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

Kelsey Moore博士获得了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的Emmy Smith和Maya Gomes教授的指导下开展研究和教育活动。在地球历史的前20 - 30亿年里，生命主要由微生物——像细菌和早期单细胞真核生物这样的简单有机体组成。这种早期生命在确定生命的进化过程中发挥了至关重要的作用，并推动了我们的大气和地球的进化。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，我们求助于化石记录。然而，这个记录是有偏见的，因为促进石化的过程需要特殊的环境，通常只会选择性地使某些生物变成化石。软质微生物的石化尤其罕见。幸运的是，一些矿物，如黄铁矿（FeS2），确实保存了古代软体微生物的记录。一个重要的例子是保存在新元古代Ikiakpuk组的黄铁矿化Obruchevella蓝藻化石组合。这些是光合细菌的化石，它们在一次全球性的冰川事件——雪球地球事件的斯图亚特冰川时期——之后繁衍生息。但是这些蓝藻是如何与环境相互作用并变成化石的还不清楚。该项目旨在更好地了解极端气候事件后的微生物生物圈，它是如何应对环境压力的，以及它是如何变成化石的。蓝藻可能在它们自己的石化过程中发挥了作用。现代蓝藻在对环境压力的反应中产生了富硫有机化合物，这些化合物可能有助于过去黄铁矿的形成和石化。该项目将通过与化石相似的生物进行化石实验来验证这一假设。这些实验将与对Ikiakpuk组及其包含的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是如何变成化石的，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，就有可能描绘出一幅更完整的全球冰期后浅海环境的图景，冰期后繁盛的微生物群落，以及这些微生物是如何进化和应对环境压力的。虽然提出了不同的黄铁矿化模型，但很少有人关注被石化的生物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，Ikiakpuk组中黄铁矿保存的化石与现代蓝藻相似，后者产生硫酸多糖，有机硫化合物，可能在局部黄铁矿形成中起关键作用。为了解决这个问题，该项目将测试有机硫酸盐对局部黄铁矿化的贡献。通过埋藏实验和黄铁矿化化石组合的化石分析相结合，本研究试图确定(1)有机硫酸盐是否可以作为MSR的硫酸盐源，(2)这种特定的局部硫酸盐源是否可以解释黄铁矿化的局部化和某些生物的优先保存，以及(3)有机硫是否赋予了化石黄铁矿的特征硫同位素组成。这项工作将在约翰霍普金斯大学与Emmy Smith和Maya Gomes教授以及外部合作者Sara Pruss（史密斯学院）和Francis Macdonald（加州大学圣巴巴拉分校）合作进行。现代微生物实验将有助于限制微生物生物地球化学组成、营养循环和生态相互作用如何驱动石化、埋藏学偏差和硫同位素分馏。然后，对类似化石组合的分析提供了一种将这些发现应用于化石记录的方法，并测试与生物多样性和丰度有关的假设，因为它们与埋藏学偏见有关。这些综合分析还提供了一种应用硫同位素指纹图谱的方法，以测试黄铁矿化模型的应用，该模型可以解释岩石记录中的有机硫酸盐。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，而不仅仅是单一化石组合提供的信息。当我们试图了解像斯图特冰川这样的全球性冰川事件后环境和生物圈的演变时，这一点尤为重要。更广泛地说，这项工作将告诉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。

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.