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Silica accumulation in the Yangtze Sea during the Ordovician–Silurian (O–S) transition appears to have coincided with global climatic fluctuations, widespread upwelling, and volcanism. There is a need to further evaluate their respective contributions to silica deposition and potential relationships among these factors. The current study selected siliceous deposits in the Wufeng and Longmaxi Formations from four sections spanning the inner to outer Yangtze Sea, South China, to gain a deeper understanding of the climatic and oceanographic evolution associated with silica enrichment. Al/(Al + Fe + Mn) values, the presence of radiolarians, and Si isotope values of samples recovered from the investigated shale successions offer compelling evidence that the silica is largely of biogenic origin with some terrigenous contributions. Further, various productivity and redox proxies suggest that biogenic silica (BSi) accumulated under conditions of enhanced marine productivity and anoxic bottom water conditions. Hg/TOC and Zr/Al 2 O 3 profiles suggest intermittent volcanism during the BSi deposition in the Yangtze Sea. However, the lack of correlation between BSi and Hg/TOC values indicates that volcanic iron fertilization was not responsible for BSi accumulation. Instead, most BSi-rich samples are dominated by low Mn EF × Co EF values (<0.5), consistent with BSi deposited in modern upwelling settings. Hydrographic reconstruction based on Mo–U covariation indicates a more open water setting in the outer Yangtze Sea, while the coeval inner Yangtze Sea was relatively restricted. Therefore, upwelling events appear to have been more vigorous in the outer Yangtze Sea. Published and new Chemical Index of Alteration (CIA), BSi, and Mn EF × Co EF data for the Wufeng and Longmaxi Formations across the inner to outer Yangtze Sea demonstrate that temporal and spatial variations of BSi were controlled by climate-driven upwelling. In particular, cool-water upwelling contemporaneous with Hirnantian glaciation may have been responsible for the establishment of the cool-water fauna of the shallow-water Guanyinqiao Bed and enhanced silica deposition in deeper water. Moreover, a moderate negative relationship between compiled CIA and BSi contents suggests that enhanced upwelling driving BSi accumulation appears to have been favored during cooling events. Integrated analysis of BSi deposits of the Laurentia and Baltica continental margins further suggests that BSi accumulation on continental margins during the O–S transition was primarily influenced by global cooling. Therefore, we suggest that wind patterns or/and thermohaline circulation, influenced by climate fluctuations, induced widespread cold water upwelling events during the O–S transition. Moreover, elevated BSi production diluted accumulating OM resulting in the observed parabolic relationship of BSi and TOC. • The O–S siliceous deposits derived from enhanced BSi productivity. • Upwelling was the first-order constraint on BSi distribution. • The upwelling events appeared to correlate with the global cooling. [ABSTRACT FROM AUTHOR]

The sulfate-methane transition zone (SMTZ) serves as an important geochemical transition especially during early diagenesis. Organoclastic sulfate reduction (OSR), sulfate-driven anaerobic oxidation of methane (SD-AOM), fermentation, and methanogenesis, the dominant diagenetic processes near the SMTZ, play key roles in marine sulfur and carbon cycling. However, the paleo-SMTZ has rarely been identified in the geologic record because of equivocal evidence. The presence of barite-calcite-pyrite assemblages (abundant nodules and pyritic laminae) concentrated over a 1-m thick interval of the Silurian Longmaxi Formation, Yangtze Block, South China, provide a unique opportunity to investigate possible vertical movements of the paleo-SMTZ in ancient sediments. The inferred paleo-SMTZ comprises nodules made up of barite (BaSO 4), calcium carbonate (CaCO 3), and pyrite (FeS 2), and pyritic laminae. Strong 34S enrichment of barite (50 to 62.1‰ VCDT) and modestly 13C-depleted calcite (−11.4‰ to −5.3 VPDB) of the nodules associated with positive δ34S values of pyritic laminae (4 to 6.2‰ VCDT) suggest precipitation of these minerals sourced by mixed carbon and sulfur sources close to and/or within the paleo-SMTZ. Nodules from center to edge display mineral zonation that may reflect the effects of depth fluctuations of the paleo-SMTZ. Initial (stage 1) barite mineralization in nodule centers appears to have occurred within the base of the sulfate reduction zone (SRZ). By the time if stage 2 mineralization, the stratigraphic locus of precipitation had migrated closer to highly alkaline SMTZ resulting in barite dissolution and calcium carbonate precipitation. Higher proportions of calcium carbonate exhibiting some enrichment of 12C associated with cubic pyrite crystals in nodule centers are consistent with the occurrence of SD-AOM within the SMTZ. Stage 3 acicular barite mineralization along nodule edges likely took place near at the base of SRZ immediately above the SMTZ, whereas the replacement of barite by calcite during stage 4 mineralization occurred again within the SMTZ. Vertical movement of paleo-SMTZs suggested by the occurrences of barite-calcite-pyrite assemblages in the Longmaxi Formation across Yangtze Block, are likely the consequence of the complex interplay of varying sedimentation rate, organic matter quality, methane flux, and perhaps seawater sulfate concentration. • Barite-calcite-pyrite assemblages of the Longmaxi Formation were formed near the sulfate–methane transition zone (SMTZ). • Nodule growth occurs concentrically so that changes in mineralization plausibly record vertical movements of SMTZ. • Multiple factors controlled the spatial distribution of SMTZ in the Longmaxi Formation across Yangtze Block. [ABSTRACT FROM AUTHOR]

