Ancient salt reveals a clear view of Earth’s atmosphere from 1.4 billion years ago, offering a glimpse into a bygone era of our planet's history. Over a billion years ago, a subtropical lake in northern Ontario resembled modern Death Valley, with heat driving evaporation and leaving salt behind. As the lake dried, halite crystals formed, trapping tiny pockets of brine and air. These pockets sealed a direct sample of Earth's atmosphere, preserving it for approximately 1.4 billion years. Researchers at Rensselaer Polytechnic Institute have now unlocked this ancient archive. Led by graduate student Justin Park and guided by geology professor Morgan Schaller, they analyzed gases trapped inside the halite, extending direct measurements of Earth's atmosphere far deeper into the past than ever before. Their work appears in the Proceedings of the National Academy of Sciences.
The period they studied falls within the Mesoproterozoic eon, spanning from 1.8 to 0.8 billion years ago. Life at that time was simple, dominated by bacteria with red algae just beginning to appear. Animals and land plants were still hundreds of millions of years away. However, the atmosphere captured in those crystals tells a more complex story.
Cracking open air older than dinosaurs is no easy feat. Fluid inclusions in halite contain both air bubbles and salty water, and gases like oxygen and carbon dioxide behave differently in water than in air, making it challenging to reconstruct the original atmosphere. Park developed innovative techniques to overcome this challenge. Using custom-built lab equipment, he separated gas signals and corrected for how each gas behaves in brine, allowing the team to extract reliable measurements from primary inclusions that formed when the salt crystals first grew.
"It’s an incredible feeling to crack open a sample of air that’s a billion years older than the dinosaurs," Park said. Schaller added that the advance goes beyond technical novelty, stating, "The carbon dioxide measurements Justin obtained have never been done before. We’ve never been able to peer back into this era of Earth’s history with this degree of accuracy. These are actual samples of ancient air."
The halite crystals were buried soon after forming and remained isolated from later contamination, giving researchers confidence that the gases reflect conditions at the time, not later changes. The measurements show that oxygen levels during this Mesoproterozoic window reached about 3.7 percent of modern atmospheric levels, higher than many scientists expected. This level of oxygen is sufficient to support simple animals from a metabolic standpoint.
Carbon dioxide levels were also striking, roughly ten times higher than preindustrial levels. This concentration would have helped warm the planet when the Sun was fainter than it is today, resulting in a climate not unlike the modern one. Schaller explained that these findings help resolve a long-standing puzzle, aligning with the absence of major ice ages during this era and pointing to a long stretch of relatively mild climate.
"They also raise a deeper question. If oxygen levels were sometimes high enough for animals, why did complex life take so long to appear?" Schaller pondered. Park cautioned against overgeneralizing from a single snapshot, suggesting that it may reflect a brief, transient oxygenation event in the long era known as the 'boring billion,' characterized by apparent stability with little change in climate or life.
Despite the name, Park emphasized the rarity and value of direct evidence from this period. "Having direct observational data from this period is incredibly important because it helps us better understand how complex life arose on the planet and how our atmosphere came to be what it is today," he said.
The team also measured the temperatures at which the fluid inclusions formed, averaging just over 31 degrees Celsius with some variation. Combined with high carbon dioxide levels, the data suggest a warm but stable world. Schaller noted that red algae emerged around this time and still play a major role in producing oxygen today. The rise of more complex algae may have pushed oxygen levels higher, at least temporarily, and plate tectonics may have contributed by altering nutrient flows and chemical cycles in the oceans.
"It’s possible that what we captured is actually a very exciting moment smack in the middle of the boring billion," Schaller said. The findings support the idea that life, climate, and the atmosphere evolved together, with oxygen potentially crossing critical thresholds earlier than once thought, even if animals did not immediately take advantage of those conditions.
The study reshapes how scientists think about Earth’s middle age, providing direct measurements that give researchers firmer ground for testing models of climate and biological evolution. It shows that stable, warm climates can persist alongside relatively low but meaningful oxygen levels. For future research, the methods open the door to studying other ancient salt deposits, offering insights into how atmospheric changes unfolded across deep time and improving models of how planets regulate climate and sustain life.
For humanity, the work offers context, demonstrating that Earth maintained habitable conditions long before complex life emerged. Understanding these balances may guide the search for life on other worlds and clarify how resilient and fragile planetary systems can be. The research findings are available online in the journal PNAS.
