Isotopes of oxygen Oxygen ( chemical symbol O) has three naturally occurring isotopes : 16 O, 17 O , and 18 O , where the 16, 17 and 18 refer to the atomic mass. The most abundant is 16 O, with a small percentage of 18 O and an even smaller percentage of 17 O. Oxygen isotope analysis considers only the ratio of 18 O to 16 O present in a sample. 18 O is two neutrons heavier than 16 O and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is required to vaporize H 2 18 O than H 2 16 O Because H 2 16 O requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H 2 16 O, and the residual liquid is enriched in H 2 18 O. Since large amounts of 16 O water are being stored as glacial ice, the 18 O content of oceanic water is high
Connection between calcite and water Limestone is deposited from the calcite shells of microorganisms. Calcite, or calcium carbonate , chemical formula CaCO 3 , is formed from water , H 2 O, and carbon dioxide , CO 2 , dissolved in the water. The carbon dioxide provides two of the oxygen atoms in the calcite. The calcium must rob the third from the water. The isotope ratio in the calcite is therefore the same, after compensation, as the ratio in the water from which the microorganisms of a given layer extracted the material of the shell. The microorganism most frequently referenced is foraminifera .
The Carboniferous Ice Age (Late Carboniferous ice house) T wo special conditions of terrestrial landmass distribution, when they exist concurrently, appear as a sort of common denominator for the occurrence of very long-term simultaneous declines in both global temperature and atmospheric carbon dioxide (CO2) : 1) the existence of a continuous continental landmass stretching from pole to pole, restricting free circulation of polar and tropical waters , and 2) the existence of a large (south) polar landmass capable of supporting thick glacial ice accumulations.
Climate change during the Carboniferous Period was dominated by the great Carboniferous Ice Age . As the Earth alternately cooled then warmed, great sheets of glacial ice thousands of feet thick accumulated, then melted, then reaccumulated in synchronous cycles. Vast glaciers up to 8,000 feet thick existed at the south pole then, moving from higher elevations to lower, driven by gravity and their tremendous weight. These colossal slow-motion tidal waves of ice destroyed and pulverized everything in their path, scraping the landscape to bare bedrock-- altering mountains, valleys, and river courses. Ancient bedrock in Africa, Australia, India and South America show scratches and gouges from this glaciation.
Although cycles of glaciation are believed to occur in response to solar input variations like the Milankovich Cycle and Precession of the Equinoxes , another important factor is the rearrangement of continental landmasses over geologic time by the processes of continental drift. Throughout the Carboniferous Period, continental drift was rearranging most (but not all) of the Earth's landmasses into a single supercontinent stretching from the south polar region to the north polar region. Although the precise mechanisms involved are still a matter of debate this appears to cause regional humidity changes and redistribution of ocean currents which in turn promote ice accumulation and glacier formation over the earth's polar continents. These glacial ice caps grow larger during periods of reduced solar input, and because ice caps are very good solar reflectors this tended to accelerate and perpetuate cyclical relapses to global cooling.
Over the past 750,000 years of Earth's history, Ice Ages have occurred at regular intervals, of approximately 100,000 years each.