击。随着时间的推移，第一块地壳结晶，海洋沉淀，大陆开始形成。然而，我们不知道这些过程是何时发生的，也不知道地球是何时变得适合居住的。这是由于缺乏地球最初约5.5亿年历史的岩石记录，即冥古宙。那个时代唯一遗留下来的物质是锆石。我们对冥古宙锆石的了解大多来自于澳大利亚的杰克山。最近，PI团队在南非发现了冥古宙的锆石。这提供了观察冥古宙的第二个镜头。这些锆石的微量元素和同位素将有助于解决地球宜居性的关键过程。这包括板块构造的开始，从地幔中放出气体，以及海平面以上地壳的出现。拟议的研究提供了整合研究和教育经验的共生机会。PI将创建一个博物馆展览，在哈佛自然历史博物馆和斯瓦蒂尼的恩格维尼亚博物馆展出。这项研究也为来自少数族裔的本科生提供了大量的研究机会。最后，将分发改进的本科和研究生教材，以供更广泛使用。该项目支持1名女性早期职业科学家、1名博士后研究员和10名本科生研究助理。这项工作将通过分析统计上可靠的锆石数量，扩大和整合锆石同位素（Hf， O, Si）以及微量元素和稀土元素的地球化学分析，创建杰克山外冥古宙锆石的第一个大型数据集。该研究检查了来自南非绿砂岩床的冥古宙锆石，这些锆石之前是通过单独的资助收集、提取和确定年代的。首先，通过对锆石Hf、O同位素、微量元素的综合研究，并结合模拟，验证锆石形成于停滞盖还是活动盖（即板块构造）环境。其次，对幔源锆石中氧化还原敏感Ce异常进行评价，评价地幔氧化状态。与兴兴克拉通和杰克山冥古宙锆石数据的比较将提供地幔非均质性的约束条件。第三，它将评估同位素（O, Si）和地球化学代用物（Al， P），以测试粘土的生成和作为大陆出现代用物的大气水的存在。研究成果将通过期刊文章、会议报告和在线数据库发布。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。

Earth is the only planet known to harbor life, but when Earth became habitable is still debated. Initially, Earth was uninhabitable because it was covered by a magma ocean and frequently struck by giant impactors. Through time, the first crust crystallized, oceans precipitated, and continents started forming. Yet, we don't know when these processes occurred and when Earth became habitable. This is due to lack of a rock record for Earth’s first ~550 million years of history, the Hadean Eon. The only remaining materials from that time are zircons. Most of our knowledge comes from a single location of Hadean zircons, the Jack Hills in Australia. Recently, the PI’s team discovered Hadean zircon in South Africa. This offers a second lens into the Hadean Eon. Trace elements and isotopes of these zircons will help address processes key to Earth’s habitability. This includes the onset of plate tectonics, outgassing from the mantle, and the emergence of crust above sea level. The proposed research offers symbiotic opportunities to integrate research and educational experiences. The PI will create a museum exhibit to be shown at the Harvard Museum of Natural History and the Ngwenya museum in Eswatini. The research also offers abundant research opportunities for undergraduates from underrepresented minorities. Lastly, improved undergraduate and graduate teaching materials will be disseminated for broader use. The project supports one female early career scientist, one postdoctoral researcher, and 10 undergraduate research assistants.The proposed work will create the first large dataset of Hadean zircon outside the Jack Hills by analyzing a statistically robust number of zircons and by broadening and integrating zircon isotopic (Hf, O, Si) and trace and rare earth element geochemical analyses. The research examines Hadean zircons from the Green Sandstone Bed, South Africa, that were previously collected, extracted, and dated through a separate grant. Firstly, the research will test whether the zircons formed in a stagnant-lid or mobile-lid (i.e., plate tectonics) environment through an integrated study of zircon Hf and O isotopes, trace elements and with modeling. Secondly, it will assess redox-sensitive Ce anomalies in mantle-derived zircons to evaluate the oxidation state of the mantle. A comparison to data from Hadean zircons of the Singhbhum Craton and the Jack Hills will provide constraints on mantle heterogeneity. Thirdly, it will assess isotopic (O, Si) and geochemical proxies (Al, P) to test for the generation of clays and the presence of meteoric waters as proxies for continental emergence through time. The research results will be disseminated through articles in journals, presentations at conferences, and submission to online databases.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.