Earths atmosphere before the age of dinosaurs
An earlier article in Chemical Innovation (1) showed that if you believe that biologys mouse-to-elephant curve also applies to the flying creatures of the past, and if you also trust aerodynamic theory (which applies equally to flying insects, birds, and airplanes), then the giant flying creatures of the dinosaur age could only fly if the atmospheric pressure was much higher than it is now: at least 3.75.0 bar.
If this is so, it raises several interesting questions. For example, how did the atmosphere get to that pressure 10065 million years ago (Mya)? What was the pressure before that? And how did it drop down to todays 1 bar? Although we have no definite answers to these questions, let us put forth reasonable possible explanations.
The third alternative seems to be the most reasonable, so let us pursue it. We will also look into the composition of Earths atmosphere, but we will first discuss Earths surface and see how it affects the atmosphere.
Because the atmosphere is largely influenced by the characteristics of Earths surface, let us consider its history. Evidence shows that fresh mantle material upwells at fractures in midocean, spreads to the continents, and then sinks back into the interior. Recent measurements (2) show surface movements of 230 cm/year or more. Although this may seem to be very slow to us on the human timescale, it is not slow on Earths timescale. For example, measurements indicate that South America and Africa separated a mere 125 Mya (3); you could have walked directly east from New York to the Sahara Desert 155 Mya (4). The Atlantic and Pacific Ocean floors are swept clean and are replaced by fresh upwelled material roughly every 200300 million years (4). Given that Earth is 4600 million years old, enough time has elapsed for >15 exposures of completely fresh solids on the ocean floors.
During this period, the crust floating on the mantle has wandered about. Current thinking is that todays continents were all part of a super landmass <200 Mya. What about the previous 4400 million years? How many times did the crust split apart and rejoin? Our clues come from the distribution of various life forms. For example, because dinosaur skeletal remains have been found on all continents, this suggests that the landmasses were joined 13565 Mya.
All of this indicates that Earths surface is plastic, deform able, and mobile, with much mixing with the interior. We believe that this movement greatly influenced the atmosphere.
Earths original atmosphere
Geologists believe that most of the carbon on the young, hot Earth, >4000 Mya, was in the form of gaseous carbon dioxide, carbon monoxide, and methane. With time, the CO and CH4 reacted with oxide minerals and were transformed into CO2. These reactions did not change the total amount of carbon in the atmosphere.
Our sister planet and nearest neighbor, Venus, has an atmosphere of 90 bar pressure, consisting of 96% CO2 (5). Why should Earth be so different? Ronov measured the equivalent of at least 55 bar of CO2 tied up as carbonates around the world (6), whereas Holland estimates that at least 70 bar of CO2 is bound as carbonate materials (7). These carbonates had to come from the atmosphere, by way of the oceans, so we propose that, after the original oxidation of CH4 and CO, Earths early atmosphere was at very high pressure, up to 90 bar, and that it consisted primarily of CO2.
If we are correct, why did Venuss atmosphere remain at 90 bar while Earths decreased to a few bar during the age of dinosaurs and then declined to the 1 bar it is today? What happened to Earths CO2 and by what mechanism did it virtually disappear?
Being thinner, Earths crust was fragile and broke up under the action of the mantles convective forces. In contrast, Venuss thicker crust remained rigid and did not permit the mechanisms that removed the CO2 from its bound state.
In addition, because Venus is closer to the Sun and hotter than Earth, free liquid water cannot exist on it, whereas Earth has giant oceans that cover two-thirds of the planet. The oceans played an important secondary role in removing CO2 from the atmosphere.
Dissolution of CO2 in Earths oceans
At an atmospheric pressure of ~90 bar, a considerable amount of CO2 would dissolve in the oceans. CO2 dissolves in water according to the equilibrium relationship
If we assume that Earths oceans are 2 km deep, the values given above imply that at equilibrium, for each mole of CO2 in the atmosphere, there is one mole of CO2 dissolved in the ocean. So an atmosphere originally consisting of 90 bar of CO2 would decrease to 45 bar of CO2 by dissolution alone; however, another factor acts to lower the concentration of CO2 in the atmosphere even further.
Reaction of CO2 with upwelling minerals
We assume from reaction kinetics that the rate-controlling step of this reaction is the upwelling of the alkaline oxides at the tectonic plate boundaries and not the solution and transport of CO2 from the atmosphere to seawater. On the basis of a conservative assumption of a roughly constant spreading rate of the seafloor, this means that the concentration of CO2 in the atmosphere decreased roughly linearly with time (zero-order kinetics). When the carbonates sink back into the mantle, they are heated and decompose, releasing captured CO2, which returns to the atmosphere through volcanic eruptions and to the ocean from vents in the ocean floor (Figure 4).
