Precise age for the Permian-Triassic boundary in South China from high precision U-Pb geochronology and Bayesian age-depth modelling

Abstract

Abstract. This study is based on zircon U-Pb ages of 12 volcanic ash layers and volcanogenic sandstones from two marine sections with conformable formational Permian-Triassic boundaries (PTB) in the Nanpanjiang Basin (South China). Our dates of single, thermally annealed and chemically abraded zircons bracket the PTB in Dongpan and Penglaitan and provide the basis for a first proof-of-concept study utilizing a Bayesian chronology model comparing the three sections of Dongpan, Penglaitan and the Global Stratotype Section and Point (GSSP) Meishan. Our Bayesian modeling demonstrates that the formational boundaries in Dongpan (251.938 ± 0.029 Ma), Penglaitan (251.982 ± 0.031 Ma) and Meishan (251.956 ± 0.033 Ma) are synchronous within analytical uncertainty of ca. 30 ka. It also provides quantitative evidence that the ages of the paleontologically defined boundaries, based on conodont Unitary Associations zones in Meishan and on macrofaunas in Dongpan, are identical and coincide with the age of the formational boundaries. The age model also confirms the extreme condensation around the PTB in Meishan, which distorts the projection of any stratigraphic points or intervals onto other more expanded sections by means of Bayesian age-depth models. Dongpan and Penglaitan possess significantly higher sedimentation rates and thus offer a greater potential for high resolution studies of environmental proxies and correlations around the PTB than Meishan. This study highlights the power of high-resolution radio-isotopic ages that allow a robust intercalibration of patterns of biotic changes and fluctuating environmental proxies and will help recognizing their global, regional or local significance.

Timing of global regression and microbial bloom linked with the Permian-Triassic boundary mass extinction: implications for driving mechanisms

“…S5b). An additional line of evidence in support of this inferred sediment accumulation rate of >0.6 cm/ka is found in the compatible >0.19 cm/ka sediment accumulation rate based on the age difference between the lithological PTB (251.959 ± 0.018 Ma) and the first basal Triassic ash bed DGP-21 (251.953 ± 0.038 Ma) in Dongpan25. In comparison to the >3.6 cm/ka obtained for the uppermost two meters of the Permian Dalong Fm.…”

“…The lithological PTB is well constrained in terms of U-Pb dates at Dongpan, Penglaitan and Meishan25. The application of probabilistic age-depth models revealed the synchronicity of the formational PTBs in these sections, allowing to calculate a weighted mean age of 251.959 ± 0.018 Ma (N = 3; MSWD = 2.2; see Supplementary Fig.…”

“…In comparison to the >3.6 cm/ka obtained for the uppermost two meters of the Permian Dalong Fm. in Dongpan25, the minimum sediment accumulation rate of the basal Triassic black shales shows a six-fold decrease across the PTB. As there are no fundamental differences in bathymetry between the Dalong Fm.…”

Reply to reviewer comments

“…However, these dates are either not sufficiently precise to allow calibrating magmatic and biological timescales at resolution adequate for both groups of processes, or are biased by a systematic age offset between the U-Pb and Ar-Ar systems of ~1.0 % (Schoene et al, 2006). In order to properly compare the two systems, all older 40 Ar/ 39 Ar data have to be corrected for the revised age of the standard Fish Canyon sanidine of 28.201 ± 0.046 Ma (Kuiper et al, 2008) and the decay constant uncertainty has to be added to U-Pb and Ar-Ar ages which would drastically expand the 40 Ar/ 39 Ar age error and recalculate the 40 Ar/ 39 Ar age of Renne et al (1995) to 252.1 ± 1.6 Ma. In a first detailed ID-TIMS study, U-Pb ages of mechanically abraded zircons were published by Bowring et al (1998) for six volcanic ash beds at Meishan, placing the PTB at 251.4 ± 11 Solid Earth Discuss., doi:10.5194/se-2016-145, 2016 Manuscript under review for journal Solid Earth Published: 28 October 2016 c Author(s) 2016.…”

“…Instrumental mass fractionation was corrected using the fractionation factor derived from the measured 202 Pb/ 205 Pb ratio relative to a true value of 0.99924. BaPO 2 interferences on mass 202 to 205 were corrected by determining 138 Ba 31 P 16 . Raw data were statistical filtered by using the Tripoli program, followed by data reduction including correct uncertainty propagation and online data visualization using U-Pb_Redux software .…”