More than a billion years ago, in a shallow basin spanning what is now northern Ontario, a subtropical lake much like today’s Death Valley evaporated under the gentle heat of the Sun, leaving crystals of halite, a mineral made of sodium chloride that we more commonly call rock salt. It was a very different world from the one we know today. Bacteria were the dominant life form. Red algae (among the oldest eukaryotic organisms) had just entered the evolutionary scene, while complex multicellular life (animals and plants) would not appear for another 800 million years.
Microscopic image of fluid inclusions in 1.4 billion-year-old halite crystals, which preserve ancient air and brine. Credit: Justin Park/Rpi
As the water evaporated into brine, some of it remained trapped in tiny pockets inside the crystals, unchanging over time, bringing with it bubbles of air that could tell us, 1.4 billion years later, the composition of the primordial Earth’s atmosphere. A team of researchers from the Rensselaer Polytechnic Institute (Rpi) in Troy, New York, analyzed them after bringing to light these ancient crystals that remained buried in the sediments. The results were published last week in Proceedings of the National Academy of Sciences (Pnas).
“It’s an incredible feeling to open a sample of air that is a billion years older than the dinosaurs,” he comments Justin Parkstudent at Rpi and first author of the article.
Researchers have long known that fluid inclusions in rock salt crystals contain samples of Earth’s early atmosphere, but obtaining accurate measurements from such inclusions is no simple task. As we said, these crystals contain both air bubbles and brine, and gases such as oxygen and carbon dioxide behave differently in water than in air. To do this, the authors of this study used equipment specially built in the laboratory.
“The carbon dioxide measurements obtained by Justin have never been done before,” he said Morgan Schallerco-author of the study and responsible for the construction of this new equipment. “We have never been able to peer into this era of Earth’s history with this degree of accuracy. These are real samples of ancient air.”
What was in the air 1.4 billion years ago? The analyzes show that the Mesoproterozoic atmosphere (this is the name of the geological era to which these samples date back) contained 3.7 percent of the oxygen present today, a surprisingly high percentage, sufficient to support the complex multicellular animal life that would only appear hundreds of millions of years later. Carbon dioxide, however, was ten times more abundant than pre-industrial levels, enough to counteract the “weak radiation” coming from a still young Sun and to create a climate similar to the modern one.
Why, if there was enough oxygen to support animal life, did it take so long for it to actually appear? According to the authors, one reason may be that the sample captures only a snapshot of geologic time, and may therefore reflect a brief, transient oxygenation event in this long era that geologists jokingly call “boring billion” (English for “boring billion”) – an era in Earth’s history characterized by low oxygen levels, widespread atmospheric and geological stability, and little evolutionary change.
“Despite the name, having direct observational data from this period is incredibly important because it helps us better understand how complex life on the planet arose and how our atmosphere became what it is today,” Park continues.
Previous indirect estimates of carbon dioxide during the period indicated lower levels and were inconsistent with other observations showing the absence of significant glaciers during the Mesoproterozoic era. The higher measurements reported in this study, however, combined with temperature estimates from the salt itself, suggest that the Mesoproterozoic climate was milder than previously thought, comparable to that of today. Precisely in this long period, red algae also appeared, organisms that still contribute significantly to the global production of oxygen today. According to the authors, the relatively high levels of oxygen found in their analysis could also be a direct consequence of the increasing abundance and complexity of algal life. They may, in other words, have hit the infamous man’s turning point boring billion.
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