Did biogeochemical methane cycling regulate the Neoarchean atmosphere?

数十亿年前，年轻的地球与我们今天居住的地球大不相同，温度剧烈波动，大气中充满了有毒气体。了解我们是如何从那个不适宜居住的地方发展到今天这个由温和气候和大型氧基生命形式主导的世界，是地球科学的一个基本问题。一个重要的转变发生在大约25亿年前，被称为大氧化事件（GOE），当时地球大气中的氧浓度首次从接近零增加到现代水平的一小部分。自然科学研究的一个主要焦点是确定地球系统（包括生命）如何在环境中产生如此巨大的变化；然而，GOE究竟是如何以及为什么发生的仍然是一个谜。要理解早期地球向含氧环境的转变，就必须对古代大气的组成进行定量估计。使用大多数地球化学工具很难作出这些估计，这些工具往往反映的是在海洋环境中发生的过程。这项研究提出将硫的四种稳定同位素（直接反映大气中发生的化学反应）与将这些地球化学特征与大气成分联系起来的数值模型联系起来。另一组地球化学分析将使我们能够确定海洋的化学成分以及生物圈在同一时期的作用。这项研究的独特之处在于它结合了这些多种技术，我们将把这些技术应用于GOE之前直接沉积的保存完好的沉积物，以确定地球大气在这段时间内是如何发展的，以及海洋和生物圈是如何贡献和回应的。在这一时期，了解大气、海洋和生命之间的相互作用尤为重要，因为它代表着地球系统处于全球表面化学重大转变的边缘。我们对~2.65-2.5 Ga的沉积物进行了一组初步的类似分析，描绘了一幅在GOE之前不寻常的地球环境的诱人画面。这些分析表明，当时的大气不仅含氧量很低，而且还周期性地由一层有机颗粒（称为“雾霾”）主导，这种有机颗粒在甲烷含量很高的情况下产生，与土星的卫星土卫六上的情况类似。我们将扩展从这些初步分析中得出的假设，并探讨它们对地球表面化学的发展和地球历史上这一关键时期生命进化的意义。

Billions of years ago the young planet Earth was much different from the one we inhabit today, with wildly fluctuating temperatures and an atmosphere filled with toxic gases. Understanding how we got from that inhospitable place to the world of today, dominated by mild climates and large oxygen-based life forms, is a fundamental question in Earth sciences. One important transition occurred approximately 2.5 billion years ago (Ga), called the Great Oxidation Event (GOE), when the oxygen concentrations in Earth's atmosphere first increased from near zero to a fraction of modern levels. A major focus of research in natural science is determining how the Earth system (including life) has acted to produce such monumental changes in the environment; however, exactly how and why the GOE occurred remains a mystery. Integral to understanding the transition to an oxygenated environment on the early Earth are quantitative estimates of the composition of the ancient atmosphere. These estimates are difficult to make using most geochemical tools, which tend to reflect processes that occurred in the marine environment instead. This study proposes to link the four stable isotopes of sulfur, which directly reflect chemical reactions that occurred in the atmosphere, with numerical models tying these geochemical signatures to atmospheric compositions. An additional set of geochemical analyses will allow us to determine the chemistry of the oceans and how the biosphere was acting at the same time. This study is unique in its combination of these multiple techniques, which we will apply to well-preserved sediments deposited directly before the GOE, to determine how the Earth's atmosphere developed during this time, and how the oceans and biosphere both contributed and responded. Understanding the interactions between the atmosphere, oceans, and life is particularly crucial during this time period, as it represents an Earth system poised at the edge of a major transition in global surface chemistry.We have performed a preliminary set of similar analyses on ~2.65-2.5 Ga sediments that paint a tantalizing picture of an unusual Earth environment directly before the GOE. These analyses point to an atmosphere that was not only very low in oxygen, but was also periodically dominated by a layer of organic particles (termed "haze") produced at high methane levels, similar to that seen on Saturn's moon Titan. We will expand upon the hypotheses developed from these preliminary analyses and explore their significance for the development of Earth surface chemistry and the evolution of life during this critical period in Earth history.