OG-9-53

lucysz

53. It can be inferred from the passage that precipitation formed from evaporated ocean water has

(A)   the same isotopic ratio as ocean water

(B)   less oxygen 18 than does ocean water

(C)   less oxygen 18 than has the ice contained in continental ice sheets

(D)  a different isotopic composition than has precipitation formed from water on land

(E)   more oxygen 16 than has precipitation formed from fresh water

答案(B)，没有问题。但是怎么理解(C)呢？

OG的解释：C is incorrect because the passage suggests that the water evaporated from the ocean contributes to the growth of continental ice sheets, which should therefore have an isotopic composition similar to that of the precipitation ice sheets, which should therefore have an isotopic composition similar to that of the precipitation formed from evaporated ocean water.

 文章的相关描述：When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it.Because heavier isotopes tend to be left behind when water evaporates from the ocean surfaces,the remaining ocean water becomes progressively enriched in oxygen 18.

 从文章中可以得到这些信息：ice age 时期，1、蒸发的水最终会回到ocean中；2、水蒸发的时候，重一点的 isotopes蒸发的少，留在ocean的多。由这两个信息可以得到(B)，没有问题。

 但是怎么比较precipitation和the ice contained in continental ice sheets的isotopic ratio呢？文章似乎没有关于the continental ice sheets 的isotopic ratio的信息啊。

[此贴子已经被作者于2006-10-17 16:08:14编辑过]

bolodong

precipitation 会 冷却成为the ice contained in continental ice sheets。 所以which should therefore have an isotopic composition similar to that of the precipitation formed from evaporated ocean water.

bolodong

说的对不对阿？

lucysz

谢谢解释，不过我还是没有理解。。。

fantasy06

以下是引用bolodong在2006-10-17 17:40:00的发言：
说的对不对阿？

I agree. Actually ice sheet was formed from precipitaton such as rain or snow, so

in the chain: evaporated ocean water--precipatation--- icesheet  O18 was remained unchanged during the change

so left ocean water has more O18 than --- O18 in evaporated ocean water=O18 in precipitation =O18 in icesheet

lucysz

以下是引用fantasy06在2006-10-17 23:06:00的发言：

I agree. Actually ice sheet was formed from precipitaton such as rain or snow, so

in the chain: evaporated ocean water--precipatation--- icesheet  O18 was remained unchanged during the change

so left ocean water has more O18 than --- O18 in evaporated ocean water=O18 in precipitation =O18 in icesheet

这个推理链没错，不过有个前提：Actually ice sheet was formed from precipitaton such as rain or snow。

我现在就是不明白这个前提是怎么得到的。我觉得湖面上结冰不都是因为下雨或下雪吧？天气冷的时候，湖水也会自动结成冰的吧？而且从文章（When an ice age begins, the continental ice sheets grow, steadily reducing the amount of water evaporated from the ocean that will eventually return to it.）来看，也看不出冰是因为降雨形成的还是由湖水结成的呀。我看文章的时候第一反应就是the continental ice sheets就是由湖水形成的，而湖水中Oxygen 18的含量是比precipitation中的含量高的，所以(C)也是对的。

OG关于(C)的解释，我最不明白的就是这句话：the water evaporated from the ocean contributes to the growth of continental ice sheets。the water evaporated from the ocean 怎么就 contributes to the growth of continental ice sheets了？

请继续讨论。。。

[此贴子已经被作者于2006-10-18 10:09:40编辑过]

shirryzhan

刚搞懂

是这样的， when an ice age begins, the continental ice sheets grow, steadily reducing the amout of wather evaporated from the ocean that will eventually return to it.

海水蒸发出更多O16，留下O18，但是往复循环， 海水里O16的数量一直是比较充裕的，因为蒸发出去的会回来。
而进入ice age, 由于蒸发出去的雨水落在地上结成了冰，不回海里去了，所以海里的O18越来越多。雨水=land ice sheet