Variable redox conditions as an evolutionary driver? A multi-basin comparison of redox in the middle and later Cambrian oceans (Drumian-Paibian)

Abstract

The middle to later Cambrian (Drumian-Jiangshanian Ages, 505–490 Ma) was a time of unique evolutionary dynamics that remain enigmatic. This interval records unusually high rates of faunal turnover that produced a “plateau” within the broader trajectory of rapidly increasing biodiversity seen across the Cambrian Explosion and Great Ordovician Biodiversification Event (GOBE). The oceans during this time are generally thought to have been less oxygenated than later in the Phanerozoic, yet knowledge of oceanic redox structure and the influence this exerted upon the biosphere remains limited. Importantly, this interval also encompasses two large carbon cycle perturbations—the DICE and SPICE events— that are thought to involve the expansion of anoxic and, more specifically euxinic regions in the ocean. Despite this supposition, direct characterization of redox conditions across this time remains limited. Here we explore these conditions using new and previously published Fe-speciation data from seven basins distributed across five paleocontinents representing a range of depositional conditions. Our analysis reveals anoxia was a common and persistent feature of deeper-water environments and that it was generally absent from shallower-waters across this timespan. An exception to this broad pattern is seen during the SPICE when these deeper-water anoxic conditions expanded into shallower-water environments. These anoxic conditions were dominantly ferruginous and rare instances of euxinia were spatiotemporally limited to environments of high productivity, low clastic sedimentation and high sulfate availability within a generally low-sulfate ocean. Intriguingly, during these events faunal turnover was concentrated in inner-shelf areas suggesting a mechanistic link to the variable redox conditions characteristic of these environments. More broadly this instability in nearshore environments appears a likely cause of the high rates of faunal turnover seen across the later Cambrian and into the Early Ordovician, but further detailed paleontological and redox investigation of these environments are needed to adequately evaluate this view.