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. 2017 Jan 24:8:14066.
doi: 10.1038/ncomms14066.

Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification

A Lindskog  1 , M M Costa  2   3 , C M Ø Rasmussen  2   4 , J N Connelly  2   3 , M E Eriksson  1
Affiliations

Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification

A Lindskog et al. Nat Commun. .

Abstract

The catastrophic disruption of the L chondrite parent body in the asteroid belt c. 470 Ma initiated a prolonged meteorite bombardment of Earth that started in the Ordovician and continues today. Abundant L chondrite meteorites in Middle Ordovician strata have been interpreted to be the consequence of the asteroid breakup event. Here we report a zircon U-Pb date of 467.50±0.28 Ma from a distinct bed within the meteorite-bearing interval of southern Sweden that, combined with published cosmic-ray exposure ages of co-occurring meteoritic material, provides a precise age for the L chondrite breakup at 468.0±0.3 Ma. The new zircon date requires significant revision of the Ordovician timescale that has implications for the understanding of the astrogeobiologic development during this period. It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Stratigraphic and geochronologic framework.
Stratigraphic column, with the stratigraphic positions and CRE ages of fossil meteorites indicated in the diagram to the right. Stippled purple lines indicate maximum (CRE Max.) and minimum (CRE Min.) sedimentation rate provided by the CRE data. A middle line (blue) corresponding to a sedimentation rate of 4 mm ka−1 is included to illustrate how the CRE data can be translated into a high-resolution timescale. The green-shaded field indicates long-term time development calculated from U-Pb data (dark=central dates of datum points; light=incl. uncertainties), with dotted line (green) indicating average. Timescales resulting from the CRE Max. and CRE Min. lines are illustrated to the right in the figure, together with a timescale produced from the average sedimentation rate indicated by U-Pb data. Asterisks indicate levels searched for zircon.
Figure 2
Figure 2. The zircon-bearing bed and zircon grains.
(a) The ‘Likhall' bed (bracketed between arrows), as seen at the Thorsberg quarry. The scale bar corresponds to 0.1 m. (b) Representative zircon grains, illustrating external (1–13, reflected light) and internal (14–17, cathodoluminescence) characteristics. The scale bar corresponds to 100 μm.
Figure 3
Figure 3. Zircon U-Pb data.
(a) Concordia diagram, with analyses used in the age calculation indicated by blue ellipses (unused by grey). Data point error ellipses are 2σ. (b) U-Pb age distribution of analysed zircon grains, with age-defining grains indicated by blue colour. Box heights are 2σ.
Figure 4
Figure 4. The timescale of the Middle Ordovician.
A revised Middle Ordovician timescale based on our CA-TIMS data and well-constrained, high-precision datum points from the literature. The resolution of tick marks in the timescale indicates confidence and precision of the calibration. GTS2012 (ref. 27) is included for comparison; differing distances between tick marks indicates the relative distortion resulting from the recalibration introduced herein. Radioisotopic dates used in the construction of the timescale are indicated by black labels, with remaining datum points in grey for reference (A1 (ref. 51); A2* (ref. 49); A3*, A6* (ref. 59); A4, A5 (ref. 23); A7 (ref. 60); A8* (ref. 61); A9 (ref. 55); A10 (ref. 50); *reported with recalculations for GTS2012 (ref. 27)). The interval corresponding to the Kunda Baltoscandian Stage is calibrated at high resolution with the aid of CRE ages of fossil meteorites. Baltoscandian brachiopod and global graptolite species diversity curves are calibrated against the biostratigraphic and geochronologic framework.
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