Earth Science and Evolution Study Guide
Earth today is the product of 4.5 billion years of changes. Life is an important player in connecting the earth's surface reservoirs of atmosphere, ocean, and soil. Here we look at questions that surround the history of the oceans, continents, and, in particular, the atmosphere. Did life play a role in the story of the air? Geology certainly played a role in the history of life, especially so when we consider mass extinctions such as the demise of the dinosaurs 65 million years ago.
History of the Continents, Oceans, and Atmosphere
Earth formed about 4.5 billion years ago. At that time, the planet was hot, too hot for liquid water, but at some point, Earth was cool enough to stabilize water, presumably in the form of oceans. Direct evidence for those oceans vanished long ago, but geologists search for indirect evidence in the form of sedimentary rock, which must form under water.
When were the first sedimentary rocks formed? Rocks on Earth are always being destroyed (and newly created) by the processes of plate tectonics. So as we look back at earlier and earlier rocks, we find fewer and fewer of them. However, most geologists would say that adequate evidence exists for liquid water by 4 billion years ago and probably earlier. Thus, Earth has had an ocean for at least 4 billion years.
What about the continents? The earliest continents are thought to have been something like the country of Iceland today. Iceland sits along the volcanic midocean ridge of the Atlantic Ocean. In fact, Iceland can be thought of as part of the mid-Atlantic ridge that simply sticks up above water. Iceland, despite its chilly latitude, is full of volcanic activity and steam vents everywhere in the country. Most of the country's energy is hydrothermal, meaning from hot water, supplied by the volcanic magic of plate tectonics.
Thus, the earliest continents were probably small. Earth's more appropriate name of "ocean" was even more true way back then, but continents, as noted earlier, are made of lighter material and tend to float on the heavier asthenosphere. Continental crust is even lighter than the oceanic crust. Continents, once formed, will stay. They might split and merge, driven by the dynamics of plate tectonics, but the continental crust rarely goes very far back down into the deep earth before melting and floating back to the surface. This means that continents have grown over time, and if the continents grew, the total area of the ocean must have shrunk.
Most continents' growth occurred during the first quarter or so of Earth history, when the interior of the earth was hotter and the dynamics of plate tectonics were more active. Geologists today are still debating the exact history of the continents, but all agree that the total area of all the continents probably began much smaller than the area today.
Probably the best understood of the histories (and most interesting for our purposes) is that of the atmosphere. The chemicals of the atmosphere react with the minerals of the soil and ocean sediments. By deciphering the chemical reactions that must have taken place long ago to create certain chemical signatures in sedimentary rocks, geologists and geochemists have been able to make some fairly definite statements about the history of Earth's atmosphere. How did the atmosphere come to be what it is today?
Evidence shows that the early earth's atmosphere contained virtually no oxygen. If you were to step out of a time machine on to an ancient shoreline, you would choke and die in a few breaths. Miniscule amounts of free oxygen were made by cosmic rays that could split molecules of water vapor in the atmosphere into hydrogen and oxygen, but these amounts were millions of times smaller than today's amount of oxygen. Recall that oxygen gas, O2, is the second most abundant gas in the atmosphere, at about 21%.
Some sedimentary rocks are known to contain layers that are ancient fossil soils. Once real soils, these zones were covered and protected by water and overlying sediments and were eventually squeezed into sedimentary rock. By analyzing the chemicals in these fossil soils, it has been discovered that a great change happened to Earth's atmosphere about 2 billion years ago, roughly at the halfway point in Earth's total history.
Two billion years ago, a dramatic increase in the level of atmospheric oxygen took place. This step did not create today's 21% oxygen level, but it did create perhaps 2% oxygen. This was a major change in the chemistry of the atmosphere and a change for the organisms that lived in the biosphere. Then, some where in the timeframe between 1 billion and 600 million years ago, a second rise in oxygen brought the atmosphere's oxygen level to a value close to that of today's oxygen level.
What could have caused these rises in oxygen? Oxygen is made as a waste product of photosynthesis. Plants get rid of the oxygen they make, just as we humans get rid of the waste carbon dioxide our cells make. So Earth's oxygen must come from photosynthesizers. At 2 billion years ago, no plants existed on land. So the oxygen for the rise at that time must have been generated by photosynthesizers in the ocean—by phytoplankton.
But remember: Other creatures in the ocean use up oxygen, just like we do, just like cats and dogs and butterflies and frogs do. In other words, just like all respiring organisms do. In particular, the bacteria in the deep ocean that feed upon and thus recycle the elements in dead organic detritus require oxygen for their recycling. So if the phytoplankton make oxygen but the bacteria consume the oxygen, then presumably there should be no oxygen left to accumulate in the atmosphere.
The only way for oxygen to accumulate in the atmosphere is if more oxygen is produced by photosynthesizers than is consumed by the oxygen users such as bacteria. In addition, certain gases given off by volcanoes (such as sulfur gases) also combine with oxygen in the atmosphere; these chemical reactions are essentially consuming oxygen as well. Therefore, for oxygen to accumulate in the atmosphere, more oxygen had to be generated by photosynthesis than consumed by the sum of oxygen-consuming organisms and natural chemical reactions.
We can conclude from this analysis that the rise in oxygen in Earth's atmosphere did not have to coincide with the beginning of photosynthesis. Indeed, most scientists who study the history of life think that photosynthesis began at least a billion or more years before the first great rise in oxygen. What happened 2 billion years ago is not yet certain. But the rise in oxygen could have been driven by an increase in the amount of photosynthesis and thus burial of organic carbon.
If the amount of photosynthesis (which creates oxygen) and the amount of respiration (which consumes oxygen) are equal, then the organic matter created by photosynthesis is all consumed by organisms that feed on the organic matter and derive their energy by performing respiration. But if some of the organic matter slips through the biosphere's recycling systems, then there is excess oxygen created that is not consumed by organisms feeding on the organic matter. Perhaps at 2 billion year ago, more organic matter started being buried, which meant more free oxygen entered the atmosphere. Whatever the answer, we do know that Earth's atmosphere has undergone a dramatic change in its level of oxygen, and furthermore, that this change involved life. Life has been a geologial force on the chemistry of the atmosphere.
Life's involvement with the carbon cycle means that life could also exert an influence on another important gas in the atmosphere, carbon dioxide (CO2). Living things, of course, create organic molecules made of carbon. When these molecules slip through the biosphere's recycling systems, as already noted, carbon is lost from the biosphere. It had been shown that 0.5 billion tons per year of carbon is actually lost from the biosphere by burial in the ocean's sediments. This amount is made up by the release of new carbon from volcanoes and the chemical dissolution of rocks.
Most of the burial of the 0.5 billion tons of carbon per year is, however, not in the form of organic matter. It is calcium carbonate, the shells of marine organisms, which eventually become carbonate sedimentary rock. The amount of the calcium carbonate that is buried is related to the atmosphere's level of carbon dioxide and to the activities of organisms in the soil, which can increase the amount of chemical weathering of soil minerals.
When earth scientists put together the story of how these influences determine the level of atmospheric CO2, it appears that the CO2 level has been declining gradually over long geological time periods. (Note: This is just the opposite from the rapid rise in CO2 that is happening today.) Apparently, over billions of years, the greenhouse effect of carbon dioxide has been decreasing. This is interesting for another reason—because at the beginning of the earth, the sun was weaker by 30%. A weaker sun meant less solar energy hitting the earth. Were a cosmic dial-turner able to turn down the sun by 30% today, the biosphere would become a solid ball of ice because the oceans would freeze. So a larger amount of carbon dioxide in the atmosphere several billion years ago is probably the reason that liquid water existed back then.
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