Nitrogen Cycle And Marine Environment During The Late Ediacaran-early Cambrian In South China

The Ediacaran-Cambrian transition is a profound period,involving significant perturbations of marine environment alongside rapid diversification of metazoans.The Yangtze block in south China crops out in well-preserved strata from late Neoproterozoic to early Cambrian,with abundant fossil records.In order to understand the co-evolution of the palaeo-ocean environment and biological innovations during this key geological time,in this thesis we selected five representative sections on the Yangtze block,followed by muti-proxy geochemical investigations.We report organic isotopic data(?13Corg)to build up a standard chronostratigraphy for basinal facies,analysis the redox sensitive trace element concentrations(Mo,U,V)and total nitrogen isotopic data(?15Nbulk)to reconstruct marine redox structure,assess the degree of paleohydrographic restriction,and evaluate the evolution of the global marine oxygenation.Based on a combination of ?13Corg data from this study,published ?13Corg data from studies on contemporaneous sections,fossil records and zircon U-Pb dating on the Yangtze block,the Ediacaran-Cambrian boundary could be placed at the negative ?13Corg excursion(BACE)in the upper Liuchapo Formation of Hunan and Guizhou Provinces,and at the top of Piyuancun Formation of Zhejiang Province.The boundary of Cambrian Fortunian Stage and Stage 2 is uncertain due to the condensed strata,could probably be placed at the positive ?13Corg excursion(ZHUCE)in the top Liuchapo Formation or lowermost Niutitang Formation.The boundary of Cambrian Stage 2 and Stage 3 could be above the Ni-Mo enriched layer and the negative ?13Corg excursion(SHICE)at the lower part of the Niutitang/Xiaoyanxi/Hetang Formation.The higher part of the Niutitang and Hetang formations belong to the Cambrian Stage 3,where ?13Corg exhibit a positive excursion(CARE).However,it is unclear whether or not early Stage 4 was covered.For the Xiaoyanxi Formation in Hunan Province,the lower-middle part the formation belongs to the Cambrian Stage 3,evidenced by a a positive ?13Corg excursion(CARE).Upward,another positive ?13Corg excursion(MICE)suggests that the middle Xiaoyanxi Formation belongs to the early Cambrian Stage 4.At the middle-upper part of the formation,?13Corg shows another negative excursion(AECE)in the middle Cambrian Stage 4.Then the upper part of the formation belongs to the late Cambrian Stage 4.It is not certain if the top of the formation covers the end of the Cambrian Stage 4.The negative?13Corg excursions are generally associated with the major animal extinction events,such as the extinction of Ediacaran fauna,some small shelly fossils(SSFs)and Archaeocyathids,probably owing to the upwelling of fatally 12C-enriched anoxic bottom water into the shallow water realms during the worldwide transgressionRedox sensitive trace element(Mo,U,V)concentrations in the sediments suggested widespread anoxic bottom water on the Yangtze block during the early Cambrian.Mid-depth euxinic waters which sandwiched between oxygenated surface water and ferruginous bottom water demonstrates variable evolutional stages.During the Cambrian Fortunian Stage-early Stage 2,low Mo enrichments(<30 ppm)and moderated U and V enrichments([U]=10-60 ppm;[V]=200-600 ppm)suggest suboxic to non-sulfidic anoxic condition and mid-depth euxinia may have not emerged.In the late Cambrian Stage 2,high Mo,U and V enrichments([Mo]and[U]over hundreds ppm;[V]over thousands ppm)indicate widespread euxinic bottom water and expanded mid-depth euxinia from shelf margin to basinal areas.In the early Cambrian Stage 3,less Mo,U and V enrichments([Mo]=50?100 ppm;[U]=10?60 ppm;[V]=200?4000 ppm)than the late Stage 2 imply suboxic to intermittently euxinic bottom water,suggesting mid-depth euxinia gradually shrank.By the time of late Cambrian Stage 3,there were not Mo,U and V enrichments in the shelf margin area,suggesting oxic bottom water.However,high Mo,U and V enrichments([Mo]=100?500 ppm;[U]=20?200;[V]=200?15000ppm)suggest euxinic bottom water in the basinal area at this period.The mid-depth eunixic waters expanded again at this time,but not influenced the shelf margin yet.In the early Cambrian Stage 4,less Mo,U and V enrichments([Mo=2?60 ppm;[U]=3?60 ppm;[V]=300?2500 ppm]in the sediment of the basinal area than the earlier time indicate suboxic to intermittently euxinic bottom water and the retreat of mid-depth euxinia.Later in the late Cambrian Stage 4,high Mo,U and V enrichments([Mo]and[U]over hundreds ppm;[V]over thousands ppm)in the sediment of the basinal area reflect that the bottom water became euxinic and mid-depth euxinia expanded again,although its impact on the