Mercury (Hg) anomalies and carbon isotope excursions as a stratigraphic marker for the Permian – Triassic mass extinction

Mercury concentration anomalies in sedimentary successions are widely considered as proxies for volcanism and together with negative carbon isotope (δ¹³C) excursions, are a common feature of many Permian-Triassic boundary (PTB) sections ^1,2. On the basis of a temporal overlap of these geochemical excursions with the Permian-Triassic mass extinction (PTME) interval and PTB at the stratigraphically condensed Meishan PTB Global Stratotype Section and Point (GSSP), Hg and/or C- isotope excursions occurring stratigraphically close to the PTB are often used as chemostratigraphic markers for the extinction interval ². However, several studies indicate that near-PTB Hg anomalies vary in their stratigraphic occurrence and expression ³; a point also noted for PTB δ¹³C records ⁴. Permian – Triassic sedimentary successions are also frequently characterized by an unconformity straddling the PTB and/or by stratigraphic condensation, questioning the robustness of PTME correlations based on these geochemical markers. This study investigates the terminal Permian to earliest Triassic Hg and δ¹³C record, coupled with U-Pb zircon geochronology, for two stratigraphically continuous deep-water marine sections in the Nanpanjiang Basin, South China. The results show an interval of significant Hg enrichment stratigraphically close to the PTB, which is coeval with the nadir of a negative δ¹³C excursion spanning the Changhsingian to Induan. U-Pb zircon geochronology of volcanic ash beds interbedded with sediments in the studied sections indicate that the onset of this Hg anomaly postdates 251.82 ± 0.060 Ma, and that the peak of the Hg anomaly (and nadir of the negative δ¹³C excursion) is of Griesbachian age (between 251.59 ± 0.052 Ma and 251.67 ± 0.079 Ma). The peak of the Hg anomaly and nadir of the δ¹³C excursion in these stratigraphically continuous marine successions post-date both the PTB (251.90 ± 0.024 Ma) and mass extinction interval (251.94 ± 0.037 Ma – 251.88 ± 0.031 Ma) as determined from the Meishan GSSP ⁵. Our results indicate that stratigraphical correlation of the extinction interval based on Hg anomalies and/or δ¹³C excursions occurring stratigraphically close to the litho- or bio-stratigraphically determined PTB should be interpreted with caution. Furthermore, this study emphasizes the importance of precise and accurate U-Pb zircon ages for stratigraphic correlation between spatially disparate localities, especially during periods of notable environmental perturbations and biotic turnover such as the Permian-Triassic transition.

References

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2             Shen, J. et al. Evidence for a prolonged Permian-Triassic extinction interval from global marine mercury records. Nat Commun 10, 1563 (2019). https://doi.org:10.1038/s41467-019-09620-0

3             Sial, A. N. et al. Globally enhanced Hg deposition and Hg isotopes in sections straddling the Permian-Triassic boundary: Link to volcanism. Palaeogeogr Palaeocl 540, 109537 (2020). https://doi.org:10.1016/j.palaeo.2019.109537

4             Shen, S.-Z. et al. A sudden end-Permian mass extinction in South China. GSA Bulletin 131, 205-223 (2019). https://doi.org:10.1130/B31909.1

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