每日新GRE之阅读能力锻炼素材四

KnightBM

这里是由KnightBM给您能带来的精选新GRE阅读能力提升素材分享贴四(From Scientific American) 理科类
A Calcium Conundrum Explained

Fish excrement could solve a decades-old ocean mystery. After marine plankton die, their calcium carbonate exoskeletons dissolve, making seawater alkaline; However, past studies found that the surface waters are more alkaline than expected from plankton. Now scientists at the University of Exeter in England and their colleagues have determined that calcium carbonate “gut rocks,” first found in toadfish intestines about 20 years ago, could account for a dramatic percentage of marine carbonate. Their computer models estimate roughly 812 billion to two trillion kilograms of bony fish swim in the ocean, producing some 110 billion kilograms of calcium carbonate annually. The amount constitutes at least 3 to 15 percent of the total ocean carbonate production and possibly up to 45 percent. Increasing sea temperature and rising carbon dioxide this century could cause fish to produce even more calcium carbonate, the researchers suggest in the January 16.



LEAVES FOR LEAVING ALONE



The vast majority of grasses retain their lifeless leaves, raising the question of why they keep dead weight that could

drain their productivity. To find out, scientists at the University of Buenos Aires removed dead leaves from grass in the Argentine pampas, where cattle graze. In the absence of cows, the pruning promoted grass growth, but in the presence of bovines, those grasses were grazed on more so than intact ones, resulting in less growth. The findings, published online January 15 by Oikos, suggest that dead leaves act as a defense against herbivores.



WHEN ORBITS COLLIDE

In an orbital traffic accident 490 miles above Siberia, a Russian satellite and a commercial satellite owned by U.S. communications firm Iridium collided in February. It wasn’t exactly unexpected, given the number of objects in orbit—the past 20 years has seen three other accidents, but they were minor, only producing a few pieces of debris. This latest impact, however, yielded hundreds, and some pieces have drifted down to the altitude of the International Space Station, posing a small but still real hazard.

竹林中人

顶！