托福阅读真题+题目+答案：The Azolla Event

托福阅读真题+题目+答案：The Azolla Event

During the early Eocene period, about 56 million years ago, Earth's atmosphere had high levels of carbon dioxide (CO2), which warmed the planet.▋But about 49 million years ago the climate began to cool, mostly as the result of activity in the Arctic. ▋In the early Eocene the Arctic Ocean, unlike today, was almost completely surrounded by land, isolating it from the other oceans. ▋One consequence of this isolation was that the Arctic Ocean was able to retain much of the freshwater that fell as rain and that entered along with the rivers that flowed into it from the nearby land.▋ This freshwater accumulated on the ocean surface as a light, slightly salty layer floating on denser, saltier water a few meters below.Because rainfall was even more intense than it is today, and because there was less disruption by waves from other oceans, this layer of surface water occasionally became completely fresh. Those were the conditions that allowed rapid growth across the Arctic of Azolla, a freshwater fern (a plant) normally incapable of living in the sea but able to thrive on this very odd, almost salt-free ocean.

1.According to paragraph 1, which of the following contributed to the formation of a slightly salty surface layer in the Eocene Arctic Ocean?

A.The warm water temperature

B.The location of surrounding land areas

C.The rapid sinking of salt to the ocean bottom

D.The flow of salt water from the Arctic Ocean to other oceans

2.According to paragraph 1, the growth of Azolla in the Eocene Arctic Ocean was a result of

A.the decrease in atmospheric CO2, levels

B.the presence of a layer of mostly freshwater in the ocean

C.Azolla's adaptation to salt water

D.the increased density of the ocean water

Azolla is a fascinating plant. Unlike the land-loving ferns most of us are familiar with, Azolla grows as free-floating mats on the surfaces of lakes and rivers. It can do this because of its mutually beneficial relationship with bacteria that are able to break apart the strong molecular bonds of atmospheric nitrogen to make the nitrate that plants need for growth. In other words, it obtains its food from the air. Azolla carries these useful companions around in specially formed growths that provide it with the nitrate fertilizer that a free-floating plant cannot get by the usual approach of sticking roots into soil. In return for this invaluable service, the plant provides its bacterial guests with food in the form of glucose, and this partnership allows Azolla to thrive in nutrient-poor waters where other aquatic plants struggle。

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