Analysis performed on thousands of ancient continental rock reveals the complex interplay of geological and biological process that prepared Earth for human life.
Multicellular life (like humans) requires abundant oxygen in the atmosphere. Geological studies indicate that oxygen was not a permanent component of Earth’s atmosphere until 2.5 billion years ago—even though photosynthetic organisms appeared hundreds of millions of years earlier. A recent study of ancient rocks bracketing this time period shows how changes in the mantle worked in concert with biological organisms to establish oxygen as a stable part of Earth’s atmosphere.
Our family swims in our backyard pool to combat the summer heat. But before we can dip a toe in the refreshing water, we must prepare the pool for use. That means assembling the frame, installing the lining, and putting the hose in to start filling the pool with water. Often, I find small holes in the bottom of the pool and, on occasion, forget to plug the drain. Left unchecked, these holes prevent the pool from filling, even with water gushing from the hose. Processes that established oxygen in Earth’s atmosphere faced a similar situation.
For nearly two billion years, Earth’s atmosphere contained no oxygen. The reasons for Earth’s lack of atmospheric oxygen are (1) oxygen reacts very rapidly with gases thought to comprise Earth’s early atmosphere; and (2) nothing operating on the early Earth produced oxygen in significant quantities. Around 2.7 billion years ago, photosynthetic organisms (those that use sunlight to produce energy and release oxygen in the process) appeared in abundance. Yet even with this potent source of oxygen, the geologic record shows no evidence for oxygen in the atmosphere for the next few hundred million years. As it turns out, geological activity below Earth’s surface likely served as a big hole, draining the oxygen from the atmosphere.
To understand the situation, two scientists assembled a database including analysis of more than 70,000 crustal rock samples with ages spanning the last four billion years.1 Such an extensive repository allows for a broader exploration of how geological processes changed over time. A geochemical timeline built using the database shows that the fraction of mantle-melt included in crustal rocks clearly changed around 2.5 billion years ago. The timing of this change corresponds to the increase of oxygen in Earth’s atmosphere.
This composition shift may result from a change in how plate tectonics operated or from an abrupt decrease in melting due to the mantle’s gradual cooling (similar to how a small change in temperature causes H2O to change from liquid to solid). While the data cannot distinguish between these two options just yet, it does reveal that the decreased melt fraction in rocks was responsible for plugging the atmospheric oxygen hole.
Volcanic gases derived from mantle rock contain more abundant quantities of reactive, iron oxide-based gases that quickly remove any oxygen in the atmosphere. The decreased fraction of mantle-melt occurring 2.5 billion years ago would lead to a decrease in the volcanic gases that react with oxygen. Consequently, the oxygen produced by photosynthetic organisms would begin to accumulate in the atmosphere.
Because these processes operate beneath miles of rock (and happened billions of years ago), geologists are still piecing together the puzzle of Earth’s history. These results add another puzzle piece revealing the dynamic relation between geological activity (plate tectonics and volcanism) and biological growth (photosynthetic life) that must happen to make Earth habitable for humanity.