Discover, January 31, 1992
New Jersey was a very different place a couple of hundred million years ago. Instead of having gentle hills, it was covered by mountains as high as the Rockies, and rather than lying on the eastern edge of North America, it lay snuggled inside the supercontinent known as Pangaea. By that time the giant landmass had begun to pull apart, and rifts had been created among the mountains. Monsoons dumped water on these rifts, and as they filled they formed a series of narrow crescent-shaped lakes, much like today’s Lake Tanganyika. The largest of them all, cutting across New Jersey, is formally known among geologists as Lake Newark. Informally, they call it the Big Blue Banana.
This year Dennis Kent and Paul Olsen, two geologists at Columbia University’s Lamont-Doherty Geological Observatory, extracted a four-mile-deep sample from the Big Blue Banana’s bottom, in the form of two-inch-wide cylinders of rock, or cores. During the lake’s 30-million-year lifetime, from 225 to 195 million years ago, the color of this rock varied as the among of water in the lake rose and fell. When the monsoons filled Lake Newark to its brim, the oxygen-free lake bottom preserved the organic matter that settled there, and this carbon-rich muck eventually turned into coal-black rock. However, when changes in the climate dried up the monsoons for thousands of years and emptied the lake, the exposed mud turned red and cracked, and stayed that way when it hardened into rock. Between these extremes, Lake Newark was often shallow and full of burrowing animals that riddled the sediment with holes and left it a light gray.
All of this makes the cores a color-coded geologic rain gauge that can be used to reveal ancient climate patterns; indeed, they make up the longest continuous climate record ever found, and they have provided the researchers with an unsurpassed wealth of data. “In six months,” says Olsen, we got more information than you could get out of several lifetimes of field geology.”
The geologists’ task was made possible by the particular orientation of the sedimentary layers that make up the lake bottom. Digging a four-mile hole through these layers would have taken an incredible amount of time and money. Fortunately, though, after the lake dried up for the last time, tectonic forces went to work and tilted the layers up at an 11-degree angle. This gave Kent and Olsen a chance to take a shortcut, drilling several short holes instead of one deep one.
To visualize their task, think of a layer cake sitting on a table. The cake’s layers are horizontal, as were Lake Newark’s rock layers. If you tilt the cake up on one end slightly and then slice off a hunk of the cake parallel to the tabletop, you expose layers that were previously masked by the layer on top. At Lake Newark, erosion acted as the knife on the tilted rock layers.
Kent and Olsen drilled straight down into these angled layers for three-quarters of a mile. Then they moved to a nearby site where the layer at the bottom of the first drill hole was exposed to the surface. There they drilled again, essentially picking up where they left off. Putting together six such cores gave them their seamless record.
Remarkably, the Earthbound geologists have been able to use the cores not just to learn more about the fluctuating Pangaean climate but also to gain insight into the heavens. Researchers have long known that many climate changes–from warm to cold, say, or from wet to dry–mirror periodic changes in Earth’s rotation and orbit. These celestial rhythms alter how much sunlight the planet gets, and thus its temperature. (The variations are too small to be solely responsible for such extreme climate shifts as ice ages. Some yet-to-be-determined feedback mechanism on Earth must amplify the orbital effects.)
Geologists had linked three orbital variations to climate changes that recur every 21,000, 40,000, and 100,000 years. But celestial mechanists who charted the orbits of the nearby planets insisted that their gravitational interaction with Earth should also create longer cycles, running every 400,000 and 2 million years. Yet such predictions were impossible to confirm: geologists didn’t have enough continuous samples of rock in which to look for evidence of these cycles.
The Lake Newark cores changed that. From their changing colors Olsen estimated when the climate was hot and wet like a rain forest, cold and dry like Antarctica, or moderate. Then with the help of a computer and a mathematical pattern-analysis technique, he was able to tease any recurring cycles out of the mass of data. And he was pleasantly surprised to see every cycle that had been predicted appear on his computer screen–nicely establishing that orbits and climates are inextricably linked.
His discovery has some far-reaching–and reassuring–consequences, indicating that our solar system is far more well-behaved and orderly than planetary scientists had thought. Since the orbits of other planets determine Earth’s longer climate cycles, the cycles can tell astronomers how these planets were behaving 200 million years ago. In recent years some computer simulations have shown that if the starting point of a planet’s motion is changed just a little, its orbital path can change radically in a few million years. Over 200 million years, celestial mechanists argued, the smallest change can have the largest effects, rendering the long-term movements of the planets completely unpredictable and chaotic.
But the Newark core shows climate cycles 200 million years ago that were predicted by using today’s orbits. “That means the solar system hasn’t changed much,” says Olsen. If the orbits had changed, it would have been next to impossible for the planets to have come back into the same arrangement as they are in now. Advocates of planetary chaos have looked at Olsen’s work and–grudgingly–agreed he is right. Chaos may still exist in space, but only over a much longer time scale or with much less effect than previously thought.
Copyright 1992 Discover Magazine. Reprinted with permission.