Biotic turnover driven by eutrophication before the Sturtian low-latitude glaciation

Abstract

A dramatic oceanic biotic shift from eukaryotic phytoplankton to bacteria occurred about 740 million years ago. Microfossil and geochemical data from the Chuar Group in the southwestern United States link this biotic turnover to widespread eutrophication of surface waters. Reconstructions of the diversity of Precambrian microorganisms suggest a pronounced biotic turnover coinciding with the onset of Neoproterozoic low-latitude glaciation, in which diverse assemblages of organic-walled microfossils known as acritarchs were replaced by assemblages of simple, smooth-walled forms called leiosphaerids, and the remnants of bacterial blooms1,2,3,4. This turnover has been interpreted as the mass extinction of eukaryotic phytoplankton1,2,3,4 and the subsequent proliferation of bacteria5. However, the causes of this mass extinction and its exact temporal relationship to the glaciations remain unclear1,2,3,4. Here we present palaeontological data from the >742±6-Myr-old Chuar Group from Arizona, which indicate that the biotic turnover occurred before the first low-latitude (Sturtian) glaciation, constrained to be between 726 and 660 Myr in age6. In our record, the turnover is associated with the appearance of abundant and diverse protozoan fossils and a shift to rising total organic carbon, suggestive of increased primary productivity spurred by the influx of nutrients. This is followed by an increase in the ratio of highly reactive iron to total iron, which we interpret as persistent water column anoxia. We therefore conclude that the biotic turnover recorded in the Chuar Group was driven by widespread eutrophication of surface waters, rather than low-latitude glaciation.