Scientists Crack Open Ancient Salt Crystals to Reveal Secrets of 1.4 Billion-Year-Old Air More than 1.4 billion years ago, in what is now northern Ontario, a shallow subtropical lake much like Death Valley evaporated under the sun's heat, leaving behind crystals of halite - ordinary rock salt. These ancient crystals, untouched for over a billion years, have now yielded remarkable insights into the composition of Earth's atmosphere during a pivotal period in the planet's history. By cracking open these primordial salt samples, a team of geologists has uncovered tiny pockets of ancient air trapped inside. Through sophisticated chemical analysis, they were able to determine the concentrations of oxygen, nitrogen, argon, and other gases present in this billion-year-old atmosphere - a feat that was simply not possible until recent advances in analytical technology. The findings, published in the Proceedings of the National Academy of Sciences, provide a rare glimpse into a time when life on Earth was still in its infancy, dominated by single-celled organisms in the ancient oceans. This was a critical juncture, just before the Great Oxidation Event, when oxygen levels in the atmosphere began to rise dramatically and fundamentally transform the planet's ecosystems. "It's really an amazing opportunity to step back in time and see what the air was like more than a billion years ago," said Benjamin Linzmeier, a geochemist at the University of Wisconsin-Madison and lead author of the study. "These tiny fluid inclusions trapped in the salt give us a direct window into the past, without the need to rely on indirect proxies or models." The ancient lake where the salt crystals formed was located in what is now the Sibley Group, a geological formation near the town of Thunder Bay. At the time, this region was part of the supercontinent Rodinia, situated near the equator in a subtropical climate. As the lake evaporated, the dissolved salts and minerals precipitated out, layer by layer, creating the distinctive banded structure characteristic of evaporite deposits. Trapped within these layers were tiny pockets of ancient liquid and gas, protected from the ravages of time by the impermeable salt. By carefully extracting and analyzing the contents of these "fluid inclusions," the researchers were able to determine the atmospheric composition with unprecedented precision. The results were surprising: Oxygen levels in the 1.4-billion-year-old atmosphere were only about 10-18% - significantly lower than the ~21% oxygen we breathe today. Nitrogen, on the other hand, made up a much larger proportion of the ancient air, at around 70-80% compared to 78% now. Argon levels were also somewhat higher. This suggests that Earth's atmosphere was profoundly different in the middle Proterozoic eon, before the Great Oxidation Event that transformed the planet's chemistry and paved the way for the rise of complex multicellular life. At that time, oxygen was still a relatively scarce commodity, constraining the evolution of larger, more energy-hungry organisms. "What's really exciting is that we can use these fluid inclusions to directly test our models and theories about the early evolution of the atmosphere," said Linzmeier. "We've long had indirect evidence from things like sedimentary rocks and molecular fossils, but being able to measure the actual gas composition is a real breakthrough." The researchers noted that the low oxygen levels would have had significant implications for the dominant lifeforms of the era. Most modern animals, for instance, would have struggled to survive in such an atmosphere - their energy-intensive metabolisms simply couldn't function with less than a fifth the amount of oxygen available. Instead, the Proterozoic world was likely dominated by microbial mats, primitive algae, and other single-celled organisms that could thrive in the oxygen-poor conditions. These early life forms played a crucial role in gradually building up oxygen levels over hundreds of millions of years, paving the way for the rise of complex multicellular life. "This was a pivotal moment in the history of our planet, when the basic chemistry of the atmosphere was being transformed," said Linzmeier. "Understanding what that air was like gives us vital clues about the environmental pressures and evolutionary challenges faced by the earliest life on Earth." The ancient salt crystals also provided insights into other aspects of the Proterozoic world. By analyzing the relative abundance of different isotopes of argon, for example, the researchers were able to estimate the atmospheric pressure at the time, which appears to have been slightly higher than today. Additionally, the team detected trace amounts of gases like methane and carbon dioxide, which likely influenced global temperature and climate patterns back then. Further analysis of these trace components could yield even more information about the environmental conditions of the distant past. "These salt deposits are truly a remarkable geological archive, preserving secrets of the ancient world that we're only just beginning to unravel," said Linzmeier. "I'm excited to see what other insights we can glean as analytical techniques continue to advance." Indeed, the discovery underscores the value of studying Earth's deep history, which can provide crucial context for understanding the planet's present and future. By piecing together the puzzle of our atmospheric evolution, scientists hope to shed light on the delicate balance of gases that sustains complex life - and how that balance might shift in the centuries to come.