EAR-PF: What drove localized pyrite formation and taphonomic bias in the fossil record?

凯尔西摩尔博士已被授予美国国家科学基金会地球科学博士后奖学金，在约翰霍普金斯大学进行研究和教育活动的指导下，教授埃米史密斯和玛雅戈麦斯。在地球历史的前20 - 30亿年，生命主要由微生物-简单的生物体如细菌和早期的单细胞真核生物。这种早期生命在设定生命的进化过程中发挥了重要作用，并推动了我们的大气层和地球的进化。为了了解这种古老的生命，它是如何进化的，以及它是如何与早期地球相互作用的，我们转向化石记录。然而，这一记录是有偏见的，因为促进微生物化的过程需要特殊的环境，这些环境往往只选择性地使某些生物体化。柔软的微生物的化石尤其罕见。幸运的是，一些矿物质，如黄铁矿（FeS 2），确实保存了古代软体微生物的记录。一个重要的例子是一个黄铁矿化的Obruchevella组合，蓝藻化石保存在新元古代Ikiakpuk组。这些是光合细菌的化石，它们在一次全球性冰川事件--雪球地球事件的斯特尔期冰川作用--的余波中茁壮成长。但是这些蓝藻是如何与环境相互作用并变成蓝藻的仍然不清楚。该项目旨在更好地了解这种极端气候事件之后的微生物生物圈，它如何应对环境压力，以及它如何变得脆弱。蓝藻可能在它们自身的生物化过程中发挥了作用。现代蓝藻产生富硫有机化合物响应环境压力，这些化合物可能有助于形成黄铁矿和黄铁矿化在过去。该项目将通过与化石相似的活生物体进行化石化实验来验证这一假设。这些实验将与对Ikiakpuk地层及其所含黄铁矿化化石的深入分析相结合。这项工作的目的是确定生物体是如何成为黄铁矿化的，以及什么生物和非生物因素促成了黄铁矿的形成。有了这些见解，就有可能描绘出全球冰川作用后浅海环境的更完整的画面，冰川作用后蓬勃发展的微生物群落，以及这些微生物如何进化和应对环境压力。虽然已经提出了不同的黄铁矿化模型，但很少关注被黄铁矿化的生物体产生的有机化合物及其在硫循环和硫化铁成核中的作用。特别是，在Ikiakpuk形成的黄铁矿保存的化石类似于现代蓝藻，产生硫酸多糖，有机硫化合物，可能在局部黄铁矿形成中发挥关键作用。为了解决这个问题，该项目将测试有机硫酸盐对当地黄铁矿化的贡献。通过对黄铁矿化化石组合的埋藏学实验和化石分析相结合，本研究试图确定（1）有机硫酸盐是否可以作为MSR的硫酸盐来源，（2）这种特定的局部硫酸盐来源是否可以解释黄铁矿化的局部化和某些生物优于其他生物的保存，有机硫是否赋予化石黄铁矿特征硫同位素组成。这项工作将在约翰霍普金斯大学进行，合作者包括Emmy Smith教授和Maya Gomes教授，以及外部合作者Sara Pruss（史密斯学院）和弗朗西斯麦克唐纳（加州大学圣巴巴拉分校）。现代微生物的实验将有助于限制微生物的生物地球化学组成，营养循环和生态相互作用如何驱动化石化，埋藏偏差和硫同位素分馏。模拟化石组合的分析，然后提供了一种手段，将这些研究结果应用到化石记录和测试假设有关的生物多样性和丰度，因为它们涉及到埋藏的偏见。这些综合分析还提供了一种应用硫同位素指纹来测试黄铁矿化模型的应用的方法，该模型将有机硫酸盐计入岩石记录。实验埋藏学和化石分析的结合提供了一种新的方法来深入了解古代微生物群落，海水化学和全球生物圈，超越了单一化石组合所提供的信息。这是特别重要的，因为我们试图了解环境和生物圈的演变后，全球冰川事件，如斯特尔蒂安冰川。更广泛地说，这项工作将告知我们如何解释地球历史上其他时期岩石记录中的黄铁矿化化石组合，并揭示同位素特征可以告诉我们关于生态学，细胞生理学和有机物保存的信息。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

Dr. Kelsey Moore has been awarded an NSF Earth Sciences Postdoctoral Fellowship to carry out research and education activities at Johns Hopkins University under the mentorship of Professors Emmy Smith and Maya Gomes. For the first 2-3 billion years of Earth history, life was dominated by microbes—simple organisms like bacteria and early unicellular eukaryotes. This early life played a fundamental role in in setting life on its evolutionary course and in driving the evolution of our atmosphere and our planet. To learn about this ancient life, how it evolved, and how it interacted with the early Earth, we turn to the fossil record. However, this record is biased because the processes that facilitate fossilization require special circumstances that often selectively fossilize only certain organisms. Fossilization of soft, microbial organisms is especially rare. Luckily, some minerals, like pyrite (FeS2), did preserve a record of ancient soft-bodied microbes. An important example is an assemblage of pyritized Obruchevella, cyanobacterial fossils preserved in the Neoproterozoic Ikiakpuk Formation. These are fossils of photosynthetic bacteria that thrived in the aftermath of a global glacial event—the Sturtian glaciation of the Snowball Earth event. But how these cyanobacteria interacted with the environment and became fossilized remains unclear. This project seeks to better understand the microbial biosphere in the aftermath of this extreme climatic event, how it coped with environmental stresses, and how it became fossilized. It is possible that the cyanobacteria may have played a role in their own fossilization. Modern cyanobacteria produced sulfur-rich organic compounds in responses to environmental stresses and these compounds