Critical new perspectives on molybdenum cycling under modern and experimental euxinic conditions: Tuning the paleoredox proxy

古代海洋化学的故事就是我们起源的故事。地质记录揭示了地球上只有10%的地方有动物的存在？S 46亿年的历史和现代人类的起源只是一瞬间的事情。大多数研究人员将第一批动物的出现与大气含氧量上升到足以支持复杂动物新陈代谢的水平联系起来。然而，地球历史的前90%是一个大气和海洋氧气状态发生深刻变化的故事，在地球历史的前半部分完全没有氧气，在接下来的一半时间里大部分时间都是没有氧气的深海。今天氧气充足，海洋中有大量的动物，但早期海洋的贫氧景观是我们祖先的摇篮。对这些早期生物及其周围环境的最佳看法来自于化石和沉积在古代海洋中的沉积物的化学性质，元素周期表中没有任何元素比钼(Mo)作为通往过去的窗口发挥了更大的作用。钼是氮生物循环中必不可少的营养物质，它肯定是最终导致海洋动物早期进化的路径和速率的决定因素，特别是因为海水中钼的浓度随着氧气的可获得性而变化很大。在硫化氢存在的情况下，这种金属和其他维持生命的金属可以从海水中清除出来，硫化氢可能会扩散到古代海洋的相当大一部分区域。最近，该研究小组和其他几个人的研究已经取得了很大进展，根据古代沉积物中钼的分布和同位素特性，为研究古代海水的化学提供了路线图，但关键问题仍然存在。我们对早期生命和海洋化学共同进化的理解，只能像对钼现在和过去如何循环的了解一样强烈--特别是在主导早期海洋的缺氧条件下。这项研究试图在实验室和现场使用新颖、尖端的分析方法，以填补我们在掌握钼生物地球化学方面的基本空白。关键问题之一是不同沉积环境中的钼吸收机制，特别是那些海水中含有丰富硫化氢的沉积环境。与有机质、有机体残留物有什么具体的关系，钼的同位素在与有机底物相互作用过程中会发生分馏吗？同位素是相同元素的原子，它们的质量和特定的反应行为不同，因此可以提供关于化学路径和控制环境因素的唯一信息，如氧气的可用性。在富含硫化氢的环境中，潜在的钼的全部范围是什么？在这些条件下，溶解的钼是如何以同位素的形式存在和分馏的？尽管这些化学问题是非常具体的，但其含义是广泛的，说明了对海洋条件进行指纹识别的能力，这些条件要么促进了早期生命的起源，要么挑战了早期生命的多样性。这项研究的影响将广泛扩展。更大的揭开生命历史的能力是预期的结果，研究人员将开发和完善与许多其他痕量金属及其生物地球化学循环更广泛相关的新分析方法，包括现代海洋中的金属。由于他们来之不易的经验，他们现在能够证明粒子加速器在地球生物学研究中的巨大作用。他们的目标是通过提供一种虚拟体验(网站和短期课程)，捕捉他们从初学者到专家的分析成熟过程，从我们经验的细节到更一般的后勤和科学细节(一种类似于傻瓜的同步加速器？)，从而为其他走同样路线的人铺平道路。他们计划高水平的本科生参与，这反映了加州大学S分校作为美国最具文化多样性的校园之一的地位。最后，为了跨越国际边界，研究人员计划与他们在北京的同事每月举行小组会议，以强调学生的陈述，促进双向的思想交流，并促进对早期海洋及其共同进化生命的更多合作探索。尽管方法多种多样，要求也很高，但他们的动机可以归结为一个简单的问题：我们从哪里来？

The story of ancient ocean chemistry is the story of our origins. The geologic record reveals the presence of animals over only the last 10% of Earth?s 4.6-billion-year history and the origins of modern humans only a flicker of time ago. Most researchers link the appearance of the first animals to a rise in atmospheric oxygen to a level high enough to support complex animal metabolisms. The first 90% of Earth history, however, is a story of profound change in the oxygen state of the atmosphere and ocean, with complete absence of oxygen over the first half of Earth history and an oxygen-free deep ocean for most of the next half. Oxygen abounds today, and the ocean teems with animals, yet the oxygen-lean landscape of the early ocean was the cradle of our ancestors. The best view of these early organisms and their surroundings comes from fossils and the chemical properties of sediments deposited within the ancient ocean, and no element in the periodic table has played a greater role than molybdenum (Mo) as a window to the past.Molybdenum, a nutrient essential in the biological cycling of nitrogen, must have been a determining factor in the paths and rates that led ultimately to the early evolution of marine animals, particularly since its concentration in seawater varies dramatically with the availability of oxygen. This and other life-sustaining metals can be scrubbed out of seawater in the presence of hydrogen sulfide, which probably spread through sizeable portions of the ancient ocean. Recent research by this group and a few others has come a long way in providing a roadmap to the chemistry of ancient seawater based on the distributions and isotopic properties of Mo in ancient sediments, but key questions remain. Our understanding of the co-evolution of early life and ocean chemistry can only be as strong as the knowledge of how Mo is cycled now and in the past?particularly under the oxygen-poor conditions that dominated the early ocean. This study seeks to use novel, cutting-edge analytical methods in the lab and in the field to fill essential gaps in our grasp of Mo biogeochemistry. Among the key issues and questions are the mechanisms of Mo uptake across diverse depositional settings, particularly those with abundant hydrogen sulfide in the seawater. What specifically are the relationships to organic matter, the remains of organisms, and are the isotopes of Mo fractionated during interactions with organic substrates? Isotopes are atoms of the same element that differ in their masses and specific reaction behaviors and so can provide unique information about chemical pathways and the controlling environmental factors, such as oxygen availability. What is the full range of potential Mo hosts in settings rich in hydrogen sulfide, and how does dissolved Mo speciate and fractionate isotopically under those conditions? Although these chemical questions are quite specific, the implications are broad and speak to the ability to fingerprint conditions in the ocean that either fostered or challenged the origins and diversification of early life. The impact of this study will extend widely. A far greater ability to unravel the history of life is an expected outcome, and investigators will develop and refine novel analytical methods with broader relevance to many other trace metals and their biogeochemical cycles, including those in the modern ocean. Thanks to their hard-earned experience they are now able to demonstrate the great utility of particle accelerators in geobiological research. They aim to smooth the path for others traveling the same route by providing a virtual experience (website and short course) that captures their analytical maturation from beginners to experts, spanning from the particulars of our experience to more general logistical and scientific details (a sort of ?Synchrotron for Dummies?). They plan a high level of undergraduate involvement that mirrors UCR?s status as one of the most culturally diverse campuses in America. Finally, in an effort to reach across international boundaries, investigators have planned monthly group meetings with their colleagues in Beijing designed to emphasize student presentations, to enhance the flow of ideas in both directions, and to foster additional collaborative exploration of the early ocean and its co-evolving life. Although the methods are diverse and demanding, their motivations distill down to a single simple question: where did we come from?