CARBONATE U-PB GEOCHRONOLOGY ESTABLISHES FLASK-SHAPED MICROFOSSILS FROM THE DZHANYTAS GROUP OF SOUTHERN KAZAKHSTAN AS MID-TONIAN AND THE OLDEST GLOBALLY

Abstract

The Proterozoic fossil record of early eukaryotic algal lineages is sparse and discontinuous. As a result, it is difficult to determine when major algal clades originated, how long they persisted, and whether complex algae survived major environmental perturbations such as the Cryogenian Snowball Earth glaciations. New, well-dated algal fossil occurrences from the Neoproterozoic are therefore essential for evaluating evolutionary continuity and resilience during this interval.

Here, we document large (31–104 μm), flask-shaped microfossils from the Dzhanytas Group, a carbonate succession in the Malyi (Lesser) Karatau Range of southern Kazakhstan. These fossils are morphologically and compositionally similar to Cryogenian and Ediacaran flask-shaped microfossils from Mongolia that have been interpreted as putative rhodophyte algal fossils. Given the overlap in size and shape, we suggest that the Dzhanytas microfossils are taxonomically similar to those reported from Cryogenian and Ediacaran strata, and therefore may also be red algal fossils. The age of the Dzhanytas Group has remained poorly constrained, with previous age interpretations ranging from Neoproterozoic to Ordovician. Here we report a new in-situ carbonate U-Pb date of 825.3 ± 29.2 Ma from a limestone horizon that is stratigraphically between two occurrences of the Dzhanytas flask-shaped microfossils and interpret this to represent a minimum depositional age for these fossils. Additionally, we report chemostratigraphic data from the Dzhanytas Group that have δ13C values ranging between 1–8‰, further supporting a mid-Tonian age for this unit, just before the onset of the c. 810 Ma Bitter Springs negative carbon isotope excursion.

Together, these data establish the Dzhanytas assemblage as the oldest known occurrence of flask-shaped microfossils globally, extending the temporal range of this morphogroup by >130 Myr. By documenting complex putative algal fossils in the mid-Tonian and linking them to similar younger Cryogenian and Ediacaran occurrences, these data fill a c. 400 Myr gap in the rhodophyte fossil record. Regardless of the taxonomic interpretation of these flask-shaped microfossils, these data, combined with the Mongolian flask-shaped microfossils, provide direct evidence for organisms originating ∼100 Ma before and persisting through the Cryogenian Snowball Earth events.