At one point they must have been thriving: fossils of rangeomorphs and others have been discovered in locations all over the world; from Russia to Australia, and Namibia to China.
However, they suddenly disappeared from the fossil record approximately 30 million years after they first arrived. Some million years ago, the Earth was thawing its way out of an ice age, and this area was flooded with glacial water, forming a shallow sea. You can walk for hundreds of miles in any direction and see records of the animals that lived there, displayed on the surface of rocks. Life became more widespread and complicated during the 'Cambrian Explosion' some million years ago Credit: Getty Images.
In , Darroch found vast fields of what looked remarkably like burrows made in the Nama sediment — signs that these younger animals were foraging, and churning up the seafloor. Some of the burrows in the sediment also looked exactly like those created by sea anemones — a predatory animal.
If predators had been present, that would surely have been the death knell for the Ediacarans, who would have been unable to escape. Geologists are able to work out what ocean oxygen levels were like millions of years ago by measuring the relative amounts of different types of uranium found in sedimentary rocks formed at that time.
The rocks soak up a heavier form of uranium when surrounded by low-oxygen waters, and a lighter form when oxygen levels are high. The Namibian landscape is offering tantalising clues as to what the environment was like more than million years ago Credit: Rachel Wood. Using this method, scientists have shown that oxygen levels in the ocean fluctuated a lot during the Ediacaran period, with levels rising shortly before the appearance of the first animals, and falling shortly before their demise.
The icy mantles of the grains begin sticking together and eventually grow to meter-sized rocky boulders called planetesimals. The planetesimals collide and accrete into larger bodies that are tens of kilometers in diameter called protoplanets. Once the protoplanets clear a gap in the disk, they become bonafide planets and their orbits begin to stabilize Figure The process of planet formation is messy.
Not all of the planetesimals are accreted into planets. Millions of planetesimals remain as the leftover debris and are now the asteroids and ice-coated comets in our solar system. In the first hundred million years after the formation of the Sun, collisions between the leftover planetesimals and the planets were common. We see evidence for heavy bombardment by planetesimals on the surfaces of the moon and Mercury Figure Most of the planetesimals were accreted into planets or moons, but some of these objects remain as meteors, asteroids, and comets in our solar system today.
The same types of collisions would have occurred on the surface of the Earth, however erosive processes have erased all except the most recent of these collisions. Pictured in Figure Such collisions are rare today. About million years after the formation of the Sun, the gravity of the planets and moons in our solar system had swept up most of the planetesimals. However, millions of these objects still remain in gravitationally stable orbits in the main asteroid belt of the solar system, in the Trojan asteroid belt, or out beyond Neptune and Pluto in the Kuiper belt.
Illustrated in the sketch below is the location of the largest reservoir of asteroids in our solar system today Figure Earth is the only object in our solar system known to support life Figure Today there are over 1 million known species of plants and animals on Earth. The materials that came together to form the Earth were made of several different chemical elements. Each element has a different density , defined as mass per volume.
Density describes how heavy an object is compared to how much space the object takes up. The lighter elements rose to the surface. You have probably seen something like this happen if you have ever mixed oil and water in a bottle. The water is denser than oil.
If you put both in a bottle, shake it up, and then let it sit for a while, the water settles to the bottom and the oil rises up over the top of the water. Today, the Earth consists of layers that represent different densities Figure The core is made of very dense metal elements called iron and nickel.
The outermost layer of the Earth is its crust. The crust is made mostly of light elements such as silicon, oxygen, and aluminum. More information on the different layers of the Earth is presented in the lesson on plate tectonics. The center of the Earth is the core, which is the densest. The outermost layer is the crust, which is the least dense. As Patterson argued, some meteorites were indeed formed about 4. But Earth continued to grow through the bombardment of planetesimals until some million to million years later.
At that time This possibility had already been suggested by Bruce R. Doe and Robert E. Zartman of the U. Geological Survey in Denver two decades ago and is in agreement with Wetherills estimates. The emergence of the continents came somewhat later. According to the theory of plate tectonics, these landmasses are the only part of Earth's crust that is not recycled and, consequently, destroyed during the geothermal cycle driven by the convection in the mantle.
Continents thus provide a form of memory because the record of early life can be read in their rocks. Geologic activity, however, including plate tectonics, erosion and metamorphism, has destroyed almost all the ancient rocks.
Very few fragments have survived this geologic machine. Nevertheless, in recent decades, several important nds have been made, again using isotope geochemistry. One group, led by Stephen Moorbath of the University of Oxford, discovered terrain in West Greenland that is between 3. In addition, Samuel A. Bowring of the Massachusetts Institute of Technology explored a small area in North America--the Acasta gneiss--that is thought to be 3.
Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dissolved during the process of erosion but is deposited in particle form in sediment. A few pieces of zircon can therefore survive for billions of years and can serve as a witness to Earths more ancient crust.
Lancelot, later at the University of Marseille and now at the University of Nmes, respectively, as well as with the efforts of Moorbath and Allgre. It was a group at the Australian National University in Canberra, directed by William Compston, that was nally successful. The team discovered zircons in western Australia that were between 4.
Zircons have been crucial not only for understanding the age of the continents but for determining when life rst appeared.
