Essays - Autumn 2018

Finding Time

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Geochronologists establish precise Dates for events that occurred eons ago

By Michael W. Robbins | September 4, 2018
The Hell Creek region of Montana, a happy hunting ground for paleontologists, features sedimentary strata from 67 to 64 million years ago. (Grant Groberg)

When did all the dinosaurs die? The short answer is that they went extinct about 66 million years ago (MYA). But no one can check that date by consulting a history book or glancing at a calendar or a clock.

So how is it possible to establish a date for the death of dinosaurs that long ago? For that matter, how was it possible to pinpoint the age of the famous fossil hominid Lucy, unearthed in East Africa in 1974, at 3.18 MYA?

Many fossil bones look much alike, whether their owners were striding around East Africa 200,000 years ago or three million years ago. But with an accurate age, a fossil bone or a stone tool can fit into a meaningful niche in our developmental timeline. Without an accurate date, it’s just another old fossil. Or another old rock.

Geochronologists are the scientists who know how to find the answers to such questions. Geochronology is a fiendishly complex subdiscipline of geology that relies upon a deep understanding of both geology and geography—and of physics, chemistry, soil science, mathematics, and computer science. Locked as we all are in our span of three-score years and 10, we cannot think realistically about a million years of time, let alone a billion years.

Geochronologists routinely deal with time spans in the millions of years. It is how they see Earth and how they think. Among the most accomplished geochronologists in the world are the scientists at the Berkeley Geochronology Center (BGC), an independent nonprofit institution located just off the campus of the University of California. They are the ones who established those dates noted above.

BGC, led by its founding geochronologist and chief scientist, Paul Renne, has five additional principal scientists— Alan Deino, Warren Sharp, Roland Mundil, Greg Balco, and David Shuster—plus several research associates, a lab manager, some technical staffers, and a few postdoctoral fellows. In their state-of-the-art laboratory, they work to establish significant dates and ages in such broad subjects as extinctions, human evolution, and climate change. They have made major contributions to archaeology, paleontology, geology, and paleoanthropology. Currently, they are addressing these challenges:

What, precisely, were the global events that led to the extinction of dinosaurs? Was it the giant meteor impact on what is now the Yucatán Peninsula 66.04 MYA? Or the massive volcanic eruptions in what is now India? Or some combination of those catastrophic events?

Where, exactly, is the San Andreas Fault located in Southern California?

What do ostrich eggshells reveal about human behavior in Middle Stone Age Africa?

Was the emergence of modern humans in East Africa related to climate changes?

Precise dates enable scientists to address larger questions of causality: not just when something happened, but what actually happened, how it happened, and even why. These questions are not merely of academic interest. The proximate cause of the extinction of dinosaurs—and of about 75 percent of all life at the time—was a climate catastrophe.


Geochronologists go about their work in the field and then in the laboratory. The field may be anywhere from California to Ethiopia, from Montana to Antarctica. Fieldwork involves seeking samples of rocks, minerals, and other materials that can be dated, often while crawling around on the ground in inhospitable—even dangerous—conditions. Think scorpions, snakes, heatstroke, and occasional boy soldiers toting AK-47s.

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