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.

寒武纪的中后期（Drumian-Jiangshanian时代，505-490 Ma）是一个独特的演化动力学时代，至今仍是一个谜。这个间隔记录了异常高的动物更新率，在整个寒武纪爆发和大奥陶纪生物多样性事件（GOBE）中看到的迅速增加的生物多样性的更广泛轨迹中产生了“高原”。人们普遍认为，这个时期的海洋含氧量低于后来的古生代，但有关海洋氧化还原结构及其对生物圈的影响的知识仍然有限。重要的是，该间隔还包括两个大的碳循环扰动-DICE和SPICE事件-被认为涉及海洋缺氧区域，更具体地说是富氧区域的扩张。尽管有这样的假设，在这段时间内，氧化还原条件的直接表征仍然有限。在这里，我们使用分布在五个古大陆代表七个沉积条件的七个盆地的新的和先前发布的铁形态数据探索这些条件。我们的分析表明，缺氧是深水环境的共同和持久特征，而在这段时间内，浅水环境通常没有缺氧。当这些深水缺氧条件扩展到浅水环境时，在SPICE期间可以看到这种广泛模式的例外。这些缺氧条件主要是铁质的，而在正常情况下，通常在低硫酸盐含量的海洋中，极少发生的游刃有余的情况被时空限制在高生产力，低碎屑沉积和高硫酸盐利用率的环境中。有趣的是 在这些事件中，动物群的更新集中在架子内部区域，这表明与这些环境的可变氧化还原条件具有机械联系。从更广泛的意义上讲，近岸环境中的这种不稳定性似乎是造成整个寒武纪晚期至奥陶纪早期的动物更新率很高的原因，但是需要对这些环境进行进一步的古生物学和氧化还原研究，以充分评估这一观点。