Answer

If methane in an atmosphere were converted to carbon dioxide, the overall greenhouse effect of the planet would decrease, the planet would trap twenty times less heat, and the planet would cool—a lot!

Answer

As you already know, oxygen-producing photosynthesis is the source of the free oxygen that aerobic organisms such as humans use to burn carbohydrates (e.g., broccoli and snickers bars). Before organisms that employed oxygen-producing photosynthesis developed (from pre-existing photosynthetic organisms that did not produce oxygen), Earth’s atmosphere contained essentially no oxygen. After cyanobacteria evolved, the poisonous byproduct of photosynthesis—oxygen—had, for the first time, a chance to begin building up in Earth’s atmosphere.

As mentioned, oxygen-producing photosynthesis originated by 2.7 Bya. However, the oxygen content of the atmosphere remained low until the modern atmosphere emerged by 600 Mya. So, why did two billion years pass before oxygen levels rose to modern levels? In other words, what is required besides oxygen production to allow free oxygen to build up in the atmosphere.

Answer

If the amount of money you are depositing remains unchanged, the only way to increase the balance in your account is to decrease withdrawals. Right?

Answer

The short answer is that oxygen withdrawals would have to diminish for oxygen levels in the atmosphere to increase. For this to occur, the elements that react with oxygen and move oxygen from the atmosphere to solid Earth would need to decrease. One way to do this would be to bury these compounds before they react with oxygen.

The diagram below illustrates the role that burying materials containing reduced iron, sulfur, and carbon plays in the buildup of free oxygen in Earth’s atmosphere. As you can see, the presence of atmospheric free oxygen requires the burial of materials containing reduced species.

Burying reduced iron and sulfur before they react with oxygen was difficult (impossible?) on early Earth, because these elements were soluble in the oceans, and sulfur remained in the oceans after it oxidize. Iron, on the other hand, can become a solid, but only after it reacts with oxygen. Thus, iron and sulfur represented huge sinks for the early oxygen produced by photosynthesis, but they could not effectively cause the buildup of oxygen in the atmosphere. Even so, once oxidized, iron and sulfur could no longer remove oxygen from the atmosphere.

Thus, after oxygen-producing photosynthesis began, there followed a long period in which the reduced iron and sulfur ‘soaked up’ oxygen. Materials containing reduced carbon—like the tissues of dead organisms—also soaked up oxygen. But these materials were different, because they could be buried before reacting with oxygen. Once the reduced species had consumed as much oxygen as they could, free oxygen could build up in the atmosphere if unoxidized carbon was buried. And this is exactly what happened—in two events. The first carbon burial event occurred in the mid-Precambrian. It raised oxygen to very low levels. Later, a period of massive carbon burial during the late Precambrian removed carbon reactants and allowed atmospheric oxygen levels to rise to modern levels. We discuss these historical processes and events in more detail below.

Here’s an interesting consequence of this reality: if all the buried carbon in Earth’s crust were returned to the atmosphere (by, for example using it to fuel vehicles and produce electricity and cement), Earth’s atmosphere would once again return to very low concentrations of atmospheric oxygen—too low to support abundant modern animal life.

In short, significant increases in atmospheric oxygen require a balance between oxygen production and consumption (by reduced iron, sulfur, or carbon) and burial of unoxidized carbon. As today, microorganisms on early Earth were susceptible to burial in large quantities. Why? Because some of the elements they need to make their bodies (e.g., nitrogen, phosphorous, & iron) are delivered to the oceans from continents. Thus, these organisms thrive in near-continent environments where nutrients are abundant. However, thriving in these nearshore environments means that they are prone to being buried by the sediments (sand, silt, mud, and lime) that are delivered to these environments by rivers. Thus, the very environment where microorganisms can proliferate is also the place where burial by sediments is most likely.

Answer

During periods of abundant coastlines, heavy sediment load in rivers, and thriving microorganisms, the rates of burial of unreacted carbon would skyrocket.

Answer

The abundance of shorelines decreases when supercontinents form and increases when supercontinents break up. So, a breaking supercontinent would increase the abundance of coastlines.

