The Influence Of Volcanic Activities On The Marine Environment And Organic Matter Accumulation During The Ordovician-Silurian Transition In South China

The Ordovician–Silurian transition interval was a time of significant environmental and ecological change in the marine system,which witnessed the first major mass extinction event of the Phanerozoic.Recent studies suggest that large-scale volcanic activity may have been an important causal factor in the abrupt climate and environmental changes.During this critical period,marine organic-rich black shales developed extensively and deposited large amounts of volcanic ash in South China,recording frequent volcanic eruption events.In-depth study of these volcanic eruption events can not only resolve the regional tectonic evolution process,but also provide new insights into the correlation between volcanic activities and the marine ecological crisis at the end of the Ordovician.In addition,assessing the impact of these volcanic eruptions on organic matter accumulation can also provide an important reference for the exploration and development of marine shale gas resources in China.In this study,the geochemical characteristics of volcanic ash,sedimentary geochemical responses to volcanic eruptions,sedimentary environmental evolution and organic matter enrichment processes were investigated by high-resolution biostratigraphy,mineralogy,elemental geochemistry,and isotope geochemistry analyses of black shales of Wufeng-Lungmachi formations in the Muchanggou section,Chongqing Municipality.Combined with regional data,this study clarifies the origin and tectonic background of volcanic ash at the Ordovician–Silurian interval in the Yangtze region,reveals the connection between volcanic activity and global climate change and marine ecological crises during this period.Additionally,the study discusses the potential correlation between volcanic activity and anoxia events,as well as the mechanisms by which volcanic activity affects the formation of high-quality source rocks.The main findings are as follows:（1）The parent magma types of the volcanic ash in the Yangtze block are mainly intermediate to acidic,with only a small amount of basic magma.They are highly consistent with the contemporaneous volcanic rocks found in the Cathaysian,which are dominated by dacite,andesite and rhyolite,with minor amounts of basalt.Most of the volcanic ash has a similar zircon Hf isotope composition to the volcanic rocks,indicating that it mainly originated from volcanic eruptions in the Cathaysian block.The parent magma of the volcanic ash is characterized by arc magma geochemistry,indicating that it formed in a subduction-related tectonic setting.These findings support the subduction-collision orogeny model during the early Paleozoic in South China.（2）The high-frequency volcanic ash layers in the Muchanggou section are concentrated in the graptolite P.pacificus（Lower Subzone）biozone of the Katian.Zircon U-Pb dating of the volcanic ash indicates that the intense volcanic eruptions occurred at～446Ma.During this period,a negative shift in organic carbon isotope and sedimentary Hg anomalies occurred synchronously,and similar carbon-mercury cycling perturbation records were observed at both regional and global scales.The Hg emitted from the volcano was rapidly sequestered by volcanic ash deposition,without exhibiting toxic impacts on the marine ecosystem.Based on volcanic Hg flux and carbon isotope mass balance calculations,it is estimated that more than 1.1×10¹³ tons of CO₂were emitted from the Katian volcanic activity in the South China,which interrupted the growth of glaciers and rapid global cooling.After the volcanic activity weakened,the massive CO₂ released by the volcano accelerated the surface silicate weathering,causing further global temperature decline,eventually leading to the Hirnantian glaciation.Both regional and global biodiversity declined rapidly with the end of volcanism,which suggests that the marine ecological crisis was related to a series of disruptions in biogeochemical cycles in the post-volcanic period.（3）During the Ordovician–Silurian transition,the marine sedimentary environment in the Upper Yangtze region underwent frequent fluctuations between oxidation,anoxia,and euxinia.The intense volcanic eruption during the Katian（～446 Ma）enhanced the sulfate supply to the seawater,but the frequent deposition of volcanic ash disrupted the water,making it unfavorable for euxinic conditions to form.The persistent anoxia during the post-volcanic period led to rapid depletion of sulfate in seawater,which limited the development of euxinic environment.At the end of Katian and the late Hirnantian,two large-scale euxinic water expansions occurred in the Upper Yangtze Sea,both of which were synchronous with volcanic eruption events.Molybdenum isotope analysis shows that the development of euxinia in the late Katian was a local event,which could be linked to the enhanced sulfate supply caused by volcanic eruptions under the ferruginous conditions（insufficient sulfate supply）.The development of euxinia in the late Hirnantian was a global marine deoxygenation event.The marine Mo cycle mass balance model calculation shows that this event led to an increase in the euxinic seafloor area by at least 16 times compared to the present day.The volcanic activity in South China during the Hirnantian a significant trigger for this global marine deoxygenation event.（4）The organic matter accumulation in the Wufeng-Lungmachi formations in the study area was influenced by the comprehensive effects of sedimentation rate,ancient ocean productivity,and redox conditions.The transient"fertilization"effect formed by the leaching of nutrients during volcanic ash deposition has a limited contribution to the enrichment of organic matter.Frequent volcanic ash deposition may repeatedly interrupt the biogeochemical cycle process,making it difficult to establish a relatively stable water column chemistry and microbial community structure,which hindered the formation of high-quality hydrocarbon source rocks.After the volcanic activity weakened,the surface nutrient supply increased,and the productivity and bottom water anoxia improved significantly,leading to highly enriched organic matter in the rock.Climate change driven by volcanic activity could affect the organic matter enrichment process over millions of years,and the post-volcanic phase was an important period for the formation of high-quality hydrocarbon source rocks.