Plants As A Control On C02

Since the beginning of photosynthetic life on earth, plants have likely had a big influence on the C02 level in the atmosphere. Green and (especially) blue-green bacteria, the precursors and distant cousins of modern-day green plants, began to spread through the oceans about 3.5 billion years ago. They were a source of oxygen, pouring out this highly reactive corrosive gas, which gives life to us but acts as a poison for many of the more primitive bacteria. At the same time, these photosynthesizers acted as a trap for carbon, but not in terms of standing biomass as in today's forests, for there would have been very little living carbon stored at any one time in all the green and blue-green bacteria in the world. Instead they left carbon in debris, dead cells buried in sediment that added up over all the millions of years to a huge amount of C02 taken out of the atmosphere. Still dispersed through the world's sedimentary rocks is a vast store of organic carbon, put there mainly by marine algae. This all adds up to an amount of carbon many times greater than the amount in C02 presently in the atmosphere. As the deep buried carbon reservoir increased in size over time, oxygen concentrations in the atmosphere would have risen. This is because carbon not buried in rocks tends to accumulate as C02 in the atmosphere, holding oxygen as well as carbon. When more carbon is buried, the oxygen is left behind. If all the dead carbon fixed by plants had quickly been able to oxidize back into C02, the oxygen left behind in photosynthesis could not have built up in the atmosphere— because when the dead plant cells decayed and oxidized back to C02, this would have taken up exactly the same amount of 02 as was initially released in photosynthesis. Balanced by only the living biomass of plants and the dead carbon in soils at the surface, the oxygen concentration in the atmosphere would be far lower: much less than one percent. As it is, with most of the organic carbon out of reach below the surface, oxygen has accumulated to very high levels—a fifth of the atmosphere.

It is likely that the buried organic carbon reservoir in rocks has also undergone significant fluctuations, sometimes storing up extra carbon and sometimes releasing it. Unlike changes in C02 brought about by volcanic output and weathering (see below), this variability in the organic carbon reservoir would have been paralleled by changes in oxygen concentration, because organic carbon released from rocks will always tend to react with oxygen in the atmosphere to form C02. So, carbon released from rocks uses up oxygen from the atmosphere. Some calculations have it that about

Models Measurements

GEOCARB III ■ Royer Compilation

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Figure 7.3.

Estimated C02 concentrations in the atmosphere over the last several hundred million years (Phanerozoic C02), using different methods. From:

200 300 400 Millions of Years Ago

Roger Rhode, after D. Royer.

200 300 400 Millions of Years Ago

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Models Measurements

GEOCARB III ■ Royer Compilation

Figure 7.3.

Estimated C02 concentrations in the atmosphere over the last several hundred million years (Phanerozoic C02), using different methods. From:

450 million years ago the oxygen concentration in the atmosphere was only 15%, instead of the present 21%, because there was so little organic carbon held in the sorts of rocks that existed at that time. At that time the C02 concentration in the atmosphere would have been something like 15 to 20 times what it is now (Figure 7.3). Over the tens of millions of years that followed, land plants evolved from seaweeds and spread across the continents. Where they formed the first forests in swampy river deltas, they laid down undecayed carbon as peats. Some of these peat layers were compressed into coal, while others were washed away and incorporated as fragments into the sediments of deep ocean floors. Geologists who study the chemical balance of rocks (geochemists) suggest that the huge amount of carbon taken out of the atmosphere by undecayed parts of land plants was enough to cause atmospheric C02 levels to plunge down to levels similar to the present. For instance, in the environment in general at that time there was a big decrease in the abundance of the carbon-12 isotope, which is preferentially taken up by plants during photosynthesis (see Chapter 8). This suggests that plants were sucking away a lot of carbon—especially carbon-12—and that it was ending up held in undecayed organic material. In contrast, the oxygen level between around 300 and 150 million years ago might have stood at 25%, or even 30% because so much of the oxygen split off in photosynthesis was unable to rejoin with carbon in decay. Fluctuations in oxygen level would have had all sorts of interesting effects on life at the surface. For instance, they would have affected the ease with which fires could start and spread on plant material. At 15% oxygen, it is hard to sustain a fire even on dry material, whereas at 30% oxygen even moist plant tissues will burn. Some geologists have claimed to find evidence of these fluctuations in oxygen in the form of changes in the frequency of charcoal layers in rocks laid down during the past several hundred million years. For example, the coal swamp forests that existed around 350 million years ago may have at least partially burned every 3 or 4 years. 0thers suggest that there are too many complexities affecting the likelihood of preservation of charcoal to reach any meaningful conclusions about fire frequency. It is also rather difficult to explain the existence of forests at times when the atmospheric oxygen level was supposedly around 30%. At this sort of concentration, a single lightning strike in even the moistest forest would cause it all to be consumed by fire, and it is rather unlikely that forest anywhere in the world would be able to grow and reach maturity. Yet, throughout the past 350 million years there is evidence of forests having existed; so, we can at least say that some of the uppermost estimates of past oxygen concentration are probably wrong. On the other hand, some independent evidence that oxygen levels were at least somewhat higher around 300 million years ago comes from the existence of huge flying insects, such as dragonflies with 70 cm wingspans. Calculations suggest that in our present atmosphere of 21% oxygen, such insects could not exist because they would not be able to get enough oxygen for their active lifestyle, due to the limits of how fast the breathing tubes (called tracheids) in their bodies can supply oxygen to their muscles. More oxygen around in the atmosphere would also have increased the density of the air, making it easier for such huge winged insects to hold themselves aloft.

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