5. How is the age of fossils determined?
Determing the age of fossils is very important to palaeontologists; it allows them to locate the species in time and study how they relate to each other and the environment. This also permits them to investigate the way in which plants and animals have evolved over time
Scientists can date rocks in an absolute manner, measuring concentrations of radioactive elements, or in a relative way, relying on the presence of defined groups of fossils in a given stratum.
The most precise way is measuring the absolute age of rocks by radiometric dating. This method uses a physical phenomenon known as the radioactive decay of atoms. Elements such as carbon, argon and uranium, among others, change thier atomic structure over time transforming into other elements.
Beginning with the work of Marie and Pierre Curie, physicists had found that this transformation happens very precisely during the lapse of time.
The concentration of Carbon-14 in tree wood, for example, dacays over 5,730 years to one-half of the amount that was present when the tree died, and to a quarter of its original value after twice that time. As a result, by comparing the normal concentration in existing wood and that observed in ancient wood its possible to know its age. The same occurs with fossils.
When these geological clocks are present, the abundance of radioactive elements allows the assignment of a precise age to when the rock was formed, and hence the fossils contained in it. However it’s not always possible or practical to use this form of measurement. Only a few types of rocks are suitable for radiometric dating.
The first geologists became aware that fossils of distinct species appeared together, forming associations that were found repeatedly in different part of the world. This is to say that these groups appeared in the same relative positions in rock stratifications in places that at times were very distant geographically. They therefore concluded that these assocciations represented finite units of geological time, and that the rocks that contained them were of the same age, although they came from distant places.
In this method of dating and correlation of geological strata, guide fossils find their use; very abundant organisms with a wide distribution and which only existed for a short time period. Their presence in a defined stratum serves as witness to this relative time period.
The use of there two methods has permitted the establishment of a chronology of the natural history of life on earth, from the appearance of the first organisms to the present. This is known as the “Geologic Time Scale.” In this way new fossils that are found each year can be dated and located in their corresponding prehistoric moments.
6. How do they study past enviroments and climate changes?
A desolate valley was not always such. With the passing of geological time periods the landscape could have changed profoundly; it once was perhaps a tropical forest environment, hot and humid, or a plain covered with glaciers. At other times it could have been a forest populated with dinosaurs or even the bottom of the ocean.
These landscapes had thier own particular climatic characteristics and animals and plants that were adapted to them. And if scientists wish to reconstruct the history of life on Earth, not only do they need to know about the species, but also the environment in which they moved. The is the study of Paleoecology.
A few words about ecosystems
Paleoecology is the study of the ecosystems of past epochs, while ecology is focused upon the present.
An existing ecosystem is an assembly of the interactions amongst living creatures, and between them and the physical environment. The organisms (flora and fauna) that belong to a an ecosystem inhabit a particular place at a defined moment in time. This “place” could be as small as a drop of water or as large as the whole planet Earth.
In terrestrial ecosystems, the most conspicuous, easily-observed characteristic is the type of vegetation. That is why when one refers to an ecosystem, it’s usually references the type of vegetation that characterizes it, such as grassland, forest, steppe, etc.
Other factors such as temperature, solar light levels, types of soil and the availability of water (humidity, rain, and bodies of water such as rivers and lakes, etc.) also exert a strong influence on the formation of the ecosystems. These factors keep changing, usually slowly, with the passage of time.
On the current Earth, the temperature decreases as we move away from the Equator and towards the poles, and as we climb in altitude above sea level. Solar radiation, along with heat from the Earth’s interior are the main sources of heat. The availability of water, on the other hand, depends on a number of variables, such as wind direction, geographic features and ocean currents.
If the changes are slow and gradual, the ecosystem is also modified, or changes over long time scales, sometimes millions of years. On the other hand, if catastrophic changes occur such as volcvanic eruptions or the sudden impact of a large meteorite, ecosystems can change rapidly and even disappear.
Palaeoecology not only draws from ecology, but also botany and zoology, since to understand the past it is necessary to make comparisons with the existing conditions in these same environments.
Some climatic characteristics (water availability, temperature,etc.) are reflected in the different types of fossils present. This provides evidence of the climatic conditions that ruled at the moment when these organisms were alive. Fossil Pollen for example allows one to deduce the type of vegetation that could have been encountered in the vicinity of the place where the fossils were found. From this, the type of environment can be inferred.
Also, the collection of fossils found in the same place but from different epochs tells us the story of climatic changes that the location has undergone.
7. If fossils were buried by sediments and lava, why are they found exposed or very close to the surface?
The ground suffers a continuous process of erosion that changes it and wears it away. In Patagonia the three important erosive factors are:
The wind, predominantly from the West;
Water, from rain, rivers and the sea;
The advance and retreat of glaciers.
These factors at work over a period of millions of years have formed valleys, gulleys and canyons in which very old geologic layers are often left exposed.
Limited tectonic activity has also been a factor that has contributed to raising parts of the Patagonian ground, and has also allowed outcropping of ancient geological strata, for example the rising of the Andean mountain range.
Paleontologists look for fossils in rocky outcrops. Naturally, all fossils are of interest but they usually prefer those which haven’t ‘been travelling,’ which is to say that they are still in thier original place. This is because the surroundings or setting of the fossils also give clues to the environment in which it once lived.
8. Why did the dinosaurs become extinct?
The extinction of the dinosuars, even though it wasn’t the greatest crisis in the hstory of life on the planet, has fascinated researchers for years. How could a kind of animal become extinct so rapidly after dominating the Earth for more than 160 million years?
The mass extinction (of dinosaurs and a huge number of other species), known as the Cretaceous- Tertiary or “K-T” eposide meant the end of the Mesozoic era and the beginning of dramatic changes on a planetary scale.
Dozens of different theories attempting to explain this exist. The most famous is that which involves a massive meteorite hitting the Earth. Others theories attribute the extinction to a marked increase in volcanic activity on a world- wide level involving diverse mountain ranges, planetwide temperature changes, sea level changes, stellar explosions (supernovae) or changes in the polarity of the Earth’s magnetic field.
A catastrophic Impact
65 million years ago, one or more meteorites several kilomteres in diameter, would have struck the Earth at speeds possibly greater than 10 kilometers per second. One of these impacts took place on the Yucatan Peninsula in the Gulf of Mexico, where part of the crater left by the colossal explosion is still visible.
Its effects were devastating: earthquakes, and/or tsunamis, at a local level, and total destruction for a radius of several hundred kilometers. As well, it raised an enormous cloud of dust produced by the meteorite and ground into the atmosphere. Driven by winds, the cloud dipsersed over the whiole planet, reducing or preventing sunlight from reaching the ground for months. For many ecosystems, the lack of light for long periods would have been catastrophic and provoked the mass death of plants and animals, ill-suited for such conditions (that is to say, a nearly complete rupture of the food chain as well as the ecosystem balance).
The main evidence in favour of this theory is the large concentration of the chemical element iridium in the geological rock strata corresponding uniquely to the “K-T” episode. Astronomers have measured similar concentrations only in meteorites. Scientists then infer that the dust cloud dispersed in the atmosphere by the impact was thus deposited over the Earth’s surface resulting in the contamination of this element.
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