Of bubbles, brines, and biology: evaporite fluid inclusions and the evolution of earth's atmosphere

Abstract

The atmosphere is a dynamic, ever changing part of the Earth system that has evolved significantly over 4.6 billion years. This evolution is closely interwoven with the planet’s climatic and biological evolution, both driving and responding to changes in the latter. For example, atmospheric oxygenation has been implicated as both a cause and a consequence of major biological evolution and diversification. Similarly, carbon dioxide, the primary atmospheric control on planetary climates, has played a central role in the regulation of Earth’s habitability through time. To properly understand the interplay among these systems, it is necessary to constrain the composition, density, and isotope ratios of the atmosphere through time. Researchers have primarily relied on proxies and Earth system models to investigate these properties, and as a result, generalized trends in the evolution of specific gases have emerged. Yet these methods are inherently indirect, relying on measurements of system properties that covary with atmospheric species or internally consistent parameterized functions. Consequently, the absolute abundance of a gas may differ substantially between reconstructions, sometimes by orders of magnitude. Often the disparity widens further back in time, leading to great ambiguity and uncertainty in our long term understanding of the atmosphere’s evolution. To address these concerns, researchers have sought to sample the evolving atmosphere directly. Evaporites have emerged as viable archives from which such samples can be retrieved. Microscopic fluid inclusions within these minerals trap and preserve aliquots of paleoatmospheric gases. Through extraction and chemical analysis of these gases, the ancient atmosphere can be directly characterized. Indeed, many constraints have already been placed using these methods, particularly on the abundance of atmospheric oxygen. Although these constraints now span over a billion years, there has been a general hesitancy to accept these records, despite the fact that they represent the only direct samples of deep-time atmosphere. Herein, the nature of these inclusions and their capacity for accurate paleoatmospheric reconstructions are investigated. These systems are first examined through the lens of phase chemistry, where the heterogeneous entrapment of gas and brine imparts compositional and isotopic variability due to the differing solubilities between gases. To address this variability, the Method for Atmospheric Gas Partitioning from fluid Inclusions (MAGPI) was developed. In this approach, the ratios of inclusion gases are used to determine the relative fractions of air and brine, from which a unique correction is applied to isolate inferred atmospheric compositions. Subsequently, the influence of alteration and mixing processes are assessed. From first principles, existing methodologies are critiqued, and then improved upon by incorporating the MAGPI framework alongside stable isotope measurements. Attention is then focused on appending new analyses to, and reassessing existing analyses of, the fluid inclusion record of atmospheric evolution. As a case study, these techniques are applied to inclusion gases from 1.4 billion year old halite. Following thorough screening of the samples and partitioning of the released gases, direct constraints are placed on the abundances of atmospheric oxygen and carbon dioxide. From these constraints, it is argued that the planet maintained a moderate climate during this interval, and that oxygen levels were sufficient to support the earliest animals, even though they evolved significantly later. To extend this work, the same techniques are applied to existing datasets, resulting in a 1.4 billion year record of atmospheric oxygenation, as well as some of the oldest direct constraints on atmospheric CO2 to date. Through the adoption of phase-partitioning techniques and internally consistent frameworks for assessing alteration, it is shown that fluid inclusions can reliably capture and preserve meaningful signatures of the atmospheres evolution.