Glamation of the Preambrium

GLAMATION OF THE PREAMBRIUM The proterozoic is a transitional period, when oxygen produced by photosynthesis replaces carbon dioxide. In the early Archeans, the Sun's output was only about 70 percent of its current value. Large amounts of atmospheric carbon dioxide 1,000 times greater than current levels keep Earth from freezing. When the first microscopic plants evolved, they began replacing carbon dioxide in the ocean and atmosphere with oxygen so that today the relative percentages of these two gases have been completely reversed. The loss of carbon dioxide, an important greenhouse gas, causes the climate to cool even as the Sun gets hotter. The drop in global temperatures soon after the beginning of the Proterozoic began the first known glacial epoch about 2.4 billion years ago when huge ice sheets nearly engulfed the entire landmass. The position of the continent also had a tremendous influence on the initiation of the ice age. Lands wandering to cooler latitudes allow for the buildup of glacial ice. Global tectonics, featuring extensive volcanic activity and seafloor spreading, may have triggered ice ages by lowering oxygen levels in the ocean and atmosphere. This will retain more organic carbon in the sediment so that living organisms cannot return it to the atmosphere. Plate tectonics also began to operate more vigorously. Plate subduction drives carbon sediments deep within the Earth along with the underlying oceanic crust. Growing continents store large amounts of carbon in thick deposits of carbonaceous rocks such as limestone. The removal of carbon dioxide in this way causes the Earth to cool dramatically. In addition to the high rate of organic carbon burial, iron deposition and intense hydrothermal activity associated with plate tectonics further increase global cooling. Although this is the first ice age the world has ever known, it is not the worst. Carbon burial in the Earth's crust may have been key to the beginning of another glacial period just before the emergence of recognizable animal life near the end of the Proterozoic about 680 million years ago, called the Varanger ice age, named after the Varanger Fjord in Norway. Huge glaciers ruled almost half of the continent for millions of years. Four periods of glaciation occurred between 850 and 580 million years ago. Even the frozen tropics, as shown by the unusual occurrence of glacial debris and deposits of iron-rich rocks on almost every continent, suggest that they formed in cold waters. The ice age almost extinguished all life, which experienced extremely difficult living conditions for millions of years. A supercontinent located on the equator splits, forming four or five large continents. The largest seems to wander into the south polar region and acquire a thick blanket of ice. This was probably the largest and longest glaciation period when ice enveloped nearly half the planet. The ice sheet is so vast that this period has been nicknamed the snowball Earth. If it were not for massive volcanic activity, which restores the carbon dioxide content of the atmosphere, the planet could still be buried under the ice. The climate is so cold that permafrost and permafrost extend to equatorial latitudes. During this time, no plants grew on land, and only simple plants and animals lived in the sea. Ice ages dealt a deadly blow to life in the oceans, and many simple organisms vanished during the world's first mass extinction. At this point in Earth's development, animal life was still scarce. The extinction destroyed a population of marine acritarchs, a community of planktonic algae that was one of the first organisms to develop an intricate cell with a nucleus. This extreme glaciation occurred just before the rapid diversification of multicellular life, culminating in a species explosion between 575 and 525 million years ago. The glacial period is characterized by deposits of moraines and tillites. The thick order of Precambrian tillites existed on every continent (Fig. 39). Tillites are a mixture of large rocks and clay deposited by glacial ice and cemented into solid rock. In North America's Superior Lakes region, tillites are 600 feet thick in some places and range from east to west for 1,000 miles. In northern Utah, tillites reach a thickness of 12,000 feet. The various layers of glacial sediments deposited show a series of ice ages that followed each other. Similar ttillites exist among Precambrian rocks in Norway, Greenland, China, India, southwest Africa, and Australia. Australia was practically buried by glaciers during the late Precambrian ice age (Fig. 40). The 680-million-year-old glacial varves in lakebed deposits north of Adelaide, South Australia, may hold evidence of solar cycles operating in the Proterozoic. Varve consists of alternating layers of mud laid annually. Every summer when glacial ice melts, sediment-laden meltwater is discharged into lakes, and sediments settle in stratified deposits. During times of intense solar activity, average global temperatures rise. As a result, glaciers melt faster, depositing thicker varve. By counting thick and thin varve layers, a stratigraphic sequence is established that mimics the 11-year sunspot cycle and the 22-year solar cycle or perhaps the early lunar cycle, which is currently about 19 years. Rocks containing carbonate minerals are located just above glacial deposits. This shows the greenhouse effect produced by the abundance of volcanic activity pumping carbon dioxide into the atmosphere, heating the Earth and melting large glaciers. Not long after the ice disappeared towards the end of the Proterozoic, the great diversity of animal life culminated with the evolution of an entirely new species, which had never existed before or since. This forever changed the biological composition of the Earth. Life forms take off in all directions, resulting in many unique and strange animals. Rapid evolution resulted in three times as many phyla, groups of organisms sharing the same common body plan, as those alive today. Many unusual creatures occupy the ocean. The highest percentage of experimental organisms, animals that evolved specific characteristics, emerged than during any other interval in Earth's history. As many as 100 phyla exist, whereas only about one-third of that number are alive today. This biological excitement sets the stage for more advanced species. Also, for the first time, the fossil remains of animals became abundant due to the evolution of limesecreting organisms that built hard shells.