Peabody Fossil Hunter

Monday, July 11

In Uncategorized on July 11, 2011 at 1:24 am

We’ve found quite a lot of fossils in the past few days from the Hoff Ranch, and paleontologists with the Marmarth Research Foundation have found even more in past years. But fossils don’t mean much without knowing when they lived, and in what kind of environment. Today we followed French geologist Antoine Bercovici as he began the process of piecing together the history of the ranch’s buttes and ravines, based on their geologic bands of varying colors and textures.

When the meteorite hit the Yucatan Peninsula 65 million years ago, it pounded the ground beneath it hard enough to melted it, sending little bits of Mexico in all directions in the form of little glass droplets. Some of the impact’s projectiles—tectites and shocked quartz—had enough velocity to make two revolutions around the planet before landing. Because the impact was instantaneous and unparalleled, these spherical glass bits are one of the best indicators for finding what geologic layer the impact occurred in. The meteorite also brought in iridium, a dense silvery metal that is very rare in the Earth’s crust but much more abundant in meteorites.

The presence of iridium and tektites would be the clearest way to identify the K-T boundary in a geologic formation, but they’re rarely preserved well enough to measure.  In the end, scientists often use something much more commonplace to date the boundary: pollen.

Bercovici is a palynologist—he studies plant microfossils including spores, fungi, algae, and pollen. He looks at the amount and types of pollen embedded in each layer to infer what the paleoenvironment was like when each was formed.

To get a hold of good pollen samples, we had to be destructive. We picked a site—a gently sloping butte face close to a T-rex fossil, with clear banding. Then we cut it up. With pickaxes and shovels, we whacked a trench into its side, cutting a 30-foot-high staircase to expose fresh sediments safe from surface impurities. We hammered nails into every point on our new dirt staircase where the rock layers appeared to change. Finally, we climbed it, taking samples, measurements, and notes for each of the 21 layers.

Antoine Bercovici stands next to our freshly dug trench.

Once a lab in Canada dissolves out impurities with acid, Bercovici will look at each one under a microscope, noting the presence of pollens and other organic inclusions that can date the layer to a specific time period.

The impact obliterated much of the Earth’s vegetation. About 36 plant taxa went extinct—as much as a third of all pollen taxa. Whenever such ground cover is destroyed, even with recent events like the eruption of Mount St. Helens, ferns are the first plants to bounce back. Palynologists like Bercovici look for a spike in fern spores, and the presence of the taxa that went extinct, to pinpoint the layer representing the K-T boundary. Eventually, we’ll be able to match each fossil with its sedimentary layer. Most importantly, we’ll know what layer represents the K-T boundary—a key step to understanding what taxa survived the asteroid impact, and where on the timeline our fossil mammals fall.

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