Nexus - 1404 - New Times Magazine-pages

Page Content (OCR)

the Earth's path of orbit, which is elliptical and can vary up to When alive, these organisms fix themselves to two types of five per cent over time. This eccentricity affects the amount oxygen atoms: the abundant and more common oxygen-16 of solar radiation that strikes the Earth's surface at aphelion isotope, and oxygen-18. Oxygen-18, the heavier isotope, is (our farthest point from the Sun) and at perihelion (our enriched in ocean water; the lighter atom is found in higher nearest point to the Sun). concentrations in snow and ice. Whenever water is extracted According to Croll's theory, a decrease in the amount of from the ocean to make more ice, the calling card is left in the solar radiation during the winter favours the accumulation of oxygen. This enrichment, from oxygen-16 to oxygen-18, is snow, but this will result in additional loss of heat by the seen in the carbonate shells of the foraminifers (made of reflection of sunlight back into space. CaCOs). The carbonate precipitates from sea water, so the If winter occurs when the Earth is close to the Sun, oxygen that builds the carbonate crystals reflects the temperatures will be naturally warmer than usual; but if composition of the sea water. winter occurs when the Sun is further away, temperatures will By analysing oxygen isotopes in foraminifers, scientists can be colder than usual. Therefore, if the polar area of a determine when the Earth produced more glaciers and the hemisphere becomes colder, trade winds will be stronger in time periods when ice ages occurred. In sea-floor sediments, that hemisphere and warm equatorial ocean currents will shift evidence for 100,000-year as well as 41,000-year and 23,000- towards the opposite hemisphere, further year cycles in climate has been augmenting the heat loss. If the Earth's discovered. But there are still unresolved orbit were circular, the slow wobble questions. would have no effect at all on climate. In glacial data, the 100,000-year cycle Each season would occur at the same seems to dominate, with the 41,000-year distance from the Sun. However, since cycle weaker and the 23,000-year weakest insolation in the northern hemisphere is of all. However, in insolation theory, it is out of phase with that of the southern In sea-floor the reverse: the 23,000-year cycle hemisphere, Croll believed that the ice . . dominates and the weakest appears to be ages would alternate from the northern to sediments, evidence the 100,000-year cycle.’ the southern hemisphere. Although the alternating ice ages theory for 100,000-year Himalayan Uplift and the Global was proved to be wrong, Croll's ideas laid as well as 41 ,000-year Climate the foundation for ice age causality. He One of the more recent theories to was the first to recognise the importance and 23,000-year cycles explain ice ages links changes in global ° ocean currents, rar radiation and in climate has been climate to one of the rans most the eccentricity of the Earth's orbit . impressive geological features: the in building an explanatory model. In discovered. Himalayas. According to the theory 1876, Croll was named a Fellow of But there are still proposed in 1988 by Maureen unresolved questions. the Royal Society of London. Raymo, an Earth sciences research Early in the 20th century Milutin professor at Boston University, as Milankovitch, a professor of physics, the Himalayas grew, a massive mathematics and astronomy at the amount of rock was exposed to the University of Belgrade, revived elements. Monsoon rains soaked the Croll's insolation theory and set out land, and the face of the exposed on the task of detailing insolation rock eroded. based on Ludwig Pilgrim's latest This process of chemical calculations of the Earth's orbit. weathering extracted so much carbon He showed that the insolation was dioxide from the atmosphere that dominated by a 23,000-year cycle global temperatures dropped, thus and concluded that ice ages would be most intense when the triggering an ice age.‘ To show that this was the case, Raymo solar radiation dropped below a certain threshold. Since the turned to the study of sea-floor sediments and strontium. insolation curve has an approximate 100,000-year cycle, he There are several types (isotopes) of strontium, each with a believed that such a cycle might be seen in the ice ages. He different atomic mass. Strontium-87, a heavier variety, is also had the insight to put forward the idea that the northern washed into the sea by the chemical weathering of rock. The hemisphere would dominate because it contained two-thirds ighter variety, strontium-86, is released by the spreading of the Earth's land mass. Driven by the amount of solar seafloor and comes from deep inside the Earth. By radiation in the north, the ice ages in both the hemispheres comparing the amounts of the isotopes in different layers, would be synchronised. Raymo believed that she would learn which process was more Milankovitch's insolation theory was abandoned when age active at any point in time. Thirty-five million years ago, estimates, made possible by radiocarbon dating, showed that strontium-87 increased dramatically, coinciding with the the timing of his ice ages calculations was incorrect. Himalayan uplift. However, isotope studies in sea-floor sediments during the With the strontium evidence, Maureen Raymo thinks she 1960s and 1970s, which focused on changes in the Earth's as solved the ice ages mystery. First, the uplift of the climate, revived his theory. Tibetan region intensified the Indian monsoon. Then the Deep-sea sediments containing the shells of small plankton- monsoon rains eroded the mountains, stripping carbon dioxide like organisms, called foraminifers, hold a history of climate from the air. Finally, with less carbon dioxide, the change. atmosphere gradually cooled. In sea-floor sediments, evidence for 100,000-year as well as 41,000-year and 23,000-year cycles in climate has been discovered. JUNE — JULY 2007 NEXUS + 43 But there are still unresolved questions. www.nexusmagazine.com