A Rock-Solid Method of Dating the Dinos
How old are the eggs?
When scientists announced the discovery last November, they could only estimate that the eggs were some 70 million to 90 million years old. That was based on the kinds of animal fossils found in the same geological formation.
The job of narrowing that uncertainty has fallen to geologist Lowell Dingus, 47, an earnest, bearded man of few words who takes the field under a cowboy hat. He’s president of the InfoQuest Foundation, based in San Diego and New York, which supports fieldwork in paleontology and geology.
On the expedition, he prowled buttes with Julia Clarke, a 25-year-old Yale graduate student, and Alberto Carlos Garrido, 27, an Argentine working on a geology thesis. They were looking for rock with a long memory.
They were hoping to exploit a curious habit of Earth. Every few hundred thousand or million years, it has flipped its magnetic field around so that sometimes a compass needle would point south instead of north. After a while, it would flip back again
As layers of mud build up underwater, tiny magnetic particles line up with the prevailing magnetic field. Once the mud hardens into rock, the particles are locked into position. So a lab test can reveal whether a particular rock formed while the Earth’s field was normal or reversed.
Scientists have been able to put dates on these field reversals. So once you can place a sample in one of the normal or reversed periods, you can date it. The problem is, if you just have one sample, or a series of samples showing the same magnetic field orientation, you can’t tell which normal or reversed period it’s from.
You need to take a lot of samples from rocks of different ages from a site, so you can discern a pattern that can be used to find the corresponding pattern in the dated geological record.
All the samples Dingus had taken before this field trip showed the reversed orientation. Now, working in younger rocks, he wanted to find some with normal orientation, a step toward finding the pattern that would let him know when all these rocks formed.
He also came here with another, greater goal: finding a layer of ash blown away from an ancient volcano. A lab test of such ash can date it to within about 200,000 years. That would give Dingus an anchor of time in these sediments and let him date the eggs within a million years or so.
One day on the expedition, while Dingus explored on his own, Clarke and Garrido carved out chunks of stone for the magnetic field analysis. Dressed in long-sleeved shirts, baseball caps, long pants and hiking boots, they climbed from spot to spot on the buttes. They looked for steep slopes; gentle slopes are covered with lots of eroded dirt that gets in the way of finding rock.
Once they found a promising place, they hacked out a small area with their picks or heels, the crumbling surface dirt rattling down the slope in tiny avalanches. If they found rock, they drew an arrow on it pointing north. Then they hacked free a chunk at least the size of a fist, containing the arrow. They wrapped the chunk in aluminum foil to keep it from crumbling, bound the foil with masking tape, and marked a sample number on the tape.
It was time-consuming work. This morning, they collected three samples before lunch. Now it’s up to a lab to determine the secrets locked inside.
Weeks later, Dingus finally got his deposit of volcanic ash. It’s not clear whether it’s well enough preserved to provide a good date, but at least it gives hope for finding better stuff when the scientists return next year.
