Evidence for Eutrophication in the >742±6 Ma Chuar Group, Grand Canyon, and Possible Implications for Neoproterozoic Glaciation

Well-preserved fossils from the >742±6 Ma Chuar Group, Grand Canyon, provide a record of life just prior to Neoproterozoic glaciation. Diverse acritarchs are found in the lower Chuar Group, but are replaced in the upper Chuar Group by abundant specimens of Bavlinella faveolata, an aggregate of <1µm spheres often interpreted to be the remains of cyanobacteria. The presence of the organic-walled fossil Chuaria in both “diverse acritarch” and “B. faveolata” assemblages suggests that the absence of acritarchs from the latter is not a preservational artifact. Likewise, the transition to “B. faveolata” assemblages is independent of water depth, suggesting the pattern is not due to changing depositional environments. This transition is stratigraphically associated with the development of water column anoxia, as suggested by Fe-speciation analyses and the presence of the biomarker gammacerane in upper Chuar strata. It is also associated with a jump in %TOC in shale from <1.0wt% to <9.4wt% and a high magnitude positive shift in δ¹³C values suggesting increased primary productivity at this time. This is also consistent with the appearance of vase-shaped microfossils at this level; VSMs are likely the remains of testate amoebae, organisms that today thrive in low-oxygen, organic-rich environments. Collectively, these data point to eutrophication of the basin in late Chuar time, where an influx of nutrients such as phosphorus favored blooms of bacteria at the expense of eukaryotic algae.

Similarities between the upper Chuar assemblages and those associated with Neoproterozoic glaciations worldwide (i.e., acritarch-poor, B. faveolata-rich) invite speculation that eutrophic conditions may have been widespread during glaciations, consistent with evidence for anoxia during this time. If true, such conditions may have played a role in the repeated initiation of global cooling by sustaining high rates of organic carbon burial and as a result, CO₂ drawdown.