What is shelf life on a breath of air? Well, it just so happens that back in northern Ontario, it’s 1.4 billion years. Trapped within crystals of rock salt formed while a subtropical lake dried up, millions of microscopic bubbles have yielded a direct analysis of Earth’s Mesoproterozoic atmosphere in other words, air older than dinosaurs, older than plant life, and even older than most eukaryotes.
The halite formed from a shallow basin of the type which exists in Death Valley today. As the water concentrated in the weak warm light of the mid-Proterozoic climate, small pockets of water and gas became incarcerated in the forming crystals. The “fluid inclusions” are time capsules of sorts, locked away beneath layers of rock and untouched until scientists from Rensselaer Polytechnic Institute found a means of releasing them with integrity intact.
pulling the data from the inclusions has been a technological challenge in terms of accurately measuring the data, says Li. Also, the inclusions contain brine as well as air, but the gases like carbon dioxide and oxygen behave differently in water than in air, says Li. However, graduate student Justin Park was able to extract the data by using specialized equipment designed by graduate student Li and professor Morgan Schaller. Justin’s measurement of the carbon dioxide has never been done before, says Schaller. These samples are actual samples of ancient air, says Schaller.
The implications of these findings are astonishing. The direct analysis of atmospheric components demonstrates that oxygen levels were then 3.7% of present concentrations, which is significantly higher than earlier estimates indicating Mesoproterozoic oxygen levels. Although this amount of oxygen would be sufficient to support complex multicellular life forms, these would not appear for at least hundred million years. Atmospheric carbon dioxide was ten times higher than present levels and would have provided enough greenhouse effect to counteract the “faint young sun” and produce an ice-free environment with mild climates.
These results contradict the common view of the ‘boring billion’, the time period spanning the 1.8 to 0.8 billion years which has been viewed as uninteresting in terms of environmental and evolutionary dynamics. This was because the geochemical proxy indicated a low level of oxygen in the atmosphere, anoxia in the deep oceans, and low nutrient supply, which would limit the evolution of life. However, the halite proxy indicates the occurrence of brief oxygenation events in the atmosphere.
These oxygen pulses could be related to the evolution of red algae, which first appear in the fossil records around this period. Modern algae produce significant amounts of Earth’s atmospheric oxygen, alongside cyanobacteria Prochlorococcus, which alone accounts for 20% of the atmospheric content of oxygen. During the Mesoproterozoic era, an accumulation of algae in the oceans could have established an “oasis” of oxygen in the surface waters, even if an anoxic ocean persisted below.
From a climate modeling perspective, the high levels of CO₂ concentration solve a long-standing problem of paradox. Previous indirect estimates of concentration levels suggested a much lower concentration, which was not compatible with a non-glacial climate during this period. These concentrations of halite, together with temperature estimates from the salt itself, imply a warm climate, more similar to the present period, rather than the very cold or very warm conditions of the past and future eras.
The engineering that made the innovation possible is equally impressive as the scientific significance. The analysis of fluid inclusions requires meticulous control of sample handling, temperature, and pressure to prevent the analysis from being tainted by contamination of the included gases. The system designed by Park enabled stepwise liberation of the included fluids and gases with concomitant analysis, thereby distinguishing terrestrial from aqueous origins. There is immense scientific potential in this technique for analysis of ancient atmospheres, which can also be done with ice cores, amber, or volcanic glass.
In terms of the larger evolution of oxygen on Earth, these halite measurements are a valuable real-world reference point in a temporal record usually inferred through geological proxies. These results show that while it’s clear that a level of oxygen sufficient for animal life continued to be present throughout Proterozoic times following the Great Oxidation Event around 2.4 Ga, it’s not a smoothly rising curve between the Great Oxidation Event and Neoproterozoic Oxidation Event around 800 Ma. This kind of complexity simply supports a holistic explanation for why animal life did not quickly appear on a planet with sufficient oxygen levels. In opening these crystals that have waited for over a billion years, scientists have not only extended the history of Earth’s atmosphere but have also changed the history of Earth from one that is uneventful to one ofEvents.