shallow platform and shelf margin areas are still unclear.In general,the mid-depth euxinic waters in the basin gradually shrank along with the increasing oxygenation of the early Cambrian ocean.The Mo-U covariation patterns of the early Cambrian sediments on the Yangtze block exhibit marked enrichments in both Mo and U and nearly constant Mo/U ratios between?1×SW and-3×SW,suggesting a weakly restricted basin.The shallow water of the basin was well connected with the open ocean,while the deep-water mass suffered from limited aqueous Mo depletion.The relatively constant Mo/U ratios between?1×SW and?3×SW imply no significant variations for the deep-water restriction since the Cambrian Stage 2 transgression.Although Mo/U ratios declined below?1×SW,indicating enhanced deep-water restriction during the early Cambrian Stage 4 regression,the following transgression increased Mo/U ratios back to?1×SW-3 xSW,and hence recoverd weakly deep-water restriction.The Mo/TOC ratios in the euxinic sediments decrease from?15 at Sancha section in the shelf margin area to?3-4 at the Yuanjia and Huitong sections in the basinal area,reflecting aqueous Mo depletion in the deep watermass owning to the weakly paleohydrographic restriction.On the other hand,sedimentary Mo/TOC ratios in Yuanjia and Huitong sections increased from?3-4 in the Cambrian Stage 2 to?15 in the Stage 4,suggesting increasing aqueous Mo concentrations in the water column.In the context of invariable weakly deepwater restriction,the open ocean Mo reservoir became larger from the Cambrian Stage 2 to Stage 4,implying increasingly oxygenated environment for the radiation of Cambrian fauna.Given that the nitrogen cycle is not only closely related to marine environmental conditions and biological metabolism,but also reflects the ocean redox structure,we carried out a comprehensive study of bulk nitrogen isotopes from four representative sections in this thesis and published data from previous studies on contemporaneous sections on the Yangtze block.During the time interval from the late Ediacaran to the Cambrian Fortunian Stage,?15Nbulk range from+2?+6‰,suggesting considerably oxygenated environment and nitrate was stable in the seawater.In the latest Ediacaran,some ?15Nbulk analyses shifted towards lower values between-2?+4‰.N2-fixation and ammonia assimilation became dominant process,indicating expanded anoxic condition and shoaling chemocline possibly into the photic zone in the basinal area.A large range of ?15Nbulk values(-2?0‰ in the slope-basin sections and-1?+9‰ in the shallow-platform section)were recorded at the end of Fortunain Stage and early Stage 2,reflecting that the dominant metabolic process shifted between N2-fixation,ammonia assimilation and denitrification.This variation could imply that deep-water anoxia intermittently expanded to the shallow water realms,resulting in the unstable chemocline in the water column.In the Late Cambrian Stage 2,?15Nbulk showed a large negative shift,ranging-6?+2‰.Ammonia assimilation and N2-fixation were the dominant process of the N cycle,suggesting that vigorous upwelling transported anoxic bottom water to the shallow water realms and shoaling chemocline into the photic zone.In the Cambrian Stage 3,?15Nbuik range from 0?+6‰,suggesting considerably oxygenated environment and nitrate was stable in the seawater.Hospitable environment and sufficient nutrient(NO3-)could have made the foundation for the radiation of Cambrian fauna.In the late Cambrian Stage 3,the lower ?15Nbulk values between-3‰ and+3‰ suggest N2-fixation and ammonia assimilation were the dominant processes,reflecting expanded anoxic condition and shoaling chemocline again,possibly into the photic zone in the basinal area.During the Cambrian Stage 4,?15Nbulk values lie between+4?+6‰ in the transitional zone,reflecting considerably oxygenated environment and high nitrate concentration in the seawater.Hospitable environment and sufficient nutrient(NO3-)benefit the flourish of Cambrian fauna,such as trilobites.However,for the basinal Huitong section during this period,?15Nbuik values were mainly around 0‰ with a negative shift towards?-3‰ in the middle Stage 4.N2-fixation was the dominant process,with occasional ammonia assimilation,suggesting expanded anoxic condition and shoaling chemocline sometimes into the photic zone.This anoxia did not influence the shallow water in the transitional zone.In summary,during the late Ediacaran-early Cambrian period,the open ocean has been considerably oxygenated,and seawater nitrate concentration was high,accommodating the Cambrian fauna.The occasionally anoxic events in the photic zone of the Yangtze basin could be linked with the mass extinctions of Ediacaran fauna,SSFs and Archaeocyathids.