Evolution and Climate Change
Drilling Deep into Pre-Dinosaurian Rocks
On a high ridge in Arizona’s Petrified Forest National Park, Lamont paleontologist Paul Olsen sits on the fallen trunk of a 215-million-year-old tree, now turned to stone. The tree once loomed 70 or 80 feet above a riverine landscape teeming with fish, turtles, giant crocodilians and tiny, early species of dinosaurs. From here, Olsen can survey the remnants of this lost world: miles and miles of surreal badlands, where sediments built up over millions of years have eroded back down to expose endless cross sections of brightly colored rocks. The layers represent tectonic movements, natural climate cycles, the growth and disappearance of lakes, and buildups of river deltas. The petrified trees scattered across the landscape are only the most obvious fossils; others are bleeding out by the ton. It is perhaps the world’s richest trove of rocks from the late Triassic, when dinosaurs, and early mammals, got their evolutionary start. The Triassic was also a hothouse world: a time of high atmospheric carbon dioxide, rapid climate shifts, and fast-moving extinctions. Olsen thinks there may be much to learn from it for our own time.
Scientists have been digging here since the 1850s, yet much remains unknown about the precise timing of events in the late Triassic, between about 235 million and 201 million years ago. That is why Olsen is here. He is co-leader of a team that is drilling a borehole deep into the rocks. By taking out a continuous core – a first for this region – they hope to assemble a record that will not only help them write a reliable history for this pivotal time, but shed light on how natural climate cycles work, and how they affect ecosystems – knowledge that, among other things, should advance scientists’ ability to assess the prospects for human-induced shifts.
“Even in this area of beautifully outcropping rocks, it’s hard to put together an exact sequence of events, based on what you see,” says Olsen, sweeping his hand over the vast maze of buttes, mesas, canyons and pinnacles. Deeper layers are inaccessible from the surface, and erosion has carried away elements from many shallower ones, making it impossible to see clearly how they relate to each other, and when each was formed. A continuous core will provide “an unimpeachable record,” he says. Because of the richness of fossils and the large number of studies that have already been done, he says, “This is one place in the world, where by resolving exact times of events, we can ask very specific questions about how Earth’s systems work. Understanding ancient environments gives us strong clues to future ones. In fact, it’s the only way to test our climate models. Other than letting the experiment [in global warming] we are in right now to run its course. I think we’d like to know that with a little more certainty.”
Olsen thinks that many types of questions can be answered here. For one, the core should allow scientists to figure out the timing of repeated shifts in temperature and precipitation caused by periodic shifts in Earth’s orbit, and whether such shifts operated on the same schedule during the Triassic as they have in more recent times. These cycles have been documented by Olsen and his colleagues in Triassic-age rocks of the Newark Basin, near New York City – but those rocks don’t contain minerals that allow the cycles to be placed precisely in absolute time. The ones at Petrified Forest do. “If we can show that the Newark timescale is correct, we can empirically calibrate the solar system’s behavior,” Olsen told the journal Nature in an article about the project.
A more specific question: whether a giant asteroid that left a crater more than 50 miles wide in what is now Quebec had anything to do with a large turnover in flora and fauna during the Triassic. The crater is precisely dated, at 215.5 million years, but the turnover is not; estimates vary by 10 million years or more. If there is a direct connection, Olsen suspects it can be made by dating what he calls an “extinction layer” that outcrops in various parts of the park, where the sediments suddenly change to red and white and fossils practically disappear, suggesting some kind of catastrophe.
Near where Olsen is sitting, on the edge of a nearby butte, a three-man crew is running a roaring diesel-powered drill. Its hollow-tip bit is boring at 1,000 revolutions per minute through one formation after another at a 30-degree angle, 24 hours a day, seven days a week. About every 20 or 30 minutes, the crew hauls out a casing containing a five-foot length of core. Geologists working with Olsen take turns carrying the plastic-swathed core to a tent, labeling it, cutting it into half-sections, and piling it in the back of pickup truck. Depending on the layer, the drill brings up mudstones or siltstones in shades of purple, red and brown; some are flecked with gray or white carbonates, and occasionally squiggles that look like they could be fossilized tree roots, or streambed ripples. At one point, the drill hits an unknown obstruction that causes it to veer off course – possibly a fossilized tree, made of extremely hard quartz. Their straight course compromised, the drillers have to pull out and start again.
The project is a collaboration among Lamont, Rutgers University, the universities of Arizona, Texas and Utah, and other institutions. The drilling, which took place in November and December 2013, took nearly a month, bottoming out at 1,706.5 feet in one hole, then at 830 feet in a second. The deeper hole appears to reach back at least 250 million years – the very start of the Triassic. In coming months, the cores will be examined at several labs with CAT scans, chemical analyses, magnetic analyses and high-resolution photography.
Ashes from repeated volcanic eruptions are known to punctuate the sedimentary layers, and those ashes contain mineral grains with radioactive isotopes that can be analyzed to yield absolute ages. Also, sporadic reversals in Earth’s magnetic field are recorded in the orientation of grains within the sediments themselves. Lamont paleomagnetism expert Dennis Kent will try to line up these reversals with the volcanic layers. Project co-leader John Geissman, a geologist at the University of Texas, Dallas, says, “It’s a unique opportunity to put together a coherent time framework for a critical [time].”
The team hopes that this will be the first of a half-dozen sites in a proposed wider study dubbed the Colorado Plateau Coring Project, which aims to study the Four Corners region straddling portions of Colorado, Arizona, Utah and New Mexico, a region that shares many of the same rock formations.