When the Med almost dried up [pg.6]

September 17, 2006 01:36
4 minute read.

A sudden drop in the level of the Mediterranean Sea six million years ago, followed by a warmer, wetter climate, wore the Alps down to a shadow of their former selves. So claims a University of Washington geologist who has studied the rugged mountain range. Typically, mountain ranges reach a sort of equilibrium, with erosion more or less keeping pace with the tectonic forces that enlarge them. But an event called the Messinian salinity crisis, precipitated by blockage of the forerunner of the Strait of Gibraltar, cut the Mediterranean off from the rest of the world's oceans. Evaporation dropped the water level as much as four kilometers below the rest of the world's oceans, says Dr. Sean Willett of the university's earth sciences and space department. The beds of rivers flowing from the Alps dropped sharply as the level of the Mediterranean basin fell, and their increased force carried away huge amounts of sediment, carving many of the deep valleys for which the Alps are known, and leaving behind nearly a dozen major alpine lakes. "The erosion rates were 10 times normal during the salinity crisis, and that correlates to the fall of the base level of the rivers," explains Willett. "We think this jump-started the erosion, but it doesn't explain why the erosion remained so rapid for three million years. The Mediterranean was low for only 20,000 to 80,000 years." The Mediterranean substantially refilled with fresh water, likely from heavy rainfall, indicating major climate change. The result was a brackish water mixture that probably continued to evaporate at high rates, Willett says. About 200,000 years later, the Atlantic Ocean finally breached Gibraltar and seawater poured in, apparently refilling the Mediterranean completely within a few years. "Probably the biggest erosion came when there was both the heavy rainfall and the low base level of the rivers," says Willett, lead author of a paper on the subject published in the August edition of Geology. His research was funded by the US National Science Foundation. Before the massive erosion, he said, the Alps were 100 to 180 kilometers wider than they are today, and 350 to 1,700 meters higher. The highest point today is Mount Blanc on the border of France and Italy, at about 5,250 meters. It is likely that erosion took a toll on the northern edges of the Alps as well, Willett said. After three million years of warm, wet conditions, the climate cooled again and glaciers formed, though they are in retreat now because of global warming. Willett noted that European precipitation is strongly linked to changes in the mean global temperature, which scientists can track through ice records. REDEFINING PLANETS Every schoolchild knows (or is supposed to know) that there are nine planets in the solar system. But things change, and now the International Astronomical Union, meeting in Prague, has redefined what constitutes a planet. The solar system thus consists of eight classical planets; three planets in a new category called "plutons" (Pluto-like objects) and Ceres. Pluto will remain a planet and become the prototype for the new category of "plutons." Astronomers and other space experts have been arguing about how large an asteroid should be before it can be considered a planet, and how large a planet can be before it is regarded as a "brown dwarf" star. According to the advisory panel, a planet is "a celestial body that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, is in orbit around a star, and is neither a star nor a satellite of a planet." "It may end up that we have hundreds of planets in the solar system, under this new definition," University of Alabama at Birmingham physics Prof. Perry Gerakines comments. "Instead of just talking about planets, we'll be talking about types of planets," he said. Gerakines, who is a member of the IAU, reminds people who would panic over the changing definition that 500 years ago, it was widely thought that the Earth was the center of the universe, and that planets were merely "wandering stars." The word "world" once meant "universe" until it was found by Galileo that the other planets were similar to Earth. Nowadays, we know of many "worlds" such as ours. "That's the nature of science," Gerakines said. "Science is about adapting our thinking according to what we observe." Several Johns Hopkins University astronomers, however, have called the IAU ruling "muddled." One of them, applied physics Prof. Harold Weaver, said: "The scientific investigation of Pluto remains an important part of our effort to understand the processes that shaped the outer solar system, even if some of the objects in that region defy efforts to categorize them... The situation is still somewhat muddled… Is that clear? Now please tell your kid's science teacher.

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