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旁白：请听一段地理课上的演讲。

Narrator: Listen to part of a lecture in a geology class.

演讲者：那么，我们继续讨论沙漠湖泊，现在我想主要说说“空白地区”。

Lecturer: So, continuing our discussion of desert lakes, now I want to focus on what's known as the “Empty Quarter”.

“空白地区”是一大块覆盖了四分之一的阿拉伯半岛的沙漠地带。

The “Empty Quarter” is a huge area of sand that covers about a quarter of the Arabian Peninsula.

如今那里很是荒凉，土地贫瘠，极其炎热。

Today it's pretty desolate, barren and extremely hot.

但在过去一段时间里，季风雨湿润了整片空白地区并将之从沙漠转变成绿地，其上湖泊星罗棋布，各种动物在此安家。

But there've been times in the past when monsoon rains soaked the Empty Quarter and turned it from a desert into grassland that was dotted with lakes and home to various animals.

事实上，这里有过两次降雨和湖泊形成期：第一次发生在35000年前，第二次可追朔到10000年前。

There were actually two periods of rain and lake formation: the first one began about 37,000 years ago; and the second one dates from about 10,000 years ago.

女学生：不好意思，老师，打断一下。

为什么沙漠里会有湖泊形成呢？

我们现在知道，像澳大利亚南部的艾尔湖，就是在正常的沙漠条件下形成了的湖泊。

We know from modern day desert lakes, like Lake Eyre in South Australia, that under the right conditions, lakes do form in the desert.

但是几千年前空白地带的湖泊就消失了。

But the Empty Quarter lakes disappeared thousands of years ago.

这些湖泊沉积矿或某些流域是作为石灰岩层而存留至今,我们仍然可以看到。

They left behind their beds or basins as limestone formations that we can still see today.

他们地处低洼地带，灰白相间，看起来像狭长而平顶的山丘，几乎不到一米高。

They look like low-lying, white or grey buttes, long, narrow hills with flat tops, barely a meter high.

最近的对某些岩层的研究中提到了关于该地区的过去形态的某些理论。

A recent study of some of the formations presents some new theories about the area's past.

记住，这次研究只是针对十九种构造进行的。

Keep in mind though that this study only looked at 19 formations.

而将近一千种构造已经被记录在册。

And about a thousand have been documented.

根据此次研究，空白地带的湖泊构造有两个重要的因素：第一，该地区的降雨是倾盆而注的。

According to the study, two factors were important for lake formation in the Empty Quarter: first, the rains that fell there were torrential.

而本来所有的雨水是不可能渗入到地底下的。

So it would've been impossible for all the water to soak into the ground.

第二，你们知道，沙丘中含有其他种类的粒子，除了沙子之外，还包括黏土和泥沙。

Second, as you know, sand dunes contain other types of particles, besides sand, including clay and silt.

如今，当下雨的时候，雨水冲刷着沙丘的两侧，携带着泥土和泥沙颗粒。

Now, when the rain fell, water ran down the sides of the dunes, carrying clay and silt particles with it.

不管这些颗粒沉降到哪里，它们都会形成一个平板隔水层。

And wherever these particles settled, they formed a pan, a layer that water couldn't penetrate.

一旦隔水平板形成，就会收集到更多的流失的泥沙，最终形成湖泊。

Once this pan formed, further run-off collected, and formed a lake.

而年代更为久远的湖泊，大概占到一半的构造，它们于35000年前就已经形成。我们所看到的石灰岩层高达一千米，而宽却只有几米。他们沿着沙漠地板和沙丘间山谷分布。

Now, the older lakes, about half the formations, the ones that started forming 37, 000 years ago, the limestone formations we see, they're up to a kilometer long, but only a few meters wide, and they're scattered along the desert floor, in valleys between the dunes.

所以，这里的理论是，湖泊沿着沙漠地板在这些狭长的山谷中形成。

So, the theory is, the lakes formed there on the desert floor, in these long narrow valleys.