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. • Generation of a multi-proxy paleoenvironmental profile of an Ordovician-Silurianorganic-rich shale succession. • The history of organic matter (OM) accumulationin studied core can be subdivided into five phases. • Hg concentrations are used to consider the contribution of volcanism to OM accumulation. • The heterogeneous nature of the marine environment played a major role on regional variations of OM accumulation. [ABSTRACT FROM AUTHOR]

The Llandovery epoch (Early Silurian) was marked by the interruption of widespread oceanic anoxia and associated oxygenation of seawater. The first oxygenation event appears to occur near the Rhuddanian–Aeronian stages boundary. However, ventilation of the marine environments is a dynamic process, differing in timing and magnitude from place to place, basic variations in the nature of marine circulation; e.g., open shelf versus restricted shelf. The correlation of oxygenation events from open to restricted shelf environments is of significance to enhancing our understanding of marine ventilation processes. Here, we describle nodular barite that accumulated in the open marine environment near the boundary at the northern margin of the Yangtze Block, South China, emphasizing its origin and environmental implications. Petrographic evidence suggests that the nodular barite is early diagenetic. Barium was likely derived from the dissolution of biogenic barite as suggested by the 87Sr/86Sr values of the diagenetic barite. The REE + Y pattern, low Y/Ho and high Ce*/Ce values of the barite nodules demonstrate that microbial degradation of organic matter in association with Ce oxidation controlled REE composition of the nodules. PAAS-normalized positive Eu anomalies of the studied barite, however, suggest an anoxic porewater environment perhaps related to bacterial sulfate reduction or the anaerobic oxidation of methane focused within the sulfate-methane transition zone (SMTZ) as also indicated by the minor contribution of radiogenic terrigenous Sr isotopes. We infer that high marine productivity and the associated transport of biogenic barite to the sea floor was critical to the formation of the studied barite nodules. Diagenetic barite formation and preservation in Aeronian stage deposits may indicate that circulation of open marine water masses had been much ameliorated. Our results are consistent with earlier suggestions that the restricted shelf experienced a transition from anoxic or even euxinic conditions to a more oxygenated (probably suboxic) condition during the early Aeronian. The approximately contemporaneous nature of this transition and global cooling associated with the glacial pulses indicate the critical role that global climate change near the Rhuddanian–Aeronian (R-A) boundary played in the redox evolution of the Yangtze Block, South China. • A diagenetic model for Llandovery nodular barite formation was established. • The different style of barite formed at a fluctuating sulfate methane transition zone. • Diagenetic barite precipitation implicates Llandovery paleoenvironmental changes. • Global climate had an important role in the Llandovery Yangtze Block redox evolution. [ABSTRACT FROM AUTHOR]

As the last part of the Hercynian cycle in the Yangtze region, the Dongwu Movement included the eruption of the Emeishan large igneous province (ELIP) and was an essential event between the Guadalupian and Lopingian, which had significant implications for the paleo-oceanic environment and paleoclimate of the area. To study the changes in the sedimentary environment in the Guadalupian and Lopingian, the Zhenba section was selected, and its trace elements and rare earth elements (REEs) were analyzed. From analysis of the trace elements and REEs in the Guadalupian, the limestone samples had prominent typical shallow-water seawater deposition characteristics and were less affected by terrigenous detritus, which was in an arid oxidation environment; Sr/Cu ranged from 4 to 1381, the Rb/Sr value was low, δCe < 1, Ceanom < −0.1, Er/Nd ranged from 0.1 to 0.27, and Y/Ho ranged from 44 to 72. However, in the Lopingian, the limestone samples were more affected by terrigenous detritus, which was in a humid climate with a weak oxidation‒reduction environment; the Sr/Cu ranged from 1 to 656, the Rb/Sr value was high, δCe ≅ 1, Ceanom ≥ −0.1, Er/Nd < 0.1, and Y/Ho ranged from 26 to 44. The geochemical diagrams of TiO2‒Al2O3, Al2O3‒REE, La‒Th‒Sc, Th‒Co‒Zr/10, Th‒Sc‒Zr/10 and 87Sr/86Sr of a sample indicated that the terrigenous detritus of the sample mainly came from the South Qinling active continental margin, rather than the weathering of Emeishan basalt. Acid bentonite came from the Paleo-Tethys island arc or the island arc formed by the subduction of the South China Plate beneath the North China Plate, rather than the ELIP. This evidence supported the view that the North China Plate potentially collided with the South China Plate in middle to late Wujiaping. [ABSTRACT FROM AUTHOR]