Late Ordovician Paleoceanographic Changing Process

The first occurrence of Phanerozoic biological extinction events is during the Late Ordovician,marine ecosystems and biodiversity are greatly affected by this process.The genetic mechanism of Late Ordovician mess extinction?LOME?has been one of the focus in geological research.It is generally accepted that glacial events and widespread anoxic event were the main reason of the LOME.However,almost all study of the ocean anoxic event during Ordovician were focus on the redox condition variations of 1-2 Ma before the end of Ordovician,little concern about the redox condition of Sandbian-Katian.The later has provided important clues for full evaluate of the redox condition changing regularity and the effect of extreme changes on biology evolution.In the meantime,it offers a forceful evidence for regular pattern of ocean chemical condition variations from Ordovician Radiation to LOME.Here,this study selected Dawangou section?Northwest Tarim?and Puxihe section?middle Yangtze region?as a research object,use size distribution of pyrite framboids,sedimentary facies analysis,carbon and strontium isotope geochemistry and redox sensitive elements analysis to reconstruct the redox condition verations of the paleo-ocean during this period,discuss the effect of environmental changes on marine organism.The Dawangou section became the auxiliary global stratotype section and point?ASSP?for the base of the Upper Ordovician?Sandbian?in 2001,with a well exposed Ordovician sedimentary,and was intensively studied on stratigraphy and paleontology.We performed a high-accuracy carbon and strontium isotope analysis from Kanling to Kepingtage Formations in the Dawangou cross section,?¹³C,?¹⁸O and ⁸⁷Sr/⁸⁶Sr curves are reconstructed.The evolution of strontium isotope in Tarim shows a good consistency with the global carbon isotope evolution,this indicate the section can be well comparable globally.The ages of the Upper Ordovician Kanling,Qilang and Yingan Formations boundaries in the Dawangou cross section are recognized by stratigraphic correlation:Qilang-Yingan boundary is 450.6 Ma,Yingan-Terekawat boundary is 450 Ma and Sandbian-Katian boundary is 453 Ma.At the meantime,the global Guttenberg isotope carbon excursion?GICE?and the Sandbian-Katian boundary were recognized.The most important is the GICE in our section is not only one peak during Sandbian-Katian boundary but three-stages.According to the peaks of GICE,it can be devided into G??G?and G?.This feature coincides with the carbon isotope curve of Puxihe section?middle Yangze region?during Late Ordovician.This research also focuses on size statistics of pyrite framboids and redox sensitive trace elements from Dawangou section and Puxihe section,reconstruct the redox history of Late Ordovician.The mean diameters and maximum diameters in our study are in inverse proportion to the ratio of redox sensitive trace elements,and are in directly proportion to the carbon isotope.These indicate that the redox histary experienced four periods during Late Ordovician in northwest Tarim and middle Yangze region:Sandbian-early Katian,there are five short-term euxinic-anoxic events,but the oxic period is longer than euxinic-anoxic;early Katian,there are short-term cycles of euxinic-anoxic-dysoxic-oxic,a long term euxinic events appeared in the bottom;early Katian-middle Katian,there are less frequency of the redox condition variations;middle Katian shows sustainable and stable euxinic event,during this interval,seawater became shallow because of marine regression,it is indicate the sulfidic water masses intrusion into the euphotic zones.This imply that the euxinic condition might be one of the main genetic mechanism of LOME.In addition to the above-mentioned contents,this study first report about microspherules from the Upper Ordovician in Dawangou section of northwestern Tarim,China.These microspherules are greatly different from previous study in distribution and chemical composition.Microspherules are divided into five major types:iron microspherules,calcareous microspherules,siliceous microspherules,titaniferous-iron microspherules and organic microspherules.Siliceous and titaniferous-iron microspherules are deposited continuously from the Qilang Formation.Based upon their surface morphology and chemical composition these microspherules are proposed as volcanic material derived from the surrounding volcanic centers in the Late Ordovician.Finally,a seriers important geological event such as enhanced siliciclastic influx,volcanism-hydrothermal activity and paleoproductivity variations have been recognized by geochemical elements analyse.Detrital siliciclastic proxies including contents of Hf?Zr?Th??REE and Eu*indicated enhanced siliciclastic influx across Middle Katian.The changes of siliciclastic influx have been through three stages:enhanced in repeated fluctuation,stable low and rapidly enhanced.The enhanced siliciclastic influx perhaps induced by intense volcanism or sea level drop caused by glaciation.Besides,all samples in this section show strong positive anomalies in Eu,the trend from strong to weak,that indicated the influence of volcanism decreased gradually.This result is consistent with the distribution of volcanic microspherules.This result suggests the volcanism and euxinic events are the key genetic mechanisms of the LOME.And the other geological events?enhanced siliciclastic influx,volcanism-hydrothermal activity and paleoproductivity variations?might have connection with the mass extinction.In summary,the volcanism and euxinic events happened in middle of Late Ordovician.Long-term environmental degradation maybe the main reason for LOME,and weathering increasing,volcanism-hydrothermal activity and paleoproductivity variation may be connected with mass extinction,but need further study.