The early Paleozoic Era (∼540–420 Ma) was an interval of profound biogeochemical changes including increasing oxygen (O) and the onset of bioturbation (sediment mixing by animals). It is hypothesized that incipient bioturbation caused a monotonic decrease in sedimentary burial of pyrite (FeS), which would have slowed atmospheric O accumulation. However, pyrite accumulation can exhibit complex responses to dynamic, low-O environmental conditions. To assess pyrite burial in a potential modern analogue to early Paleozoic environments, we collected sediment cores from the Chesapeake Bay, an estuary with multiple gradients in sulfate concentration, hypoxia intensity, organic carbon flux and lability, and bioturbation. Results indicate that pyrite accumulation is maximized not under strong sulfate depletion in highly reducing sediments, but rather in sediments that occupy the mid-range of sulfate–chloride ratios. This probably occurs through efficient replenishment of pore water sulfate and/or through the generation of sulfur redox intermediates, which promote pyrite formation via the polysulfide reaction pathway. In light of these results and in contrast to earlier models, we hypothesize that mild early Paleozoic bioturbation temporarily increased pyrite burial efficiency by stimulating higher sulfate reduction rates and increasing sedimentary sulfide retention. Compiled sulfur and carbon data from a geochemical database indicate that median sulfur-carbon ratios of fine-grained marine siliciclastic rocks increased from the Ediacaran through the Ordovician, then decreased and became much less variable from the Silurian onward. Thus, the Cambrian and Ordovician Periods may constitute a distinct interval of the Proterozoic-Phanerozoic transition in which bioturbation temporarily accelerated O buildup. This transition probably ended in the Silurian, when O rose to sufficient levels to homogenize sedimentary carbon–sulfur cycling.

中文翻译：

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河口黄铁矿形成的趋势表明古生代黄铁矿埋藏的替代模型


早期古生代（∼540–420 Ma）是一段深刻的生物地球化学变化时期，包括氧气（O）的增加和生物扰动的开始（动物的沉积物混合）。据推测，早期的生物扰动导致黄铁矿（FeS）的沉积埋藏量单调减少，这会减缓大气中氧气的积累。然而，黄铁矿的积累会对动态、低氧环境条件表现出复杂的反应。为了评估与早期古生代环境潜在的现代模拟类似的黄铁矿埋藏，我们从切萨皮克湾收集了沉积岩芯，切萨皮克湾是一个在硫酸盐浓度、缺氧强度、有机碳通量和不稳定性以及生物扰动方面具有多个梯度的河口。结果表明，黄铁矿积累最大化不是在高度还原性沉积物中硫酸盐强烈消耗的情况下，而是在硫酸盐-氯化物比率处于中等范围的沉积物中。这可能是通过有效补充孔隙水硫酸盐和/或通过生成硫氧化还原中间体而发生的，这些中间体通过多硫化物反应途径促进黄铁矿的形成。根据这些结果并与早期的模型相比，我们假设早期古生代的温和生物扰动通过刺激更高的硫酸盐还原率和增加沉积物硫化物的保留来暂时提高黄铁矿埋藏效率。从地球化学数据库收集的硫和碳数据表明，细粒海洋硅质碎屑岩的中位硫碳比从埃迪卡拉纪到奥陶纪不断增加，然后从志留纪开始下降，变化幅度大大减小。 因此，寒武纪和奥陶纪可能构成元古代-显生宙过渡的一个独特时期，其中生物扰动暂时加速了O的积累。这种转变可能在志留纪结束，当时 O 上升到足以均匀沉积碳硫循环的水平。