Origin & Evolution of the Atmosphere

Where did our atmosphere come from and how did it arrive at its present state? These are huge questions with huge and fascinating answers; here, we'll briefly explore these questions.

The Earth formed about 4.5 billion years ago, along with the other planets in our solar system as a cloud of gas and dust cooled and contracted and condensed, leading to the formation of many small particles that then accreted together to form the planets. During the later stages of formation, as the planets grew larger and larger, their gravitational pulls increased and they attracted large and larger fragments that collided with the planets in explosive impact events. The heat from these explosions was tremendous, and it led to the melting of much of the material on the surface of the planet -- perhaps to a depth of several hundred kilometers! This melting released the volatile, lighter elements that were locked up in the pre-existing rocks and these volatiles stayed in a gaseous state, bound to Earth by its gravity, thus forming a primitive atmosphere that was probably rich in CO₂ and H₂O with some N₂. Much of this early atmosphere was probably blown away when our Sun really lit up, but continued volcanic activity and asteroid impacts quickly built up a new atmosphere, again consisting primarily of CO₂ and H₂O with some N₂. The radioactive decay of an isotope of potassium (⁴⁰K) in rocks slowly added argon (Ar), an inert gas, to the atmosphere. Some of the water was broken down in the upper atmosphere by solar radiation, leaving a small amount of oxygen (the hydrogen is so light it escapes). This early atmosphere was probably much denser than today's, primarily due to the fact that now, there is a tremendous amount of CO₂ tied up in the form of limestone, which consists of CaCO₃.

This early atmosphere suffered frequent and catastrophic shocks due to the high frequency of asteroid impacts until about 3.8 billion years ago. Many of these impacts delivered enough energy to the Earth's surface to effectively sterilize it through intense heating, thus it is not surprising to see that life on Earth does not appear (there is no record of it) until after the bombardment subsided, around 3.5 Ga (Giga-annum = billion years). At that time, we see the first indications of life, fossilized in the form of stromatolites, irregularly-shaped, laminated build-ups of carbonate that are formed by cyanobacteria (modern equivalents of these stromatolites are seen today in Australia and the Bahamas). These cyanobacteria were (and still are today) photosynthesizers, meaning they utilized the sun's energy to combine CO₂ and H₂O to make the organic molecules needed for their growth, giving off oxygen as a by-product. The basic reaction describing photosynthesis is one of the most important of all chemical reactions:

CO₂ + H₂O (+light energy) ð CH₂O + O₂

The cyanobacteria live on the shallow sea floor, forming mats that trap sediment; the bacteria then grow up through the newly accreted sediment so they can photosynthesize some more and trap more sediment. This process goes on and on, leading to the construction of a stromatolite.

Figure 1.2 shows a possible history of the build-up of atmospheric oxygen. The photosynthesizers carried on for more nearly a billion years without altering the global atmosphere too much. Either they didn't produce much oxygen, or the oxygen was used up, preventing it from accumulating in the atmosphere.

This matter is at least partially resolved by the fact that a unique kind of sediment was forming at this time - banded iron formations (BIF for short). BIFs are beautiful, finely layered sedimentary rocks that consist of alternations of iron oxide minerals and silica-rich sediment. BIFs are unusual because they represent the results of a process that does not occur on Earth today, which makes it a little more difficult to understand how they formed. A variety of evidence leads to the following interpretation. In the absence of atmospheric oxygen, iron is dissolved by rain water as rocks undergo weathering; this iron ends up in the oceans, where it stays in solution as long as there is no oxygen. In shallow, warm parts of the oceans, cyanobacteria live and produce oxygen. This oxygen, also in solution, combines readily with the dissolved iron to form particles of iron oxide, that, being very dense, settle out onto the seafloor. This process thus removes both iron and oxygen from sea water, limiting the build-up of oxygen in the atmosphere. Interestingly, there are many stromatolitic structures in many BIFs. The banded nature of the BIF means that conditions were changing back and forth over time. Perhaps the cyanobacteria fluctuated seasonally and in their off-seasons, some other sediment was deposited. Some BIFs even show millimeter-scale laminations that may represent daily variations in the photosynthetic activity of the cyanobacteria.

The net result of the formation of BIF was to sequester an amount of oxygen equal to 25 times the present content of our atmosphere. To put this in perspective, it helps to look at the present day rate of oxygen production through photosynthesis. That value is about 1.9 X 1019 g/yr. At present, our atmosphere contains about 1.2 X 1021 g of oxygen. That means that at the present rate of oxygen production, it would take about 1600 years to produce an amount of oxygen that is tied up in BIF - this is a brief moment in geologic time, which makes it seem likely that early photosynthesis did not produce oxygen at anything near the rate that it does today.

The slow build-up of oxygen in the atmosphere is interesting for a couple of reasons. The first reason concerns ozone, which today forms a shield against harmful ultra-violet radiation from the Sun. Ozone (O3) is produced by reactions that start with oxygen, so with no atmospheric oxygen, there would have been no shield against ultra-violet radiation. Intense UV-radiation may have made for a more hostile surface environment that would have slowed the evolution of life. The second reason has to do with the fact that oxygen itself is a kind of poison for organisms since it attacks the bonds of organic molecules. So before life forms developed antioxidant mechanisms, an oxygen-rich environment would have been deadly. The deposition of BIF is therefore a very important part of the whole evolution of the atmosphere and biosphere.

Most of the BIF appears to have formed around 2 to 2.5 Ga; after that time, oxygen climbs slowly, and steadily to present values, with a jump around the time of the Cambrian explosion. It should be emphasized that the precise history of oxygen build-up in our atmosphere is not well known; Figure 1.2 is based on a few control points, some theory, and some speculation. The same is true of Figure 1.3, which shows the history of atmospheric CO2. In fact, the details of the CO2 history are even less well known, although there is general agreement that the basic story is one of declining atmospheric CO2 over time. Why does it decline? It is mainly due to the deposition of carbonate sediments, which is mainly controlled by chemical weathering of exposed rocks and transport of the weathering products to the oceans, where organisms aid in the formation and deposition of carbonate, thus locking up some atmospheric CO2 into long-term geologic storage. Here again, we see the important role that life has played in the evolution of our atmosphere.

This decline is especially interesting when you begin to think about the importance of CO2 in our greenhouse along with the fact that early in Earth's history, our Sun did not send as much solar energy to us. As can be seen in Figure 1.4, if we combine the present atmosphere with the changing solar luminosity, our planet should have remained frozen solid up until about 1.5 Ga. But, if you look at ancient sedimentary rocks, there is clear evidence of abundant running water and warm, tropical oceans, so something must have been different early in our history to prevent an icehouse condition. The most widely accepted answer to the problem of the faint young sun is greatly elevated CO2 levels, and in fact Figure 1.3, which shows the history of CO2 over time, is partly drawn such that the earth's climate would have been similar to today's.

Clearly, our atmosphere has undergone some profound changes during the last 4.5 billion years. But the atmosphere experiences change on much shorter timescales too and the processes that produce these variations are the ones that we are most concerned with when we study the climate changes that are critical to our future. Furthermore, the composition of the atmosphere also changes spatially. Our next goal, then, is to examine briefly some of the changes that occur on shorter timescales.