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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. • Anoxia was common in mid- to later Cambrian deeper marine environments. • These anoxic conditions were dominantly ferruginous and with occasional euxinia. • Shallow waters were generally oxygenated during this time interval. • Anoxia developed in shallower-water environments at the start of the SPICE. • Redox volatility in shallow environments was potentially linked to faunal turnover. [ABSTRACT FROM AUTHOR]

A growing body of evidence suggests that the deep oceans during the early Paleozoic Era were widely oxygen deficient, despite evidence for increased marine oxygenation during the Neoproterozoic. However, the temporal and geographic extents and dynamics of reducing marine conditions within these oceans are not well understood. Here, we investigate marine redox history during the Drumian through the earliest Jiangshanian International Stages of the Cambrian Period, using concentrations of redox-sensitive metals (vanadium, uranium, and molybdenum), iron speciation, and Mo isotope stratigraphy of the Alum Shale Formation of Scandinavia. These data suggest a major perturbation occurred in trace metal cycling during the later Cambrian Period that was linked to a transient change in marine redox conditions coincident with the well-known Steptoean Positive Isotope Excursion or SPICE. The δ98Mo measurements of the Alum shale show systematic variations during the interval that contains the SPICE which are broadly consistent with a transient expansion of sulfidic, reducing marine environments — indicating a significant exacerbation of an already-common condition during the Cambrian Period. Additionally, iron speciation data record a local transition from predominantly anoxic, ferruginous (Fe+2 containing) to anoxic, euxinic (sulfide containing) water column conditions near the initiation of the SPICE. Trace metal abundances, however, appear to decline well before the start of the SPICE, suggesting an earlier initiation of the global expansion of reducing environments. More broadly, our data and modeling support the notion that significant portions of the oceans remained oxygen deficient throughout the later portion of the Cambrian, and that these oceans were also prone to transient intervals of more reducing conditions similar to the Oceanic Anoxic Events of the Mesozoic. • Multi-redox proxy study of the Cambrian Alum Shale of Sweden. • Mo isotopes suggest a transient expansion of euxinia across the SPICE. • Fe speciation data indicate a local switch from ferruginous to euxinic conditions. • Metal contents suggest expansion of reducing marine conditions before the SPICE. • Later Cambrian ocean prone to transient intervals of even more reducing conditions. [ABSTRACT FROM AUTHOR]