Did biogeochemical methane cycling regulate the Neoarchean atmosphere?

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.

Beneficiaries and Relevance of Research:

The beneficiaries of this research beyond the scientific community will be the public sector and the general public, through outreach activities planned by the principal investigators. The question of how life evolved on Earth and how the planet became habitable are topics of great interest to the general public, and are amenable to outreach activities that can easily be presented to a broad audience. In turn, this fosters increased interest in and understanding of environmental science by the broader community. In addition, this research focuses on a critical time period in Earth history that is associated with the run-up to one of the most fundamental changes in the surface of our planet, the Great Oxidation Event. An increased understanding of the interactions between the atmosphere, oceans, and biosphere from this time period is critical to recognizing and interpreting the driving forces and response to major environmental change, and as such is of great interest to the general public as well. This work will additionally strengthen the quality and international recognition of research in England by fostering strong international collaborations with North America (the US), and by promoting interdisciplinary research within the environmental sciences (geochemistry and atmospheric chemistry). The post-doctoral researcher and student will receive a wide range of training that will enhance their skills in fields including analytical geochemistry, biogeochemical cycling, atmospheric and biogeochemical modelling, and Earth system history. These are ideal transferable skills to the employment sector, in either the academic realm or in industry, in consideration of global issues related to environmental changes on Earth.

Engagement with Users and Beneficiaries:

The principal investigators plan to highlight their work to the public wherever possible via public talks, popular science articles, and public outreach activities. The researchers involved in this study all have excellent records in this regard, and will continue to build upon this important area of activity. For example, PI Zerkle has contributed to numerous outreach programs aimed at the general public, including Space Day (at Pennsylvania State University) and Maryland Day (at the University of Maryland), and is planning a public exhibit at the Hancock Great North Museum in Newcastle on Earth surface oxygenation. Co-I Poulton gave a keynote lecture on the oxygenation history of the ocean at the 2009 British Science Festival and has given several other public lectures. Co-I Claire has developed planetarium shows for the Pacific Science Center (Seattle, WA) and is a founding member of Blue Marble Space Institute, which has partnered with NASA to bring the Cheltenham Festival's flagship science communication activity - FameLab - to the United States.

In particular, as part of this project we will host two workshops highlighting research on the Precambrian Earth, as discussed in Academic Beneficiaries above. Each of these workshops will entail a public lecture to be delivered by an international guest speaker (PP Farquhar is confirmed for one of these lectures, see letter of support). We are further committed to advertising our findings through the media and through popular articles. We also propose to highlight major outcomes of this proposal, as they are generated, through press releases at Newcastle University and the University of East Anglia. In addition we will engage with colleagues at these workshops and at international and national scientific meetings and foster future scientific development and additional collaborations.