而我们知道，正因为我们了解相类似的古代沙漠湖泊，我们才知道这些湖泊存在的时间不长，平均是几个月到几年时间。

And we know, because of what we know about similar ancient desert lakes, we know that the lakes didn't last very long, from a few months to a few years on average.

而对于比较近期才形成的湖泊，那些10000年前形成的湖泊，嗯，他们似乎更为小，所以可能会更快干涸。

As for the more recent lakes, the ones from 10,000 years ago, well, they seemed to have been smaller, and so may have dried up more quickly.

另一个区别，一个今天我们要谈到的，区分年代久远和近期形成的湖泊沉积矿，就是石灰岩层的所在地。

Another difference, very important today for distinguishing between older lake beds and newer ones, is the location of the limestone formations.

近期的沉积矿在沙丘中位置更高。

The more recent beds are high up in the dunes.

嗯，这里有一些说法，这些沉积矿跟沙丘的形状有关。

Well, there are some ideas about that, and they have to do with the shapes of the sand dunes, when the lakes were formed.

在37000年前湖泊形成的时候，沙丘的顶部很可能是圆状的，因此雨水就只是从两边顺势留下到沙漠地板上。

37, 000 years ago, the dunes were probably nicely rounded at the top, so the water just ran right down their sides to the desert floor.

而在两个雨季之间几千年来刮风不止啊，这就重新将沙丘塑形了。

But there were thousands of years of wind between the two rainy periods, reshaping the dunes.

那么在第二次雨季期间，沙丘的顶部有点被分割开了，满是凹陷的空洞和隆起线，而这些洞就足以储存从顶部留下的雨水。

So, during the second rainy period, the dunes were kind of chopped up at the top, full of hollows and ridges, and these hollows would've captured the rain right there on the top.

如今，在湖泊生态系统的草地上，我们期望发现各种动物的化石，而至少在某些特定的地方我们已经发现了很多化石。

Now, in a grassland of Lake Ecosystem, we'd expect to find fossils from a variety of animals, and numerous fossils have been found at least at these particular sites.

但是，这些动物是从哪里来的呢？

嗯，该理论一直以来说的就是动物从附近的栖息地迁徙而来。

Well, the theory that has been suggested is that they migrated in from nearby habitats where they were already living.

Then as the lakes dried up, they died out.

此次研究中有几个有趣的点是关于化石的，我希望这些点在未来的研究中会被重视。

The study makes a couple of interesting points about the fossils, which I hope will be looked at in future studies.

在年代久远的湖泊遗址中，发现的化石是河马，水牛这些在水中长时间生活的生物，还有，牛的化石。

At older lake sites, there’s fossil remains from hippopotamuses, water buffalo, animals that spend much of their lives standing in water, and also, fossils of cattle.

然而，在较为近期形成的湖泊遗址中，却只发现了牛的化石，对于地理学家来说，这是另外的证据，证明湖泊很可能更小更浅，因为牛只饮水。

However, at the sites of the more recent lakes, there’s only cattle fossils, additional evidence for geologists that these lakes were probably smaller, shallower, because cattle only use water for drinking.

所以他们存活的数量要少很多。

有趣的是，还有蛤蜊和蜗牛壳，但是没有鱼的化石。

Interestingly, there are clams and snail shells; but, no fossils of fish.

Um...Maybe there was a problem with the water.

其他的沙漠湖泊湖水确实如此。

That's certainly true of other desert lakes.

What is the professor's opinion about the conclusions of the recent study of the limestone formations in the Empty Quarter?

According to the professor, what feature of the sand dunes made the formation of the lakes possible?

How is it possible to determine in which rainy period a lake was formed?

What does the professor imply about the lack of water buffalo and hippopotamus fossils in the more recent lakes?

What possible explanation does the professor give for the apparent absence of fish in the most ancient lakes?