may have contributed to the formation of pyrite and fossilization in the past. This project will test this hypothesis by conducting fossilization experiments with living organisms that are similar to the fossils. These experiments will be paired with in-depth analysis of the Ikiakpuk Formation and the pyritized fossils that it contains. The aim of this work is to determine how the organisms became fossilized and what biological and abiotic factors contributed to pyrite formation. With these insights, it may be possible to paint a more complete picture of the shallow marine environments after this global glaciation, the microbial communities that thrived in the aftermath of the glaciation, and how those microbes evolved and coped with environmental stresses. While different models for pyritization have been suggested, little attention has been given to the organic compounds produced by the organisms being fossilized and their role in sulfur cycling and iron sulfide nucleation. In particular, the fossils preserved by pyrite in the Ikiakpuk Formation are similar to modern cyanobacteria that produce sulfated polysaccharides, organosulfur compounds that may play a key role in localized pyrite formation. To address this, this project will test the contribution of organosulfates to local pyritization. Through a combination of taphonomy experiments and fossil analysis of pyritized fossil assemblages, this study seeks to determine (1) whether or not organosulfates can be used as a sulfate source for MSR, (2) whether or not this specific localized sulfate source can account for localization of pyritization and preferential preservation of some organisms over others, and (3) whether organosulfur imparts a characteristic sulfur isotope composition in fossil pyrite. This work will take place at Johns Hopkins University in collaboration with Professors Emmy Smith and Maya Gomes, as well as external collaborators Sara Pruss (Smith College) and Francis Macdonald (University of California at Santa Barbara). Experiments with modern microbes will help constrain how microbial biogeochemical makeup, nutrient cycling, and ecological interactions drive fossilization, taphonomic bias, and sulfur isotope fractionation. Analysis of analog fossil assemblages then provides a means of applying these findings to the fossil record and testing hypotheses related to organism diversity and abundance as they relate to taphonomic bias. These combined analyses also provide a means of applying sulfur isotope fingerprints to test the application of a pyritization model that accounts for organosulfates to the rock record. The combination of experimental taphonomy and fossil analyses provides a novel approach to gain insight into ancient microbial communities, seawater chemistry, and the global biosphere beyond the information offered by a single fossil assemblage. This is especially important as we attempt to understand the evolution of environments and the biosphere following a global glacial event like the Sturtian Glaciation. More broadly, this work will inform how we interpret pyritized fossil assemblages in the rock record during other intervals in Earth history and reveal what the isotopic signatures can tell us about ecology, cell physiology, and preservation of organic matter.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.