Earth and Planetary Science Letters | Volume 482 | 15 January 2018 | Pages 591–606

Astrochronology of the Anisian stage (Middle Triassic) at the Guandao reference section, South China

Highlights

•Milankovitch cycles are detected in gamma ray and magnetic susceptibility series.
•Anisian reference section at Guandao (South China) is astronomically tuned.
•Magnetic polarity zones, conodont biozones and inferred Anisian substages are calibrated.
•Anisian Stage spans 5.3 Myr from 246.8 Ma to 241.5 Ma.
•This provides a 10-Myr timescale for the early Triassic climatic-biotic crises and recoveries.

Abstract
A high-precision global timescale for the Early and Middle Triassic is the key to understanding the nature, pattern and rates of biotic recovery following the end-Permian mass extinction. The Guandao section of Guizhou Province of South China is an important reference section for the magnetic polarity pattern, conodont datums, geochemical anomalies and interpreted temperature history through the Anisian (Middle Triassic). We analyzed the high-resolution gamma-ray and magnetic susceptibility series from the complete Anisian stage. Intensity variations are indicative of fluctuating terrestrial clay influxes showing strong signals that match predicted astronomical solutions for eccentricity and precession. Astronomical tuning of these series to interpreted 405-kyr long-eccentricity cycles yields a 5.3 Myr duration for the Anisian at Guandao. When combined with the astrochronology of the Early Triassic, then the projected age of the Anisian–Ladinian boundary relative to the base-Triassic date of 251.9 Ma is 241.5±0.1 Ma. This provides a 10-Myr reference timescale for other key geological events, including conodont zones, geomagnetic polarity chrons, rates of marine carbon- and oxygen isotope excursions and global sea-level changes, that were associated with the repeated biotic crises and recovery episodes after the end-Permian mass extinction. The middle Anisian humid phase in ca. 244–244.5 Ma was probably a global event, which may have been linked to the middle Anisian warming event and sea-level change. Sea-level fluctuations at Guandao generally correlate with those in western Tethyan and Boreal regions in time, confirming sea-level changes during the Anisian were of eustatic origin.

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