Direct high-precision U–Pb geochronology of the end-Cretaceous extinction and calibration of Paleocene astronomical timescales

doi:10.1016/j.epsl.2016.07.041

Earth and Planetary Science Letters

Volume 452, 15 October 2016, Pages 272-280

https://doi.org/10.1016/j.epsl.2016.07.041 Get rights and content

Highlights

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K–Pg boundary in Denver Basin (USA) dated to be $66.021 \pm 0.024$ using U–Pb geochronology.
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New Paleocene U–Pb timescale agrees with recent astrochronological calibrations.
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Rapid rates of biotic extinction and initial recovery at K–Pg boundary in Denver Basin.

Abstract

The Cretaceous–Paleogene (K–Pg) boundary is the best known and most widely recognized global time horizon in Earth history and coincides with one of the two largest known mass extinctions. We present a series of new high-precision uranium–lead (U–Pb) age determinations by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) method from volcanic ash deposits within a tightly constrained magnetobiostratigraphic framework across the K–Pg boundary in the Denver Basin, Colorado, USA. This new timeline provides a precise interpolated absolute age for the K–Pg boundary of $66.021 \pm 0.024 / 0.039 / 0.081 Ma$ , constrains the ages of magnetic polarity Chrons C28 to C30, and offers a direct and independent test of early Paleogene astronomical and ⁴⁰Ar/³⁹Ar based timescales. Temporal calibration of paleontological and palynological data from the same deposits shows that the interval between the extinction of the dinosaurs and the appearance of earliest Cenozoic mammals in the Denver Basin lasted ∼185 ky (and no more than 570 ky) and the ‘fern spike’ lasted ∼1 ky (and no more than 71 ky) after the K–Pg boundary layer was deposited, indicating rapid rates of biotic extinction and initial recovery in the Denver Basin during this event.

Introduction

The geological timescale provides the framework for interpreting Earth history. Major subdivisions of the timescale correspond to mass extinctions such as the K–Pg boundary, one of the two largest extinctions in earth history (e.g. Schulte et al., 2010). Calibration of the timescale in absolute time is critical for examining rates of geological and biological processes. Recent advances in radioisotope geochronology and astrochronology have led to proposed timescale calibrations of unprecedented precision. Astrochronological models are based on the interpretation of sedimentary cyclicity as being driven by astronomical forcing of climate. However, because of uncertainties associated with the chaotic nature of planetary dynamics, astronomical timescales older than ∼50 Ma are not considered robust unless tested by independent geochronological methods (Laskar et al., 2011a). Such tests are often difficult because astrochronologies are typically derived from deep sea records where dateable volcanic deposits are rare. Here we use high-precision U–Pb geochronology (CA-ID-TIMS technique) of intercalated ash beds to date the K–Pg boundary and surrounding magnetic polarity reversals in fossiliferous continental rocks from the Denver Basin of Colorado (USA) to calibrate the early Paleogene astronomical timescale and precisely constrain the tempo of extinction and recovery.

The Geomagnetic Polarity Timescale (GPTS) is commonly used to sequence events in geological time yet was traditionally calibrated in absolute time by only a handful of ⁴⁰Ar/³⁹Ar age determinations from widely separated geographic locations and had relatively large intercalibration uncertainties (e.g. Cande and Kent, 1995, Cande and Kent, 1992). The most recent GPTS calibrations (GTS2004 – Gradstein et al., 2004 and GTS2012 – Gradstein et al., 2012) have capitalized on advances in geochronology and used a numerical calibration that integrates both radioisotopic and astrochronological constraints. However, stand-alone astronomical timescales prior to the Neogene remain uncertain due to the chaotic nature of planetary interactions (Hinnov and Hilgen, 2012) and varied interpretations of local cyclostratigraphic data (e.g. Dinarès-Turell et al., 2014). Therefore, these timescales must be tested and refined using independent geochronological methods that can attain higher resolution than the shortest astronomical frequency used. Uranium–lead zircon geochronology by the ID-TIMS method is particularly suitable for high-resolution calibration of geologic time because it takes advantage of two independent radioactive decay schemes with precisely measured decay constants (e.g. Bowring et al., 2006). In addition, the accuracy of U–Pb dates does not rely on the age of mineral standards and the effects of open system behavior in zircon can be readily detected and mitigated. By directly calibrating individual globally correlated stratigraphic events (e.g. K–Pg boundary, Chron boundaries) using multiple high-resolution U–Pb ages in a single integrated stratigraphic framework, we are able to achieve higher precision than most previous studies without reliance on models of sea-floor spreading or long-term sedimentation rates.

