Table of Contents

An earlier article in Chemical Innovation (1) showed that if you believe that biology’s 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 3.7–5.0 bar.

If this is so, it raises several interesting questions. For example, how did the atmosphere get to that pressure 100–65 million years ago (Mya)? What was the pressure before that? And how did it drop down to today’s 1 bar? Although we have no definite answers to these questions, let us put forth reasonable possible explanations.

Figure 1. Three possible alternatives for the atmospheric pressure early in Earth’s lifetime, given that it was at ~5 bar, ~100 Mya.

What was the air pressure for the 97% of Earth’s life before the age of dinosaurs? We have three possible alternatives, as shown in Figure 1.

• The pressure could have been at 1 bar throughout Earth’s earlier life, risen to 4–5 bar ~100 Mya (just at the time when the giant fliers needed it), and then returned to 1 bar (curve A).
• The pressure could have been ~4–5 bar from Earth’s beginning, 4600 Mya; and ~65 Mya, it could have begun to come down to today’s 1 bar (curve B).
• The atmosphere could have started at higher pressure and then decreased continuously through Earth’s life to ~4–5 bar ~100 Mya and down to 1 bar today (curve C).

The third alternative seems to be the most reasonable, so let us pursue it. We will also look into the composition of Earth’s atmosphere, but we will first discuss Earth’s surface and see how it affects the atmosphere.

Earth’s surface

Because the atmosphere is largely influenced by the characteristics of Earth’s 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 2–30 cm/year or more. Although this may seem to be very slow to us on the human timescale, it is not slow on Earth’s 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 200–300 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 today’s 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 135–65 Mya.

All of this indicates that Earth’s surface is plastic, deform able, and mobile, with much mixing with the interior. We believe that this movement greatly influenced the atmosphere.

Earth’s 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, Earth’s early atmosphere was at very high pressure, up to 90 bar, and that it consisted primarily of CO₂.

If we are correct, why did Venus’s atmosphere remain at 90 bar while Earth’s decreased to a few bar during the age of dinosaurs and then declined to the 1 bar it is today? What happened to Earth’s CO₂ and by what mechanism did it virtually disappear?

Figure 2. Comparison of Venus with Earth. p = atmospheric pressure, r = radius, and ρ = density.

We compare Venus and Earth in Figure 2. The two planets are about the same size; however, Venus has no moon, whereas Earth has one of the largest moons in the solar system. Our moon has the same density as Earth’s crust, which suggests that the moon was formed by stripping Earth of some of its then fluid crust. If not for this loss, Earth’s crust, now only 5–30 km thick, might be 42 km thicker. Therefore, Earth’s crust should be thinner than that of Venus.

Being thinner, Earth’s crust was fragile and broke up under the action of the mantle’s convective forces. In contrast, Venus’s 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.

where H is the Henry’s Law constant. H depends on temperature, but is ~876–1000 bar per unit mol fraction of CO₂ (8, 9).

If we assume that Earth’s 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

As

Figure 3. The upwelling of Earth’s mantle material causes rejuvenation of the ocean bottoms.

mentioned earlier, our picture today is of continents “floating” above a circulating layer of mantle, which rises at mid ocean ridges and sinks elsewhere (Figure 3). The upwelling brings fresh minerals, including oxides of calcium, magnesium, and other elements, up from Earth’s interior. The oxides dissolve in ocean water and then combine with dissolved CO₂ to form carbonate deposits.

Today, vast deposits of sedimentary carbonate rocks are found on land and on ocean bottoms, >1,000,000 km³ throughout Earth’s 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. History of deposition of CO2 as carbonates. The red area represents continental deposits that “float” on denser material and are not subducted. The blue area represents ocean deposits. These are frequently subducted and therefore relatively young.

A detailed analysis by Hay (6) of the extensive measurements taken from around the world by Ronov and Yareshevsky (10) is summarized in Figure 5. Hay’s analysis shows that today the continents contain at least 2.82 × 10⁶ km³ of limestone, which are the remains of deposits over the past 570 million years that have not been washed to sea or subducted back into Earth’s interior. This is equivalent to a CO₂ atmospheric pressure of 38 bar. If we add the carbonates found on the ocean floor, the equivalent CO₂ atmospheric pressure rises to 55 bar. Integrating the values plotted in Figure 5 gives the progressive depletion of CO₂ from the atmosphere (Figure 6). Thus, CO₂ is recycled: 55–70 bar or more is accounted for on the surface of Earth (6, 7), and ~30 bar is in the process of being recycled in the planet’s interior.

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, 100–65 Mya, the oceans and the atmosphere shared the free CO₂ equally. Consequently, the pressure of CO₂ in the atmosphere was ~8–10 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 Earth’s history, how do you explain where the measured 55–70 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 Earth’s surface did not freeze in its early days, given a 25–40% lower solar luminosity at that time (11). These values represent the range of five estimates. With these lower luminosities, Earth’s average temperature would have been somewhere between –5 and –21 °C instead of the present 13–15 °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 atmosphere’s efficient greenhouse gas, was present at high concentration in those early times. Kasting and co-authors suggest a factor of 100–800 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.

Earth’s history

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 Earth’s 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 cyanobacteria—organisms 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 Earth’s 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, 350–280 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 10–20-m layer of dead organic matter (14), so that today’s 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.

Figure 7. Earth's proposed atmospheric history.

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).

Limestone caves

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.

Experimental verification

Besides the general findings supporting the theory of plate movement, perhaps the most tangible and unambiguous evidence for the recycling of Earth’s 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 Earth’s 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 Michigan’s 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 today’s 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 3000–27,000 times greater than normal, or 50–90% 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 planet’s evolution.

Making sense of it all

If we assume that Earth’s early atmosphere was very different, both in composition (mainly CO₂) and total pressure, that would answer some puzzling questions from a variety of disciplines.

• How did the flying creatures from the age of dinosaurs have enough energy to fly when physiology, biology, and aeronautics say that this was impossible?
• How could life have developed on Earth when astronomy says that Earth was too cold to sustain life?
• If Earth’s atmosphere had stayed at ~1 bar throughout its history, where did the equivalent of 50–70 bar of CO₂ in limestone and other carbonates on Earth’s surface come from?

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.

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Thomas J. Fitzgerald is a senior scientist in the space technology division of TRW Inc. (Redondo Beach, CA 90278; 562-596-8674).

Donald Pettit is a mission specialist (astronaut) at the National Aeronautics and Space Administration, Johnson Space Center (Houston, TX 77058-3696; 281-461-0630).