Enhanced productivity and/or widespread anoxia have been postulated as the dominant controls on organic matter (OM) accumulation in South China during the Ordovician-Silurian (O-S) transition. However, their roles appear to have varied in space and time, and causal mechanisms linking OM accumulation with changing paleoenvironmental conditions remain poorly constrained. Here we report on results of a multi-faceted investigation of the Wufeng and Longmaxi formations in a drilled core from Xianfeng County, western Hubei Province, South China. The roles played by paleoredox conditions, paleoproductivity, paleoclimate, terrigenous input, upwelling, and volcanism on OM accumulation and preservation during the O-S transition are considered. Our results suggest that the depositional history of OM accumulation as recorded by the studied succession can be subdivided into five phases, each one represented by a lithostratigraphic unit. Unit 1 (average TOC = 3.28%) comprises siliceous shale deposited under generally suboxic conditions interrupted by brief episodes of anoxia beneath very productive surface water during the late Katian time (~447.62 Ma). Seasonal upwelling-driven productivity appears to have exerted fist-order control on OM accumulation. Overlying siliceous organic-rich shale of unit 2 (average TOC = 4.93%) records anoxic bottom water conditions from the latest Katian to Hirnantian time. Enhanced productivity and OM preservation may have been maintained by phosphorous cycling. Calcareous shale of the overlying Guanyingqiao unit 3 (TOC = 2.8%) accumulated under suboxic conditions and low to moderate water column productivity during the late Hirnantian Stage. Climate-driven sea level fall was responsible for diminished OM accumulation at this time. Siliceous clay-rich shale of unit 4 (average TOC = 5.53%) was deposited beneath highly productive surface water under anoxic bottom water conditions at the beginning of the Rhuddanian. The presence of volcanic ash and enriched Hg concentrations in these deposits suggests that productivity was enhanced by volcanism. Overlying silty shale of unit 5 (average TOC = 1.71%) was deposited in the middle to upper Rhuddanian under dominantly suboxic conditions, declining productivity levels, and increased delivery of terrigenous sediments. Accumulation of these deposits was contemporaneous with a period of falling sea level related to the Kwangsian Orogeny that would have curbed OM accumulation. Our results, considered in tandem with those of studies from shallow to deep water regions of the Yangtze Block, reveal a level of spatial heterogeneity of redox conditions and productivity levels across the Yangtze Sea during the O-S transition manifested by regional differences in OM accumulation mechanisms and histories.

在奥陶纪-志留纪 (OS) 过渡期间，生产力的提高和/或广泛的缺氧被认为是华南有机质 (OM) 积累的主要控制因素。然而，它们的作用似乎在空间和时间上有所不同，将 OM 积累与古环境变化联系起来的因果机制仍然缺乏约束。在这里，我们报告了对华南湖北省西部咸丰县钻孔岩心中五峰组和龙马溪组的多方面调查结果。考虑了古氧化还原条件、古生产力、古气候、陆源输入、上升流和火山作用对 OS 过渡期间 OM 积累和保存的作用。我们的结果表明，所研究的序列所记录的 OM 积累的沉积历史可以细分为五个阶段，每个阶段由一个岩石地层单位表示。单元 1（平均 TOC = 3.28%）包括沉积在一般缺氧条件下的硅质页岩，在卡提安晚期（~447.62 Ma）期间，在非常丰富的地表水下被短暂的缺氧中断。季节性上升流驱动的生产力似乎对 OM 积累施加了一级控制。单元 2 上覆硅质富含有机质页岩（平均 TOC = 4.93%）记录了从最近的 Katian 到 Hirnantian 时间的缺氧底水条件。磷循环可能保持了生产力的提高和 OM 的保存。上覆官营桥单元 3 钙质页岩（TOC = 2. 8%) 在低氧条件下积累，在 Hirnantian 阶段后期，水柱生产力低到中等。气候驱动的海平面下降是造成此时 OM 积累减少的原因。4 号单元的富含硅质粘土的页岩（平均 TOC = 5.53%）在 Rhuddanian 初期在缺氧底水条件下沉积在高产地表水之下。这些矿床中存在火山灰和富集的汞，表明火山活动提高了生产力。单元 5 的上覆粉质页岩（平均 TOC = 1.71%）沉积在中上至上鲁丹阶，主要是在低氧条件下，生产力水平下降，陆源沉积物的输送增加。这些沉积物的积累与与广西造山运动相关的海平面下降时期同时发生，这将抑制 OM 的积累。我们的结果与扬子地块浅水区到深水区的研究结果相结合，揭示了 OS 转变期间整个长江的氧化还原条件和生产力水平的空间异质性水平，表现为 OM 积累机制的区域差异和历史。