The base of the Lystrosaurus Assemblage Zone, Karoo Basin, predates the end-Permian marine extinction

The current model for the end-Permian terrestrial ecosystem crisis holds that systematic loss exhibited by an abrupt turnover from the Daptocephalus to the Lystrosaurus Assemblage Zone (AZ; Karoo Basin, South Africa) is time equivalent with the marine Permian–Triassic boundary (PTB). The marine event began at 251.941 ± 0.037 Ma, with the PTB placed at 251.902 ± 0.024 Ma (2σ). Radio-isotopic dates over this interval in the Karoo Basin were limited to one high resolution ash-fall deposit in the upper Daptocephalus AZ (253.48 ± 0.15 (2σ) Ma) with no similar age constraints for the overlying biozone. Here, we present the first U-Pb CA-ID-TIMS zircon age (252.24 ± 0.11 (2σ) Ma) from a pristine ash-fall deposit in the Karoo Lystrosaurus AZ. This date confirms that the lower exposures of the Lystrosaurus AZ are of latest Permian age and that the purported turnover in the basin preceded the end-Permian marine event by over 300 ka, thus refuting the previously used stratigraphic marker for terrestrial end-Permian extinction.


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Data collection
Timing and spatial scale U-Pb CA ID-TIMS analyses of zircon grains in an ash fall depositand related concentrations of mercury in a lithostratigraphic and magnetostratigraphic context.
Geochronology: Volcanic ash beds are uncommon in any stratigraphic succession and when found, are sampled as availability of exposure allows. A 1-cm-thick, light gray to white ash bed, exposed for less than 10 m of lateral extent, was excavated and sampled as conditions allowed. About~800 grams of very fine-grained, light green ash from a~1 cm-thick ash-fall layer was sampled with care taken to exclude material from adjacent beds and/or loose detritus that could contain zircon grains. Zircon grains average 200 micrometers in size, and yield of sufficient numbers for analyses would be possible from less than 1 kg of sediment. The layer is hosted in a~0.5-m-thick, flat-lying, massive, reddish-gray siltstone unit on Farm Nooitgedacht 68, Bethulie District (S30.32616º, E025.93242º). For comparison of zircon populations, similar-sized samples of the massive red siltstone were collected from immediately above and below the ash bed.
Magnetostratigraphy: In general, only competent lithologies are sampled using a coring device. Due to the friable conditions of the weathered and exposed rock, we sampled weathered siltstone beds with a technique applied to unconsolidated sediment. Samples were taken 5 cm above and below the~1 cm ash-fall bed sampled for geochronology, within the massive,~0.5 m thick red siltstone interval. These were supplemented with ceramic-box samples obtained five meters below the ash horizon and eight meters above. Mercury: Samples of approximately 50 gm were taken at 5 cm intervals below, at, and above the ash fall deposit, and at 10 cm intervals across an interval identified by Botha-Brink et al. (2014) as the end-Permian extinction event, and equated to the critical interval in the marine realm. The marine crisis is associated with several Hg excursions and a similar pattern might be represented in the terrestrial record if both turnovers were coincident. Both TOC and Hg analyses are conducted on milligrams of sediment.
Zircons: About~800 grams of very fine-grained, light green ash from a~1 cm-thick ash-fall layer was sampled with care taken to exclude material from adjacent beds and/or loose detritus that could contain zircon grains. Magnetostratigraphy: Siltstone/mudstone intervals are highly weathered and fragmentary. After cleaning off the exposure with nonmagnetic implements, small (<0.7 cm) chips of thinly bedded siltstone were carefully placed into ceramic boxes (measuring 1.7 cm on a side; Beijing Eusci Technologies Ltd.) with ceramic lids, keeping each chip upright and roughly oriented with respect to Geographic north. Chips were removed using non-magnetic tweezers, and, if needed, they were shaped into appropriate size using non-magnetic (Cu-Be) tools. Glass wool or cotton was used to pack the chips in the ceramic boxes, to prevent movement or fragmentation during transport, and the boxes taped shut. Mercury: see above Zircons: U and Pb were isolated from the zircon using 50 #l anion exchange columns using HCl, deposited onto outgassed rhenium filaments with silica gel(63), and analyzed with a VG354 mass spectrometer using a single Daly detector in pulse counting mode for Pb, and three Faraday cups in static analysis mode for U or Daly detector if the signal was <300KCps. Corrections to the 206Pb-238U ages for initial 230Th disequilibrium in the zircon have been made assuming a Th/U ratio in the magma of 4.2. All common Pb in each analysis was assigned the isotopic composition of procedural Pb blank. Dead time of the measuring system for Pb was 16 nanoseconds. The mass discrimination correction for the Daly detector is constant at 0.05% per atomic mass unit; the thermal mass fractionation correction for Pb was 0.10% per atomic mass unit (± 0.076%, 2$); and the U thermal mass fractionation correction was measured and corrected within each measurement block for static runs. Amplifier gains and Daly characteristics were monitored using the SRM 982 Pb standard. Decay constants are those of Jaffey et al. (64). Magnetics: In the laboratory, the glass wool or cotton was removed and the ceramic cubes filled with Zircar alumina cement, which is completely non-magnetic. The ceramic boxes were labeled using a soft aluminum rod, and then subjected to progressive thermal demagnetization using an ASC TD48 thermal demagnetization unit. Magnetizations were measured on a pulse-cooled DC SQUID 2G Enterprises magnetometer. Demagnetization data were inspected using orthogonal demagnetization diagrams(66) and directions of components of magnetization were determined using principal components analysis(67). The general dispersion (in declination) of magnetizations isolated in these materials is largely attributed to the nature of the sampling procedure, necessitated by the very fissile and friable nature of the hematitic siltstone.
Sampling of the ash occurred on two occasions. A pilot project sample was collected in 21 January 2017 to determine if zircon grains were present in the ash by RAG. These were sent to S. Kamo for disaggregation and inspection. Upon discovery that pristine zircon A second sampling of the horizon occurred on 15-17 January 2018 by RAG, SK, and JWG. The entire bed thickness was sampled over a