ENVIRONMENT OF THE EARLIEST EUKARYOTIC MICROFOSSILS: INSIGHTS FROM THE PALEOPROTEROZOIC CHUANLINGGOU FORMATION

The earliest unambiguously eukaryotic fossils appear around 1650 Ma, but little is known about their early diversity, abundance, and habitats. Oxygen levels during this time were lower than the modern, with oceans largely anoxic and ferruginous below the mixed layer.

The 1678-1634 Ma Changcheng Group, North China Craton (Hebei Province), hosts one of the oldest assemblages of eukaryotic microfossils. Samples of shales and siltstones through 80m of the Chuanlinggou Formation (lower Changcheng Grp.), representing subtidal to intertidal environment, were split for geochemical and palynological analysis. To better understand the environment in which these early complex cells evolved and provide a paleoecological context for the Chuanlinggou Formation, we combined a reconstruction of paleoredox conditions (using an iron speciation proxy), δ¹³C analyses of organic matter, and quantitative analysis of the organic-walled microfossil assemblage.

The assemblage is dominated by sphaeromorphic and fusiform microfossils, but also includes complex (process-bearing) taxa, as well as those with long stratigraphic ranges that extend into the end-Tonian Period (~750 Ma). The presence of morphologically distinct long-ranging acritarch Valeria lophostriata on two paleocontinents around this time, suggests a wide dispersal by the late Paleoproterozoic. Iron speciation data indicate the microfossils and sediments of the Chuanlinggou Fm. were mainly deposited under ferruginous, anoxic conditions, with rare instances of local oxia. Organic matter content (TOC) is low throughout the unit. Morphologically complex microfossils are common in anoxic samples (~3/4 of the assemblage per anoxic sample), while rare samples interpreted as oxic (FeHR/FeT< 0.21) are dominated by microfossils of bacterial coccoidal colonies. No differences in preservation of prokaryotic and eukaryotic microfossils were observed, so this pattern likely reflects true microbial distribution.