Abundant mountain belts increase the amount of sediments in rivers. Glaciers do this as well: as ice flows over continents, it drags rocks that pulverize underlying rock, producing abundant sediment. Glaciers move this material towards coastlines and, when glaciers melt, there is an overabundance of finely-ground sediment on continents. In addition, the warm periods that cause glaciers to melt promote the proliferation of microorganisms in nearshore environments. These factors combine to make the ends of ice ages periods in which tremendous quantities of unreacted carbon can be buried.

Answer

Skyrocketing rates of unreacted carbon burial over an extended period would lead to increasing levels of free oxygen in Earth’s atmosphere. And this is exactly what produced Earth’s modern atmosphere. We explore these events below.

Answer

As you know, both methane and carbon dioxide are greenhouse gases, but methane absorbs about 20 times more heat than carbon dioxide.

Answer

The efficient burial of dead organisms in shallow near-shore environments removed carbon that would have recombined with atmospheric oxygen. As a result, oxygen concentrations increased—to the low levels that characterized Earth’s boring billion atmosphere.

Answer

The first eukaryotic cells emerged no later than the end of the Huronian ice age. Chemical evidence suggests that stem eukaryotes may have emerged much earlier.

Genetic evidence indicates that eukaryotic cells developed from symbiotic relationships between an anaerobic organism that engulfed an aerobic organism. The symbiosis involved the anaerobic organism providing nutrients, a hospitable environment, & protection and the aerobic organism providing food (energy). This origin explains why eukaryotes have two sets of DNA: one set builds the host cell and the other set builds the food (energy) producer.

The descendants of the aerobic bacterium in animal cells are the mitochondria that live inside and provide energy for most of the cells in your body. Did you know that you have a symbiotic bacterium living in nearly all the cells in your body? Pretty cool, isn’t it? (Incidentally, can you see where George Lucas, the creator of Star Wars, came up with the idea for midichlorians—the intelligent microscopic life forms that inhabited the cells of Jedi knights in sufficient concentrations to allow them to sense the force? Mitochondria ≈ midichlorians. Kind of fun to understand stuff like this, right?!)

Answer

As methane was converted to carbon dioxide by the oxygen that cyanobacteria were producing, the greenhouse effect of Earth’s atmosphere decreased (by about 20 times). As a result, Earth’s surface temperature cooled (A LOT!).

Earth has experienced five major ice ages. Although a minor glaciation occurred earlier (at 2.9 Bya), Earth’s first major ice age was produced by the conversion of atmospheric methane to carbon dioxide described above. The most intense portion of this first ice age (known as the Huronian Ice Age) extended from ~2.4–2.1 Bya. Glacial deposits of this age show that ice existed at sea level at the equator; that is, they indicate that Earth was covered by ice. This kind of an extreme ice age is often referred to as a ‘Snowball Earth’ event.

As a warming Earth brought the ice age to an end, microscopic organisms flourished in shallow near-continent environments. As sediment produced by glaciers during the ice age was carried to the oceans, it efficiently buried dead microorganisms.

Answer

Volcanoes eventually ended Earth’s first Snowball Earth event, the Huronian Ice Age. In general, volcanic activity is the process responsible for not allowing Earth to become uninhabitably cold (and stay that way). Why? Because volcanoes pump carbon dioxide into the atmosphere, which increases the greenhouse effect and warms Earth’s surface. Thus, so long as Earth is tectonically activity, volcanoes will prevent our planet from becoming uninhabitably cold.

In short, the climate consequences of the Great Oxidation event resulted from the oxygen released by cyanobacteria. This oxygen converted atmospheric methane to carbon dioxide, significantly decreased the greenhouse effect of Earth’s atmosphere, and caused global cooling. This cooling initiated an extreme ice age, the Huronian Snowball Earth event. As carbon dioxide released by volcanoes warmed Earth, microorganisms flourished in nearshore environments where they were efficiently buried by abundant sediments being washed into these environments by rivers. This allowed the atmosphere’s oxygen levels to increase.