Probing Earth's earliest ecosystems: a multi-proxy study of the ~2.7 Ga Belingwe Greenstone Belt, Zimbabwe

自地球历史的最早阶段以来，生物学一直是全球变化的主要驱动力。早在38亿年前，生命就在地球上首次进化，但在最初的30亿年里，它完全由缺乏细胞核的小型单细胞生物（原核生物）组成。与主要使用氧气和有机碳作为呼吸燃料的大型真核生物不同，原核生物具有多种代谢方式，可以从多种化合物（包括硫化物、甲烷甚至有毒金属）中产生能量。这些代谢催化的化学反应只有在生物干预下才能迅速进行，其产物可能对环境的化学性质产生不可挽回的影响。 现代地球环境带有当前和过去生物化学反应的无可辩驳的印记，过去环境的地质记录也是如此。大约24亿年前，地球的大气层主要由二氧化碳和甲烷（几乎没有氧气）组成，海洋富含溶解的铁，并定期含有硫化物。这种环境不适合大型多细胞生物，但原核生物（“微生物”）生态系统蓬勃发展。大约27亿年前，被称为蓝藻的生物体（现代植物的前身）进化出了通过在阳光下将H2O与CO2结合来产生能量和生物质的能力。这种新发展的代谢，称为产氧光合作用，是一项重大的生物创新，大大提高了全球碳循环的效率。对我们的行星变化历史特别重要的是-这种新陈代谢的废物是分子氧，随后开始在环境中积累，这是有史以来第一次。大气中氧气的最终积累，被称为大氧化事件，是动物和多细胞生物进化的先决条件，并最终使我们今天居住的全球生物圈成为可能。 尽管在早期地球上逐渐氧化的重要性，地球科学家仍然缺乏对古代生态系统如何促进氧气生产和对环境中分子氧的反应的基本了解。解开全球碳固定和氧气生产之间的反馈的核心是了解与O2反应的其他生物所需营养物质循环的变化。氮，特别是，是无处不在的生命和需要形成几乎所有的生物分子，包括核酸（DNA和RNA）和蛋白质。海洋氮循环在很大程度上是由生物过程驱动的，生物过程产生的变化可以在岩石记录中保存的含氮化合物和同位素中测量。 这项研究旨在研究早期微生物生态系统中元素转化的相互作用，使用对约27亿年前形成的原始沉积物的地球化学分析。对该项目至关重要的是新的岩心，这些岩心对这一时期的岩石保存得非常好，并包括一些最早的微生物生态系统（可能包括蓝藻）的证据。我们将测量地球化学N、C和S循环的替代物，沿着额外的氧可用性地球化学分析，以研究早期生物圈演化过程中氧、碳、硫和氮循环之间的相互作用。这些来自约27亿年前岩石的记录将有助于我们对地球表面环境的化学和生物进化的基本理解，这一时期与蓝藻进化密切相关，是生物圈氧化和地球上复杂生命扩散的先决条件。

Biology has been a major driver for global change since the very earliest stages of our planetary history. Life first evolved on Earth as early as 3.8 billion years ago, but for the first ~3 billion years it was composed entirely of small unicellular organisms lacking a nucleus (prokaryotes). Unlike their larger eukaryotic counterparts which mainly use oxygen and organic carbon as fuel for respiration, prokaryotes have diverse metabolisms that produce energy from a wide array of chemical compounds, including sulfide, methane, and even toxic metals. These metabolisms catalyse chemical reactions that only proceed rapidly with biological intervention, and their products can have an irrevocable effect on the chemistry of the environment. The modern Earth environment carries the irrefutable imprint of current and past biochemical reactions, as does the geologic record of past environments. Before ~2.4 billion years ago, Earth's atmosphere was dominated by carbon dioxide and methane (with little to no oxygen), and the oceans were rich in dissolved iron and, periodically, sulfide. This environment was inhospitable to large multi-cellular organisms, but prokaryotic ("microbial") ecosystems thrived. Sometime around ~2.7 billion years ago, organisms called cyanobacteria (the precursors to modern plants) evolved the ability to generate energy and biomass by combining H2O with CO2 in the presence of sunlight. This newly developed metabolism, termed oxygenic photosynthesis, constituted a major biological innovation and significantly increased the efficiency of global carbon cycling. Of particular significance to our history of planetary change - the waste product of this metabolism was molecular oxygen, which subsequently began to accumulate in the environment for the first time ever. The eventual buildup of oxygen in the atmosphere, termed the Great Oxidation Event, was a prerequisite for the evolution of animals and multi-cellular organisms, and eventually enabled the global biosphere that we inhabit today. Despite the importance of progressive oxygenation on the early Earth, geoscientists still lack a fundamental understanding of how ancient ecosystems contributed to oxygen production and responded to molecular oxygen in the environment. Central to unravelling feedbacks between global carbon fixation and oxygen production is understanding the changes in the cycling of other biologically-required nutrients that react with O2. Nitrogen, in particular, is ubiquitous to life and required for the formation of nearly all biomolecules, including nucleic acids (DNA and RNA) and proteins. The marine nitrogen cycle is driven largely by biological processes which produce changes that can be measured in nitrogen-bearing compounds and isotopes preserved in the rock record. This research seeks to investigate the interplay of elemental transformations in early microbial ecosystems, using geochemical analyses of pristine sediments that formed ~2.7 billion years ago. Of central importance to this project are new drill cores that are extremely well-preserved for rocks of this time period, and include some of the earliest evidence for fossilized microbial ecosystems (possibly including cyanobacteria). We will measure proxies for biogeochemical N, C, and S cycling, along with additional geochemical analyses for oxygen availability, to examine interactions between the oxygen, carbon, sulfur, and nitrogen cycles during early biospheric evolution. These records from ~2.7 billion year old rocks will contribute to our fundamental understanding of the chemical and biological evolution of Earth's surface environments during the time period most closely associated with cyanobacterial evolution, a prerequisite to biospheric oxygenation and the proliferation of complex life on Earth.