The Earth’s climate changes naturally and gradually over long periods of time due to geological forces. The climate can also change abruptly. These abrupt changes can be due to geological forces or several other reasons, such as asteroid impacts.
When the climate changes over longer periods of time plant and animal species are able to evolve along with these gradual processes, resulting in the wide number of species and diverse ecosystems that the world is filled with today. In the case of humans, gradual reductions in food availability probably forced our distant primate relatives out of the trees in search of food. Millions of years later we humans can now walk around upright while some of our other primate cousins never had reason to come down out of the trees and can still swing effortlessly through the tree tops but only walk upright in limited amounts.
When the climate is changed abruptly all bets are off from an evolutionary standpoint. Rapid changes in climate don’t afford species the necessary time to adjust to changes. If some event caused our distant relatives’ food supply to disappear in say under a year rather than over generations, they would have starved to death before gradually adapting to walking upright. Survival at this point is largely luck.
There have been five counts of global climate change events that were shocking enough to drastically wipe out life on Earth. These events are called mass-extinctions. We know they happened because of evidence provided by fossils records; there are dinosaur fossils in Utah but no dinosaurs walking through Utah or anywhere else.
The timeline chart shows the approximate number of different families of species. The increasing number shows the variation of species as evolution occurs. The sharp drops show sudden losses families of species.
Polar ice cores can provide yearly data and some records span periods of hundreds of thousands of years. As layer upon layer of new snow builds up, older layers on the bottom get compressed transforming into solid ice. The layers of ice have alternating light and dark rings that indicate one year of snowfall. We simply count the layer colors to determine how long ago the snow fell, similar to the way we can count tree rings to determine a tree’s age. These ice layers have air pockets trapped in them that are preserved for hundreds of thousands or millions of years. By analyzing the composition of those air pockets we can determine past atmospheric composition. Because large parts of the ocean’s carbon dioxide (CO2) concentration remains roughly in equilibrium with the atmosphere’s CO2 concentration, the ocean CO2 content and
acidity level at the time of freezing
can be calculated.
Layers of sediments that are deposited at the lake or ocean bottom provide insight to past climate conditions. Sediment is washed or blown in bodies of water. It eventually travels in rivers, which flow into lakes and oceans. When the moving water reaches a calmer environment, it deposits sediments that fall to the bottom, gradually building up layer upon layer. Winds can bring sediments, volcanic ash, and pollen to the water. These items fall to the water surface and sink to the bottom. Scientists drill into the sediment to retrieve sediment cores that are used to analyze past geological and environmental compositions. These sediment rings usually show climate and geographic variation by centuries or more, rather than yearly as rings in ice cores do.
Corals turn out to be remarkable record keepers. Most coral species live in colonies that build massive limestone skeletons over many years. The coral live on the surface of the skeleton and lay down layer upon layer of calcium carbonate beneath them for centuries or more. The structures that corals deposit as they grow have ring patterns similar to those found in tree rings, ice cores, and lake sediments. Sometimes coral grows in ways that we can detect changes on a seasonal basis. The small chemical differences of the layers provide scientists with clues about past climates and tells us about sea temperatures, while the layered structure allows them to date the changes they find.
Black coral fossils are estimated to be 4,200 YO.
Corals have a narrow range of environmental conditions that they can tolerate. Like Goldilocks, coral like it only when conditions are just right. Tropical or subtropical oceans must be just the right temperature, with clear, shallow water and precise acidity levels. Sensitivity to environmental factors makes coral a good gauge of local climate conditions. Coral has almost completely disappeared from the fossil records several times. These mass disappearances are excellent indicators of abrupt climate changes and are perfect for measuring the changes that occurred.
So now we know that extinctions occurred, and what the climate was like before and after the extinctions. Now we need to figure out when it happened and what caused the climate to change.
Scientists have uncovered a wide array of fossils or stones that are millions or billions of years old, respectively. To determine the ages of these specimens, scientists need an isotope (a radioactive element) with a very long half-life. A half-life is the rate at which it takes half of an amount of one type of isotope to decompose into another type of element. An isotope of potassium will eventually decompose into argon for example. Once we can predict how long it will take an amount of isotopes to decay into another element and we can discern the ratio of the original isotopes to the newly formed element, we can rewind time and find out the date the rock was made. This is called radiometric or radioactive dating.
As you may have guessed, radioactive elements don’t typically exist in plant or animal species or in fossils themselves. But some amounts of them typically exist in rocks called igneous rocks, which are formed by cooled magma. Fossils form in the most common type of rock called sedimentary rock. This is the type of rock formed by the gradual covering and compression of surface minerals. Sediment and the bones gradually turn into rock, but this sediment doesn’t typically include the necessary isotopes in measurable amounts either.
Bracketing fossils
To determine the age of sedimentary rock layers and the fossils within them, we have to find neighboring layers of Earth that include igneous rock types, such as volcanic ash. These layers are time brackets. By using radiometric dating to determine the age of igneous layers, researchers can accurately determine the age of the sedimentary layers between them. Using the basic ideas of bracketing and radiometric dating, researchers have determined the age of rock layers all over the world. Figuring out what caused these events has been the topic of much discussion among scientists from varying fields. Geologists, chemists, biologists, and more have all contributed to the conversation. There is no debate on whether or not these events occurred, the debate is on how they occurred. Each mass-extinction event has its own unique proposed theories but they also have much in common. My following diaries will cover each of the mass extinction events and some of the leading scientific discussions and theories that explain how and why these events occurred. First up will be the Ordovician-Silurian mass extinction during which some 85 percent of sea life was wiped out.
-Cheers, Greg.
Work cited
Kolbert, Elizabeth. The Sixth Extinction: An Unnatural History. Henry Holt, 2014. Print.
Bryson, Bill. The Short History of Nearly Everything. Broadway Books, 2004. Print