The Darriwilian (Middle Ordovician) is marked by a striking peak of the Great Ordovician Biodiversification Event (GOBE) followed by an abrupt ∼50 % decline in invertebrate species. Understanding the ultimate driving mechanism behind this biotic turnover and its correlation with those proximate climatic-oceanic changes that directly result in elevated biotic mortality have garnered considerable attention. One hypothesis posits that oceanic redox evolution actively influenced biodiversification, and progressive expansion of marine anoxia caused the termination of this biotic event. In this study, we present sedimentological and geochemical profiles for three outcrops of different lithofacies across the Middle-Upper Ordovician transition, ranging from carbonate platform to deep-water slope-basinal facies of the Tarim and South China cratons. Mercury isotopes reveal that recurrent and spatially dynamic photic-zone euxinia (PZE) occurred in the slope-basinal facies while being absent in platform facies. Mass balance model results suggest that ∼23 % of local atmospheric Hg was sequestered in marine sediments due to elevated dissolved H2S in surface waters. This finding suggests that destabilized oceanic redox conditions developed during climatic cooling, particularly shoaling and upwelling of deep-marine euxinic waters into the photic zone, coupled with global carbon-cycle disturbances, resulted in biodiversity decline following the peak of the GOBE.

中文翻译：

---

中奥陶纪光带真西离子的汞同位素证据：对终止大奥陶纪生物多样化事件的影响


达里维利亚（中奥陶纪）的特点是奥陶纪大生物多样性事件 （GOBE） 的显著高峰，随后无脊椎动物物种突然下降约 50%。了解这种生物周转背后的最终驱动机制及其与直接导致生物死亡率升高的那些邻近气候-海洋变化的相关性已经引起了相当大的关注。一种假设认为海洋氧化还原进化积极影响了生物多样化，而海洋缺氧的逐渐扩大导致了这一生物事件的终止。在这项研究中，我们展示了中上奥陶统过渡中不同岩相的三个露头的沉积学和地球化学剖面，范围从碳酸盐台地到塔里木和华南克拉通的深水斜坡-盆地相。汞同位素显示，循环和空间动态光带 euxinia（PZE）出现在斜坡盆地相中，而在台地相中不存在。质量平衡模型结果表明，由于地表水中溶解的 H2S 升高，大约 23% 的当地大气汞被封存在海洋沉积物中。这一发现表明，在气候冷却过程中形成的不稳定的海洋氧化还原条件，特别是深海 euxinic 水域的浅滩和上升流进入光带，再加上全球碳循环干扰，导致生物多样性在 GOBE 达到峰值后下降。