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Time traps

The whole world felt the effects of the dinosaur-killing mass extinction 65 million years ago. But a spot in Colorado may have the best record of it. Rex Dalton reports from Denver.

It's no wonder palaeontology is so popular in Colorado: fossils practically permeate the landscape. Hiking trails run through hills known for Triceratops. Part of the local interstate is named after Tyrannosaurus rex. And around the east of the city of Denver you can drive out into the grassy plains and walk the landscape where dinosaurs once lumbered.

Artist's impression of West Bijou valley soon after the asteroid strike 65 million years ago. Credit: J. VRIESEN/DENVER MUSEUM OF NATURE & SCIENCE

But for some, the thrill isn't just in finding the giants — it's in dating precisely when they disappeared. Here, in the rolling expanses east of the city, lies what could be the world's best spot for understanding what transpired 65 million years ago, when the Cretaceous geological period ended and the Tertiary one began. This 'K/T boundary' marks a crucial moment when some cataclysmic event — probably an asteroid impact (see page 48), or massive volcanic eruptions — wiped out most creatures worldwide.

The going date for this event is 65.5 million years ago, with a margin of error of 100,000 years1. But one group of scientists is aiming to get this margin down to just 25,000 years. Their work is part of a major international geochronology effort called EARTHTIME, which aims to calibrate the geological timescale for all of Earth's history2. “We hope to use the hundreds of ash layers to develop an unprecedented time sequence that will allow us to read the geological record of deep time like never before,” says palaeobotanist Kirk Johnson, leader of the team and head curator at the Denver Museum of Nature and Science.

Colorado has taken centre stage in geology after some serendipitous discoveries in the Denver basin over the past 15 years. In 1994, plant fossils of an ancient rainforest were unearthed as a highway was being built near the town of Castle Rock, about 40 kilometres south of Denver. Dating techniques pegged them at 64.1 million years old. That suggests that a tropical rainforest had established itself less than a million-and-a-half years after the cataclysmic extinction — earlier than some had suspected3.

More discoveries followed, defining the Cretaceous underbelly of Denver. Then in 2003, Johnson and his co-workers published a report4 that detailed the wealth of K/T sediments at a site called West Bijou Creek, about 65 kilometres east of Denver. Suddenly, Colorado took on world-class importance. Sediments marking the K/T boundary can be found across the globe, but the West Bijou site is one of the most complete locations, with plant and animal fossils encased within layers of volcanic ash.

In an eroded knob along a gully overlooking the grassy plains, Johnson points to a layer just below the surface that is rich in the element iridium, which is often used as an indicator of an extraterrestrial impact. Below the iridium layer marking the K/T boundary are dinosaur fossils; above it there are none.

The region's rich geological history makes it ideal for studying the events leading up to, at and just after the K/T extinction. Volcanoes once dotted the eastern side of the Front Range of the Rocky Mountains, showering ash over the countryside. Eventually, the volcanoes eroded away, but they left behind thick layers of sediment that piled up in the basin. It is these alternating white and dark layers that provide geologists with clues about the past.

Leaves are a fossil thermometer, rain gauge and chalk board for insect tracks. Kirk Johnson

On past field trips, Johnson and his crew scoured the sites for plant fossils — finding about 500 leaves and 2,000 pollen samples, which together show what the environment was like around the cataclysmic transition. The sizes of leaves can indicate rainfall5, and the edges of leaves denote temperature — the smoother the edge, the warmer the temperature must have been6.

“Leaves are a fossil thermometer and rain gauge,” says Johnson. “They also are a chalk board for insect tracks.” By studying insect markings on the leaves, scientists can see which species went extinct and when. For instance, the team found little damage due to insect feeding on leaves dated at 63.8 million years old — suggesting that food webs were still out of kilter between a million and 2 million years after the extinction event7.

Dating service

Now, though, the researchers are moving beyond fossils and into geochemical, magnetic and other evidence to help them improve the chronology of what happened across the K/T boundary. Johnson has been joined by Sam Bowring, a specialist in geochemical dating at the Massachusetts Institute of Technology in Cambridge and William Clyde, a geologist from the University of New Hampshire in Durham who analyses palaeomagnetic fields.

Kirk Johnson (left) and Sam Bowring trawl through sediment in search of zircon crystals. Credit: R. DALTON

Their quarry is tiny zircon crystals in the layers of ash. By measuring the ratio of uranium to lead in individual zircon crystals, Bowring can date the ash layers and help the team pinpoint the time at which the K/T extinction occurred.

Hiking to the bottom of a gorge called My Lucky Number, Johnson leads Bowring and Clyde to an eroded outcrop. Here they dig a shallow trench, revealing a dozen layers of ash. “You are looking at the surface of the swamp where the ash fell” about 500,000 years after the K/T boundary, says Johnson, sweeping clean the first whitish layer.

William Clyde checks the magnetic polarity of ash layers in the Denver Basin. Credit: R. DALTON

Every time a new layer of ash is uncovered, Bowring kneels — trowel in hand — and scoops out samples for the collection bag. “Oh, baby,” is his frequent refrain. A field crew can work for years before finding a single ash layer. Here, more and more appear beneath the trowel.

Meanwhile, Clyde uses clues from the periodic reversals in Earth's magnetic field. He takes a wood rasp, flattens a section of the soft ash layer, and aligns a compass along its plane. Then he digs out a fist-sized piece and tucks it away in the collection bag. These pieces will later be tested to see whether they fall in the same polarity as occurs today, or in the 'reversed' direction. By working out the date of the reversals around the K/T layer, the team can narrow down the time at which the extinction event must have occurred.

Back in time

For the rest of the day the researchers work their way up the gorge, scaring off deer as they go. They repeat the exercise half-a-dozen times, eventually taking readings on some 40 layers of ash. As they sample the rising sediments, Clyde may find five consecutive normal palaeomagnetic readings, then five consecutive reversed ones. Bowring's zircon dates can perhaps be used to bracket and date that reversal point.

By comparing the findings to a research core stored at the US Geological Survey's office in Denver, near-surface dates can be correlated with the same sediments found deeper at other locations. And by comparing the new Denver dates to records from the ocean cores, the researchers might even be able to improve the correlation of sediment layers found around the globe. For instance, European researchers have found evidence8 in Spain for changes in Earth's orbital cycles near the K/T boundary, which they date to 65.8 million years ago. The Denver studies will test that result, and help to refine the chronology of the past climate changes caused by changes in the Earth's orbit.

These observations will be plugged into the larger EARTHTIME project, which is just beginning to get under way. But already, Johnson is looking at short-term benefits of the research — in particular, tying it back to issues concerning the health and development of the Denver area.

His team's work on local aquifers, which run through sediments from the Cretaceous period, has helped to clarify the region's geology and hydrology. And that, in turn, could help city planners make better decisions about how to manage water supplies for the rapidly growing metropolis — to keep itself from going extinct.


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Dalton, R. Time traps. Nature 449, 20–21 (2007).

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