passage 60 新仙女木事件的起因

★★★

文章分析：

①For years, the leading theory for what caused the Younger Dryas (a dramatic reversal, about 12,900 years ago, in a global warming trend) was a release of water from Glacial Lake Agassiz. ②The theory posited that this meltwater flooded into the North Atlantic, lowering the salinity and intensity of surface waters enough to prevent them from sinking. ③Ocean currents were changed in such a way that northward transport of heat in the ocean diminished, and the North Atlantic regions plunged back into near-glacial conditions. ④However, evidence has emerged that the Younger Dryas began long before freshwater flooded the North Atlantic. ⑤Additionally, the temperature changes included by a shutdown in the North Atlantic heat conveyor system are too small to explain the Younger Dryas.

S1 观点

release water 导致  YD

S2 顺承观点

水导致表面水域盐度和浓度的变化使之不下沉

S3 顺承事实

洋流改变

S4 转折事实

YD早就发生

S5 顺承事实

温度变化不足以解释YD

① 观点v1=release water -->YD

↓

②+③ 顺承观点+事实∈v1 ↔ ④+⑤ 转折事实，×v1

①

↓          ↔  ④+⑤

②+③

题型解析：

1. The author of the passage implies which of the following about the release of water from glacial Lake Agassiz? 又是一题定位失效，考场上先做下一题

D

A. The notion that the release occurred has been challenged by more recent findings.（相反，的确发生了，我要看的是因果关系，不是这件事本身）

B.The release probably occurred much earlier than scientists have generally assumed.（occur earlier的是YD，不是release，相反）

C.The release would not have been sufficient to cause anytemperature change in the North Atlantic.（最后一句说了温度发生过变化，相反）

D.The timing of the release is such that it probably did not trigger the onset of the Younger Dryas.

E.The release was probably unrelated to the global warming trend that was taking place.（没有证据，无法证明或证伪，不算implication）

2. The passage is primarily concerned with

A

A. presenting evidence that undermines an explanation

B. explaining the nature of a climatological phenomenon（无关，只是细节）

C. questioning the timing of a particular event（无关，只是论据）

D. discussing a new explanation for a phenomenon（没有证据，不存在新解释）

E. suggesting revisions to apopular theory（态度相反/没有证据）

背景拓展：

新仙女木事件

The Younger Dryas (c. 12,900 to c. 11,700 years BP) was a return to glacial conditions which temporarily reversed the gradual climatic warming after the Last Glacial Maximum started receding around 20,000 BP. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala, as its leaves are occasionally abundant in the Late Glacial, often minerogenic-rich, like the lake sediments of Scandinavian lakes. 

冰河世纪结束以后，地球气候于大约1.7万年前开始变暖，气温逐渐地回升。两极、北美和北欧的冰川开始消融，海平面逐渐上升，渤海、黄海、挪威海的草原被水淹没。到了1.3万年前，北美和北欧的冰雪已经融化了相当大一部分，南北半球春暖花开，一片繁荣景象。但是，就在这时，在12640年前，气温又骤然下降了，世界各地转入严寒，两极和阿尔卑斯、青藏高原等地的冰盖扩张，许多本来迁移到高纬度地区的动植物大批死亡。这一次降温是很突然的，在短短十年内，地球平均气温下降了大约7、8℃。这次降温持续了上千年，直到11500年前，气温才又突然回升。这就是地球历史上著名的新仙女木事件。它的得名由来是：在欧洲这一时期的沉积层中，发现了北极地区的一种草本植物——仙女木的残骸。更早的地层里也有同样的两次发现，分别称为老仙女木事件和中仙女木事件。

蔷薇科仙女木属 

Evolution of temperatures in the Post-Glacial period, after the Last Glacial Maximum (LGM), showing very low temperatures for the most part of the Younger Dryas, rapidly rising afterwards to reach the level of the warm Holocene. Based on Greenland ice cores.

仙女木事件发生非常突然，但是至今仍未有足够的证据证明其发生的原因。比较流行的假说是彗星撞击地球导致温度下降。