Sharpening the tools of time: A Q&A with dendrochronologist Charlotte Pearson

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By dating relics of the past, researchers can string together stories of the past make predictions about the future. This technique, called radiocarbon dating, just became better than ever, thanks to a new international research effort that included the University of Arizona.

Here's how it works: At any given time period, a specific ratio of stable carbon and radioactive carbon, also called C14, atoms exists in the atmosphere. Organisms continuously take up new carbon until they die, after which the radioactive carbon begins to decay at a known rate. By measuring the ratio of radioactive carbon to stable carbon remaining in an object, the date of its death can be estimated back 60,000 years at most.

If the level of atmospheric radioactive carbon were constant, determining an exact date of death would be easy. However, atmospheric carbon is always fluctuating. To date organisms precisely and translate C14 measurements into calendar ages, scientists need a reliable historical record of its variation through time and in different regions.

In a series of three papers, an international team of researchers, including some from the University of Arizona, has done just that. The team's members recalculated and adjusted the international radiocarbon calibration curves for the Northern and Southern hemispheres and the marine record to be more precise than ever.

The papers were published in a special issue of the journal Radiocarbon, which has been headquartered on campus since its founding in 1959 and has been published by the University since 1989.

Lo Que Pasa spoke with Charlotte Pearson, assistant professor of dendrochronology, anthropology and geosciences, and a member of the international radiocarbon calibration working group, about what it takes to refine the curve, what the consequences are and what this work means to her.

Q: Why is the calibration curve so important?

A: Today we are used to hearing the past described in very precisely dated terms. When we read that a pristine-looking artifact revealed by a melting glacier is 5,350 years old, for example, we might think, "Wow, that's a long time ago," but we're not surprised to see the date. But before the 1940s and the development of the radiocarbon dating method, there would be no way to date an artifact in such exact terms. That's the difference that radiocarbon dating makes, and calibration is the step that fine-tunes that date to be most precise and accurate date possible. The more accurate the calibration curve, the more accurate the date.

Q: What did you do to improve it?

A: Scientists from around the world voluntarily contribute new C14 measurements on things of known age. These measurements are reviewed by other scientists for quality control and experts in statistics work out evermore effective ways to combine them all together. It's an international and interdisciplinary effort to create a freely available, peer-reviewed resource for the global scientific community. The curves make it possible for dating results generated by any radiocarbon study anywhere in the world to be directly studied in terms of a single continuous timeline.

Q: How have you used this resource?

A: I trained first as a geoarchaeologist, so I've been using the calibration curve for dating organic materials from sediments and archaeological sites since I was an undergraduate. Since falling in love with tree rings as the ultimate method for dating human activity, I've also been fascinated by tree rings also as the ultimate time capsules of past C14.

Dated blocks of tree rings were used in the earliest calibration curves and the first 13,900 years or so of the new international radiocarbon calibration curves is entirely based on calendar-dated tree-ring C14. A lot of these new data are now generated from single tree rings, rather than blocks, because improvements in instrumentation have made it feasible to generate long series of single-year data from very small amounts of material.

It is all really exciting. As new annual C14 data are generated, aside from making great quality calibration datasets to feed into international radiocarbon calibration curves, we are seeing patterns of solar activity emerging from thousands of years ago that have never been seen before, and by comparing how these look in different trees in different regions, we are learning things about the carbon cycle and developing new ways to refine dating, synchronize timelines and understand past events.

Q: What are some other insights the curve provides?

A: The new IntCal curves will provide opportunities to recalibrate all existing radiocarbon dates. This will allow dating frameworks for numerous projects to be refined and improved and, hopefully, people will learn something new. Although the impacts of the new curves will likely be quite subtle in many cases, changes of just a few years can actually make a huge difference for researchers who are trying to tie down rapid events or figure human and environmental interactions in a very narrow time window; going back to the older part of the curve, the changes will likely get larger, up to a few hundred years' difference.

One of the projects our University of Arizona group is working on is trying to refine how we use radiocarbon dating and other records to date the massive volcanic eruption of Thera that happened in the Mediterranean around 3,500 years ago. The eruption gives us this marker horizon all across the ancient Aegean and Near Eastern region at this time. It represents an exact point in time, a single year, around which to synchronize the timelines of Egypt, Turkey, Greece and the Levant.

The time period possible for this eruption is now the best replicated section of the entire IntCal20 curve, with over 800 high-precision measurements on tree rings from different regions produced by different laboratories, including several hundred from our AMS (Accelerator Mass Spectrometry) laboratory. So, we have a precisely defined time window. Unfortunately, it happens to be quite a wide window because dates for samples from the eruption calibrate to a period where radiocarbon production plateaued out. So, we need to delve further into tree rings, ice cores, and cave and lake sediments to try and get this event pinned down to an exact calendar date.

Q: The University's Laboratory of Tree-Ring Research has a long history with radiocarbon dating. Can you give a few notable examples?

When radiocarbon dating was first developed and the need to test the accuracy of some of the first dating results using measurements of samples on "known age" came up, the LTRR provided some of the first-ever calibration material. If you visit the lab today, you can see samples on our walls from Centennial Stump (PDF) and Broken Flute Cave (PDF). These were used by Willard Libby in his Nobel Prize presentation as part of the radiocarbon Curve of Knowns. 

It's also fun to think how Andrew Ellicott Douglass (the University's acting president from 1910-11) – who was developing tree-ring science at the University of Arizona in the early 1900s, inspired by his interest in tracking solar activity in tree rings – would feel about these latest developments in annual radiocarbon measurement. He was right: Tree rings were the key to high-resolution solar data – not through the growth patterns but through the annual C14 time capsule, a capsule that could not be unpacked until technology was ready for it many decades later. I think he'd be pretty excited.

Q: How did you get into this area of study?

A: Actually, it was the eruption I mentioned, the Thera eruption, and the story of tree rings, radiocarbon and a dating controversy that has raged over decades. The idea that trees around the world could contain a record for a volcano sort of blew me away as an undergraduate and I was hooked on the mystery of a puzzle to be answered. I love this iterative process of science –  looking at problems in different ways, evaluating new data, coming up with new approaches. That's why being part of the IntCal working group is really important to me as well; it's a chance to play a tiny role in contributing to an ongoing continuum of discovery that helps us know more about who we are and where we are going.