Digging for zircon – time capsules from the past

An image displaying detrital zircon, that were directly entomed alongside Moros intrepidus.

A geologist from Stellenbosch University was part of an international team of scientists who have just announced the discovery of Moros intrepidus, the oldest Cretaceous tyrannosauroid species yet discovered in North America.

“It’s like searching for buried treasure. When you finally find that zircon in the rocks entombing a fossil, you know you have in your hands a time-capsule from the prehistoric past.”

Speaking is Dr Ryan T. Tucker, a sedimentologist and taphonomist in the Department of Earth Sciences at Stellenbosch University.

He was part of an international team of scientists, lead by Dr Lindsay Zanno of the North Carolina Museum of Natural History (NCMNH), who have just announced the discovery of Moros intrepidus, a small tyrannosauroid who lived about 96 million years ago in what is now Utah, during the Cretaceous period.

According to a media release issued by NCMNH, it is “the oldest Cretaceous tyrannosaur species yet discovered in North America, narrowing a 70 million year gap in the fossil record of tyrant dinosaurs on the continent.

While Dr Lindsay Zanno from NCMCH and other paleontologists have been working on the site for nearly a decade, Dr Tucker was brought on board in 2015 to provide the geological context.

“These bones are way too old for carbon-dating, so we look for minerals such as zircons in the sediments and rocks in which the fossil bones have been entombed. Zircons are like time radiometric clocks. Because they contain uranium, lead and thorium, we use the ratio of these particular isotopes preserved within the zircon to determine the possible age of the rock.”

But this is easier said than done. Like a gold digger from the 1800s, Dr Tucker first had to break up the rock into individual sand grains, separate the unwanted sediment and then with a needle, hand-pick individual zircons for analysis, often with the Foo Fighters blaring in the background. The next step was to try and analyse the isotopes, and their decay, preserved within the zircon.

This work was done at Stellenbosch University’s Central Analytical Facility (CAF), which is one of a handful of institutions in the world to house a Laser Ablation Inductively Coupled (LA-ICP) Mass Spectrometer. This is a powerful analytical technology that enables highly sensitive elemental and isotopic analysis to be performed directly on solid samples.

“A few decades ago it was a very time-consuming and expensive way of radiometrically age dating minerals such as zircon. But now we can readily process and analyse zircon samples collected from various fossil assemblages around the world,” he adds.

Beyond Dr Tucker’s ongoing work in Utah, postgraduate students in the Department of Earth Sciences are undertaking a similar study just south of Utah in the Zuni Basin of New Mexico.