介绍:
TPO 9 Lecture 3 Geology
Lecturer: 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, barren and extremely hot. 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. 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.
Female Student: Excuse me, Professor. But I'm confused. Why would lakes form in the desert? It's just sand, after all.
Lecturer: Good question! We know from modern day desert lakes, like Lake Eyre 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 beds or basins as limestone formations that we can still see today. They look like low-lying, white or grey buttes, long, narrow hills with flat tops, barely a meter high.
A recent study of some of the formations presents some new theories about the area's past. Keep in mind though that this study only looked at 19 formations. And about a thousand have been documented. So there's a lot more work to be done.
According to the study, two factors were important for lake formation in the Empty Quarter: first the rains that fell there were torrential. So it would've 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 run-off collected, and formed a lake.
Now, the older lakes, about half the formations, the ones that started forming 37000 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. 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 10000 years ago, well, they seemed 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 are some ideas about that, and they have to do with the shapes of the sand dunes, when the lakes were formed. 37000 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've captured the rain right there on the top.
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. But, where did these animals come from? 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.
The study makes a couple of interesting points about the fossils, which I hope will be looked at in future studies. 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. 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. So they survive on much less. Interestingly, there are clams and snail shells; but, no fossils of fish. We're not sure why. Maybe there was a problem with the water. Maybe it was too salty. That's certainly true of other desert lakes.
