We are what we breathe

Oxygen: where would we be without it? 

The air we breathe today—inhaled from the Earth's lower atmosphere—is composed predominantly (about 78%) of nitrogen and a nice, agreeable amount (about 21%) of oxygen. Small amounts of argon, carbon dioxide, and other gases make up the rest. But this plentiful supply of oxygen— sufficient to sustain humans and the other oxygen-breathing species with which we share the biosphere—has not always enveloped the planet. Indeed, Earth's atmosphere has been evolving since it first formed early in the planet's history (more than 4 billion years ago). Scientists believe that Earth's earliest atmosphere was created from the outgassing associated with the volcanic activity that was prevalent during the planet's early differentiation (cooling of the molten Earth that formed the core, mantle, and crust). This earliest iteration of the atmosphere likely contained abundant carbon dioxide and water vapor (common volcanic gases), as well as other relatively heavy molecules (e.g., nitrogen and sulfur gases). Additional, lighter gases such as hydrogen would have escaped the Earth's gravitational field and escaped to space.

Artist's impression of the Earth's early atmosphere and oceans. Outgassing volcanoes released a mixture of gases (mostly carbon dioxide and water vapor) from the planet's interior to the atmosphere. As the surface of the Earth slowly cooled, water vapor condensed to form some part of the oceans. Credit: Lunar and Planetary Institute

During this period it is thought that the atmosphere contained less than 0.001% of the amount of oxygen that we find in our air today. Any 'free' molecules of this rare oxygen would have been chemically captured by dissolved iron or organic matter. Meanwhile, the oldest known fossils show that life has existed on Earth for at least 3.5 billion years (and possibly longer). These earliest preserved lifeforms are cyanobacteria fossilized within Archean-aged rocks from western Australia.

Two forms of fossilized cyanobacteria from the Bitter Springs chert (Australia).
Credit: J. William Schopf 

Cyanobacteria, which still exist today, are peculiar because they obtain their energy from photosynthesis. This process—more commonly known as the way plants use sunlight to convert water and carbon dioxide into chemical energy—meant that the cyanobacteria could thrive in Earth's early anoxic environment. Importantly, the photosynthesis of the ancient cyanobacteria also produced oxygen as a by-product. With the rise of the cyanobacteria, therefore, the amount of oxygen in the atmosphere steadily began to rise. For the first time, the previous 'oxygen sinks' (i.e., organic matter and iron) became saturated. In other words, the overall rate of oxygen production surpassed the rate of its removal by iron oxide sedimentation (a result of both increased bacterial colonization and of decreased volcanic activity) and the oxygen could accumulate in the atmosphere. By about 2.4 billion years ago, the Great Oxidation Event had occurred and the geologic record indicates that the concentration of atmospheric oxygen had significantly increased. 

Although there are various strands of evidence for the Archean's low atmospheric oxygen levels (e.g., the lack of oxidized iron in fossilized soils and large volumes of banded iron formations in Archean sedimentary rocks), the geologic information all relates to the lower atmosphere in particular. Until now, there has been no way to examine the makeup of Earth's upper atmosphere during the Archean. A new paper published last week in Nature, however, has provided a new twist to the story of our atmosphere's evolution.

In this new study, led by Andrew Tomkins from Monash University in Australia, 60 'fossil' micrometeorites were extracted from layers of limestone in the 2.7-billion-year-old Tumbiana Formation (in the Pilbara region, northwest Australia). Micrometeorites are extraterrestrial dust particles (up to about 2 mm in size) that survive entry through the atmosphere and are collected on the Earth's surface. As these particles fall through atmosphere, they experience maximum temperatures at altitudes between about 75 and 90 km (i.e., within the upper layers of the modern atmosphere). The small size of the micrometeorites means that many will completely melt during this passage and then rapidly re-crystallize. This 'quench-crystallization' occurs over a timespan of just a couple of seconds and the micrometeorites therefore chemically interact only with the upper layers of the atmosphere. Tomkins and his colleagues have thus used this understanding of modern micrometeorite behavior and applied it to their fossil samples so that they can probe the Earth's ancient upper atmosphere. 

Scanning electron microscope images of a selection of the fossil micrometeorites extracted from the Tumbiana limestone formation. Credit: Tomkins et al., 2016, Nature

Upon examination of the 60 sampled micrometeorites, it was found that they had diameters of between 8.6 and 50 micrometers and 'cosmic spherule' morphologies. The rounded form of the particles indicates that they all had fully melted during their journey through the atmosphere. In addition, all but one of the samples had compositions that were almost entirely iron-nickel metal (with no silicate minerals). Analyses for the interiors of 11 of the spheres were also conducted as part of the study and they revealed compositions dominated by the iron oxides magnetite (Fe₃O₄) and wüstite (FeO), as well as minor amounts of iron-nickel metal. These results thus demonstrate that most of the micrometeorites' original iron-nickel material had been oxidized during their encounters with the Archean upper atmosphere.

Given the long-held view that the Archean atmosphere had a very low oxygen content, these new results are quite a surprise. The highly oxidized nature of the micrometeorites suggests that there must have been abundant oxygen at altitudes above 75 km, at the time the micrometeorites fell to Earth. Tomkins et al., therefore propose a model for the Archean atmosphere in which an oxgyen-rich upper layer—with a similar oxygen concentration to today's—experienced minimal levels of mixing with an oxygen-poor lower atmospheric layer.

So for this postcard, I propose we send a piece of the Tumbiana Formation—along with its hidden treasure of micrometeorites—into space, as a representative of our special Earth and the life that it contains. Not only do the microscopic spherules tell an important story about the history of our life-sustaining atmosphere, but the limestone formation itself may be uniquely capable of telling this tale. The mineral wüstite is rarely found on Earth's surface, and is crucial to interpreting the extraterrestrial origin of the micrometeorites. Luckily, these microscopic particles were laid down in a rock formation that was once a system of highly alkaline lakes. In such pH conditions, wüstite has a low solubility and was thus able to survive in these micrometeorites for 2.7 billion years. Unfortunately, such conditions are rarely found within the geologic record and this set of micrometeorites from the Pilbara region may indeed be unique. Surely, therefore, they deserve the adventure of our interplanetary mission.