In the previous post, we established that the different rock types, namely sedimentary, igneous and metamorphic rocks, reveal information about the processes involved in shaping our planet’s exterior environments as well as the interior conditions. Sedimentary rocks, in particular, are a fantastic rock record, known to capture indirect relations between the atmosphere (air) and the water bodies (lakes, rivers, seas, oceans) through time. Although, as rocks “age,” they mature (a process called diagenesis) and ultimately enter the destructive phase of the rock cycle that can change or erase the original environmental information captured at the time of sedimentation. As we go further back in time, well-preserved sedimentary rocks are rare, often difficult to access, and they undergo different degrees of alteration. That is why reconstructing the early evolution of Earth’s atmosphere and oceans is a difficult task. Nevertheless, researchers have been tackling this problem and developing a general narrative of how the Earth’s atmosphere has evolved through time by combining different scientific tools.
The Earth’s evolving atmosphere
There is no way of knowing if Earth had an atmosphere when the planet first formed. Although, if the early atmosphere did indeed exist, it didn’t stick around for long. The main reason why the primary atmosphere would have escaped to space was a weak magnetic field on primitive Earth. The magnetic field acts as a shield against Solar wind from the Sun and protects the atmosphere from being swept away (Figure 1). The primitive Earth would have been too hot for the so called ‘geodynamo‘ to operate and produce a strong magnetic field. First, the planet had to cool down and form segregated layers in its interior, so that heavier elements like iron and nickel moved towards the center, while the lighter elements, like silica, remained in the planet’s outer layers, and gases were expelled from the planet’s interior to the surface by volcanoes.
Over time, those gases accumulated and formed a second type of atmosphere composed of various greenhouse gases and water vapor but lacked free oxygen (O2). After the planet’s surface cooled enough to create a crust, water rained down to form the first oceans. As the Earth’s interior ‘differentiated‘ into the mantle, the outer core, and the inner core, the geodynamo produced a strong magnetic field, providing more protection from the Solar wind. The formation of the core and the mantle also affected the composition of the gases which were expelled from volcanos. It is hard to tell the exact composition of those volcanic gases, but likely resembled modern ones, such as water vapor (H2O), carbon dioxide (CO2), nitrogen (N2), sulfate (SO2), hydrogen gas (H2), and carbon mono-oxide (CO).
This second type of atmosphere existed until at least ca. 2.4 billion years ago, when photosynthesizing organisms started to release more O2 into the environment than was consumed by various chemical reactions. The first organisms to release oxygen as a by-product of their “bodily workings” were cyanobacteria. It took hundreds-of-millions-of-years from the first appearance of cyanobacteria to reach measurable amounts of O2 in the atmosphere at ca. 2.4 billion years ago. The time interval over which O2 levels reached at least 0.001% of present atmospheric levels (PAL) is called the Great Oxidation Event (GOE). This marked change in the atmospheric composition helped establish modern-like surface environments and ecosystems, however, evidence from the sedimentary rock record suggests that it may have taken another billion years for oxygen concentrations to reach present levels. One thing is certain: for the past ca. 500 million years, O2 has remained a significant and unique component of the Earth’s atmosphere that distinguishes it from other planets in our Solar system.
Text: Kärt and Ichiko
- Holland, H.D., 2006. The oxygenation of the atmosphere and oceans. Philos T R Soc B 361, 903–915.
- Kasting, J., 1993. Earth’s early atmosphere. Science 259, 920–926.
- Reddy and Evans, 2009. Palaeoproterozoic Supercontinents and Global Evolution. Geological Society, London 323, 1–26.