Today, vast deposits of sedimentary carbonate rocks are found on land and on ocean bottoms, >1,000,000 km3 throughout Earths crust. Above the continents, the CO2 was taken up by rainwater and by groundwater. This CO2-rich water reacted with rocks to form bicarbonates, followed by transport to the ocean and precipitation as calcium and magnesium carbonates. In the ocean, dissolved CO2 combined with the calcium hydroxide to form deposits of chalk, or it was taken up by coral, mollusks, and other living creatures to form giant reefs. A study of the distribution through time of these deposits gives us clues to the history of CO2 in the atmosphere.
Figure 5 verifies the earlier statement that the present oceans are relatively young because they contain limestone not older than 200 million years. On the other hand, the continental landmasses are much older because, 10065 Mya, the oceans and the atmosphere shared the free CO2 equally. Consequently, the pressure of CO2 in the atmosphere was ~810 bar in the age of the flying creatures (Figure 6).
The geological evidence is consistent with and lends support to the physiological and aerodynamic arguments (1) that the atmospheric pressure was definitely higher in the age of dinosaurs than it is today. If you reject this argument and if you prefer to believe that the atmosphere was at 1 bar throughout Earths history, how do you explain where the measured 5570 bar of CO2 in limestone and other carbonates came from?
The astronomical argument
From the viewpoint of the modern theory of stellar evolution, Sagan and Mullen discussed the faint early sun paradox, which asks why Earths surface did not freeze in its early days, given a 2540% lower solar luminosity at that time (11). These values represent the range of five estimates. With these lower luminosities, Earths average temperature would have been somewhere between 5 and 21 °C instead of the present 1315 °C. With frozen oceans covering our planet, how could life have established itself and thrived under these inhospitable conditions?
One reasonable answer to this question is that CO2, the atmospheres efficient greenhouse gas, was present at high concentration in those early times. Kasting and co-authors suggest a factor of 100800 times as high as today, all at 1 bar (12, 13); however, he did not consider the possibility of a higher total pressure of the atmosphere, as we do here.
As our young planet cooled and condensed into a solid, it was surrounded by a thick, soupy atmospheric mixture. Hydrogen and hydrogen-containing compounds combined with oxygen to form the water that became oceans, while the carbon-containing compounds, principally CO and CH4, combined with oxygen to form CO2 at high pressure. All this took about half of Earths lifetime, and it left the atmosphere depleted of oxygen.
Life probably got its foothold in the oceans of the barren planet as blue-green algae and cyanobacteriaorganisms that did not need oxygen to live. Photosynthesis had not yet been invented by plant life. So the reaction that sustained these life forms was
These algae spread throughout Earths oceans.
After about a billion years of additional experimentation, life came up with its most important invention, photosynthesis, and so learned to live off the abundant CO2 of the atmosphere plus sunlight and thereby invade the landmasses. Land plants evolved and lived by the reaction
During the Carboniferous period, 350280 Mya, these plants proliferated widely, covering the land surfaces with lush forests of giant ferns, trees, and plants of all types. Because the atmosphere was rich in CO2, but very poor in oxygen, dead plant material did not decompose rapidly, so layer upon layer of it was laid down in thick blankets that would transform over time to coal.
It is estimated that each 1-m thickness of coal comes from the compression of a 1020-m layer of dead organic matter (14), so that todays 10-m thick coal seam represents an original 100 m of decayed material. Such a thick layer of decaying matter is something that we do not see anywhere today. Tropical forests today only support a very thin layer of decaying matter because of rapid oxidation. Thus, 100-m-thick layers can only occur if the atmosphere discourages oxidation. This is additional strong evidence that the atmosphere in those distant times was rich in CO2, but poor in oxygen.
With time, the concentration of CO2 steadily decreased, primarily because of the formation and deposition of limestone and other carbonaceous materials. CO2 was also lost by photosynthesis followed by the deposition of carbonaceous substances such as coal, petroleum, peat, oil shale, and tar sands; however, this loss was quite minor. Calculations show that the deposit of what are now considered fuel reserves lowered the atmospheric CO2 by <<1 bar.
