GTS2020: The Paleogene Period

It’s nearly forty years ago that ‘A Geologic Time Scale 1982’ appeared (Harland et al. 1982); it was succeeded by major updates in 1989 (Harland et al. 1990), 2004 and 2012 (Gradstein et al. 2004, 2012 – known as GTS2004 and GTS2012, respectively). The primary rationale was “to show as clearly as we can how such a scale has been constructed” (Harland et al. 1982). Each update was about twice the length of the previous version. Consistently aiming to achieve a common language with respect to chronostratigraphic units and geological time, these books have served as state-of-the-art summaries for the entire geological community, both in academia and industry. The last two time scale books contained a discrete and extensive chapter devoted entirely to the stratigraphy of the Paleogene, summarizing information on all stages, established GSSPs, various biozonations and the creation of the time scale  (Luterbacher et al. 2004; Vandenberghe et al. 2012). After a three-year-long preparation GTS2020 was published in November 2020.

All Paleocene and Oligocene stages (Danian, Selandian, Thanetian, resp. Rupelian, and Chattian) have formally ratified definitions and so have the Ypresian, Lutetian, and Priabonian stages of the Eocene. We anticipate that the Global Boundary Stratotype Section and Point (GSSP) for the Bartonian Stage still requires more research before all stages of the Paleogene (66-23 Ma) are formally defined. Paleogene marine microfossil groups (planktonic and larger benthic foraminifera, calcareous nannofossils, radiolarians, organic-walled dinoflagellate cysts) provide robust zonation schemes for regional to global correlation and are integrated within the magneto-biochronological framework. Since land mammal faunas are also increasingly being studied with an integrated magnetostratigraphic and/or chemostratigraphic and geochronologic approach, their age calibrations have considerably been improved since GTS2012. Stable isotope analysis and XRF (X-ray fluorescence) scanning have become key tools in Paleogene high-resolution stratigraphy, correlation, and time scale construction. Stable oxygen and carbon isotope records also provide insight into trends in paleoclimate and carbon cycling, such as the warming trend starting in the middle Paleocene and culminating during the Early Eocene Climatic Optimum, and the subsequent cooling leading to a change from greenhouse to icehouse conditions at the onset of the Oligocene. Numerous short-term isotope excursions mark high climatic variability, expressed in hyperthermal (transient global warming) events (62-40 Ma) and cooling/glaciation events (38-23 Ma). At the same time, these stable isotope excursions provide accurate stratigraphic constraints and enable land-sea correlations, such as for the Paleocene-Eocene Thermal Maximum, the “Mother of all hyperthermals.” Orbital tuning of sedimentary cycles, calibrated to the geomagnetic polarity and biostratigraphic scales, has greatly improved the resolution of the Paleogene time scale over the last two decades. We now have astronomical age control for almost all geomagnetic polarity reversals, but differences between published age models still persist through the “Eocene astronomical time scale gap” spanning Chrons C20r through C22n (43.5-49.5 Ma).