U–Pb geochronology of the Malokaroy Series (Kazakhstan) constrains the global appearance of vase-shaped microfossils to > 759 million years ago

The appearance of vase-shaped microfossils (VSMs) in the sedimentary record evidences the acquisition of tests by eukaryotes in the late Tonian Period. However, the timing of this biological innovation is uncertain, in part because of a paucity of high-precision radiometric age constraints on VSM-bearing sedimentary successions outside of Laurentia. Here, we investigate the chronostratigraphy of one such succession, the Malokaroy Series in Kazakhstan. The Malokaroy Series was recently interpreted to record deposition in a c. 810–730 Ma continental forearc basin. It includes the Aktugai, Chichkan and Kurgan formations, the middle of which preserves a fossil assemblage with vase-shaped microfossils (VSMs). Outside of Kazakhstan, the appearance of the oldest known VSM taxon is constrained to between 789–752 Ma. The age of the Chichkan assemblage has been variably interpreted, but was recently suggested to be < 737 Ma. Here, we produce a U–Pb CA-ID-TIMS magmatic zircon age of 759.4 ± 2.5 Ma from an ash collected near the top of the Kurgan Formation. It demonstrates that the top of the Malokaroy Series is ∼ 30 Myr older than most recently suggested and that the Chichkan VSMs are > 759 Ma. We also present zircon trace element ratios from the dated ash consistent with crystallization in an intra-plate setting. Given these data, as well as sedimentological observations, we suggest that the Malokaroy Series records a distinct tectonostratigraphic event, and definitively extend Tonian VSMs to > 759 Ma.