P: So continuing our discussion of desert lakes, now I want to focus on what’s known as the Empty Quarter. The Empty Quarter is a huge area of sand that covers about a quarter of the Arabian Peninsula. Today, it’s pretty desolate and, barren and extremely hot. But there’ve been times in the past when monsoon rains soaped the Empty Quarter and turned it from a desert into grassland. That was dotted with lakes, and homed various animals. There were actually two periods of rain and lake formation. The first one began about 37 thousand years ago. And the second one dates from about 10 thousand years ago. 
A( girly): Excuse me professor. But I’m confused. Why would lakes form in the desert? It’s just sand after all. 
P: Good question. 
We know from modern day desert lakes like Lake Air in South Australia that under the right conditions, lakes do form in the desert. But the Empty Quarter lakes disappeared thousands of years ago. They left behind their bed or basins as limestone formations that we can still see today. They look like low line white or grey buits, long narrow hills with flat tops barely a meter high. A recent study of some of the formations present some new theories about the areas past. Keep in mind though that this study only looked at 19 formations and about 1000 had been documented. So there’s a lot more work to be done. 
Uh..according to the study, two factors were important for lake formation in the Empty Quarter. Uh..first, the rains that fell there were torrential ,so it would have been impossible for all the water to soak into the ground. Second, as you know sand dunes contain other types of particles besides sand including clay and silt. Now, when the rain fell, water ran down the sides of the dunes carrying clay and silt particles with it. And wherever these particles settled, they formed a pan, a layer that water couldn’t penetrate. Once this pan formed, further runoff collected and formed a lake. Now, the older lakes, about half the formations, the ones started forming 37 thousand years ago, the limestone formations we see, there up to a kilometer long but only a few meters wide. And they’re scattered along the desert floor and valleys between the dunes. So the theory is the lakes formed there on the desert floor in these long narrow valleys. And we know because of what we know about similar ancient desert lakes, we know that the lakes didn’t last very long, from a few months to a few years on average. 
As for the more recent lakes, the ones from ten thousand years ago, well, they seem to have been smaller and so may have dried up more quickly. Another difference, very important today for distinguishing between older lake beds and newer ones, is the location of the limestone formations. The more recent beds are high up in the dunes. Why these differences? Well, there’re some ideas about that. And they have to do with the shapes of the sand dunes when the lakes were formed. Thirty-seven thousand years ago, the dunes were probably nicely rounded at the top so the water just ran right down their sides to the desert floor. But there were thousands of years of wind between the two rainy periods reshaping the dunes. So, during the second rainy period, the dunes were kind of chopped up at the top. Full of hollows and ridges. And these hollows would have catch the rain right down at the top. 
Now, in a grassland in lake ecosystem, we expect to find fossils from a variety of animals. And numerous fossils have been found, at least at these particular sights, but where did these animals come from? Well, the theory that have been suggested is that they migrated in from nearby habitats where they already living. Then, as the lakes dried up, they died out. The study makes a couple of interesting points about the fossils which I hope will be looked at future studies. At older lake sights, there’s fossil remains from hippopotamuses, water buffalo, animals that spend much of their lives standing in water and also fossils of cattle. However, at the sights of the more recent lakes, there’s only cattle fossils. 
However, at the sights of the more recent lakes, there’s only cattle fossils. 
Additional evidence for geologists that these lakes were probably smaller and shallower because cattle only use water for drinking. So they survived on much less. Interestingly, there are T. and snail shells but no fossils of fish. We’re not sure why, um..maybe there was a problem with the water. Maybe it was too salty. That’s certainly true of other desert lakes. 
Additional evidence for geologists that these lakes were probably smaller and shallower because cattle only use water for drinking. So they survived on much less. 
Interestingly, there are T. and snail shells but no fossils of fish. We’re not sure why, um..maybe there was a problem with the water. Maybe it was too salty. That’s certainly true of other desert lakes.