The Evolution Of Paleoceanic Conditions In Early Cambrian Niutitang Formation,Yangtze Platform

The Ediacaran–Cambrian transition is a critical period in Earth history,during which both marine environment and life had experienced drastic changes.It is arguably suggested that pervasive oxygenation and associated geochemical changes in the ocean had potentially driven the rapid diversification of Early Cambrian metazoans,the timing and detailed process of ocean oxygenation,however,have not been well constrained.In this dissertation,an integrated study was conducted on the lower Cambrian Niutitang Fomation（NTT）in multi-sections,including Siduping,W.Hunan,Hejiapu,NE Guizhou and Dalongtan sections,SW Hunan,which were situated through the shelf margin to upper slope–deep basin settings of the Yangtze Block.In combination with the data published previously from other sections of the same period from south China,integrated studies of systematic pyrite morphology,organic carbon and nitrogen isotopes,and multi-proxy geochemical analyses were conducted on theblack shales from the Lower Cambrian NTT,to further constrain the paleoceanic redox conditions,temporal and spatial evolutionary trends,and their relationships with the early metazoan diversification.Based on the study of RSEconcentrations and the relationships between Mo and TOC,Mo_EF and U_EF,Siduping,Hejiapu and Dalongtan sections were not seriously affected by the restriction,they were possibly connected to the open ocean directly.The abundance of framboidal pyrites,enrichment of RSEs,TOC,Fe,Mn,S,and relatively ratiosindicated bottom seawaters experienced a complicated evolution,with a dynamic alternation of three ferruginous intervals with few pyrite in Siduping section.The bottom seawaters experienced a secular and persistent euxinic period in the lower and middle NTT of Hejiapu section,and in the upper part of which,turned to a ferruginous–suboxic conditions.Dalongtan section was located in the deep basin where the bottom seawaters were frequently fluctuated between ferruginous and euxinic conditions.The TOC and C:N:P molar ratios both decrease in the NTT of Siduping section and Hejiapu section while the C:N:P ratios approached to the redfield ratio of the normal seawater.Bothδ¹⁵N andδ¹³C were high in the bottom,and increased again after decreased in the middle part of Hejiapu section.These geochemical index indicated that the ocean in early Cambrian was increasingly oxidizing,while the primary productivity and terrigenous inputs enhanced relatively in the shelf–margin and upper–slope area.The deep water was dominated by anoxic condition with fluctuated euxinic condition.The ocean redox structure on the early Cambrian Yangtze Platform likely showed temporally and spatially heterogeneity.The onshore area in east Yunnan may become oxic in the early Cambrian Stage 3 when offshore area was still dominated by alternating ferruginous and oxic conditions,oxic environments was not achieved until the late Cambrian Stage 3.Ferruginous and euxinic conditions coexisted in outer shelf area at the early stage of the Niutitang Formation,which changed to suboxic–oxic environment at the late stage.The occurrence of oxic condition in the shelf margin–upper slope may postdate that of the shelf,possibly at the latest Cambrian Stage 3following alternating ferruginous and euxinic conditions for most time of the Niutitang Formation.Persistent ferruginous conditions with occasionally euxinia prevailed deep basin throughout the Cambrian Stage 2 to Stage 3.This stepwise oxygenation from shelf to deep basin during the Cambrian Stage 2–Stage 3 implies an expansion of oxic environment and the deepening of chemocline,providing evidence for the rise of oxygen level during the early Cambrian.Our data of the Siduping and Hejiapu sections in combination with previously published data from other sections of the Lower Cambrian,showed strong Mo enrichments and high Mo/TOC ratios in the euxinic black shales of NTT.The early Cambrian oceanic Mo concentration was estimated as⁵0 nM,which was about half of the modern ocean Mo concentration,this might further imply that the minimum oxygen level of early Cambrian may be 50%PAL.The increasing trend of Mo concentrations and Mo/TOC ratios in the NTT of Siduping,Hejiapu and Dalongtan sections also suggested the early Cambrian oxygenation may be a unidirectional process towards stable ocean oxygenation as we saw today.Metazoan fossil records from the lower Cambrian showed temporal diachroneity and spatial heterogeneity possibly due to different oxygen demand of different taxa.Sponges and worms which had low oxygen demand were the earliest metazoan fossils known from the early Cambrian of the Yangtze Platform.The occurrence of more diversified taxa during the Cambrian Stage 3 were broadly synchronous with the enhanced oxygenation.As a bio-essential element,Mo is important for many bio-enzymes,particularly for the dinitrogenase.Widespread anoxic–euxinic environments in early ocean severely depleted seawater Mo and accordingly limited biological N fixation,suppressing the animal evolution.Thus,the enhanced seawater oxygenation and elevated micronutrients such as Mo may have played important roles in triggering the“Cambrian Explosion”.