Researchers use radiometric dating to distinguish the timing of one of Earth’s most pivotal timescale boundaries.
Zircon crystals from a volcanic ash layer located about 6 meters above the base of the Massignano section in the Umbria-Marche sedimentary basin, which hosts the global standard of the Eocene-Oligocene boundary. Layers in the crystals are seen here using cathodoluminescence. New radiometric dating indicates that the volcanic ash is 35.47 ± 0.05 million years old. Credit: Diana Sahy
Source: Paleoceanography

The transition between the Eocene and Oligocene epochs, which occurred about 34 million years ago, is marked in the geologic record by large-scale extinctions and planetwide cooling that abruptly triggered the growth of the Antarctic ice sheet. However, this dramatic change has not yet been tied to a single event, such as a volcanic eruption, which means that a precise understanding of its timing is necessary to evaluate the potential causes. Despite considerable research, however, there is still a 600,000-year discrepancy between the two principal methods—astronomical tuning and radiometry—used to date this pivotal boundary.

To get a handle on this disparity, Sahy et al. used state-of-the-art uranium-lead radiometric techniques to date zircon crystals extracted from 11 volcanic ash layers at three locations in central Italy’s Umbria-Marche sedimentary basin, which hosts the global stratotype section that’s used as a reference point for this transition. The results indicate that previously published dates from two of these locations, which underpin the Eocene-Oligocene portion of the Geologic Time Scale 2012, are up to 500,000 years too old. The new data pinpoint the Eocene-Oligocene boundary at 34.09 ± 0.08 million years, a date that is in good agreement with the astronomically tuned timescale derived from deep-sea sediment cores when magnetic proxies for the transition are taken into account.

This research, which resolves one of the fundamental outstanding questions involving the current Cenozoic era, highlights the importance of integrating the results of multiple techniques to ensure the accuracy of the dates underpinning the geologic timescale. The study’s results will have broad implications for researchers in multiple disciplines, including paleoclimatology, paleoceanography, and chronostratigraphy. (Paleoceanography,, 2017)

—Terri Cook, Freelance Writer


Cook, T. (2017), Resolving a mystery of the ages, Eos, 98, Published on 29 November 2017.

Text © 2017. The authors. CC BY-NC-ND 3.0
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