The earliest fossils of undisputed age were found in Australia and South Africa. These relics of blue-green algae are about 3. Manfred Schidlowski of the Max Planck Institute for Chemistry in Mainz studied the Isua formation in West Greenland and argued that organic matter existed as long ago as 3. Because most of the record of early life has been destroyed by geologic activity, we cannot say exactly when it rst appeared--perhaps it arose very quickly, maybe even 4. Stories from gases ONE OF THE MOST important aspects of the planet's evolution is the formation of the atmosphere, because it is this assemblage of gases that allowed life to crawl out of the oceans and to be sustained.
Researchers have hypothesized since the s that the terrestrial atmosphere was created by gases emerging from the interior of the planet. When a volcano spews gases, it is an example of the continuous outgassing, as it is called, of Earth. But scientists have questioned whether this process occurred suddenly--about 4.
These gases--including helium, argon and xenon--have the peculiarity of being chemically inert, that is, they do not react in nature with other elements. Two of them are particularly important for atmospheric studies: argon and xenon. Argon has three isotopes, of which argon 40 is created by the decay of potassium Xenon has nine, of which xenon has two different origins. Xenon arose as the result of nucleosynthesis before Earth and solar system were formed. It was also created from the decay of radioactive iodine , which does not exist on Earth anymore.
This form of iodine was present very early on but has died out since, and xenon has grown at its expense. Like most couples, both argon 40 and potassium 40 and xenon and iodine have stories to tell. They are excellent chronometers. Although the atmosphere was formed by the outgassing of the mantle, it does not contain any potassium 40 or iodine All argon 40 and xenon , formed in Earth and released, are found in the atmosphere today.
Xenon was expelled from the mantle and retained in the atmosphere; therefore, the atmosphere-mantle ratio of this element allows us to evaluate the age of differentiation. Argon and xenon trapped in the mantle evolved by the radioactive decay of potassium 40 and iodine Thus, if the total outgassing of the mantle occurred at the beginning of Earths formation, the atmosphere would not contain any argon 40 but would contain xenon The major challenge facing an investigator who wants to measure such ratios of decay is to obtain high concentrations of rare gases in mantle rocks because they are extremely limited.
Fortunately, a natural phenomenon occurs at mid-ocean ridges during which volcanic lava transfers some silicates from the mantle to the surface. The small amounts of gases trapped in mantle minerals rise with the melt to the surface and are concentrated in small vesicles in the outer glassy margin of lava ows.
This process serves to concentrate the amounts of mantle gases by a factor of 10 4 or 10 5. Collecting these rocks by dredging the seaoor and then crushing them under vacuum in a sensitive mass spectrometer allows geochemists to determine the ratios of the isotopes in the mantle. The results are quite surprising. Calculations of the ratios indicate that between 80 and 85 percent of the atmosphere was outgassed during Earths rst one million years; the rest was released slowly but constantly during the next 4.
The composition of this primitive atmosphere was most certainly dominated by carbon dioxide, with nitrogen as the second most abundant gas. Trace amounts of methane, ammonia, sulfur dioxide and hydrochloric acid were also present, but there was no oxygen. Except for the presence of abundant water, the atmosphere was similar to that of Venus or Mars.
The details of the evolution of the original atmosphere are debated, particularly because we do not know how strong the sun was at that time. Some facts, however, are not disputed. It is evident that carbon dioxide played a crucial role. In addition, many scientists believe the evolving atmosphere contained sufficient quantities of gases such as ammonia and methane to give rise to organic matter.
Still, the problem of the sun remains unresolved. One hypothesis holds that during the Archean eon, which lasted from about 4. This possibility raises a dilemma: How could life have survived in the relatively cold climate that should accompany a weaker sun? A solution to the faint early sun paradox, as it is called, was offered by Carl Sagan and George Mullen of Cornell University in The two scientists suggested that methane and ammonia, which are very effective at trapping infrared radiation, were quite abundant.
These gases could have created a super-greenhouse effect. The idea was criticized on the basis that such gases were highly reactive and have short lifetimes in the atmosphere. What controlled co? They postulated that there was no need for methane in the early atmosphere because carbon dioxide was abundant enough to bring about the super-greenhouse effect. Again this argument raised a different question: How much carbon dioxide was there in the early atmosphere?
Terrestrial carbon dioxide is now buried in carbonate rocks, such as limestone, although it is not clear when it became trapped there.
Today calcium carbonate is created primarily during biological activity; in the Archean eon, carbon may have been primarily removed during inorganic reactions. The rapid outgassing of the planet liberated voluminous quantities of water from the mantle, creating the oceans and the hydrologic cycle. The acids that were probably present in the atmosphere eroded rocks, forming carbonate-rich rocks.
The relative importance of such a mechanism is, however, debated. Heinrich D. Holland of Harvard University believes the amount of carbon dioxide in the atmosphere rapidly decreased during the Archean and stayed at a low level.
Understanding the carbon dioxide content of the early atmosphere is pivotal to understanding climatic control. Two conicting camps have put forth ideas on how this process works. The rst group holds that global temperatures and carbon dioxide were controlled by inorganic geochemical feedbacks; the second asserts that they were controlled by biological removal.
James C. Walker, James F. Kasting and Paul B.
0コメント