Section snippets

Geological background and sampling sites

The Denver Basin is an ideal place for developing a high-resolution timescale for the latest Cretaceous and early Paleogene because it has a thick sequence of fossiliferous synorogenic sediments that are well suited for paleomagnetic analysis and include abundant interbedded volcanic ash deposits, all of which are accessible via correlative core and outcrop records (Fig. 1). All sampling for this study was carried out in the upper Cretaceous–Paleocene D1 sequence. The D1 sequence is an

Paleomagnetic methods

Twenty-three oriented paleomagnetic samples were collected from the Kiowa core at the USGS Core Repository and subsequently cut into 8 cm³ cubes at the University of New Hampshire (UNH). Two of the samples were large enough to create separate sub-samples which allowed us to perform duplicate analyses at these two levels. Sampling levels in the core were selectively chosen to increase the magnetostratigraphic resolution achieved by Hicks et al. (2003) and thus were concentrated near previously

Paleomagnetism

All but one (KJ08-59) of the 25 samples from the Kiowa core exhibited clear demagnetization behavior and could be characterized by least squares analysis (Table S1, Fig. S1). Initial AF demagnetization up to 100 mT successfully isolated the ChRM for 19 of the 25 samples. Samples KJ08-31B, KJ08-50A, KJ08-59A, KJ08-61A, KJ08-62A, and KJ08-63A were subjected to additional thermal demagnetization as well. ChRM inclinations separate into two distinct groups with means of 50.0° (normal polarity) and

Time scale implications

The Geomagnetic Polarity Timescale (GPTS) plays a primary role in interpreting stratigraphic records of Earth history because polarity reversals are geologically rapid and globally distributed making them ideal timelines for correlating between different depositional environments and geographic locations. Accurate and precise calibration of the GPTS in absolute time is critical for resolving rates of geological and biological processes which in turn help constrain the underlying causes of those

Conclusions

A new U–Pb age model from a series of volcanic ash beds within a well-constrained latest Cretaceous–Paleocene magnetobiostratigraphic framework in the Denver Basin provides an age of $66.021 \pm 0.024 / 0.039 / 0.081 Ma$ for the K–Pg boundary and allows precise temporal calibration for this part of the Geomagnetic Polarity Timescale. These new results closely corroborate the most recently proposed marine-based astronomical timescale for the same time interval and agree well with a recent ⁴⁰Ar/³⁹Ar age

Acknowledgments

Property access to the West Bijou and My Lucky Number sites was provided by the Plains Conservation Center; to the Easy to Reach section by Toby Pippin; to the Hass Ranch section by Robert and Dale Haas. The project was supported by NSF grants EAR 9805474 to KRJ and R. Raynolds, EAR 0643158 to SAB, EAR 0642291 to WCC and EAR 0642838 to KRJ. We thank R. Barclay, T. Barnum, R. Dodds, B. Ellis, I. Miller, R. Raynolds and K. Tsukui, for help with field work, lab work and helpful discussions. Thanks

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Direct high-precision U–Pb geochronology of the end-Cretaceous extinction and calibration of Paleocene astronomical timescales

Highlights

Abstract

Introduction

Section snippets

Geological background and sampling sites

Paleomagnetic methods

Paleomagnetism

Time scale implications

Conclusions

Acknowledgments

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Chem. Geol.

Earth Planet. Sci. Lett.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Glob. Planet. Change

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Chem. Geol.

Extraterrestrial cause for the Cretaceous–Tertiary extinction

Science

Stratigraphy and megaflora of a K–T boundary section in the eastern Denver Basin, Colorado

Rocky Mt. Geol.

High-precision U–Pb zircon geochronology and the stratigraphic record: progress and promise

A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

J. Geophys. Res.

Revised calibration of the geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

J. Geophys. Res.

Late Cretaceous dinosaurs from the Denver Basin, Colorado

Rocky Mt. Geol.

Reconciling astrochronological and 40Ar/39Ar ages for the Matuyama–Brunhes boundary and late Matuyama Chron

Geochem. Geophys. Geosyst.

Reconciling astrochronological and ⁴⁰Ar/³⁹Ar ages for the Matuyama–Brunhes boundary and late Matuyama Chron