At the same time, the concentration of oxygen slowly rose. These two changes, the decrease in CO2 and the rise in oxygen, thinned the forests and the dead material began to be oxidized more rapidly, so that dense layers of dead organics were no longer deposited. Evidence of this change in atmospheric conditions is that we cannot find any massive coal deposits younger than 65 million years.
Animal life found this changed atmosphere to its liking, so mammals and dinosaurs flourished, first as very small creatures but then increasing in size as a result of evolutionary competition. This led to the giant flying creatures close to the end of the dinosaur age. It could be that these creatures died out as the total pressure of the atmosphere dropped below their sustainable level (Figure 7).
There are many limestone caves throughout the world, some of which are several kilometers long. These caves are all relatively young, most of them <100 million years old, and were carved by running water, which dissolved the limestone. This tells us something about our atmosphere as well.
Because of its high concentration in the atmosphere, CO2 dissolved in rainwater and groundwater, and the reaction
was driven to the right. When the atmosphere becomes lean in CO2, the reaction shifts to the left. The fact that the limestone caves were formed relatively recently indicates that the CO2 concentration in the atmosphere was very high long ago, leading to the deposits of limestone, but became very low recently, allowing limestone to dissolve.
Besides the general findings supporting the theory of plate movement, perhaps the most tangible and unambiguous evidence for the recycling of Earths material from crust to deep mantle and back again comes from a recent report by Daniels and co-authors (15).
In high-CO2 atmospheres and other hostile environments, life forms can take advantage of free energy in an amazing range of environments: above the boiling point and below the freezing point of water, in pressures as high as 300 bar, in oxygen-rich and oxygen-poor environments, and in the presence and absence of sunlight (16, 17). On a more familiar level, the microbe that produces champagne bubbles operates at pressures up to 7 bar of CO2.
Other estimates of CO2 concentrations
Researchers have speculated that the CO2 concentration may have been somewhat higher in the past than it is today. Studying carbon exchange between mantle and crust, Des Marais suggests that 3000 Mya, the atmosphere contained at least 100 times as much CO2 (or 0.03 bar) as it does today (18).
Holland (7) estimates that Earths earliest atmosphere contained up to 20 bar of CO2 and that ~10 bar could conceivably have persisted for several hundreds of millions of years (19). Many other such proposals have been put forth.
Plant growth at high CO2 concentrations
It is pertinent to ask whether any experiments have been performed to suggest whether life could thrive at higher CO2 concentrations. Pine and aspen trees grown at the University of Michigans biological station at Pellston, were found to respond dramatically to elevated CO2 levels. They grew 30% faster than normal trees at about double the normal CO2 level (700 ppm) (20).
However, to test our speculation we need to see if plants can survive, not at double todays CO2 concentration, but at thousands of times higher. We put this proposal to the test by growing plants in 32 sealed containers (1- and 2-L plastic soda bottles containing weighed amounts of CO2) at pressures from 2 to 10 bar. These conditions gave CO2 partial pressures 300027,000 times greater than normal, or 5090% CO2.
Of the species tested, Taxodium, Metasequoia, Araucaria, Equisetum, and Sphagnum grew best at these higher pressures; one specimen of Taxodium grew 7 cm over 2 years at 2 bar (50% CO2). In general, however, plant growth was considerably slower than at 1 bar. Mosses, ferns, and flowering plants died within a month at these high CO2 levels.
The poor growth observed in these experiments is most likely due to the buildup of product gases in the sealed containers, rather than high CO2 pressure, and therefore these results could be flawed. We would expect that vigorous growth would be observed in a continually rejuvenated atmosphere. Although present-day plant life is probably not adapted to living at the very different atmospheres and pressures of the past, our preliminary experiments do suggest that a dense CO2 atmosphere could have existed on early Earth without violating any known constraints on the planets evolution.
Making sense of it all
If we assume that Earths early atmosphere was very different, both in composition (mainly CO2) and total pressure, that would answer some puzzling questions from a variety of disciplines.
This picture of high CO2 concentration and high pressure in the past also explains why most massive coal seams are older than 65 million years and why most limestone caves are younger than 100 million years.
Although we do not know the values for the atmospheric pressure in those early times, and although each of the arguments in this paper only leads to suggestions, when taken together, the evidence from these various sources leads to the same conclusion: The atmospheric pressure was higher in the past than it is today and consisted primarily of CO2. This hypothesis presents a picture of our evolving planet that should be examined and that could have interesting consequences.
Octave Levenspiel is professor emeritus of chemical engineering at Oregon State University (Corvallis, OR 97331-2702; 541-753-9248; firstname.lastname@example.org).