Paleoenvironmental Reconstruction In The Middle-Upper Yangtze Block During The Late Ordovician To Early Silurian Time

In thesedimentary record,key elements-enriched marine sedimentsrecorded the paleo-environmentchange.Since the late Paleozoic,the enrichment process of most elements in the ocean is usually controlled by both sedimentary environment and diagenetic conditions.Therefore,the biogeochemical cycles of Si,C_org and Ba are closely related to marine productivity,redox environment and feedback with climate change.Understanding their enrichment processes and the details of their relationships are the key to the evolution of the Earth’s oceans in deep time.During the Late Ordovician Early Silurian period（O-S transition period）,the Yangtze Sea was bounded by the Yichang underwater uplift,with the open Yangtze Outer Sea connected to the ocean to the north and the Yangtze Inner Sea enclosed by the peripheral uplift to the south.During this period,geological events in the Yangtze Sea occurred frequently,such as the Gondwana Glacier movement,large-scale ocean anoxia,biological extinction,volcanic activity,upwelling currents,and Kwangsian Orogeny.In this regard,The Wufeng-Longmaxi Formations siliceous and organic-rich shales were deposited in the Yangtze Sea.Moreover,barium-rich intervals are wildly distributed in the Longmaxi Formation.Clarifying the relationship between geological events,ancient marine environment,and element enrichment can reveal the details of ancient marine evolution.More importantly,previous studies have mostly focused on the co-evolution of ancient marine environments and organisms,and there is still a lack of comprehensive and systematic research on the enrichment process of Si,Corg,and Ba elements in the Yangtze Sea during this period.Therefore,the XY-1 well in the inner Yangtze Sea,Caijiacao,Huolongping sections,and Mingtong and Bailu sections in the Outer Yangtze Sea were selected to carry out detailed sedimentological and geochemical research to obtain the details of the enrichment process of silica,organic and barium,and their implications.（1）Three mainlithofacies can be identifiedin the Wufeng-Longmaxi Formationsinthestudiedsections:the deepwater shelf siliceous shale+carbonaceous shalefacies;theshallowwatersiltyshale+siltstonefacies;The shallow shelf calcareous siltstone+sandy limestone+bioclastic limestone facies.The microtopography pattern of the Yangtze Sea controlled the spatiotemporal distribution of lithofacies in the Wufeng Longmaxi Formation.Due to the influence of peripheral tectonic activities,the sedimentary patterns in the outer and inner Yangtze Sea are slightly different,manifested by differences in the thickness of various lithofacies.The temporal and spatial evolution of the Wufeng-Longmaxi Formation lithofacies in the Yangtze Seamayrespond to the global climate and ocean-regional tectonic processes.（2）The silica enrichment in the Wufeng Longmaxi Formation was controlled by upwelling events caused by climate change during the O-S transition.A chain of evidence indicates that the silicawas mainly of biologicalorigin.The extremely high marine productivity is the main reason for the enrichment of biogenic silica.The elements and biological characteristics of the studied sections are similar to those of the modern upwelling region,the Cariaco Basin,reflecting that the upwelling activity is the main cause of biogenic silica enrichment.The hydrogeologic condition may affect the range of upwelling currents.During the O-S transition period,frequent climate transition seems to be the main driving factor for the upwelling events in the Yangtze Sea.The activity range ofupwelling controlled the spatiotemporal distribution of silica deposition.（3）The enrichment of organic matter in the Wufeng Longmaxi Formation cannot be generalized and is the result of multiple stages and factors.Taking the XY-1 well in the inner Yangtze Sea as an example,the organic-rich process canbedivedinto five stages with different driving factors.The average TOC in the first stage is 3.28%,and the seasonal upwelling-driven productivity increase and anoxic seawater seem to be the first driving factor.The average TOC in the second stage is 4.93%.The ocean P cycle may have maintained high productivity and anoxicwater during this stage.The TOC of the third stage is 2.80%.In this stage,the bottom water is suboxic,and the marine productivity decreased significantly.The fall in sea level driven by the cold climate is the main reason for the decrease in organic matter content during this stage.The average TOC in the fourth stage is 5.53%.The presence of volcanic ashes and the enrichment of Hg elements in the samples at this stage indicate that volcanic ash promoted marine productivity.The average TOC in the fifth stage is 1.71%.This stage may be related to the sea level decline caused by Kwangsian Orogeny.Comparing studies of different geographical locations,there is a certain degree of spatial heterogeneity in the redox conditions and productivity levels in the Yangtze Sea.This may also be the reason for the regional differences in organic matter enrichment mechanisms and history.（4）The mineral composition,barite crystal morphology,and related isotopes in the barium-rich layer of the Longmaxi Formation are similar to the mineralization near the sulfate methane transition zone（SMTZ）in modern marine sediments,recording methane activity in the sediments.The formation process of barium-rich layers indicates the evolution history of SMTZ in sediments,manifested as small-scale fluctuations of SMTZ in sediments.Before the formation of barium-rich layers,high marine productivity was crucial for the accumulation of biological barium in sediments.The oxygenationof bottom water may promote the formation of barite in a diagenetic environment.On the other hand,the organic sulfate reduction（OSR）and sulfate-driven methane oxidation（SD-AOM）near SMTZ in the diagenetic environment have an impact on theδ³⁴S_pyof Longmaxi Formationshale.Therefore,utilizing the bulk rockδ³⁴S_pyto reconstruct the sedimentary environment needs to judge the origin of pyrite.（5）The silica,organic matter,and barium enrichmentprocesses can provide details for the marine environment during the O-S transition period.The enrichment of biogenic silica records the frequent activity of upwelling in the Yangtze Sea.It exhibits strong upwelling activity during climate transitions（Hirantanian and Aeronian periods）.On the contrary,during the greenhouse period of the deposition of the Longmaxi Formation（Rhuddanian）,the upwelling events significantly weakened from the outer to the inner sear.On the other hand,due to the obstruction of the underwater Yichang highlands,the upwelling seems to be limited in its activity in the inner Yangtze Sea.In addition,the upwelling also promoted the high flux of C_organd biological Ba to the sediment.However,organic matter accumulation is also controlled by the development of anoxic water and the input of volcanic ashes.With the uplift of the Yangtze basement in the late Rhuddanian andearly Aronian period and the pulse activity of glaciers,the regional sea level dropped,and the bottom water was generally in a low oxygen oxidation environment.It may result in diagenetic barite formation above SMTZ in sediments.